Interaction of cationic gemini surfactant tetramethylene-1,4-bis(dimethyltetradecylammonium bromide) with anionic polyelectrolyte sodium carboxymethyl cellulose, with two different molar masses, in aqueous and aquo-organic (isopropanol) media

Abstract

A comparative study of the interactions of the cationic gemini surfactant tetramethylene-1,4-bis(dimethyltetradecylammonium bromide) (14-4-14) with anionic polymer sodium carboxymethylcellulose (NaCMC), with two different molar masses, was performed in water and isopropanol (IP)–water media. The interaction process was studied in detail using conductometry, tensiometry, turbidimetry and viscometry. At very low concentration, 14-4-14 monomers interact with the polymer. Above the critical aggregation concentration (cac), small micelle-like aggregates form complexes with the polymer. During the interaction process, coacervates are formed beyond C_s (polymer saturation concentration), which initially grow by aggregation and stay in the solution throughout the process. The interaction process is affected by addition of isopropanol into the medium. The intrinsic viscosities of the two NaCMCs were determined by using Huggins and Kraemer equations. Dynamic light scattering (DLS) study helps to determine the hydrodynamic size of the dispersed polymer and its surfactant-interacting complexes. The hydrodynamic size of the dispersed polymer and its interacting complex in IP–water media is lower than that obtained in aqueous solution. The surface morphology of the solvent removed polymer and its surfactant-interacting complex were examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. The pattern of the morphologies depends on the polymer–surfactant composition and the solvent environment.

---

1. Introduction

The amphiphilic nature of surfactants makes them essential ingredients of age old technological processes, such as soap, cosmetics, and paper making, etc. Today they are also widely used in gene transfection, preparation of mesoporous materials, phase transfer catalysts, etc.^1–5 In addition to this, complex formation between ionic surfactant and oppositely charged polyelectrolyte is a fascinating area of research.^6–13 The ternary system of polymer–surfactant–water has drawn considerable attention due to the wide range of domestic and industrial applications such as, detergency, food formulations, paints, drug delivery systems, coatings, cosmetics, enhanced oil recovery, water purification etc.^14–23 Polymers are generally used as viscosity modifier and binder. The mixture of polyelectrolytes and surfactants with opposite charges often shows strong tendency to form insoluble complexes and leads to phase separation which may be either liquid–liquid phase separation (coacervation) or solid–liquid phase separation (precipitation).^6,9,23 In coacervation process, the colloidal or polymer solution separates into two immiscible liquid phases. This process can be divided into two distinct classes, simple and complex coacervation. Addition of additives (salt, alcohols etc.) to single component colloid solution produced simple coacervation while addition of oppositely charged macromolecular component produced complex coacervation. The important factors for coacervation are polyelectrolyte composition, charge density, chain length, chain flexibility, polymer molecular weight, micelle surface charge density, micelle size, shape, pH and ionic strength of the medium, and so on.^6,9,24–26 Coacervation is mainly dominated by suitable balance among electrostatic, hydrophobic and solvent interaction.^27,28 The field is enriched everyday by using different combinations of additives, like alcohols (especially, in pharmaceutics and cosmetics) and salts etc. to modify and control the solubility, durability and dispersity of the system. Alcohols are frequently used as cosolvents as well as cosurfactants. As a cosurfactant, alcohol increases the cmc, but, as a cosolvent they increase the hydrophobicity of the medium and hence cmc increases. Isopropanol is used as additives in pharmaceutical and medicinal preparation as it is nontoxic, biofriendly and fairly soluble in water.^29,30 All the above factors mutually control the interaction between polyelectrolyte and ionic surfactant. Here is the importance of focusing on the effect of cosolvent (IP) and polymer molar mass in the interaction process.

In this work, we have used sodium carboxymethyl cellulose (NaCMC), one of the ionic derivatives of cellulose.³¹ It is a water soluble linear polymer having β-1,4-linkage between the glycane unit. The structure of NaCMC was shown in Scheme 1 of ESI.†¹⁰ It acts as a polyanion at pH > 4. NaCMC is a useful component of many food systems and is widely used as a thickener and binding agent in pharmaceutical applications. Interaction of NaCMC with alkyl ammonium bromide was widely studied;^10,32–34 but the interaction of NaCMC with cationic gemini surfactant is rare. Recently, Kabir-ud-Din et al.³⁵ have reported the interaction of cationic gemini surfactants with NaCMC, but their observations were quite different from our study. Actually, gemini surfactants are a novel class of compounds. This class of surfactants impart better surface properties as compared to the conventional single chain surfactants,^36–39 such as remarkably low critical micelle concentration (cmc), high surface activity, better wetting ability, unusual aggregation morphologies, low Kraft temperature, unusual rheological properties etc. Having such characteristics, gemini surfactant is superior than conventional single chain surfactant. The interaction between NaCMC and gemini surfactant in aquo-IP medium may be helpful from application point of view. There have been reports on interaction between gemini surfactant and polyelectrolyte, but report on the effect of polymer molar mass and polar organic solvent (isopropanol, IP) on the interaction is rare.

In this work, we have attempted a detailed study on the interaction of cationic gemini surfactant 14-4-14 with anionic polyelectrolyte NaCMC having two different molar masses in aqueous and IP–water medium by tensiometric, conductometric, fluorimetric, turbidimetric and viscometric methods. The hydrodynamic size of the surfactant alone and also the polymer interacted complex were determined by dynamic light scattering (DLS) method. The morphology of the polymer and surfactant–polymer interacted complex were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. We also try to focus on the effect of molar mass of the polymer on the interaction process.

2. Experimental section

2.1. Materials

Sodium carboxymethylcellulose (NaCMC) was used AR grade product of Aldrich, with a viscosity-average molecular weight 700 000 (NaCMC-1) and 208 000 (NaCMC-2) and average degree of substitution (average number of carboxymethyl groups per cellulose unit), S = 0.9 and S > 0.4 respectively. The structure of NaCMC was given in the ESI.†

The gemini surfactant tetramethylene-1,4-bis(dimethyltetradecylammonium bromide) (14-4-14) (GS) was synthesized in our laboratory.⁴⁰ The structure was confirmed by ¹H NMR spectroscopy. The purity of the GS was further confirmed by the absence of minima in surface tension (γ) vs. log[surfactant] plot. The structure of the gemini surfactant was depicted in Scheme 1. 1,4-Dibromo butane and N,N-dimethyltetradecyl amine were purchased from Aldrich and used without any further purification. AR grade ethanol (purity = 99.9%) was purchased from Changshu Yangyuan Chemical. Isopropanol (IP) used was an AR grade product of SRL, India. Double distilled water of specific conductance of 2–4 μS cm⁻¹ was used for the preparation of all sample solutions at 298 K.

Scheme 1 Tetramethylene-1,4-bis(dimethyltetradecylammonium bromide) (14-4-14).

2.2. Methods

2.2.1. Tensiometry. Tensiometric experiments were studied with a du Noüy tensiometer (Krüss, Germany) by the platinum ring detachment method. 5 mL volume of water or polymer solution of the desired concentration was taken in a double-wall jacketed container placed in a thermostated water bath (accuracy, ±0.1 K) at the requisite temperature and surfactant solution of known concentration (∼30 times the cmc) was added progressively. The cmc values of the surfactant were estimated from the break points in the surface tension (γ) vs. log[surfactant] plots. The determined surface tension (γ) values were accurate within ±0.1 mN m⁻¹. The detailed procedure was described elsewhere.^10,41,42

2.2.2. Conductometry. The conductance measurements were performed with Eütech (Singapore) (cell constant = 1 cm⁻¹). 6 mL volume of water or polymer solution of desired concentration was placed in a double-wall jacketed container and temperature was controlled by a Hahntech DC-2006 circulating bath with an accuracy of ±0.1 K. The specific conductance (κ) was measured after each addition of surfactant solution of known concentration followed by thorough mixing and attaining temperature equilibrium. The cmc values were estimated from the break points in the specific conductance (κ)–[surfactant] plots. The detailed method is available in ref. 42.

2.2.3. Turbidimetry. The turbidity of 14-4-14/NaCMC solution was measured in a Shimadzu, 1601 (Japan) UV-Vis spectrophotometer operating in dual beam mode under a thermostated condition (298 ± 0.1 K). A matched pair of quartz cuvettes of path length 1 cm was taken for this measurement. The measurements were taken in transmittance (% T) mode where turbidity of the said system was reported as 100 − % T. In the turbidimetric titration, 2.5 mL of NaCMC solution in water/water–IP solution was taken; 14-4-14 in the respective medium was added with the help of a microsyringe. Then the solution was stirred and left for 5 min before taking the measurement to obtain a steady value. The plot of (100 − % T) was drawn against [14-4-14].

2.2.4. Viscometry. The viscosity measurements were performed in a two limbed Ostwald viscometer (placed in a thermostated water bath of accuracy ±0.1 K) with a flow time of 102.3 second for 15 mL of water. For the determination of intrinsic viscosity, polymer solution of different concentrations was taken in the viscometer and the flow time was recorded.

2.2.5. Dynamic light scattering (DLS). DLS experiments were done in a Malvern Zetasizer Nano-zs apparatus with a He–Ne laser (λ = 632 nm) at 298 K. Before the experiment, the polymer solutions and surfactant solutions were filtered separately through cellulose acetate paper of pore size 0.45 μm.

2.2.6. Scanning electron microscopy (SEM). The study of SEM measurements was employed with a FEI scanning electron microscope (Quanta 200, The Netherlands). A drop of sample solution after solvent removal was spin coated on a slab followed by gold–palladium coating under a pressure of 10⁻¹ mbar.

2.2.7. Transmission electron microscopy (TEM). Investigation of TEM images of polymer and polymer–surfactant interacted complex were studied with a JEOL JEM 2010 (Tokyo, Japan) HRTEM. One or two drops of freshly prepared solution were kept on the carbon-coated copper grid. The measurements were taken after vacuum evaporation of the solvent.

3. Results and discussions

3.1. Interaction of 14-4-14 with sodium carboxymethylcellulose (NaCMC) in aqueous and aqueous-IP medium

3.1.1. Tensiometry. Tensiometry is an important technique which can distinctly determine the bulk and interfacial behavior of polymer–surfactant interaction. NaCMC has no surface activity below 0.7 g%.³² Concentration of NaCMC used in our study is 0.005–0.01 g% which is much lower than the reported value. For pure surfactant, the surface tension showed the expected behavior, with an abrupt change of slope at cmc.⁴³ After progressive addition of gemini surfactant [14-4-14] to NaCMC solution, four distinct regions have been evidenced in the tensiometric profile (Fig. 1(a)). In the initial stage, after addition of small amount of surfactant solution, binding of surfactant with the anionic sites of the polymer molecule starts and the γ value decreases up to a certain concentration. After that, the surfactant molecules start to form small assemblies, called critical aggregation concentration (cac). In this region hydrophobic interaction between the tails of the 14-4-14 with the polymer lipophilic domain plays a significant role. in this region. Schwuger and Lange reported that, for NaCMC-SDS interaction, the attractive hydrophobic interaction between the DS⁻ ions and polymeric site overcomes the coulombic repulsion created from the similar charged surfactant-head group and polymer sites.⁴⁴ Identical types of polyion-oppositely charged amphiphile interaction are available in literature.^10,32,45,46

Fig. 1 Tensiometric profile for (a) interaction of NaCMC-1 with 14-4-14 in water, (b) comparison of the interaction between 0.005 g% of NaCMC-1 and NaCMC-2 with 14-4-14 in water, (c) interaction of NaCMC-1 with 14-4-14 in different vol% of IP, (d) interaction of NaCMC-1 with 14-4-14 in 10% IP.

Table 1 showed that the cac values were slightly affected by the concentration of polymer. The cac for the interaction of 14-4-14/NaCMC-1 is lower than that of 14-4-14/NaCMC-2 (Fig. 1(b)). In the post-cac region, the aggregates continue to bind with the anionic polymer sites and do not allow the transfer of the cation to the interface. As a result, the constant γ value is maintained. In this stage, the assembly of the surfactant is attached to the backbone of the polymer molecule; only the configuration of the polymer becomes changed. Such type of binding is continued up to C_s, called the polymer saturation concentration. At this higher conc, the micellar aggregates interact strongly with the polymer and the complex goes to the bulk of the solution. After that, the further addition of 14-4-14 helped the amphiphilic molecules to get adsorbed at the interface causing a reduction in γ, which became complete (i.e. saturation of the interface) at C_f (extended cmc). Beyond this point, free micelles were formed in solution and further addition of surfactant molecule does not affect the γ value. In literature, such types of phenomena of polymer–surfactant systems are available.^{10,32–34,47} The values of C_s and C_f increase with increasing concentration of polymer (Table 1). Again, with increasing the molar mass and hence the degree of substitution (S), the cac value decreases and distance between cac and C_s increases. That was shown in Fig. 1(b). The results were given in Tables 1 and S1.†

Table 1 Interaction characteristics of 14-4-14 with NaCMC-1 in aqueous and aqueous-IP medium obtained from conductometry and tensiometry methods at 298 K

% of IP	[NaCMC-1]/g%	Conductometry		Surface tension			α1	α2	−ΔG^0m kJ mol^−1
		Cs (mM)	Cf (mM)	cac (mM)	Cs (mM)	Cf (mM)
0	0.005	0.060	0.264	0.005	0.058	0.270	1.33	0.56	57.1
	0.0075	0.073	0.306	0.006	0.069	0.280	1.24	0.63	52.2
	0.01	0.082	0.325	0.012	0.092	0.337	1.11	0.67	49.6
5	0.005	0.061	0.301	0.007	0.054	0.286	1.23	0.60	53.9
	0.0075	0.077	0.314	0.007	0.075	0.302	1.21	0.63	51.9
	0.01	0.097	0.330	0.010	0.100	0.360	1.20	0.67	49.3
7	0.005	0.067	0.310	0.010	0.055	0.297	1.24	0.62	52.5
	0.0075	0.083	0.318	0.020	0.086	0.324	1.23	0.64	51.2
	0.01	0.118	0.459	0.030	0.125	0.471	1.13	0.70	46.2
10	0.005	0.070	0.340	0.013	0.063	0.314	1.20	0.63	51.4
	0.0075	0.084	0.373	0.021	0.085	0.357	1.16	0.66	49.3
	0.01	0.128	0.483	0.036	0.129	0.482	1.12	0.71	45.3

---

Now interaction of GS with NaCMC-1 is shown in presence of IP in Fig. 1(c) and (d). Fig. 1(c) shows the higher surface activity of the system of 0.005% NaCMC-1 with 14-4-14 in presence of higher concentration of IP resulting the lower surface tension of the solution. The values of cac, C_s and C_f are dependent on solvent composition and increase with increasing composition of IP content in water. The C_s and C_f were more influenced by IP compared with cac. The interaction between cationic gemini surfactant and anionic polymer form a complex which can self-assemble to form a turbid coacervate phase.⁶ Coacervates appear after C_s and exist in the solution. These are not dissolved even at higher concentration of surfactant. The rate of formation of coacervation decreases with increasing concentration of IP. The tendency towards phase separation and coacervation region increases with increasing molar mass and surface charge density of the polyelectrolyte. Interfacial parameters, e.g. Gibbs surface excess , the area of per monomer of 14-4-14 and the surface tension value of the solution at saturated air solution interface, for the free micelle formation of GS between C_s and C_f in the polymer solution are calculated from the following relations:^10,42,47

andwhere n denotes the number of species per molecule of 14-4-14 (here n =) in solution, γ is the surface tension of the solution, N is Avogadro's number and the limiting concentration is considered as C_f. The values of , and γ_Cf for both systems are presented in Tables 2 and S2.†

Table 2 Various interfacial parameters of 14-4-14 with NaCMC-1 in aqueous and aqueous-IP medium at 298 K

% of IP	[NaCMC-1]/g%	10^6 mol m^−2	nm^2 mol^−1	ΠCf mN m^−1	kJ mol^−1
0	0.005	0.30	5.53	37.9	183.4
	0.0075	0.32	5.12	37.6	169.7
	0.01	0.27	6.15	33.6	174.0
5	0.005	0.28	5.93	15.9	110.7
	0.0075	0.27	6.15	16.8	114.1
	0.01	0.26	6.39	15.8	110.1
7	0.005	0.25	6.64	12.4	102.1
	0.0075	0.23	7.22	13.0	107.7
	0.01	0.24	6.92	12.0	96.2
10	0.005	0.23	7.21	8.9	90.1
	0.0075	0.23	7.21	8.8	87.6
	0.01	0.21	7.91	9.4	90.1

---

3.1.2. Conductometry. The bulk property of interaction between the GS and anionic polymer NaCMC was determined from conductometry (Fig. 2). The conductometric profile of GS with NaCMC shows two distinct breaks which fairly agreed with the C_s and C_f of tensiometry. In the conductometric plot, no break is observed near cac. Initially, the conductance values increase linearly with the concentration of GS and after the first break (C_s), the slope increases owing to the conductance of coacervate containing solution.⁴⁷ The final break corresponds to the micellization of free GS (C_f), where bromide ion is condensed from the bulk to the ionic micellar atmosphere, resulting the formation of micellization by the free 14-4-14 surfactants.¹⁰

Fig. 2 Conductometric profile for the interaction of NaCMC-1 with 14-4-14 in water.

At all concentrations of NaCMC, the C_f values are higher than the cmc values in absence of polymer, signifying higher propensity towards micellization of the surfactant with polymer molecules. With increasing molecular weight of the polymer, the C_f values increase. This might be due to the availability of more and more reactive binding sites present in the polymer to the surfactant monomers.⁴⁸ Thus, more amount of surfactant is required to bind the polymer. After C_s, coacervation starts and the system becomes a colloidal solution.⁴⁷

Beyond C_f, free micelle formation starts with a fair degree of counter ion binding. At higher concentration of GS, the ion transport is obstructed e.g. colloidal coacervatate.⁴⁷ Owing to this reason, the slope of the post C_f region decreases. The degree of ionization α₁ and α₂ both increase with increasing concentration of polymer, α₁ decreases where as α₂ increases with increasing molecular weight of the polymer. The standard Gibbs free energy changes for the micellization process at C_f were evaluated from eqn (3) and presented in Tables 2 and S2.†

where X_Cf is the concentration of free micelle formation (C_f) expressed in mole fraction unit. R and T are the universal gas constant and absolute temperature respectively. The spontaneity of micellization decreased with increasing concentration of polymer and also with increasing molecular weight of the polymer. The standard Gibbs free energy of adsorption at the air/solution interface, was calculated from the eqn (4) and presented in Tables 2 and S2.†where Π_Cf denotes the surface pressure at C_f, i.e., γ_water − γ_Cf at the experimental temperature.

3.1.3. Turbidimetry. Interaction of cationic gemini surfactant 14-4-14 with anionic polymer, NaCMC produces turbidity due to coacervation. The turbidimetric profile for gemini-NaCMC interaction in aqueous solution for different concentrations of polymer was shown in Fig. 3.

Fig. 3 Turbidimetric profile for the interaction of NaCMC-1 with 14-4-14 in water: □, 0.005 g%, ○, 0.0075 g% and △, 0.01 g%. Inset: turbidimetric profile for the interaction of 0.005 g% NaCMC-1 with 14-4-14 in water.

The turbidimetry showed almost sigmoidal pattern at all NaCMC concentrations. Similar type of turbidimetric profile was obtained in case of inulin-octadecyl trimethyl ammonium bromide system.⁴⁷ There were three inflection points, T₁, T₂ and T₃ in the turbidimetric profile (Fig. 3, inset). For GS-NaCMC system, the solution remains clear upto [GS] = T₁. Beyond T₁, the turbidity is clearly visible and increases sharply. The values were listed in Table 3. The values of T₁ and T₃ are very close to C_s and C_f respectively in the tensiometric profile. The inflection point T₂ corresponds to the completion of adsorption of surfactants. T₂ is quite lower than C_f value obtained tensiometrically. The turbidity of the solution slightly increases beyond T₂ and then leveled off at T₃ ≈ C_f. T₃ values increase with increasing concentration of polymer and also with increasing concentration of IP. The turbidity slightly reduced beyond T₃. There are reports of vanishing of visible turbidity due to the formation of free micelles which solubilise the coacervates.^8,49 In our investigation, the coacervates are only partially solubilised even at higher concentration of polymer; but with increasing concentration of IP, turbidity decreases slightly i.e., the coacervates are partially soluble also in IP. From the solution, a small amount of the GS-NaCMC interacted complex was separated after standing for a long time.

Table 3 Interaction characteristics of 14-4-14 with NaCMC in aqueous and aqueous-IP media obtained from turbidimetry method at 298 K

% of IP	[NaCMC]/g%	NaCMC-1			NaCMC-2
		T1 (mM)	T2 (mM)	T3 (mM)	T1 (mM)	T2 (mM)	T3 (mM)
0	0.005	0.059	0.110	0.263	0.070	0.086	0.233
	0.0075	0.076	0.147	0.307	0.071	0.136	0.277
	0.01	0.084	0.178	0.314	0.086	0.141	0.293
5	0.005	0.062	0.114	0.290	0.062	0.091	0.237
	0.0075	0.082	0.146	0.317	0.075	0.164	0.295
	0.01	0.094	0.184	0.328	0.107	0.185	0.350
7	0.005	0.076	0.110	0.307	0.062	0.092	0.260
	0.0075	0.090	0.145	0.324	0.107	0.176	0.314
	0.01	0.104	0.181	0.367	0.114	0.185	0.362
10	0.005	0.072	0.112	0.328	0.070	0.105	0.275
	0.0075	0.094	0.159	0.354	0.080	0.180	0.329
	0.01	0.120	0.182	0.447	0.118	0.193	0.420

---

3.1.4. Viscosity behavior of NaCMC in IP–water medium. Viscosity is a convenient method to characterize the dimension and configuration of a polymer in solution. The relative viscosity (η_r) of NaCMC in aqueous and IP–water media can be determined by measuring the flow time, t of dilute polymer solution with respect to the time of flow for the solvent, t₀. Using relative viscosity, specific as well as intrinsic viscosities can be determined. Intrinsic viscosity [η] was determined by using Huggins⁵⁰ and Kraemer⁵¹ equationswhere η_SP [η_SP = η_r − 1] is the specific viscosity of the polymer solution and C is the concentration of NaCMC in g L⁻¹. k_H and k_M are the Huggins constant and the Kraemer's constant, respectively which are the characteristics for a given polymer system. [η]_H and [η]_M can be evaluated from the intercept of the plot of and against C. The intrinsic viscosity values obtained from these two equations were very close and enlisted in Table 4.

Table 4 Intrinsic viscosity ([η]), Huggins constant (k_H), Kraemer's constant (k_M), voluminosity (V_E) and Simha shape factor (ν) of NaCMC-1 (NaCMC-2) in water and IP–water media at 298 K

% of IP	[η]H/L g^−1	[η]M/L g^−1	kH	kM	VE	ν
0	3.10	3.18	0.57	−0.84	1.23	2.52
	(0.96)	(0.94)	(−0.41)	(−0.59)	(0.38)	(2.52)
5	2.90	2.94	0.47	−0.89	1.14	2.55
	(0.89)	(0.87)	(−0.45)	(−0.54)	(0.35)	(2.53)
7	2.75	2.76	0.45	−0.82	1.07	2.57
	(0.84)	(0.82)	(−0.42)	(-0.47)	(0.33)	(2.50)
10	2.50	2.54	0.52	−0.56	0.99	2.52
	(0.78)	(0.77)	(−0.27)	(−0.37)	(0.31)	(2.51)

---

The profile of intrinsic viscosity of NaCMC-1 and NaCMC-2 are shown in Fig. 4(a) and (b). There is a gradual decrease in [η]_H with increase in concentration of IP. As the concentration of the ions in the solutions increases, counter ion binding onto the polyion chain is enhanced considerably. This causes the coiling of macromolecules which is reflected in the decrease in the intrinsic viscosity of the polyelectrolyte solution.⁵²

Fig. 4 Profile of intrinsic viscosity of (a) NaCMC-1 and (b) NaCMC-2. (c) Profile of Y vs. C for the determination of Simha shape factor of NaCMC-1 and NaCMC-2 (Inset).

The molecular weight of the polymer can be determined from Mark–Houwink equation

[η]_H = KM_w^a (7)

where, M_w is the weight average molecular weight of the polymer and K and a are the Mark–Houwink constants. K and a of the polymer depend on the temperature, type of solvent etc. The values of K and a for NaCMC in water are 0.73 × 10⁻⁵ L g⁻¹ and 0.96 respectively (obtained from the extrapolation of the plot of K against concentration of NaCl).⁵³ On the basis of these values, the calculated [η]_H for molar masses, M = 700 000 and 208 000 of NaCMC in aqueous medium are 3.1 and 0.96 L g⁻¹ respectively. There is a fair agreement between the calculated values with the observed values. In good solvent, K value is ∼0.35 for flexible and globular polymer.⁵⁴ Huggins constant (k_H) for NaCMC-1 is positive, while the value is negative for NaCMC-2 (in the parenthesis). Higher values of k_H suggested some non-globular configuration of the polymer.

The voluminosity (V_E) of a polymer solution was calculated from the relative viscosity data at different concentrations of polymer (C) at a given temperature^55–57 by using the following equation

V_E is determined from the intercept of the plot where Y is plotted against C. Voluminosity is the measure of volume of the solvated polymer molecules. Using the value of voluminosity and intrinsic viscosity, we can determine the shape factor (ν) from Simha equation⁵⁸

[η]_H = V_Eν (9)

The Simha shape factor gives an idea about the shape of the polymer in the solution.⁵⁹ The values of V_E and ν were given in Table 4. The plots were shown in Fig. 4. Lower values of ν proposed spherical configuration of the polymer.^55,60 In our study, the obtained values of shape factor are ∼2.5. Such low value of ν indicates the spherical configuration of the polymer.

3.1.5. Dynamic light scattering. The hydrodynamic diameter and polydispersity indices (PDI) of the polymer–surfactant interacted complex were determined from DLS measurement. The DLS results for 0.0075 g% NaCMC and its interacted complex with GS in water and IP–water media are presented in Table S3.† The surfactant concentration used in the measurements was equal to cac. DLS measurements were not taken at C_s and C_f, because coacervation starts from C_s and exists beyond C_f. The distributions were shown in Fig. S1.† The hydrodynamic diameter of both polymer and polymer interacted surfactant complex decreased with increasing percentage of IP in mixed solvent. The PDI values for the polymer are quite low, indicating monodispersity whereas PDI values at cac are much higher, indicating the multidispersity of the system. The SEM data shows much larger sizes compared to the DLS results. DLS represents native dispersion of the species in the solvent media, whereas SEM indicates the aggregated species after removal of solvent.

3.1.6. Scanning electron microscopy. Morphology of the NaCMC and NaCMC-GS interacted complexes were determined by SEM technique. Fig. 5 shows the images of 0.0075 g% of NaCMC and NaCMC-GS complex in water and 10% IP–water. In water, NaCMC-1 forms only flowerlike clusters but, NaCMC-2 forms both flowerlike¹³ and globular clusters (Fig. S2†). At cac, the floral pattern is destroyed. New fern like aggregation was grown up at C_s and C_f for NaCMC having two different molar masses. In 10% IP, NaCMC-1 shows smaller polymer cluster of nearly globular geometry. On further addition of surfactant, tubular network was obtained at C_f.

Fig. 5 SEM images of (A) pure NaCMC-1 (0.0075 g%) in water; (B) interaction with 14-4-14 at cac; (C) at C_s; (D) at C_f; (E) pure NaCMC-1 (0.0075 g%) in 10% IP; (F) interaction with 14-4-14 in 10% IP at C_f.

Under similar condition, pure NaCMC-2 shows the isolated clusters and later fern-like aggregates (Fig. S2,† E′/F′).

3.1.7. Transmission electron microscopy. Morphology of the NaCMC and NaCMC-14-4-14 interacted complexes were also determined by using TEM technique. Pure NaCMC-1 (0.0075 g%) shows crystalline morphology (Fig. 6(A)) in aqueous solution which changes to flower like aggregates on addition of 14-4-14 at C_f (Fig. 6(B)). Such aggregate of NaCMC-14-4-14 interacted complexes changes to small globular aggregates on addition of 10% IP at C_f (Fig. 6(C)). Such type of small globular aggregates of inulin-OTAB complex by TEM technique has been obtained in our earlier study.⁴⁷

Fig. 6 TEM images of (A) pure NaCMC-1 (0.0075 g%) in water; (B) interaction with 14-4-14 at C_f; (C) interaction with 14-4-14 in 10% IP at C_f.

4. Conclusion

The interaction of GS with NaCMC of two different molecular weights shows that NaCMC of higher molecular weight interacts strongly than NaCMC of lower molecular one. Like normal findings, the cac (origin of small polymer induced micelles), binding of the small aggregates with the polymer that maximizes at C_s and formation of free micelle in the solution at C_f were evaluated. With increasing concentration of polymer, C_s and C_f increase. With increasing the degree of substitution (S), the cac value decreases and distance between cac and C_s increases. The values of cac, C_s and C_f are dependent on solvent composition and increased with increasing composition of IP content. Coacervation is observed in the system without any additives. They do not dissolve even at higher concentration of surfactant. Formation of coacervation decreased with increasing concentration of IP. The nature and dimension of the NaCMC-GS interacted complex can be determined from viscosity, turbidity, DLS, SEM and TEM measurements. Spherical configuration of the polymer was obtained from viscosity measurement. The SEM measurements produce differences between the morphology of NaCMC and NaCMC-GS interacted complexes in water and IP–water media.

Acknowledgements

S. D. and S. M. thank CSIR and UGC, Government of India, for Senior Research Fellowship. We also thank Prof. S. C. Bhattacharya, Department of Chemistry, Jadavpur University and Mr S. C. Mycap, Bose Institute, for DLS and SEM measurements, respectively.

References

1. A. J. Kirby, P. Camilleri, J. B. F. N. Engberts, M. C. Feiters, R. J. M. Nolte, O. Söderman, M. Bergsma, P. C. Bell, M. L. Fielden, C. L. G. Rodŕıguez, P. Guédat, A. Kremer, C. McGregor, C. Perrin, G. Ronsin and M. C. P. van Eijk, Angew. Chem., Int. Ed., 2003, 42, 1448.
2. A. Bhaumik and T. Tatsumi, Catal. Lett., 2000, 66, 181.
3. D. Chandra and A. Bhaumik, Ind. Eng. Chem. Res., 2006, 45, 4879.
4. D. Chandra, S. K. Das and A. Bhaumik, Microporous Mesoporous Mater., 2010, 128, 34.
5. C. Borde, V. Nardello, L. Wattebled, A. Laschewsky and J.-M. Aubry, J. Phys. Org. Chem., 2008, 21, 652.
6. H. B. Bohidar, J. Surf. Sci. Technol., 2008, 24, 105.
7. E. D. Goddard, Colloids Surf., 1986, 19, 301.
8. Y. Li, P. L. Dubin, H. A. Havel, S. L. Edwards and H. Dautzenberg, Langmuir, 1995, 11, 2486.
9. Y. Wang, K. Kimura and P. L. Dubin, Macromolecules, 2000, 33, 3324.
10. T. Chakraborty, I. Chakraborty and S. Ghosh, Langmuir, 2006, 22, 9905.
11. Y. Pi, Y. Shang, H. Liu, Y. Hu and J. Jiang, J. Colloid Interface Sci., 2007, 306, 405.
12. C. D. Bain, P. M. Claesson, D. Langevin, R. Meszaros, T. Nylander, C. Stubenrauch, S. Titmuss and R. V. Klitzing, Adv. Colloid Interface Sci., 2010, 155, 32.
13. S. Mukherjee, A. Dan, S. C. Bhattacharya, A. K. Panda and S. P. Moulik, Langmuir, 2011, 27, 5222.
14. S. M. Saleem and A. Hernandez, J. Surf. Sci. Technol., 1987, 3, 1.
15. D. Ali, S. Bolton and G. Gaylord, J. Appl. Polym. Sci., 1991, 42, 947.
16. A. Bhattacharyya and J. F. Argillier, J. Surf. Sci. Technol., 2005, 21, 161.
17. B. Jönsson, B. Lindman, K. Holmberg and B. Kronberg, Surfactants and Polymers in Aqueous Solution, John Wiley & Sons Ltd., West Sussex, UK, 1998.
18. S. Ghosh and S. P. Moulik, Indian J. Chem., Sect. A: Inorg., Bio-inorg., Phys., Theor. Anal. Chem., 1999, 38, 10.
19. S. Ghosh and S. P. Moulik, Indian J. Chem., Sect. A: Inorg., Bio-inorg., Phys., Theor. Anal. Chem., 1999, 38, 201.
20. A. Chilkoti, T. Christensen and J. A. Mackay, Curr. Opin. Chem. Biol., 2006, 10, 652.
21. Y. Yeo, E. Bellas, W. Firestone, R. Langer and D. S. Kohane, J. Agric. Food Chem., 2005, 53, 7518.
22. S. Ghosh and S. P. Moulik, J. Surf. Sci. Technol., 1998, 14, 110.
23. P. Ilekti, L. Piculell, F. Torunilhac and B. Cabane, J. Phys. Chem. B, 1999, 103, 9831.
24. A. Das Burman, S. Ghosh and A. R. Das, J. Nanofluids, 2014, 3, 140.
25. S. Mondal, S. Das and S. Ghosh, J. Surfactants Deterg., 2015, 18, 471.
26. H. Wang and Y. Wang, J. Phys. Chem.B, 2010, 114, 10409.
27. A. Vanerek and T. G. M. van de Ven, Colloids Surf., A, 2006, 273, 55.
28. S. Das, I. Mukherjee, B. K. Paul and S. Ghosh, Langmuir, 2014, 30, 12483.
29. B. Naskar, A. Dan, S. Ghosh and S. P. Moulik, J. Chem. Eng. Data, 2010, 55, 2424.
30. B. Mandal, S. P. Moulik and S. Ghosh, J Surface Interfac Mater, 2013, 1, 77.
31. E. K. Just and T. G. Majewicz, Encyclopedia of Polymer Science and Engineering, John Wiley and Sons, New York, 1985, vol. 3, p. 226.
32. S. Guillot, M. Delsanti, S. Désert and D. Langevin, Langmuir, 2003, 19, 230.
33. J. Mata, J. Patel, N. Jain, G. Ghosh and P. Bahadur, J. Colloid Interface Sci., 2006, 297, 797.
34. S. Trabelsi, E. Raspaud and D. Langevin, Langmuir, 2007, 23, 10053.
35. N. Sardar, M. Kamil and Kabir-ud-Din, Colloids Surf., A, 2012, 415, 413.
36. F. M. Menger and J. S. Keiper, Angew. Chem., Int. Ed., 2000, 39, 1906.
37. K. Tsubone and S. Ghosh, J. Surfactants Deterg., 2003, 6, 225.
38. R. Zana and J. Xia, Gemini Surfactants: Synthesis, Interfacial and Solution-phase Behavior and Applications, Marcel Dekker, Inc., New York, 2004.
39. Y. Han and Y. Wang, Phys. Chem. Chem. Phys., 2011, 13, 1939.
40. F. M. Menger, J. S. Keiper and V. Azov, Langmuir, 2000, 16, 2062.
41. S. Ghosh, J. Colloid Interface Sci., 2001, 244, 128.
42. S. Das, S. Maiti and S. Ghosh, RSC Adv., 2014, 4, 12275.
43. S. Das, B. Naskar and S. Ghosh, Soft Matter, 2014, 10, 2863.
44. M. J. Schwuger and H. Lange, Tenside, 1968, 5, 257.
45. X. Wang, Y. Li, J. Li, J. Wang, Y. Wang, Z. Guo and H. Yan, J. Phys. Chem. B, 2005, 109, 10807.
46. C. Wang and K. C. Tam, Langmuir, 2002, 18, 6484.
47. A. Dan, S. Ghosh and S. P. Moulik, J. Phys. Chem. B, 2009, 113, 8505.
48. A. P. Romani, M. H. Gehlen and I. Rosangela, Langmuir, 2005, 21, 127.
49. M. Lundin, L. Macakova, A. Dedinaite and P. Claesson, Langmuir, 2008, 24, 3814.
50. M. L. Huggins, J. Am. Chem. Soc., 1942, 64, 2716.
51. E. O. Kraemer, Ind. Eng. Chem., 1938, 30, 1200.
52. R. Sharma, B. Das, P. Nandi and C. Das, J. Polym. Sci., Part B, 2010, 48, 1196.
53. T. E. Eremeeva and T. O. Bykova, Carbohydr. Polym., 1998, 36, 319.
54. C. Tanford, Physical Chemistry of Macromolecules, John Wiley and Sons, New York, 1961, p. 391.
55. S. Ghosh, Colloid Surface Physicochem Eng Aspect, 2005, 264, 6.
56. A. Rangaraj, V. Vangani and A. K. Rakshit, J. Appl. Polym. Sci., 1997, 66, 45.
57. V. Vangani and A. K. Rakshit, J. Appl. Polym. Sci., 1996, 60, 1005.
58. R. Simha, J. Phys. Chem., 1940, 44, 25.
59. H.-H. Kohler and J. Strnad, J. Phys. Chem., 1990, 94, 7628.
60. A. S. Narang and U. C. Garg, J. Indian Chem. Soc., 1989, 66, 214.

---

Footnote

† Electronic supplementary information (ESI) available: The interfacial parameters for micellization and interaction characteristics of 14-4-14 with NaCMC-2 in aqueous and aqueous-IP medium obtained from conductometry, tensiometry and DLS methods, structure of NaCMC and particle size distribution and SEM images have been provided. This material is available free of charge via the internet. See DOI: 10.1039/c6ra00640j

---