Volumetric, acoustic, viscometric, calorimetric and spectroscopic studies to elucidate the effects of citrate and tartrate based food preservatives on the solvation behaviors of acidic amino acids at different temperatures

Aashima Beri , Gagandeep Kaur , Palwinder Singh , Parampaul K. Banipal and Tarlok S. Banipal *
Department of Chemistry, UGC Sponsored Center for Advanced Studies-1, Guru Nanak Dev University, Amritsar-143005, Punjab, India. E-mail: tsbanipal@yahoo.com

Received 25th April 2019 , Accepted 20th November 2019

First published on 20th November 2019


Abstract

The effects of trisodium citrate (TSC) and disodium tartrate (DST) based food preservatives on the hydration behaviors of the amino acids L-aspartic acid (ASP) and L-glutamic acid (GLU) have been studied using thermodynamic, transport, calorimetric and spectroscopic studies. The volumetric, acoustic and viscosity data suggest that hydrophilic–ionic/hydrophilic interactions are predominant in these systems. The calculated parameters are worthwhile for exploring the solutes as structure-breakers, and the solutes undergo pairwise interactions with the co-solutes. The sweetness of both amino acids decreases in the presence of the preservatives. The hydration number and solvation data suggest that these solutes are more hydrated in water. The dominance of dehydration effects in relation to TSC is observed from the positive enthalpy changes in calorimetry studies and the negative chemical shifts in 1H NMR studies.


1. Introduction

Food preservation is carried out to maintain the quality and physicochemical properties of raw food.1 Preservatives are generally additives, which enhance the life spans of foods and drinks through preventing microorganism growth.2 As legislation has restricted the use and the levels of some preservatives in different foods, these preservatives must be approved by food safety authorities.3 Organic acids (citric and tartaric acid) are traditional preservatives. Organic acids are added to foods as acidulants, flavorants, or preservatives, inactivating the growth of spoilage microorganisms and foodborne pathogens.4 Citric and tartaric acid show various biological activities. They exhibit metal ion complexing properties with amino acids and, thus, they are preferably used for some common applications in food and agricultural industries as sequestrants, antioxidants, etc. The use of disodium tartrate (DST) and trisodium citrate (TSC) as food additives has been approved by the European Food Safety Authority. They have been assigned with the E numbers 335 and 331, respectively, on the list of antioxidants. It has been mentioned in the literature that sodium salts of citrate and tartrate have been used as anticaking agents, flavour enhancers, etc.5,6 Salts are effective preservatives because they reduce the water activity of foods. As well as their food-related value, DST can extend the range of plant growth promoters and TSC can help in the production of soil amendments for organic crop production.

L-Glutamate and L-aspartate are key molecules in cellular metabolism. Dietary proteins are broken down in humans through digestion into amino acids, which act as metabolic fuel for other functional activities in the body.7 Casein, lactalbumin, Promine-D (soy protein), and wheat gluten contain significant amounts of racemized L-aspartic acid (ASP) and L-glutamic acid (GLU).8 GLU is an important nonessential amino acid that acts as a neurotransmitter. It is not considered to be an essential nutrient because it can be manufactured in the human body from simpler compounds. GLU is a building block in protein synthesis. This dicarboxylic amino acid is available in a wide variety of foods and is an important molecule for the metabolism of sugars and fats.9 It is also used to enhance the flavour of foods.10 ASP is used for the generation of adenosine triphosphate, which is a required fuel for cell activity.11 Water is ubiquitous in biochemical processes because its presence generates hydrophobic forces.12 The physicochemical properties of studied solutes have been used to provide insights into hydrophobicity, hydration properties and solute–solvent interactions.13 The use of citrate and tartrate based salts, along with other additives, not only makes flavours sharper but also improves the stability of food products in many cases. The present report aims to study the solvation and taste quality behaviors of acidic amino acids (ASP and GLU) in the presence of citrate (TSC) and tartrate (DST) food preservatives.

2. Experimental

2.1. Materials

All the chemicals used in the present study are of high-purity analytical grade. The purity of the studied chemicals is ≥99%. Details relating to the chemicals are given in Table 1. Before using the chemicals, they have all been dried in vacuo over anhydrous CaCl2.
Table 1 Sources and solubilities of the chemicals used
S. no. Compound Structure Molar mass (g mol−1) Source Puritya CAS number Solubility in watera (mg ml−1)
a As provided by the supplier.
1 L-Aspartic acid image file: c9fo00872a-u1.tif 133.10 Siso Research Laboratories, India 99% 56–86–0 ≈4.5
2 L-Glutamic acid image file: c9fo00872a-u2.tif 147.13 Siso Research Laboratories, India 99% 2419–94.5 ≈7.5
3 Di-sodium tartrate image file: c9fo00872a-u3.tif 230.08 Central Drug House, India 99% 6132–24–7 ≈290
4 Tri-sodium citrate image file: c9fo00872a-u4.tif 294.10 Siso Research Laboratories, India 99% 6132–04–3 ≈92


2.2. Equipment and procedures

All stock solutions have been prepared using deionised, double-distilled and degassed water with a specific conductance less than 1.3 × 10−4 S m−1. The solutions were prepared on a mass basis using a Mettler balance (Model: AB265-S) with a precision of ±0.01 mg. The standard uncertainty in molality is ±2.8 × 10−4 mol kg−1.
2.2.1. Density and speed of sound. The densities and speeds of sound were measured simultaneously using a vibrating-tube digital density and sound velocity meter (DSA 5000M, Anton Paar). The two-in-one instrument is designed with two cells, i.e., a density cell and a sound velocity cell. Both cells have a set temperature, adjusted using a built-in Peltier thermostat (PT-100) having an accuracy of ±0.001 K. The speed of sound is measured via a propagation time technique. Calibration adjustments were performed at 293.15 K with triple-distilled degassed water, and with dry air at atmospheric pressure. The uncertainties in the temperature, density and speed of sound measurements are 0.03 K, 0.06 kg m−3 and 0.6 m s−1, respectively.
2.2.2. Viscosity. Viscosities, η, were determined using a suspended level Ubbelohde-type capillary viscometer having temperature stability within ±0.01 K (model: MC 31A Julabo/Germany). Flow time measurements were done using a stopwatch with a resolution of ±0.01 s. The viscometer has been calibrated with deionized, double-distilled and degassed water. The efflux time data for water was recorded at T = 288.15, 298.15, 308.15 and 318.15 K. Final efflux times are the means of at least six readings. The standard uncertainty in viscosity, u(η), on an average basis is ±0.012 mPa s. The reliability of the apparatus was checked through measuring the viscosities of aqueous solutions of glycine at T = 298.15 K, and the results agreed very well with the literature values.14 The viscosities of pure water have been taken from the literature.15
2.2.3. Enthalpy of dilution. An isothermal titration calorimeter, (iTC200, MicroCal USA) was used to perform calorimetric titrations at 298.15 ± 0.005 K for the determination of enthalpy of dilution values. 100, 250, and 500 mM DST/TSC solutions were added to a sample cell with a capacity of 200 μl and the syringe was filled with 40 μl of 30 mM ASP/GLU. A total of 19 injections of 2 μl of ASP/GLU was titrated into the sample cell containing DST/TSC and the contents of the cell were stirred at a fixed speed of 500 rpm to ensure complete mixing. The time between successive injections was 120 s, which was managed using the software provided with the instrument. The standard uncertainty in the enthalpy values is ±0.5 kJ mol−1.
2.2.4. Chemical shift measurements. 1H NMR spectra were obtained using a Bruker spectrometer (AVANCE-III, HD 500 MHz) with a probe temperature of 300.15 K. The centre of the HDO signal (4.650 ppm) is considered as the internal reference for the other nuclei. NMR spectra of ASP and GLU in water and in the presence of aqueous solutions of DST and TSC (mB = 0.1–0.75 mol kg−1) have been studied in 9[thin space (1/6-em)]:[thin space (1/6-em)]1 (w/w) H2O–D2O solution. From these spectra, the chemical shift changes (Δδ) were recorded.

3. Results and discussion

3.1. Volumetric measurements

3.1.1. Apparent molar Volumes, V2,ϕ. The densities, ρ, of the studied amino acids (ASP and GLU) as a function of molality (mA) in water, and in aqueous solutions of DST and TSC at mB (molality of DST/TSC) values of 0.10, 0.25, 0.50, and 0.75 mol kg−1 over a temperature range from 288.15 to 323.15 K are given in Table 2. The ρ values directly rise with the mA and mB values and decrease with T. The ρ values of aqueous DST and TSC solutions are in good agreement with the literature values,16–23 and a comparison is shown graphically in Fig. 1. The density values of aqueous solutions of ASP and GLU are also in line with the literature values9,24–28 and are represented in Fig. 2. The variations in the ρ values of ASP in aqueous DST solutions at different temperatures are shown in Fig. 3(a) (density, ρ, versus molality, mA, for ASP in DST at mB = 0.1 mol kg−1 as a function of temperature). In the present study, apparent molar volumes, V2,ϕ, have been calculated from the experimentally measured densities using the relation:
 
V2,ϕ = {M/ρ} − {(ρρ0)/(mAρρ0)}(1)
where ρ and ρ0 are the densities of the solution and the solvent (H2O or H2O + DST/TSC), M is the molar mass, and mA is the molality of the solute. The V2,ϕ values are positive for the studied amino acids both in aqueous and in mixed aqueous solutions. The V2,ϕ values increase with the mA, mB and T values. The V2,ϕ values of the studied solutes in the absence, as well as in the presence, of aqueous solutions of DST and TSC at mB values of 0.10, 0.25, 0.50, and 0.75 mol kg−1 and at temperatures from 288.15 to 323.15 K are given in ESI Table S1. A representative plot of V2,ϕversus mA for GLU in aqueous TSC solutions at mB = 0.25 mol kg−1 and different T values is shown in Fig. 3(b).

image file: c9fo00872a-f1.tif
Fig. 1 (a) Plots of density, ρ, versus molality, mB, for aqueous solutions of TSC and comparisons with the literature values at different temperatures. (b) Plots of density, ρ, versus molality, mB, for aqueous solutions of DST and comparisons with the literature values at different temperatures.

image file: c9fo00872a-f2.tif
Fig. 2 (a) Plots of density, ρ, versus molality, mA, for aqueous solutions of ASP and comparisons with the literature values at different temperatures. (b) Plots of density, ρ, versus molality, mA, for aqueous solutions of GLU and comparisons with the literature values at different temperatures.

image file: c9fo00872a-f3.tif
Fig. 3 Representative plots of: (a) density, ρ, versus molality, mA, for L-aspartic acid in aqueous DST solutions (mB = 0.1 mol kg−1) as a function of temperature, T; (b) apparent molar volume, V2,ϕ, versus molality, mA, for L-glutamic acid in aqueous TSC solutions (mB = 0.25 mol kg−1) as a function of temperature, T; (c) speed of sound, u, versus molality, mA, for L-aspartic acid in aqueous TSC solutions (mB = 0.1 mol kg−1) as a function of temperature, T; and (d) viscosity, η, versus molality, mA, for L-glutamic acid in aqueous DST solutions (mB = 0.1 mol kg−1) as a function of temperature, T.
Table 2 Densities, ρ, of L-aspartic acid and L-glutamic acid in water and in aqueous solutions of di-sodium tartrate and tri-sodium citrate at T values from 288.15 to 323.15 K and P = 101.3 kPa
m A/mol kg−1 10−3·ρ/kg m−3
288.15 K 293.15 K 298.15 K 303.15 K 308.15 K 313.15 K 318.15 K 323.15 K
m A is the molality of the solute in water or water + DST/TSC. mB is the molality of DST/TSC in water. Standard uncertainties, u, are u(T) = 0.03 K, u(P) = 0.5 kPa, u(m) = 2.8 × 10−4 mol kg−1, u(ρ) = 0.06 kg m−3.
L-Aspartic acid in water
0.00000 0.999102 0.998224 0.997047 0.995670 0.994040 0.992238 0.990220 0.988069
0.00517 0.999445 0.998547 0.997360 0.995990 0.994375 0.992558 0.990553 0.988378
0.00712 0.999563 0.998665 0.997477 0.996108 0.994492 0.992674 0.990669 0.988494
0.00921 0.999690 0.998791 0.997602 0.996232 0.994617 0.992799 0.990794 0.988618
0.01109 0.999803 0.998904 0.997715 0.996345 0.994729 0.992911 0.990905 0.988729
0.01300 0.999918 0.999018 0.997829 0.996460 0.994843 0.993024 0.991018 0.988842
0.01506 1.000042 0.999141 0.997952 0.996582 0.994965 0.993147 0.991140 0.988964
L-Aspartic acid in an aqueous solution of di-sodium tartrate at mB= 0.1 mol kg−1
0.00000 1.014500 1.012800 1.010169 1.008856 1.006932 1.004950 1.002974 1.000950
0.00502 1.014789 1.013089 1.010457 1.009144 1.007220 1.005238 1.003261 1.001237
0.00741 1.014926 1.013226 1.010594 1.009280 1.007356 1.005374 1.003398 1.001374
0.01180 1.015178 1.013477 1.010730 1.009531 1.007607 1.005624 1.003648 1.001624
0.01306 1.015249 1.013548 1.010844 1.009603 1.007678 1.005695 1.003719 1.001695
0.01732 1.015492 1.013791 1.010916 1.009845 1.007921 1.005938 1.003960 1.001936
L-Aspartic acid in an aqueous solution of di-sodium tartrate at mB= 0.25 mol kg−1
0.00000 1.032450 1.030457 1.028594 1.026540 1.025089 1.023450 1.020973 1.018500
0.00584 1.032778 1.030784 1.028920 1.026866 1.025415 1.023775 1.021298 1.018824
0.00817 1.032908 1.030914 1.029050 1.026995 1.025544 1.023904 1.021427 1.018953
0.01106 1.033069 1.031075 1.029211 1.027156 1.025704 1.024064 1.021586 1.019112
0.01337 1.033198 1.031203 1.029338 1.027283 1.025832 1.024191 1.021713 1.019239
0.01555 1.033319 1.031323 1.029459 1.027404 1.025952 1.024311 1.021833 1.019358
0.01667 1.033380 1.031385 1.029520 1.027465 1.026013 1.024372 1.021894 1.019419
L-Aspartic acid in an aqueous solution of di-sodium tartrate at mB= 0.50 mol kg−1
0.00000 1.063200 1.061500 1.059301 1.057450 1.055350 1.053240 1.050971 1.048200
0.00638 1.063544 1.061843 1.059644 1.057793 1.055692 1.053582 1.051313 1.048542
0.00782 1.063620 1.061920 1.059721 1.057870 1.055769 1.053659 1.051389 1.048619
0.00982 1.063726 1.062027 1.059828 1.057976 1.055876 1.053765 1.051495 1.048725
0.01307 1.063899 1.062200 1.060001 1.058150 1.056049 1.053938 1.051667 1.048897
0.01509 1.064006 1.062307 1.060109 1.058257 1.056156 1.054044 1.051774 1.049004
0.01539 1.064074 1.062375 1.060178 1.058325 1.056225 1.054113 1.051842 1.049072
L-Aspartic acid in an aqueous solution of di-sodium tartrate at mB= 0.75 mol kg−1
0.00000 1.093900 1.091750 1.090008 1.087450 1.085611 1.083560 1.080970 1.078900
0.00547 1.094183 1.092032 1.090289 1.087731 1.085892 1.083841 1.081250 1.079180
0.00739 1.094282 1.092131 1.090387 1.087829 1.085990 1.083938 1.081348 1.079278
0.01025 1.094429 1.092278 1.090532 1.087975 1.086135 1.084084 1.081493 1.079423
0.01347 1.094595 1.092443 1.090697 1.088138 1.086298 1.084247 1.081656 1.079585
0.01664 1.094758 1.092605 1.090856 1.088298 1.086458 1.084407 1.081816 1.079743
L-Aspartic acid in an aqueous solution of tri-sodium citrate at mB= 0.1 mol kg−1
0.00000 1.017603 1.01651 1.015194 1.013658 1.011929 1.010035 1.007541 1.005738
0.00576 1.017926 1.016832 1.015515 1.013979 1.012250 1.010356 1.007861 1.006058
0.00754 1.018025 1.016931 1.015614 1.014077 1.012348 1.010454 1.007960 1.006157
0.01002 1.018164 1.017069 1.015752 1.014214 1.012485 1.010592 1.008097 1.006294
0.01189 1.018267 1.017173 1.015855 1.014317 1.012588 1.010695 1.008200 1.006397
0.01520 1.018451 1.017357 1.016038 1.014500 1.012770 1.010878 1.008383 1.006579
L-Aspartic acid in an aqueous solution of tri-sodium citrate at mB= 0.25 mol kg−1
0.00000 1.044743 1.043390 1.041855 1.040130 1.038238 1.036425 1.033088 1.030213
0.00624 1.045084 1.043730 1.042194 1.040469 1.038576 1.036762 1.033425 1.030549
0.00784 1.045171 1.043816 1.042281 1.040555 1.038662 1.036848 1.033510 1.030635
0.01080 1.045332 1.043976 1.042441 1.040715 1.038821 1.037007 1.033668 1.030794
0.01208 1.045401 1.044045 1.042510 1.040784 1.038889 1.037075 1.033736 1.030862
0.01490 1.045554 1.044197 1.042663 1.040936 1.039040 1.037225 1.033886 1.031012
0.01825 1.045735 1.044377 1.042843 1.041116 1.039218 1.037403 1.034064 1.031189
L-Aspartic acid in an aqueous solution of tri-sodium citrate at mB= 0.50 mol kg−1
0.00000 1.089975 1.088190 1.086290 1.084250 1.082085 1.078250 1.075665 1.072453
0.00597 1.090285 1.088499 1.086598 1.084557 1.082393 1.078558 1.075973 1.072761
0.00809 1.090395 1.088608 1.086707 1.084666 1.082501 1.078666 1.076081 1.072869
0.00985 1.090485 1.088699 1.086797 1.084757 1.082591 1.078756 1.076171 1.072959
0.01284 1.090639 1.088852 1.086951 1.084910 1.082744 1.078909 1.076323 1.073111
0.01520 1.090760 1.088973 1.087071 1.085030 1.082863 1.079029 1.076443 1.073232
0.01810 1.090908 1.089120 1.087219 1.085177 1.083010 1.079176 1.076589 1.073378
L-Aspartic acid in an aqueous solution of tri-sodium citrate at mB= 0.75 mol kg−1
0.00000 1.135208 1.132990 1.130725 1.128370 1.125933 1.122547 1.118243 1.115480
0.00626 1.135513 1.133294 1.131029 1.128673 1.126237 1.122852 1.118548 1.115786
0.00788 1.135591 1.133373 1.131107 1.128752 1.126315 1.122930 1.118627 1.115864
0.01082 1.135733 1.133514 1.131249 1.128893 1.126457 1.123073 1.118769 1.116006
0.01180 1.135779 1.133562 1.131296 1.128940 1.126505 1.123120 1.118816 1.116052
0.01461 1.135914 1.133696 1.131431 1.129075 1.126640 1.123255 1.118952 1.116188
0.01732 1.136042 1.133825 1.131560 1.129204 1.126769 1.123384 1.119082 1.116318
L-Glutamic acid in water
0.01104 0.999776 0.998875 0.997680 0.996307 0.994687 0.992865 0.990860 0.988682
0.01308 0.999894 0.998992 0.997796 0.996423 0.994802 0.992978 0.990973 0.988794
0.01522 1.000019 0.999115 0.997919 0.996544 0.994921 0.993096 0.991091 0.988912
0.02078 1.000341 0.999434 0.998235 0.996857 0.995233 0.993404 0.991399 0.989217
0.02517 1.000595 0.999686 0.998485 0.997104 0.995479 0.993645 0.991640 0.989457
0.03030 1.000890 0.999978 0.998775 0.997390 0.995765 0.993927 0.991922 0.988735
L-Glutamic acid in an aqueous solution of di-sodium tartrate at mB= 0.1 mol kg−1
0.01288 1.015222 1.013518 1.010883 1.009568 1.007642 1.005658 1.003681 1.001656
0.01523 1.015352 1.013648 1.011011 1.009697 1.007770 1.005787 1.003808 1.001782
0.01930 1.015577 1.013873 1.011235 1.009919 1.007992 1.006010 1.004028 1.002001
0.02496 1.015890 1.014184 1.011546 1.010228 1.008300 1.006318 1.004335 1.002307
0.03127 1.016238 1.014530 1.011890 1.010571 1.008642 1.006659 1.004674 1.002645
0.03591 1.016492 1.014782 1.012142 1.010819 1.008890 1.006909 1.004920 1.002892
L-Glutamic acid in an aqueous solution of di-sodium tartrate at mB= 0.25 mol kg−1
0.01410 1.033219 1.031223 1.029358 1.027302 1.025850 1.024208 1.021730 1.019255
0.01537 1.033286 1.031291 1.029426 1.027369 1.025917 1.024276 1.021797 1.019322
0.01930 1.033499 1.031503 1.029638 1.027580 1.026127 1.024485 1.022006 1.019531
0.02391 1.033747 1.031750 1.029885 1.027826 1.026372 1.024730 1.022250 1.019775
0.03000 1.034075 1.032075 1.030211 1.028150 1.026695 1.025053 1.022572 1.020096
0.03235 1.034200 1.032200 1.030334 1.028271 1.026819 1.025176 1.022695 1.020217
L-Glutamic acid in an aqueous solution of di-sodium tartrate at mB= 0.50 mol kg−1
0.01228 1.063847 1.062143 1.059940 1.058089 1.055985 1.053871 1.051601 1.048830
0.01354 1.063912 1.062208 1.060005 1.058153 1.056049 1.053935 1.051666 1.048892
0.01654 1.064069 1.062363 1.060160 1.058307 1.056202 1.054088 1.051818 1.049043
0.02018 1.064258 1.062551 1.060347 1.058492 1.056388 1.054274 1.052003 1.049227
0.02585 1.064553 1.062844 1.060638 1.058782 1.056675 1.054561 1.052290 1.049512
0.03703 1.065130 1.063412 1.061210 1.059350 1.057240 1.055124 1.052854 1.050070
L-Glutamic acid in an aqueous solution of di-sodium tartrate at mB= 0.75 mol kg−1
0.01159 1.094489 1.092337 1.090661 1.087995 1.086187 1.084131 1.081540 1.079468
0.01375 1.094597 1.092445 1.090767 1.088101 1.086294 1.084236 1.081645 1.079572
0.01765 1.094792 1.092640 1.090960 1.088293 1.086486 1.084426 1.081835 1.079760
0.01995 1.094907 1.092755 1.091073 1.088405 1.086599 1.084538 1.081945 1.079870
0.02676 1.095245 1.093095 1.091407 1.088740 1.086931 1.084867 1.082275 1.080198
L-Glutamic acid in an aqueous solution of tri-sodium citrate at mB= 0.1 mol kg−1
0.01247 1.018281 1.017186 1.015870 1.014333 1.012603 1.010708 1.008213 1.006409
0.01463 1.018396 1.017301 1.015986 1.014449 1.012719 1.010823 1.008329 1.006524
0.01634 1.018487 1.017392 1.016077 1.014541 1.012811 1.010913 1.008420 1.006615
0.02218 1.018800 1.017706 1.016391 1.014854 1.013124 1.011224 1.008729 1.006926
0.02857 1.019140 1.018044 1.016732 1.015194 1.013464 1.011563 1.009067 1.007265
0.03390 1.019423 1.018324 1.017015 1.015478 1.013747 1.011844 1.009348 1.007545
L-Glutamic acid in an aqueous solution of tri-sodium citrate at mB= 0.25 mol kg−1
0.01213 1.045383 1.044029 1.042489 1.040763 1.038868 1.037053 1.033716 1.030842
0.01438 1.045500 1.044146 1.042605 1.040879 1.038983 1.037169 1.033832 1.030956
0.01771 1.045674 1.044320 1.042778 1.041051 1.039155 1.037340 1.034000 1.031126
0.02308 1.045954 1.044600 1.043055 1.041328 1.039430 1.037615 1.034274 1.031400
0.02531 1.046069 1.044715 1.043170 1.041442 1.039542 1.037728 1.034386 1.031512
0.03401 1.046520 1.045167 1.043619 1.041888 1.039985 1.038168 1.034828 1.031953
L-Glutamic acid in an aqueous solution of tri-sodium citrate at mB= 0.50 mol kg−1
0.01104 1.090520 1.088735 1.086830 1.084790 1.082623 1.078787 1.076200 1.072989
0.01239 1.090585 1.088800 1.086896 1.084854 1.082687 1.078851 1.076265 1.073054
0.01547 1.090735 1.088950 1.087047 1.085003 1.082836 1.078999 1.076412 1.073202
0.02012 1.090960 1.089176 1.087272 1.085226 1.083059 1.079222 1.076634 1.073426
0.02521 1.091205 1.089423 1.087517 1.085470 1.083303 1.079465 1.076874 1.073670
0.03006 1.091438 1.089657 1.087749 1.085700 1.083535 1.079694 1.077100 1.073900
L-Glutamic acid in an aqueous solution of tri-sodium citrate at mB= 0.75 mol kg−1
0.01088 1.135703 1.133485 1.131216 1.128861 1.126424 1.123036 1.118734 1.115969
0.01408 1.135848 1.133629 1.131360 1.129005 1.126566 1.123180 1.118877 1.116112
0.01532 1.135903 1.133683 1.131415 1.129059 1.126620 1.123235 1.118932 1.116166
0.02000 1.136113 1.133892 1.131625 1.129267 1.126827 1.123443 1.119140 1.116373
0.02799 1.136469 1.134249 1.131980 1.129622 1.127179 1.123796 1.119494 1.116725
0.03107 1.136605 1.134385 1.132115 1.129756 1.127314 1.123930 1.119629 1.116855


3.1.2. Partial molar volumes, V2,ϕ0. The V2,ϕ0 values have been evaluated by fitting the corresponding data to the given equation as follows:
 
V2,ϕ = V2,ϕ0 + Svm(2)

The volumetric virial coefficient, i.e., Sv, is the experimental slope in the given equation. The V2,ϕ0 values and the standard deviation values are tabulated in Table 3. The V2,ϕ0 values for aqueous solutions of amino acids agree well with those reported in the literature (Table 3).24–30

Table 3 Partial molar volumes, V2,ϕ0, of L-aspartic acid and L-glutamic acid in water and in aqueous solutions of di-sodium tartrate and tri-sodium citrate at T values from 288.15 to 323.15 K and P = 101.3 kPa
m B/mol kg−1 106V2,ϕ0 m−3 mol−1
  288.15 K 293.15 K 298.15 K 303.15 K 308.15 K 313.15 K 318.15 K 323.15 K
a Ref. 24. b Ref. 25. c Ref. 26. d Ref. 27. e Ref. 28. f Ref. 29. g Ref. 30; parentheses contain Sv (m3 kg mol−2) values; standard uncertainties, u, are u(T) = 0.03 K, u(p) = 0.5 kPa, u(m) = 2.8 × 10−4 mol kg−1.
L-Aspartic acid in an aqueous solution of di-sodium tartrate
0.0 71.73 ± 0.01 72.02 ± 0.04 72.35 ± 0.04 72.55 ± 0.03 72.74 ± 0.01 72.98 ± 0.01 73.25 ± 0.01 73.46 ± 0.02
71.8a, 72.21b (46.19 ± 5.10) 72.30a, 73.14b, 73.34c, 73.33d 74.13c, 74.13d 72.78a, 74.17b, 75.04c, 75.03d 76.01c, 76.01d 73.23a (28.61 ± 2.80)
(45.37 ± 1.47) (42.67 ± 4.86) (34.31 ± 3.92) (31.06 ± 1.65) (26.65 ± 1.77) (28.96 ± 1.71)
0.1 75.06 ± 0.03 75.19 ± 0.02 75.44 ± 0.01 75.50 ± 0.02 75.59 ± 0.02 75.62 ± 0.01 75.68 ± 0.02 75.75 ± 0.02
(25.48 ± 2.70) (23.27 ± 2.25) (12.64 ± 1.18) (14.64 ± 2.36) (14.35 ± 1.97) (17.57 ± 1.30) (20.99 ± 2.31) (19.66 ± 2.38)
0.25 76.04 ± 0.02 76.24 ± 0.03 76.48 ± 0.01 76.54 ± 0.01 76.65 ± 0.02 76.74 ± 0.02 76.85 ± 0.01 76.93 ± 0.01
(26.40 ± 2.07) (25.89 ± 2.66) (19.64 ± 1.52) (23.10 ± 1.49) (20.60 ± 2.26) (25.38 ± 1.75) (26.12 ± 1.01) (32.56 ± 0.59)
0.50 77.27 ± 0.03 77.39 ± 0.02 77.60 ± 0.03 77.69 ± 0.04 77.75 ± 0.02 77.79 ± 0.02 77.93 ± 0.03 77.98 ± 0.01
(44.41 ± 3.76) (34.32 ± 2.84) (19.91 ± 3.35) (20.86 ± 4.10) (24.55 ± 2.27) (33.46 ± 2.14) (33.36 ± 3.21) (32.24 ± 1.61)
0.75 78.46 ± 0.002 78.56 ± 0.02 78.71 ± 0.03 78.82 ± 0.01 78.90 ± 0.008 78.95 ± 0.03 79.11 ± 0.02 79.16 ± 0.01
(4.23 ± 0.33) (11.50 ± 1.97) (27.31 ± 3.02) (24.49 ± 1.40) (28.81 ± 0.85) (31.10 ± 3.68) (28.16 ± 2.30) (38.43 ± 1.57)
L-Aspartic acid in an aqueous solution of tri-sodium citrate
0.1 76.51 ± 0.02 76.71 ± 0.01 76.92 ± 0.01 76.98 ± 0.03 77.02 ± 0.03 77.12 ± 0.01 77.22 ± 0.007 77.26 ± 0.01
(22.38 ± 2.38) (17.03 ± 1.23) (16.05 ± 1.44) (24.68 ± 3.87) (28.24 ± 3.78) (15.76 ± 1.46) (18.43 ± 0.90) (23.00 ± 1.39)
0.25 77.20 ± 0.005 77.40 ± 0.02 77.61 ± 0.01 77.72 ± 0.01 77.78 ± 0.03 77.95 ± 0.02 78.12 ± 0.05 78.18 ± 0.01
(17.99 ± 0.52) (22.63 ± 2.29) (10.25 ± 1.13) (12.55 ± 0.90) (28.84 ± 2.86) (27.47 ± 2.32) (31.73 ± 4.68) (30.23 ± 1.34)
0.50 78.26 ± 0.01 78.48 ± 0.01 78.71 ± 0.01 78.82 ± 0.02 78.84 ± 0.02 79.00 ± 0.02 79.05 ± 0.02 79.11 ± 0.02
(22.48 ± 1.16) (20.19 ± 1.23) (13.71 ± 1.08) (15.64 ± 2.03) (25.68 ± 1.57) (21.00 ± 1.76) (27.61 ± 1.83) (28.47 ± 2.25)
0.75 79.23 ± 0.02 79.42 ± 0.01 79.64 ± 0.02 79.75 ± 0.02 79.79 ± 0.02 79.81 ± 0.02 79.88 ± 0.01 79.94 ± 0.03
(33.88 ± 2.43) (24.78 ± 1.52) (17.23 ± 1.73) (18.05 ± 1.71) (16.40 ± 2.20) (18.86 ± 2.20) (21.13 ± 1.20) (27.03 ± 3.29)
L-Glutamic acid in an aqueous solution of di-sodium tartrate
0.0 88.34 ± 0.01 88.78 ± 0.04 89.70 ± 0.02 90.01 ± 0.01 90.64 ± 0.03 91.19 ± 0.03 91.46 ± 0.03 91.90 ± 0.02
88.35a, 88.88b 86.68e 89.65a, 90.19b, 87.84e, 89f, 90.06f, 89.64g, 89.65g 89.25e 90.54a, 91.05b 89.87e 91.20a 90.64e
(17.83 ± 0.51) (22.72 ± 2.42) (11.13 ± 1.08) (19.88 ± 0.86) (12.77 ± 1.80) (20.83 ± 1.91) (16.57 ± 1.49) (18.29 ± 1.26)
0.1 90.37 ± 0.03 90.62 ± 0.01 91.10 ± 0.04 91.23 ± 0.02 91.46 ± 0.03 91.72 ± 0.03 91.86 ± 0.03 92.01 ± 0.06
(16.23 ± 1.31) (18.16 ± 0.54) (14.97 ± 2.09) (19.55 ± 1.23) (19.40 ± 1.29) (13.04 ± 1.30) (21.12 ± 1.63) (23.46 ± 2.93)
0.25 91.16 ± 0.03 91.32 ± 0.01 91.57 ± 0.03 91.73 ± 0.04 91.92 ± 0.03 92.17 ± 0.01 92.31 ± 0.02 92.52 ± 0.03
(13.60 ± 1.74) (17.47 ± 0.62) (13.77 ± 1.55) (18.93 ± 2.12) (17.61 ± 1.49) (15.50 ± 0.58) (17.96 ± 1.29) (18.35 ± 1.96)
0.50 91.59 ± 0.03 91.86 ± 0.03 92.35 ± 0.02 92.41 ± 0.06 92.75 ± 0.04 93.17 ± 0.01 93.33 ± 0.02 93.49 ± 0.07
(14.53 ± 1.57) (20.06 ± 1.32) (11.94 ± 0.78) (19.37 ± 2.88) (18.96 ± 1.89) (13.09 ± 0.68) (13.05 ± 1.14) (21.08 ± 3.37)
0.75 91.76 ± 0.01 92.05 ± 0.04 92.50 ± 0.04 92.71 ± 0.05 93.06 ± 0.01 93.56 ± 0.02 93.77 ± 0.03 93.95 ± 0.06
(23.58 ± 1.05) (17.30 ± 3.03) (24.03 ± 3.00) (24.22 ± 4.05) (18.56 ± 0.63) (19.44 ± 1.54) (19.24 ± 2.84) (25.87 ± 4.95)
L-Glutamic acid in an aqueous solution of tri-sodium citrate
0.1 91.83 ± 0.07 92.00 ± 0.04 92.14 ± 0.02 92.25 ± 0.01 92.36 ± 0.01 92.50 ± 0.05 92.59 ± 0.03 92.87 ± 0.03
(23.01 ± 3.57) (22.73 ± 2.30) (14.79 ± 1.09) (14.51 ± 0.75) (14.67 ± 0.54) (21.27 ± 2.74) (23.33 ± 1.76) (16.12 ± 1.57)
0.25 92.28 ± 0.03 92.44 ± 0.03 92.90 ± 0.03 93.01 ± 0.03 93.28 ± 0.03 93.50 ± 0.02 93.60 ± 0.07 93.70 ± 0.06
(16.13 ± 1.84) (13.40 ± 1.49) (12.20 ± 1.89) (15.75 ± 1.74) (19.07 ± 1.39) (16.54 ± 1.35) (23.30 ± 3.89) (23.59 ± 3.50)
0.50 93.16 ± 0.03 93.35 ± 0.03 93.74 ± 0.02 93.83 ± 0.04 94.18 ± 0.03 94.40 ± 0.03 94.54 ± 0.03 94.80 ± 0.02
(25.23 ± 2.01) (17.86 ± 1.69) (13.98 ± 1.20) (24.25 ± 2.27) (16.30 ± 2.00) (20.15 ± 1.60) (27.98 ± 1.56) (13.29 ± 1.05)
0.75 94.09 ± 0.01 94.23 ± 0.05 94.65 ± 0.01 94.74 ± 0.03 94.86 ± 0.05 95.17 ± 0.01 95.30 ± 0.03 95.51 ± 0.04
(16.50 ± 0.48) (17.87 ± 2.86) (11.21 ± 0.60) (15.94 ± 1.76) (21.36 ± 2.53) (14.19 ± 0.79) (15.59 ± 1.39) (21.00 ± 2.14)


The V2,ϕ0 values of both amino acids increase with an increase in the concentrations of the studied salts and also with an increase in temperature. As the V2,ϕ0 values increase with an increase in molecular weight, the V2,ϕ0 values are higher in GLU than ASP, because of the extra –CH2 group. The V2,ϕ0 values for both ASP and GLU are higher in aqueous solutions of TSC than in DST.

3.1.3. Apparent specific volume, vϕ. The apparent specific volume, vϕ, is calculated as follows
 
vϕ = V2,ϕ/M.(3)

The apparent specific volume, vϕ, bears a relationship to taste quality in the order salty < sour < sweet < bitter and, probably, the entire range of human taste perception is confined to molecules with apparent specific volumes between 0.1 and 0.95 cm3 g−1.31 The vϕ values for the studied systems are given in Table S2. In water, the vϕ values for ASP and GLU fall within the sweet taste range32 (0.52–0.71 cm3 g−1). ASP is sweeter than GLU in water. However, in aqueous solutions of DST and TSC, the vϕ values increase and, therefore, the sweetnesses of both ASP and GLU decrease as the mB values increase.

3.1.4. Partial molar volumes of transfer, ΔtrV2,ϕ0. The ΔtrV2,ϕ0 values of the studied solutes, from water to aqueous solutions of DST and TSC, are calculated as follows:
 
ΔtrV2,ϕ0 = V2,ϕ0 (in aqueous solutions of DST/TSC) − V2,ϕ0 (in water)(4)

The ΔtrV2,ϕ0 values are positive for the studied amino acids and increase with the co-solute concentrations at all temperatures, as shown in Fig. 4(a, d, g and j). The ΔtrV2,ϕ0 values of ASP and GLU decrease with temperature. The ΔtrV2,ϕ0 values of both amino acids are greater in the case of TSC compared to DST, due to the high charge density.5 The magnitude of transfer is greater in the case of ASP compared to GLU.


image file: c9fo00872a-f4.tif
Fig. 4 (a, d, g and j) Partial molar volume of transfer values, ΔtrV2,ϕ0, of L-aspartic acid and L-glutamic acid vs. mB for aqueous solutions of di-sodium tartrate and tri-sodium citrate at T = (◆) 288.15 K; (■) 293.15 K; (▲) 298.15 K; (×) 303.15 K; (*) 308.15 K; (●) 313.15 K; (+) 318.15 K; and (−) 323.15 K. (b, e, h and k) partial molar isentropic compression of transfer values, ΔtrKs,2,ϕ0, of L-aspartic acid and L-glutamic acid vs. mB for aqueous solutions of di-sodium tartrate and tri-sodium citrate at T = (◆) 288.15 K; (■) 293.15 K; (▲) 298.15 K; (×) 303.15 K; (*) 308.15 K; (●) 313.15 K; (+) 318.15 K; and (−) 323.15 K. (c, f, i and l) Viscosity B-coefficient of transfer values, ΔtrB, of L-aspartic acid and L-glutamic acid vs. mB for aqueous solutions of di-sodium tartrate and tri-sodium citrate at T = (◆) 288.15 K; (■) 298.15 K; (▲) 308.15 K; and (×) 308.15 K.

The magnitude of ΔtrV2,ϕ0 initially increases sharply, and the increase is less at higher co-solute concentrations. A co-sphere overlap model33 is used to interpret these results. According to this model, hydrophilic (the –NH2 and –COOH parts of the solute)-ionic (the Na+ and COO parts of the co-solute) and hydrophilic (the –NH2 and –COOH groups of the solute)-hydrophilic (the –OH groups of the co-solute) interactions contribute positively, and hydrophobic (the alkyl groups of the solute)-hydrophilic (the –OH groups of the co-solute) and hydrophobic (the alkyl groups of the solute)-ionic (the Na+ and COO parts of the co-solute) interactions contribute negatively to the ΔtrV2,ϕ0 values. Therefore, in the present system, the positive ΔtrV2,ϕ0 values for these amino acids over the co-solute concentration range studied suggest that hydrophilic–ionic and hydrophilic–hydrophilic interactions dominate over hydrophobic–hydrophilic and hydrophobic–ionic interactions. Furthermore, the decrease in the magnitude of ΔtrV2,ϕ0 from ASP to GLU indicates the building up of hydrophobic–ionic interactions due to the extra –CH2 group. The additional –CH2 group exerts a destructive effect on the polar and charged group hydration spheres of the amino acids. The low ΔtrV2,ϕ0 values for GLU are in line with the observation that ΔtrV2,ϕ0 values decrease with an increase in the lengths of the alkyl side chains of amino acids.18

3.1.5. Hydration number, Nh. The hydration number, Nh of the amino acids in water and in aqueous DST/TSC solutions can be calculated by using the following equation.34
 
Nh (in water) = Velect/(V0eV0b)(5)
In the above equation, V0e = molar volume of electrostricted water and V0b = molar volume of bulk water/solvent. The (V0eV0b) values are approximately −2.9 cm3 mol−1 at 288.15 K, −3.3 cm3 mol−1 at 298.15 K and −4.0 cm3 mol−1 at 308.15 K.35 The Velect values have been calculated from the corresponding V2,ϕ0 values using the equation:
 
Velect = V2,ϕ0V0int(6)
where the intrinsic molar volume, V0int, can be estimated from the crystallographic volume, Vcryst as follows:36
 
V0int = (0.7/0.634)Vcryst.(7)

The Nh data in Table S3 indicate that the dehydration of ASP and GLU increases with the co-solute molality. The Nh values for ASP are higher than for GLU and, also, the decreases in the Nh values with mB are larger in the case of ASP compared to GLU. TSC is found to be an effective preservative for dehydration processes, therefore, its use in the food industry is more preferred than DST.37 TSC has also been found to be a better food emulsifier than DST in the literature.5 TSC helps control the stickiness of some doughs through removing extra water and creating a firmer texture. Thus, it helps in the processing of baked foods.38,39 Therefore, the Nh data also indicate that TSC is a better food preservative than DST.

3.1.6. Partial molar expansion, E2,ϕ0. To discuss the effects of temperature on the V2,ϕ0 values, the partial molar expansion values, E2,ϕ0 (E2,ϕ0 = (∂V2,ϕ0/∂T)P), along with the 2nd order derivatives, (∂2V2,ϕ0/∂T2)P, have been calculated by the method of least-squares using the following equation:
 
V2,ϕ0 = a + bT + cT2(8)
where a, b and c are constants, and T is the temperature in Kelvin. The (∂V2,ϕ0/∂T)P values are positive for the studied solutes (Table S4), except for ASP in TSC at mB = 0.75 mol kg−1 and at T = 323.15 K. The (∂V2,ϕ0/∂T)P values decrease with temperature, except for GLU in DST at mB = 0.25 mol kg−1 at higher temperatures and GLU in TSC at mB = 0.1 mol kg−1 at all temperatures. However, the (∂V2,ϕ0/∂T)P values do not follow any regular trend in relation to the mB values.

Hepler40 used a general thermodynamic equation, {(∂CP,20/∂P)T = −T(∂2V2,ϕ0/∂T2)P}, to determine the capacity of a solute to act as a structure-maker or a structure-breaker in solution. If the term (∂2V2,ϕ0/∂T2)P < 0, the solute is a structure-breaker and if (∂2V2,ϕ0/∂T2)P > 0, the solute is a structure-maker. The negative values of (∂2V2,ϕ0/∂T2)P in aqueous solutions of the co-solutes suggest that ASP and GLU act as structure-breaking solutes. However, at mB = 0.1 mol kg−1, GLU acts as a structure-maker in the case of TSC.

3.1.7. Interaction coefficients. Interaction coefficients have been calculated using the McMillan–Mayer equation41 from the transfer volume, ΔtrV2,ϕ0, data for the studied solutes as follows:
 
ΔtrV2,ϕ0 = 2VAB·mB + 3VABB·mB2 + …(9)
where A stands for the solute and B for the co-solute, and VAB and VABB are the volumetric pair and triplet interaction coefficients, respectively (Table S5). The higher magnitudes of VAB (positive values) in comparison to VABB (negative values, except for GLU in DST at T = 323.15 K) for the studied solutes at all temperatures suggest the predominance of pair-wise interactions. This suggests the dominance of hydrophilic–ionic/hydrophilic interactions, which again strengthens the above-mentioned views.

3.2. Acoustic measurements

3.2.1. Speed of sound, u. The speed of sound, u, of amino acids (ASP and GLU) in water and in aqueous solutions of DST and TSC at mB = 0.10, 0.25, 0.50 and 0.75 mol kg−1 and at temperatures from 288.15 to 323.15 K are given in Table 4. The u values of water are in agreement with the literature values.42 The u values in aqueous solutions of DST and TSC match well with the available literature data,16–21,23 as seen in Fig. 5. The u values of ASP and GLU in water are in close agreement with the available literature data, as seen in Fig. 6.25–27 The u values increase with the mA, mB and T values, as is evident from the 3-D plot (Fig. 3(c)) of u versus mA for ASP at different temperatures in TSC at mB = 0.1 mol kg−1.
image file: c9fo00872a-f5.tif
Fig. 5 (a) Plots of speed of sound values, u, versus molality, mB, for aqueous solutions of TSC and comparisons with the literature values at different temperatures. (b) Plots of speed of sound values, u, versus molality, mB, for aqueous solutions of DST and comparisons with the literature values at different temperatures.

image file: c9fo00872a-f6.tif
Fig. 6 (a) Plots of speed of sound values, u, versus molality, mA, for aqueous solutions of ASP and comparisons with the literature values at different temperatures. (b) Plots of speed of sound values, u, versus molality, mA, for aqueous solutions of GLU and comparisons with the literature values at different temperatures.
Table 4 The speed of sound values, u, of L-aspartic acid and L-glutamic acid in water and in aqueous solutions of di-sodium tartrate and tri-sodium citrate at T values from 288.15 to 323.15 K and P = 101.3 kPa
m A/mol kg−1 u/m sec−1
288.15 K 293.15 K 298.15 K 303.15 K 308.15 K 313.15 K 318.15 K 323.15 K
m A is the molality of the solute in water or water + DST/TSC. mB is the molality of DST/TSC in water. Standard uncertainties, u, are u(T) = 0.03 K, u(P) = 0.5 kPa, u(m) = 2.8 × 10−4 mol kg−1, u(u) = 0.6 m s−1.
L-Aspartic acid in water
0.00000 1466.83 1483.03 1497.10 1510.02 1519.83 1529.98 1536.40 1542.99
0.00517 1467.53 1483.68 1497.71 1510.59 1520.34 1530.45 1536.84 1543.38
0.00712 1467.78 1483.92 1497.94 1510.81 1520.54 1530.62 1536.99 1543.52
0.00921 1468.04 1484.17 1498.18 1511.04 1520.75 1530.80 1537.15 1543.67
0.01109 1468.28 1484.39 1498.40 1511.25 1520.94 1530.96 1537.30 1543.80
0.01300 1468.51 1484.62 1498.62 1511.46 1521.13 1531.12 1537.45 1543.94
0.01506 1468.76 1484.86 1498.85 1511.68 1521.33 1531.29 1537.60 1544.08
L-Aspartic acid in an aqueous solution of di-sodium tartrate at m B = 0.1 mol kg −1
0.00000 1478.73 1494.57 1511.87 1522.45 1534.08 1543.75 1550.05 1557.45
0.00502 1479.25 1495.10 1512.40 1522.98 1534.59 1544.18 1550.48 1557.85
0.00741 1479.50 1495.34 1512.65 1523.23 1534.83 1544.38 1550.68 1558.04
0.01180 1479.95 1495.80 1513.28 1523.69 1535.27 1544.75 1551.05 1558.39
0.01306 1480.07 1495.93 1513.34 1523.82 1535.39 1544.85 1551.16 1558.48
0.01732 1480.51 1496.36 1514.04 1524.26 1535.81 1545.21 1551.51 1558.82
L-Aspartic acid in an aqueous solution of di-sodium tartrate at m B = 0.25 mol kg −1
0.00000 1504.97 1519.22 1533.45 1544.85 1554.33 1561.75 1569.19 1577.89
0.00584 1505.61 1519.86 1534.09 1545.48 1554.91 1562.23 1569.64 1578.31
0.00817 1505.86 1520.11 1534.34 1545.73 1555.14 1562.42 1569.82 1578.47
0.01106 1506.17 1520.43 1534.65 1546.04 1555.42 1562.65 1570.03 1578.67
0.01337 1506.42 1520.67 1534.90 1546.28 1555.64 1562.83 1570.20 1578.82
0.01555 1506.65 1520.91 1535.13 1546.51 1555.85 1563.00 1570.36 1578.97
0.01667 1506.77 1521.03 1535.25 1546.63 1555.95 1563.08 1570.43 1579.04
L-Aspartic acid in an aqueous solution of di-sodium tartrate at m B = 0.50 mol kg −1
0.00000 1549.36 1558.64 1569.41 1579.85 1588.09 1595.45 1601.10 1609.00
0.00638 1550.10 1559.37 1570.12 1580.54 1588.70 1595.96 1601.62 1609.47
0.00782 1550.27 1559.53 1570.28 1580.69 1588.84 1596.07 1601.74 1609.57
0.00982 1550.50 1559.76 1570.50 1580.89 1589.02 1596.22 1601.90 1609.72
0.01307 1550.87 1560.12 1570.85 1581.23 1589.33 1596.47 1602.16 1609.95
0.01509 1551.10 1560.35 1571.07 1581.44 1589.52 1596.62 1602.31 1610.09
0.01639 1551.24 1560.49 1571.20 1581.57 1589.63 1596.72 1602.41 1610.18
L-Aspartic acid in an aqueous solution of di-sodium tartrate at m B = 0.75 mol kg −1
0.00000 1588.75 1597.00 1605.38 1615.75 1621.84 1628.04 1633.00 1641.75
0.00547 1589.45 1597.68 1606.03 1616.37 1622.42 1628.51 1633.50 1642.22
0.00739 1589.69 1597.92 1606.26 1616.58 1622.62 1628.67 1633.67 1642.38
0.01025 1590.05 1598.27 1606.59 1616.90 1622.91 1628.90 1633.92 1642.62
0.01347 1590.45 1598.66 1606.97 1617.25 1623.26 1629.16 1634.20 1642.89
0.01664 1590.84 1599.04 1607.33 1617.60 1623.57 1629.41 1634.48 1643.15
L-Aspartic acid in an aqueous solution of tri-sodium citrate at m B = 0.1 mol kg −1
0.00000 1487.73 1503.57 1517.21 1529.07 1539.24 1548.45 1555.13 1562.78
0.00576 1488.28 1504.12 1517.76 1529.62 1539.76 1548.87 1555.53 1563.15
0.00754 1488.45 1504.29 1517.93 1529.79 1539.91 1549.00 1555.65 1563.27
0.01002 1488.69 1504.52 1518.16 1530.02 1540.13 1549.18 1555.82 1563.42
0.01189 1488.86 1504.70 1518.34 1530.19 1540.30 1549.31 1555.94 1563.54
0.0152 1489.17 1505.01 1518.65 1530.49 1540.59 1549.55 1556.16 1563.75
L-Aspartic acid in an aqueous solution of tri-sodium citrate at m B = 0.25 mol kg −1
0.00000 1517.97 1533.22 1545.99 1557.02 1566.44 1574.85 1580.99 1587.00
0.00624 1518.60 1533.83 1546.57 1557.57 1566.95 1575.25 1581.36 1587.34
0.00784 1518.76 1533.98 1546.71 1557.71 1567.08 1575.35 1581.45 1587.42
0.01080 1519.05 1534.27 1546.98 1557.97 1567.31 1575.53 1581.62 1587.58
0.01208 1519.18 1534.39 1547.10 1558.08 1567.41 1575.61 1581.69 1587.65
0.01490 1519.46 1534.66 1547.35 1558.32 1567.63 1575.78 1581.85 1587.79
0.01825 1519.79 1534.97 1547.65 1558.61 1567.90 1575.99 1582.03 1587.96
L-Aspartic acid in an aqueous solution of tri-sodium citrate at m B = 0.50 mol kg −1
0.00000 1568.36 1582.64 1593.94 1603.61 1611.76 1618.75 1624.10 1630.01
0.00597 1569.02 1583.27 1594.55 1604.20 1612.28 1619.16 1624.48 1630.34
0.00809 1569.26 1583.49 1594.77 1604.40 1612.46 1619.30 1624.61 1630.46
0.00985 1569.45 1583.67 1594.95 1604.57 1612.61 1619.42 1624.72 1630.56
0.01284 1569.77 1583.98 1595.26 1604.86 1612.85 1619.62 1624.91 1630.73
0.01520 1570.03 1584.22 1595.50 1605.08 1613.05 1619.77 1625.05 1630.86
0.0181 1570.34 1584.46 1595.78 1605.35 1613.28 1619.95 1625.22 1631.03
L-Aspartic acid in an aqueous solution of tri-sodium citrate at m B = 0.75 mol kg −1
0.00000 1618.75 1632.06 1641.90 1650.20 1657.08 1662.45 1667.20 1673.45
0.00626 1619.55 1632.82 1642.62 1650.89 1657.68 1662.89 1667.60 1673.84
0.00788 1619.75 1633.01 1642.80 1651.06 1657.83 1663.00 1667.70 1673.94
0.01082 1620.12 1633.36 1643.13 1651.38 1658.10 1663.21 1667.89 1674.13
0.01180 1620.24 1633.47 1643.24 1651.48 1658.18 1663.28 1667.95 1674.19
0.01461 1620.59 1633.80 1643.55 1651.77 1658.44 1663.48 1668.13 1674.36
0.01732 1620.92 1634.12 1643.85 1652.05 1658.68 1663.67 1668.30 1674.53
L-Glutamic acid in water
0.01104 1467.12 1483.26 1497.44 1510.18 1520.06 1530.36 1536.68 1543.40
0.01308 1467.16 1483.31 1497.49 1510.23 1520.12 1530.42 1536.73 1543.45
0.01522 1467.21 1483.36 1497.54 1510.28 1520.17 1530.47 1536.79 1543.51
0.02078 1467.34 1483.49 1497.68 1510.43 1520.32 1530.62 1536.94 1543.66
0.02517 1467.45 1483.60 1497.79 1510.54 1520.44 1530.74 1537.06 1543.79
0.03030 1467.57 1483.72 1497.92 1510.68 1520.57 1530.88 1537.21 1543.93
L-Glutamic acid in an aqueous solution of di-sodium tartrate at m B = 0.1 mol kg −1
0.01288 1479.08 1494.93 1512.24 1522.82 1534.46 1544.13 1550.43 1557.83
0.01523 1479.15 1495.00 1512.31 1522.89 1534.53 1544.20 1550.50 1557.91
0.01930 1479.27 1495.11 1512.43 1523.01 1534.65 1544.32 1550.63 1558.03
0.02496 1479.43 1495.28 1512.59 1523.18 1534.82 1544.49 1550.80 1558.20
0.03127 1479.61 1495.46 1512.78 1523.37 1535.01 1544.68 1550.99 1558.40
0.03591 1479.74 1495.60 1512.92 1523.52 1535.16 1544.82 1551.14 1558.55
L-Glutamic acid in an aqueous solution of di-sodium tartrate at m B = 0.25 mol kg −1
0.01410 1505.42 1519.67 1533.90 1545.31 1554.79 1562.22 1569.66 1578.36
0.01930 1505.89 1520.15 1534.39 1545.80 1555.29 1562.71 1570.16 1578.86
0.02391 1506.10 1520.36 1534.60 1546.01 1555.50 1562.92 1570.37 1579.07
0.03000 1506.41 1520.67 1534.92 1546.33 1555.82 1563.25 1570.69 1579.40
0.03535 1506.52 1520.79 1535.03 1546.45 1555.94 1563.37 1570.82 1579.52
L-Glutamic acid in an aqueous solution of di-sodium tartrate at m B = 0.50 mol kg −1
0.01104 1549.82 1559.11 1569.89 1580.33 1588.57 1595.94 1601.59 1609.49
0.01239 1549.87 1559.16 1569.94 1580.38 1588.62 1595.99 1601.64 1609.54
0.01547 1549.99 1559.28 1570.05 1580.50 1588.74 1596.11 1601.76 1609.67
0.02012 1550.13 1559.42 1570.20 1580.64 1588.89 1596.26 1601.91 1609.82
0.02521 1550.34 1559.64 1570.42 1580.87 1589.12 1596.49 1602.14 1610.05
0.03006 1550.78 1560.09 1570.86 1581.32 1589.57 1596.95 1602.60 1610.51
L-Glutamic acid in an aqueous solution of di-sodium tartrate at m B = 0.75 mol kg −1
0.01159 1589.26 1597.51 1605.79 1616.32 1622.37 1628.57 1633.54 1642.29
0.01375 1589.35 1597.60 1605.89 1616.42 1622.47 1628.67 1633.64 1642.39
0.01765 1589.52 1597.78 1606.07 1616.60 1622.64 1628.86 1633.82 1642.58
0.01995 1589.63 1597.88 1606.17 1616.71 1622.75 1628.97 1633.93 1642.69
0.02676 1589.93 1598.18 1606.49 1617.02 1623.07 1629.29 1634.25 1643.01
L-Glutamic acid in an aqueous solution of tri-sodium citrate at m B = 0.1 mol kg −1
0.01247 1488.11 1503.95 1517.59 1529.46 1539.63 1548.84 1555.52 1563.17
0.01463 1488.18 1504.02 1517.66 1529.52 1539.67 1548.91 1555.59 1563.23
0.01634 1488.23 1504.07 1517.72 1529.58 1539.75 1548.97 1555.65 1563.29
0.02218 1488.41 1504.26 1517.90 1529.76 1539.94 1549.16 1555.84 1563.47
0.02857 1488.62 1504.47 1518.11 1529.97 1540.14 1549.36 1556.05 1563.67
0.03390 1488.79 1504.64 1518.28 1530.14 1540.32 1549.54 1556.22 1563.84
L-Glutamic acid in an aqueous solution of tri-sodium citrate at mB= 0.25 mol kg−1
0.01213 1518.40 1533.65 1546.43 1557.46 1566.88 1575.30 1581.44 1587.45
0.01438 1518.48 1533.73 1546.52 1557.55 1566.97 1575.39 1581.53 1587.54
0.01771 1518.60 1533.86 1546.64 1557.67 1567.10 1575.51 1581.66 1587.66
0.02308 1518.79 1534.05 1546.84 1557.87 1567.31 1575.72 1581.86 1587.87
0.02531 1518.88 1534.13 1546.92 1557.96 1567.39 1575.80 1581.95 1587.96
0.03401 1519.19 1534.45 1547.24 1558.29 1567.73 1576.14 1582.28 1588.29
L-Glutamic acid in an aqueous solution of tri-sodium citrate at mB= 0.50 mol kg−1
0.01104 1568.85 1583.14 1594.44 1604.12 1612.26 1619.26 1624.61 1630.52
0.01239 1568.92 1583.20 1594.51 1604.18 1612.33 1619.32 1624.67 1630.58
0.01547 1569.06 1583.34 1594.65 1604.32 1612.47 1619.47 1624.82 1630.72
0.02012 1569.27 1583.55 1594.86 1604.54 1612.69 1619.69 1625.04 1630.94
0.02521 1569.51 1583.79 1595.10 1604.78 1612.93 1619.93 1625.28 1631.17
0.03006 1569.73 1583.96 1595.33 1605.02 1613.16 1620.16 1625.52 1631.40
L-Glutamic acid in an aqueous solution of tri-sodium citrate at mB= 0.75 mol kg−1
0.01088 1619.34 1632.66 1642.51 1650.81 1657.69 1663.06 1667.81 1674.06
0.01408 1619.52 1632.83 1642.69 1650.99 1657.87 1663.24 1667.99 1674.24
0.01532 1619.59 1632.90 1642.75 1651.06 1657.94 1663.31 1668.06 1674.31
0.02000 1619.85 1633.17 1643.02 1651.32 1658.21 1663.57 1668.32 1674.58
0.02799 1620.29 1633.61 1643.47 1651.78 1658.66 1664.03 1668.77 1675.03
0.03107 1620.47 1633.79 1643.64 1651.95 1658.84 1664.20 1668.95 1675.21


3.2.2. Apparent molar isentropic compression, Ks,2,ϕ. The Ks,2,ϕ values for solutes in water and in aqueous solutions of DST and TSC at mB = 0.10, 0.25, 0.50, and 0.75 mol kg−1 and at temperatures from 288.15 to 323.15 K (given in Table S6) have been calculated using the following relationship:
 
Ks,2,ϕ = (κsM/ρ) − {(κsoρκsρ0)/(mAρρ0)}(10)

The Ks,2,ϕ values are negative, and their magnitudes decrease with increases in the temperature and in the concentrations of both the solute and co-solute. ρ and ρ0, and κs and κso are the densities and isentropic compressibilities of the solution and solvent, respectively. The isentropic compressibilities (κs) were calculated from the measured speed of sound (u) as follows:

 
κs = 1/u2ρ.(11)

3.2.3. Partial molar isentropic compression, Ks,2,ϕ0. The Ks,2,ϕ0 values have been calculated using the least squares fitting of the respective data to the given equation:
 
Ks,2,ϕ = Ks,2,ϕ0 + Skm.(12)

At infinite dilution, Ks,2,ϕ0, the apparent molar isentropic compression, and the partial molar isentropic compression, Ks,20, become the same. The Ks,2,ϕ0 values of the studied solutes in water agree well with the literature values.25–27,29,30 The Ks,2,ϕ0 values of the solutes are negative in water as well as in aqueous solutions of the co-solutes, which may be due to the hydration of the solutes, as the hydrated water molecules are already compressed and are thus less compressible than those present in the bulk. The magnitudes of the Ks,2,ϕ0 values decrease with temperature and with the co-solute concentrations (Table 5).

Table 5 Partial molar isentropic compression values, Ks,2,ϕ0, of L-aspartic acid and L-glutamic acid in water and in aqueous solutions of di-sodium tartrate and tri-sodium citrate at T values from 288.15 to 323.15 K and P = 101.3 kPa
m B/mol kg−1 1014·Ks,2,ϕ0/m3 mol−1 Pa−1
288.15 K 293.15 K 298.15 K 303.15 K 308.15 K 313.15 K 318.15 K 323.15 K
a Ref. 25. b Ref. 26. c Ref. 27. d Ref. 29. e Ref. 30; parentheses contain Sk (m3 mol−2 kg Pa−1) values; standard uncertainties, u, are u(T) = 0.03 K, u(P) = 0.5 kPa, u(m) = 2.8 × 10−4 mol kg−1.
L-Aspartic acid in an aqueous solution of di-sodium tartrate
0 −7.51 ± 0.01 −7.09 ± 0.02 −6.63 ± 0.01 −6.26 ± 0.01 −5.60 ± 0.84 −4.50 ± 0.01 −4.11 ± 0.02 −3.64 ± 0.01
(11.45 ± 0.80) (16.68 ± 1.44) −3.61a, −6.67b, −7.14c (18.28 ± 0.88) −3.06 a (12.76 ± 0.52) (17.70 ± 2.30) (13.34 ± 0.63)
(17.12 ± 0.28) −6.24b,c (19.59 ± 0.84) −4.53b,c
−5.60b, −5.61c
0.1 −5.39 ± 0.01 −5.29 ± 0.01 −5.19 ± 0.02 −5.08 ± 0.01 −4.75 ± 0.01 −3.86 ± 0.01 −3.81 ± 0.01 −3.50 ± 0.01
(7.39 ± 0.89) (7.88 ± 0.50) (7.62 ± 1.33) (6.25 ± 0.35) (7.20 ± 0.75) (8.42 ± 0.95) (7.48 ± 0.56) (8.02 ± 0.98)
0.25 −5.09 ± 0.01 −4.94 ± 0.01 −4.82 ± 0.01 −4.63 ± 0.01 −4.18 ± 0.01 −3.23 ± 0.02 2.96 ± 0.01 −2.63 ± 0.01
(11.00 ± 0.42) (8.48 ± 0.57) (10.92 ± 0.44) (8.86 ± 0.84) (13.29 ± 0.45) (11.71 ± 1.23) (12.61 ± 0.69) (12.95 ± 0.88)
0.50 −4.40 ± 0.01 −4.27 ± 0.01 −4.05 ± 0.01 −3.82 ± 0.01 −3.23 ± 0.01 −2.50 ± 0.01 −2.56 ± 0.01 −2.19 ± 0.01
(8.57 ± 0.66) (10.48 ± 1.03) (10.40 ± 1.22) (13.37 ± 0.64) (8.73 ± 0.86) (12.56 ± 1.16) (9.18 ± 0.56) (9.62 ± 0.58)
0.75 −4.10 ± 0.01 −3.88 ± 0.01 −3.60 ± 0.01 −3.34 ± 0.01 −3.01 ± 0.01 −2.18 ± 0.01 −2.39 ± 0.01 −2.16 ± 0.01
(9.10 ± 0.50) (7.71 ± 0.36) (7.67 ± 0.74) (9.43 ± 0.55) (8.06 ± 0.33) (10.92 ± 0.25) (10.13 ± 0.81) (8.75 ± 0.43)
L-Aspartic acid in an aqueous solution of tri-sodium citrate
0.1 −4.66 ± 0.01 −4.50 ± 0.01 4.37 ± 0.01 −4.29 ± 0.01 −3.89 ± 0.01 −2.99 ± 0.01 −2.77 ± 0.01 −2.47 ± 0.01
(8.65 ± 0.43) (7.58 ± 1.05) (8.51 ± 0.54) (12.76 ± 0.81) (9.43 ± 0.61) (9.30 ± 0.98) (11.02 ± 1.23) (7.58 ± 1.05)
0.25 −4.20 ± 0.01 −3.89 ± 0.01 −3.56 ± 0.01 −3.30 ± 0.01 −2.89 ± 0.01 −2.01 ± 0.01 −1.79 ± 0.02 −1.54 ± 0.01
(8.33 ± 1.12) (7.91 ± 0.96) (9.29 ± 0.72) (8.03 ± 0.55) (8.54 ± 0.62) (9.38 ± 0.80) (10.16 ± 1.24) (8.07 ± 0.52)
0.50 −3.61 ± 0.01 −3.27 ± 0.01 −3.04 ± 0.02 −2.87 ± 0.02 −2.39 ± 0.02 −1.59 ± 0.02 −1.40 ± 0.02 −1.04 ± 0.01
(9.93 ± 0.74) (10.41 ± 0.81) (2.97 ± 1.65) (10.01 ± 1.44) (13.60 ± 1.70) (8.54 ± 1.65) (8.33 ± 1.46) (1.07 ± 0.37)
0.75 −3.31 ± 0.01 −3.01 ± 0.01 −2.74 ± 0.01 −2.50 ± 0.01 −2.01 ± 0.01 −1.10 ± 0.01 −0.88 ± 0.01 −0.84 ± 0.01
(9.49 ± 0.62) (9.41 ± 0.30) (9.33 ± 0.69) (8.00 ± 0.59) (10.11 ± 1.13) (0.61 ± 0.39) (11.17 ± 0.72) (1.11 ± 0.37)
10 16 ·K s,2,ϕ 0 /m 3 mol −1 Pa −1
L-Glutamic acid in an aqueous solution of di-sodium tartrate
0 −3.86 ± 0.01 −3.50 ± 0.01 −3.18 ± 0.01 −2.75 ± 0.02 −2.46 ± 0.01 −2.18 ± 0.01 −2.06 ± 0.02 −1.92 ± 0.01
−3.37a (5.81 ± 0.60) −3.20a, −3.18d, −3.92e (5.79 ± 1.08) −2.20a (6.80 ± 0.54) (6.58 ± 0.89) (8.64 ± 0.61)
(5.87 ± 0.67) (6.25 ± 0.56) (7.16 ± 0.68)
0.1 −2.58 ± 0.02 −2.44 ± 0.01 −2.30 ± 0.01 −2.16 ± 0.01 −2.03 ± 0.01 −1.85 ± 0.01 −1.80 ± 0.02 −1.71 ± 0.01
(4.57 ± 0.63) (4.63 ± 0.44) (4.42 ± 0.59) (4.91 ± 0.26) (5.26 ± 0.52) (4.91 ± 0.33) (4.81 ± 0.67) (5.47 ± 0.44)
0.25 −2.45 ± 0.01 −2.34 ± 0.01 −2.13 ± 0.01 −2.02 ± 0.01 −1.94 ± 0.01 −1.80 ± 0.01 −1.72 ± 0.01 −1.60 ± 0.01
(4.13 ± 0.20) (4.10 ± 0.53) (4.86 ± 0.47) (3.95 ± 0.28) (4.30 ± 0.54) (6.42 ± 0.53) (4.91 ± 0.46) (4.51 ± 0.36)
0.50 −2.33 ± 0.02 −2.23 ± 0.01 −2.07 ± 0.02 −1.93 ± 0.03 −1.75 ± 0.02 −1.63 ± 0.02 −1.57 ± 0.03 −1.46 ± 0.01
(4.55 ± 0.93) (4.83 ± 0.61) (5.03 ± 0.74) (5.28 ± 1.13) (4.76 ± 0.80) (5.48 ± 0.73) (4.85 ± 1.26) (5.25 ± 0.60)
0.75 −2.28 ± 0.02 −2.16 ± 0.02 −2.00 ± 0.01 −1.87 ± 0.02 −1.73 ± 0.02 −1.58 ± 0.02 −1.47 ± 0.02 −1.40 ± 0.01
(6.44 ± 1.12) (6.33 ± 0.81) (5.86 ± 0.44) (6.13 ± 1.23) (6.46 ± 1.04) (7.82 ± 1.12) (6.15 ± 1.23) (6.94 ± 0.58)
L-Glutamic acid in an aqueous solution of tri-sodium citrate
0.1 −2.19 ± 0.02 −2.04 ± 0.02 −1.94 ± 0.02 −1.77 ± 0.02 −1.69 ± 0.01 −1.47 ± 0.02 −1.43 ± 0.01 −1.33 ± 0.02
(5.49 ± 0.67) (4.35 ± 0.71) (5.21 ± 0.96) (5.62 ± 0.74) (5.27 ± 0.59) (4.55 ± 0.66) (5.82 ± 0.61) (5.91 ± 0.80)
0.25 −2.05 ± 0.02 −1.90 ± 0.02 −1.77 ± 0.01 −1.62 ± 0.02 −1.49 ± 0.02 −1.31 ± 0.02 −1.27 ± 0.02 −1.15 ± 0.01
(5.71 ± 0.68) (6.24 ± 1.06) (4.68 ± 0.56) (6.34 ± 0.87) (4.84 ± 0.91) (6.43 ± 0.91) (6.40 ± 0.87) (4.43 ± 0.62)
0.50 −1.90 ± 0.01 −1.77 ± 0.01 −1.60 ± 0.02 −1.46 ± 0.01 −1.35 ± 0.01 −1.23 ± 0.02 −1.08 ± 0.01 −1.03 ± 0.01
(4.97 ± 0.71) (6.04 ± 0.54) (6.09 ± 0.83) (6.47 ± 0.56) (5.19 ± 0.66) (3.18 ± 1.05) (1.61 ± 0.43) (1.04 ± 0.16)
0.75 −1.76 ± 0.02 −1.65 ± 0.01 −1.49 ± 0.01 −1.36 ± 0.01 −1.23 ± 0.02 −1.07 ± 0.01 −1.01 ± 0.01 −1.00 ± 0.01
(5.66 ± 0.89) (5.70 ± 0.67) (6.84 ± 0.68) (5.43 ± 0.60) (4.89 ± 0.81) (0.59 ± 0.12) (0.15 ± 0.01) (0.45 ± 0.22)


3.2.4. Partial molar isentropic compression of transfer, ΔtrKs,2,ϕ0. The partial molar isentropic compression of transfer values (ΔtrKs,2,ϕ0) were calculated using the following equation:
 
ΔtrKs,2,ϕ0 = Ks,2,ϕ0 (in aqueous solutions of DST/TSC) − Ks,2,ϕ0 (in water).(13)

The ΔtrKs,2,ϕ0 values are positive for all solutes; this suggests the predominance of hydrophilic–ionic/hydrophilic interactions and the strengthening of these interactions over the entire studied concentration range. The magnitudes decrease with an increase in temperature, as shown in Fig. 4(b, e, h and k). Due to interactions between the hydrophilic sites of the amino acids and the citrate and tartrate ions of the co-solutes, the less compressible water molecules in the hydration shells of the solute molecules come out in the bulk water, hence exhibiting positive ΔtrKs,2,ϕ0 values. Higher ΔtrKs,2,ϕ0 values are observed for the studied solutes in the case of TSC compared to DST.

3.3. Viscosity measurements

Viscosities have been determined for ASP and GLU in water and in aqueous solutions of DST and TSC, over an mB value range from 0.1–0.75 mol kg−1 and from T = 288.15 to 318.15 K, from flow time measurements using the following expression:
 
η/ρ = AtB/t(14)
where ρ is the density of the solution, t is the flow time, and A and B are viscometric constants (A = 0.0000669 mPa m3 kg−1 and B = 0.2244 mPa m3 s2 kg−1). The η values of TSC and GLU in water at 298.15 K matched well with the literature data (Fig. S1, ESI).22,28 The η values increase with an increase in the concentration of amino acid in water and in aqueous solutions of DST/TSC, and decrease with temperature (Table 6). The variation in η values for solutions of GLU in aqueous DST at different temperatures are shown in Fig. 3(d) (viscosity, η, versus molality, mA, for GLU in DST at mB = 0.1 mol kg−1 as a function of temperature).
Table 6 Viscosities, η, of L-aspartic acid and L-glutamic acid in water and in aqueous solutions of di-sodium tartrate and tri-sodium citrate at T values from 288.15 to 318.15 K and P = 101.3 kPa
η/mPa s
m A T/K T/K T/K T/K
mol kg−1 288.15 298.15 308.15 318.15
m A is the molality of the solute in water or water + DST/TSC (solvent). mB is the molality of DST/TSC in water. Standard uncertainties are u(T) = 0.03 K, u(P) = 0.5 kPa, u(m) = 2.8 × 10−4 mol kg−1 and u(η) = 0.012 mPa s.
L-Aspartic acid in water
η 0 = 1.1382 mPa s η 0 = 0.8904 mPa s η 0 = 0.7194 mPa s η 0 = 0.5963 mPa s
0.00517 1.1390 0.8910 0.7197 0.5967
0.00712 1.1392 0.8912 0.7200 0.5968
0.00921 1.1395 0.8915 0.7202 0.5970
0.01109 1.1397 0.8917 0.7205 0.5972
0.01300 1.1400 0.8919 0.7206 0.5973
0.01506 1.1404 0.8921 0.7208 0.5975
L-Aspartic acid in an aqueous solution of di-sodium tartrate at mB= 0.10 mol kg−1
η 0 = 1.1631 mPa s η 0 = 0.9518 mPa s η 0 = 0.7783 mPa s η 0 = 0.6644 mPa s
0.00502 1.1637 0.9525 0.7787 0.6647
0.00741 1.1640 0.9528 0.7790 0.6650
0.01180 1.1643 0.9529 0.7792 0.6651
0.01306 1.1646 0.9531 0.7794 0.6653
0.01732 1.1651 0.9534 0.7797 0.6655
L-Aspartic acid in an aqueous solution of di-sodium tartrate at mB= 0.25 mol kg−1
η 0 = 1.2425 mPa s η 0 = 1.0248 mPa s η 0 = 0.8544 mPa s η 0 = 0.7244 mPa s
0.00584 1.2434 1.0255 0.8550 0.7247
0.00817 1.2436 1.0257 0.8553 0.7250
0.01106 1.2439 1.0260 0.8555 0.7253
0.01337 1.2442 1.0264 0.8556 0.7255
0.01555 1.2446 1.0266 0.8559 0.7256
0.01667 1.2451 1.0269 0.8561 0.7258
L-Aspartic acid in an aqueous solution of di-sodium tartrate at mB= 0.5 mol kg−1
η 0 = 1.4178 mPa s η 0 = 1.1996 mPa s η 0 = 0.9924 mPa s η 0 = 0.8446 mPa s
0.00638 1.4186 1.2005 0.9931 0.8448
0.00782 1.4188 1.2008 0.9933 0.8453
0.00982 1.4193 1.2011 0.9936 0.8456
0.01307 1.4199 1.2013 0.9939 0.8460
0.01509 1.4203 1.2017 0.9942 0.8462
0.01639 1.4209 1.2021 0.9944 0.8465
L-Aspartic acid in an aqueous solution of di-sodium tartrate at mB= 0.75 mol kg−1
η 0 = 1.6087 mPa s η 0 = 1.3980 mPa s η 0 = 1.1562 mPa s η 0 = 0.9866 mPa s
0.00547 1.6099 1.3990 1.1570 0.9873
0.00739 1.6102 1.3992 1.1573 0.9875
0.01025 1.6107 1.3999 1.1577 0.9878
0.01347 1.6111 1.4001 1.1580 0.9881
0.01664 1.6115 1.4005 1.1583 0.9885
L-Aspartic acid in an aqueous solution of tri-sodium citrate at mB= 0.1 mol kg−1
η 0 = 1.2165 mPa s η 0 = 0.9856 mPa s η 0 = 0.8023 mPa s η 0 = 0.6650 mPa s
0.00576 1.2170 0.9862 0.8029 0.6655
0.00754 1.2172 0.9865 0.8031 0.6656
0.01002 1.2175 0.9868 0.8033 0.6658
0.01189 1.2178 0.9871 0.8035 0.6660
0.0152 1.2180 0.9874 0.8037 0.6662
L-Aspartic acid in an aqueous solution of tri-sodium citrate at mB= 0.25 mol kg−1
η 0 = 1.4206 mPa s η 0 = 1.1424 mPa s η 0 = 0.9460 mPa s η 0 = 0.7902 mPa s
0.00624 1.4217 1.1431 0.9467 0.7906
0.00784 1.4220 1.1434 0.9469 0.7908
0.01080 1.4224 1.1438 0.9473 0.7912
0.01208 1.4228 1.1442 0.9475 0.7914
0.01490 1.4231 1.1445 0.9478 0.7917
0.01825 1.4234 1.1448 0.9480 0.7921
L-Aspartic acid in an aqueous solution of tri-sodium citrate at mB= 0.5 mol kg−1
η 0 = 1.7327 mPa s η 0 = 1.4619 mPa s η 0 = 1.2096 mPa s η 0 = 1.0194 mPa s
0.00597 1.7340 1.4631 1.2103 1.0199
0.00809 1.7344 1.4634 1.2107 1.0204
0.00985 1.7347 1.4638 1.2111 1.0207
0.01284 1.7352 1.4640 1.2114 1.0209
0.01520 1.7356 1.4644 1.2118 1.0212
0.01810 1.7361 1.4649 1.2120 1.0215
L-Aspartic acid in an aqueous solution of tri-sodium citrate at mB= 0.75 mol kg−1
η 0 = 1.9928 mPa s η 0 = 1.7555 mPa s η 0 = 1.4983 mPa s η 0 = 1.2697 mPa s
0.00626 1.9941 1.7569 1.4992 1.2703
0.00788 1.9946 1.7574 1.4999 1.2709
0.01082 1.9952 1.7578 1.5003 1.2712
0.01180 1.9958 1.7582 1.5007 1.2717
0.01461 1.9964 1.7587 1.5011 1.2720
L-Glutamic acid in water
0.01104 1.1404 0.8921 0.7205 0.5974
0.01308 1.1407 0.8923 0.7209 0.5976
0.01522 1.1411 0.8927 0.7214 0.5978
0.02078 1.1417 0.8932 0.7218 0.5982
0.02517 1.1422 0.8937 0.7221 0.5986
0.03030 1.1429 0.8942 0.7225 0.5990
L-Glutamic acid in an aqueous solution of di-sodium tartrate at mB= 0.1 mol kg−1
0.01288 1.1646 0.9525 0.7789 0.6650
0.01523 1.1653 0.9534 0.7797 0.6655
0.01930 1.1661 0.9543 0.7803 0.6661
0.02496 1.1669 0.9549 0.7810 0.6666
0.03127 1.1676 0.9556 0.7815 0.6670
0.03591 1.1681 0.9564 0.7820 0.6675
L-Glutamic acid in an aqueous solution of di-sodium tartrate at mB= 0.25 mol kg−1
0.01410 1.2443 1.0262 0.8556 0.7250
0.01537 1.2450 1.0268 0.8563 0.7257
0.01930 1.2459 1.0275 0.8567 0.7262
0.02391 1.2464 1.0281 0.8571 0.7268
0.03000 1.2471 1.0287 0.8577 0.7273
0.03235 1.2478 1.0296 0.8583 0.7278
L-Glutamic acid in an aqueous solution of di-sodium tartrate at mB= 0.5 mol kg−1
0.01228 1.4199 1.2015 0.9935 0.8452
0.01354 1.4209 1.2023 0.9944 0.8462
0.01654 1.4217 1.2029 0.9952 0.8469
0.02018 1.4223 1.2034 0.9958 0.8475
0.02585 1.4230 1.2043 0.9963 0.8481
0.03703 1.4238 1.2052 0.9971 0.8488
L-Glutamic acid in an aqueous solution of di-sodium tartrate at mB= 0.75 mol kg−1
0.01159 1.6114 1.4004 1.1582 0.9877
0.01375 1.6125 1.4011 1.1588 0.9886
0.01765 1.6131 1.4017 1.1592 0.9895
0.01995 1.6139 1.4027 1.1602 0.9904
0.02676 1.6149 1.4036 1.1608 0.9908
L-Glutamic acid in an aqueous solution of tri-sodium citrate at mB= 0.1 mol kg−1
0.01247 1.2182 0.9874 0.8036 0.6660
0.01463 1.2188 0.9878 0.8039 0.6663
0.01634 1.2193 0.9882 0.8043 0.6667
0.02218 1.2198 0.9889 0.8050 0.6671
0.02857 1.2205 0.9893 0.8055 0.6676
0.03390 1.2212 0.9899 0.8060 0.6682
L-Glutamic acid in an aqueous solution of tri-sodium citrate at mB= 0.25 mol kg−1
0.01213 1.4232 1.1442 0.9474 0.7914
0.01438 1.4237 1.1446 0.9479 0.7919
0.01771 1.4243 1.1457 0.9484 0.7924
0.02308 1.4250 1.1462 0.9490 0.7929
0.02531 1.4257 1.1467 0.9497 0.7934
0.03401 1.4271 1.1475 0.9507 0.7938
L-Glutamic acid in an aqueous solution of tri-sodium citrate at mB= 0.5 mol kg−1
0.01104 1.7359 1.4646 1.2114 1.0211
0.01239 1.7362 1.4652 1.2122 1.0217
0.01547 1.7370 1.4659 1.2129 1.0223
0.02012 1.7381 1.4670 1.2138 1.0227
0.02521 1.7392 1.4677 1.2144 1.0239
0.03006 1.7410 1.4686 1.2154 1.0246
L-Glutamic acid in an aqueous solution of tri-sodium citrate at mB= 0.75 mol kg−1
0.01088 1.9955 1.7581 1.5003 1.2716
0.01408 1.9973 1.7596 1.5010 1.2726
0.01532 1.9978 1.7604 1.5017 1.2735
0.02000 1.9991 1.7614 1.5034 1.2740
0.02799 2.0009 1.7626 1.5050 1.2752
0.03107 2.0019 1.7636 1.5057 1.2766


3.3.1. Viscosity B-coefficient. The relative viscosities are given by
 
ηr = η/η0(15)
where, η and η0 are the viscosities of the solution and solvent, respectively. The B-coefficients were evaluated by fitting the ηr values to the Jones–Dole equation43via a least squares method as follows:
 
ηr = 1 + BC(16)
where C is the molar concentration (calculated from the molality and density data). The viscosity B-coefficient or B value provides knowledge about the solvation behaviour of solutes. The B values of ASP and GLU in water at 298.15 K are in line with the literature values.14 The B values for the studied amino acids in water and in aqueous solutions of DST and TSC increase with temperature and with the molality of the co-solutes and they are given in Table S7. The increase is greater in the case of TSC compared to DST. As the B-coefficient is directly dependent upon the size of the solute, the B value of GLU is slightly greater compared to ASP. Moreover, due to the greater ionic strength of TSC, the observed B values are larger compared to DST. The increase in the B values with the DST and TSC solution concentrations indicates a progressively more structured environment.

The derivative of the B-coefficient with respect to T (dB/dT) is a far better parameter for investigating the effects of additives on the structure of water, because it provides better information about the structure-making or structure-breaking behavior of the solute compared with the B-coefficient. A positive value of dB/dT indicates that a solute is a structure-breaker, whereas a negative value indicates that a solute is a structure-maker.44 The dB/dT values for ASP and GLU in water and in aqueous solutions of DST and TSC at different temperatures are given in Table S7. The dB/dT values for ASP and GLU are positive in water and in aqueous DST and TSC solutions, which classifies these solutes as structure-breakers due to the dominating effects of the hydrophilic groups (–NH2 and –COOH) over the R-groups.

3.3.2. Viscosity B-coefficient of transfer, ΔtrB. The viscosity B-coefficients of transfer, ΔtrB, from water to aqueous DST and TSC solutions have been calculated as follows:
 
ΔtrB = B-coefficient (in aqueous DST/TSC solution)-B- coefficient (in water).(17)

The B-coefficient values for the ASP and GLU amino acids in aqueous solutions of DST and TSC are higher than the values in water, which results in positive ΔtrB values. Plots of the ΔtrB values as a function of the DST and TSC molality are shown in Fig. 4(c, f, i, l). The transfer values increase with an increase in the concentration of DST/TSC. For both solutes, the ΔtrB values decrease with temperature in both aqueous DST and TSC solutions. The ΔtrB values have greater magnitudes in the case of TSC, which may again be attributed to the high charge density.

3.3.3. Ratio of the B-coefficient to the partial molar volume, B/V2,ϕ0. The solvation can be judged by the ratio of the B-coefficient to the partial molar volume (B/V2,ϕ0). Unsolvated spherical species have B/V2,ϕ0 values ranging between 0 and 2.5.14 Solvated spherical species have B/V2,ϕ0 values higher than 2.5. In the present study, the lesser magnitudes of the B/V2,ϕ0 values of the studied amino acids in the presence of the co-solutes compared to in water indicate that these amino acids are more solvated in water (Table S7).

3.4. Enthalpy measurements from calorimetry

The q values for ASP/GLU (30 mmol kg−1) in the absence, as well as in the presence, of aqueous solutions of DST and TSC over an mB range from 100–500 mmol kg−1 at T = 298.15 K have been measured using ITC. Positive q values for the solutes have been observed in water and in the presence of TSC/DST, whereas in some cases involving DST, negative q values have also been observed (Table 7). These values decrease with the molality of the solutes in water and increase in the presence of the co-solutes. The ΔdilH0 values (Table S8) have been calculated by fitting the q and mA data to the following equations:
 
q = ΔdilH0 + mASw(18)
 
q = ΔdilH0 + mASw1 + mA2Sw2 + ...(19)
where Sw is the slope obtained via linear regression. In some cases, ΔdilH0 values have been obtained using high order polynomial fitting with the constants Sw1, Sw2, etc. Positive ΔdilH0 values, except for ASP in 100 mM DST and GLU in 100 and 250 mM DST, are observed for the studied amino acids in aqueous media and in the presence of the co-solutes. The positive ΔdilH0 values indicate the dominance of dehydration effects over hydrophilic–ionic/hydrophilic interactions. These dehydration effects encourage their use to prevent against microbial activity. Negative ΔdilH0 values indicate the predominance of hydrophilic–ionic/hydrophilic interactions. The magnitude of endothermicity for ASP/GLU is greater in the case of TSC than in the case of DST, which is in accordance with the greater dehydration effects of TSC compared to DST. This shows that hydrophilic–ionic/hydrophilic interactions are more predominant in the case of DST.
Table 7 Enthalpy of dilution values, q, of aqueous solutions of L-aspartic acid and L-glutamic acid in water and in aqueous solutions of di-sodium tartrate and tri-sodium citrate at a T value of 298.15 K and P = 101.3 kPa
m A/mmol kg−1 q/J mol−1
m B = 0 mmol kg−1 m B = 100 mmol kg−1 m B = 250 mmol kg−1 m B = 500 mmol kg−1 m B = 100 mmol kg−1 m B = 250 mmol kg−1 m B = 500 mmol kg−1
The standard uncertainties, u, are u(m) = 2.8·10−4 mol kg−1, u(q) = 0.5 kJ mol−1, u(T) = 0.03 K, and u(P) = 0.5 kPa.
L-Aspartic acid in an aqueous solution of di-sodium tartrate L-Aspartic acid in an aqueous solution of tri-sodium citrate
0.05897 2331.13 −445.97 1478.99 1412.85 2981.24 3639.50 5972.42
0.35207 1928.97 116.10 1553.81 1366.52 3057.06 3758.56 6016.78
0.64226 1120.55 607.86 1638.12 1309.33 3208.70 3843.82 6063.55
0.92955 707.04 961.64 1708.83 1264.60 3285.24 3932.01 6113.76
1.21394 538.85 1196.41 1772.46 1249.85 3345.96 3995.34 6156.42
1.49542 450.80 1354.93 1822.87 1227.90 3481.98 4032.45 6214.78
1.77400 393.72 1449.52 1871.82 1215.09 3571.76 4136.79 6251.32
2.04968 351.35 1514.27 1926.90 1244.20 3661.63 4165.05 6306.43
2.32246 317.49 1549.66 1986.88 1273.96 3736.21 4183.07 6347.57
2.59233 286.92 1587.42 2050.69 1335.26 3839.65 4259.48 6396.35
2.85929 263.99 1618.50 2123.13 1445.36 3905.29 4307.43 6452.41
3.12336 238.77 1655.39 2188.19 1518.47 3987.36 4361.31 6488.21
3.38452 228.07 1678.46 2299.31 1603.81 4084.03 4485.43 6537.78
3.64278 209.37 1677.83 2398.92 1708.21 4179.06 4562.62 6577.24
3.89813 198.41 1677.24 2456.84 1802.71 4256.64 4622.75 6627.50
4.15058 183.65 1697.28 2512.35 1915.64 4342.19 4726.03 6665.68
4.40013 168.35 1698.79 2561.23 2008.51 4432.38 4812.32 6705.08
4.64677 164.13 1698.79 2606.28 2090.75 4508.28 4868.91 6749.22
4.89051 154.10 1704.97 2703.05 2171.57 4565.64 4934.77 6798.33
L-Glutamic acid in an aqueous solution of Di-sodium tartrate L-Glutamic acid in an aqueous solution of tri-sodium citrate
0.05897 397.36 −953.53 −868.11 345.92 1126.24 3303.28 4329.99
0.35207 339.42 −479.12 −816.65 423.73 1194.15 3343.50 4411.15
0.64226 274.18 −67.00 −772.47 469.41 1256.93 3391.24 4510.22
0.92955 251.38 145.23 −714.45 555.31 1314.97 3441.71 4593.43
1.21394 216.36 248.50 −662.62 644.28 1414.14 3491.56 4659.79
1.49542 179.98 306.75 −628.30 697.72 1465.35 3537.03 4727.20
1.77400 160.58 352.91 −592.38 753.07 1556.91 3589.88 4792.46
2.04968 141.98 394.64 −536.11 850.51 1635.86 3641.27 4837.82
2.32246 122.06 417.21 −502.91 902.46 1692.17 3696.22 4900.68
2.59233 105.23 431.29 −475.78 949.48 1768.96 3750.99 4945.44
2.85929 94.52 456.15 −437.47 999.15 1852.31 3807.83 5001.87
3.12336 72.25 487.66 −384.81 1045.31 1917.93 3849.69 5042.00
3.38452 58.34 490.60 −355.74 1094.73 1973.39 3908.89 5115.39
3.64278 42.74 493.88 −315.97 1190.22 2064.57 3953.95 5176.35
3.89813 36.68 478.74 −280.04 1237.70 2142.39 3998.64 5224.55
4.15058 28.84 499.50 −236.68 1275.55 2256.46 4032.62 5272.35
4.40013 19.63 501.58 −194.97 1326.44 2323.28 4079.62 5345.51
4.64677 10.21 504.97 −153.45 1382.78 2392.10 4130.85 5397.24
4.89051 4.70 504.22 −125.31 1448.78 2468.88 4180.52 5466.93


The ΔtrΔdilH0 values, i.e., the standard molar enthalpy of transfer, for the studied amino acids, from water to DST/TSC(aq.) are evaluated using the equation:

 
ΔtrΔdilH0 = ΔdilH0 (in DST/TSC(aq.)) − ΔdilH0 (in water).(20)

Negative ΔtrΔdilH0 values for ASP in 100 mM TSC, ASP in DST and GLU in DST at all molalities, and positive ΔtrΔdilH0 values for ASP in 250 mM and 500 mM TSC and GLU in TSC at all molalities have been observed (Fig. S2, ESI). The more positive magnitudes of the ΔtrΔdilH0 values in the case of GLU than ASP are due to the presence of an extra –CH2 (hydrophobic) group. The negative ΔtrΔdilH0 values suggest the dominance of hydrophilic-ionic/hydrophilic interactions and positive ΔtrΔdilH0 values may be due to the predominance of co-solute dehydration effects. Overall, the positive ΔtrΔdilH0 values in the case of TSC (greater dehydration effects) and the negative ΔtrΔdilH0 values in the case of DST coincide with the results obtained from the volumetric studies.

3.5. Analysis of NMR spectra

1H spectra have been recorded for ASP and GLU in the absence, as well as in the presence, of DST and TSC using a Bruker (AVANCE-III, HD 500 MHz) spectrometer at a probe temperature of 300.15 K. D2O was used for the deuterium lock and its signal at 4.650 ppm was taken as the internal reference for the other nuclei. The NMR spectra of ASP and GLU in water (mA = 0.01 and 0.02 mol kg−1) and in the presence of aqueous solutions of DST and TSC (mB = 0.75 mol kg−1) have been examined in 9[thin space (1/6-em)]:[thin space (1/6-em)]1 H2O–D2O solution. The changes in chemical shifts (δ) in ppm are summarized in Table 8.
Table 8 1H data for L-aspartic acid and L-glutamic acid in water and in aqueous solutions of di-sodium tartrate and tri-sodium citrate
T = triplet, Q = quartet, M = multiplet.
image file: c9fo00872a-u5.tif


In the 1H NMR spectra of ASP and GLU in aqueous solutions of DST and TSC (mB = 0.75 mol kg−1), the protons go upfield compared to in the D2O[thin space (1/6-em)]:[thin space (1/6-em)]H2O system (Fig. S3). This shows that the hydrogen bonding of the solutes with water molecules decreases in the presence of DST/TSC owing to the dominance of dehydration effects over hydrophilic–ionic/hydrophilic interactions.

4. Conclusions

Volumetric, acoustic, viscometric, calorimetric and NMR spectroscopic studies have been performed in the present study. The values of ΔtrV2,ϕ0, ΔtrK2,ϕ0 and ΔtrB suggest that hydrophilic–ionic/hydrophilic interactions dominate in the present study. The sweet tastes of both amino acids decrease in the presence of the studied preservatives. The values of the interaction parameter predict pairwise interactions between the amino acids and preservatives. Volumetric and viscometric results indicate that the amino acids behave as structure-breakers in both salt solutions, except in the case of GLU in TSC at mB = 0.1 mol kg−1. The hydration number and B/V2,ϕ0 values suggest that these amino acids are more solvated in water. Calorimetric and spectroscopic results support the fact that these preservatives stimulate the effects of dehydration on the studied amino acids.

Abbreviations

GLU L-Glutamic acid
ASP L-Aspartic acid
DSTDisodium tartrate
TSCTrisodium citrate
DSADensity and sound velocity meter
ITCIsothermal titration calorimetry
NMRNuclear magnetic resonance

Conflicts of interest

The authors declare that there are no conflicts of interest.

Acknowledgements

One of the authors (Aashima Beri) is grateful to the Department of Science and Technology, New Delhi for the award of an Inspire Fellowship. The authors also acknowledge the DST-PURSE scheme of the Department of Science and Technology, New Delhi and the UPE-Scheme of the University Grants Commission, New Delhi for research facilities.

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Footnote

Electronic supplementary information (ESI) available: DETAILS. See DOI: 10.1039/c9fo00872a

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