Synergistic interactions of surfactant blends in aqueous medium are reciprocated in non-polar medium with improved efficacy as a nanoreactor

Abstract

Organized assemblies in aqueous and non-aqueous media based on mixed surfactants are one of the desired areas for experimental studies and carrying out chemical reactions due to synergistic performance and efficient solvents for various substrates. In this report, a model C–C cross coupling Heck reaction between n-butyl acrylate and 4-iodo-toluene is performed both in micelles and water/oil microemulsion systems as reaction media using a similar set of surfactants, [cetyltrimethylammonium bromide (CTAB) and polyoxyethylene (20) cetyl ether (C₁₆E₂₀)], in their single and mixed states for the first time. In order to explain the possible location and mechanism of reaction in these media, multitechnique approaches are employed to understand the mutual interactions between surfactant(s) and other constituents in pure and mixed states at air–water as well as oil–water interfaces. A synergistic interaction is evidenced experimentally for a mixed CTAB/C₁₆E₂₀ micellar system, which is also supported by theoretical calculations using density functional theory (DFT). The yield of Heck product in different media follows the order water < pure micelle < mixed micelle, which indicates a significant role of the confined environment of the aggregated systems. Further, mixed microemulsions including constituents of the formulations 1-pentanol and n-heptane or n-decane are explored as nanoreactors for carrying out such a reaction. Reaction yield in mixed water-in-oil (w/o) systems as a function of different hydration levels has been correlated with formation and microstructural characteristics of these systems. Further, mixed microemulsions at lower hydration levels produce synergistic performance compared to micelles and individual constituents in terms of reaction yield. These results reveal that the reaction occurs in neither the water nor oil domain, evidently in the micelle/water pseudo-phase and at the palisade layer of the oil/water interface of microemulsions. Moreover, reaction yields in the studied media are rationalized in terms of interaction parameters, spontaneity of micellization, interfacial population of 1-pentanol, and spontaneity of formation of w/o microemulsions.

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1. Introduction

In recent years, the study of the physicochemical properties of mixed surfactant solutions has become a topic of interest in the area of self-assembly of molecular systems.¹ Organized assemblies viz. micelles and microemulsions,² based on mixed surfactants offer substantial modification in solubilisation behaviour, enzyme kinetics, nanoparticle synthesis and chemical activity.^3–7 The interaction between surfactants in mixtures produces marked interfacial effects due to changes in adsorption as well as in the charge density of the surface.⁸ Earlier, Azum and his co-workers explored various binary and ternary mixtures of amphiphiles in aqueous media, which were found to exhibit different surface and colloidal properties from those of the pure individual components.^9–12 In most cases different types of surfactants are purposely mixed for synergistic performance and this is utilized to reduce the total amount of surfactant used in particular applications in order to reduce cost and environmental impact.^13,14 However, a proper rationale of such modified mixing behaviour is still required and needs a more generalized approach to optimize mixing stoichiometry in order to obtain a maximum effect. Research to date includes numerous attempts to explore combined physicochemical studies of micro-heterogeneous assemblies with attention paid not only to their fundamental aspects but also their wider applications.¹⁵

The Heck reaction^16,17 has received much attention in recent years as it offers a versatile method for the generation of new carbon–carbon bonds,¹⁸ and holds much promise in many industrial processes, especially in the synthesis of fine chemicals and active pharmaceutical intermediates.¹⁹ In the view of current economic and environmental concerns, continuous efforts are now being made to develop new templates for carrying out the Heck reaction. One approach is to consider inexpensive amphiphiles as potential additives that upon self-assembly into micelles accommodate otherwise insoluble organic substrates and catalysts within lipophilic cores.²⁰ Further, microemulsions might be another, much cheaper and potentially more universal approach to this problem, based on well-known solubilisation phenomena.²¹ Very recently, the Heck reaction has been successfully performed in water/cetyltrimethylammonium bromide (CTAB)/1-propanol/1-dodecene based oil-in-water (o/w) microemulsion and two phase region as well at 353 K.²²

These findings prompted us to extend the use of micelle as well as water-in-oil (w/o) microemulsion based on mixed cationic/non-ionic surfactants as a chemical reaction media for the Heck reaction at ambient temperature. To fulfil this goal, a comprehensive study of the formation behaviour and physicochemical properties of micellization and microemulsion using blends of CTAB and polyoxyethylene (20) cetyl ether (C₁₆E₂₀) including individual constituent is carried out. The structural features of these systems are investigated via interfacial and bulk routes by employing tensiometry, conductometry, and also zeta potential, viscosity, dynamic light scattering (DLS), fluorescence lifetime, Fourier transform infrared spectroscopy (FTIR) and field emission scanning electron microscopy (FESEM) measurements. The choice of surfactants in this study is not arbitrary. Although the literature is dominated by studies of microemulsions formed with anionic sodium bis(2-ethylhexyl) sulfosuccinate (AOT), there is interest in microemulsions formed with other surfactants, particularly cationic CTAB. CTAB is of increasing interest because the head group is a good model for the lipid phosphatidylcholine.²³ Further, commercially available non-ionic surfactants such as Brijs are extensively used in pharmaceutical formulations as solubilizers and emulsifiers to improve the dissolution and absorption of poorly soluble drugs.²⁴ Heptane (Hp) and decane (Dc) were used as oil and 1-pentanol (Pn) was used as cosurfactant. After careful evaluation of various physicochemical and thermodynamic parameters, a model C–C cross coupling reaction (Heck reaction) was performed between n-butylacrylate and 4-iodo-toluene in the presence of palladium acetate and triethylamine, TEA (base), in mixed surfactant based micellar and w/o microemulsion media. To provide a proper justification for this study, we also performed the Heck reaction in media of individual constituents, which are used for the formation of these organized systems. An attempt is also made to rationalize the yields of the Heck products from the viewpoint of the physicochemical and thermodynamic parameters of their formation during the course of the reaction. It is expected that the findings of this study would improve the basic understanding of the formation, characterization and application of mixed micelles and w/o microemulsions.

2. Materials and methods

2.1. Materials

CTAB (>99%) and C₁₆E₂₀ (>98.5%) were obtained from Sigma Aldrich, USA and Fluka, Switzerland, respectively. The cosurfactant [1-pentanol (Pn, >98%)] and oils [Hp (>98%), and Dc (>98%)] were obtained from Fluka, Switzerland, Lancaster, England and E. Merck, Germany, respectively. The dye, 7-hydroxycoumarin (HCM, >99%) was obtained from Chem. Service, West Chester, USA. Palladium acetate [Pd(OAc)₂, ≥99.98%] and 4-iodo-toluene (CH₃C₆H₄I, ≥99%) were purchased from Sigma Aldrich, USA. TEA (≥99.5%) and n-butyl acrylate were purchased from Merck, Germany. All these chemicals were used without further purification. Doubly distilled water with a conductivity of less than 3 μS cm⁻¹ was used in the experiments.

2.2. Methods

2.2.1. Tensiometry. Surface tension was measured using a K9 tenisometer (Kruss, Germany) by a platinum ring detachment method. A concentrated surfactant solution was added to a known amount of water, and the surface tension values were measured after thorough mixing and temperature equilibration with an accuracy of ±0.1 mN m⁻¹.

2.2.2. Conductance measurements. Conductivity measurements were made using an automatic temperature-compensated conductivity meter, Mettler Toledo (Switzerland) Conductivity Bridge, with a cell constant of 1.0 cm⁻¹. The instrument was calibrated with a standard KCl solution. A constant temperature (303 ± 0.1 K) was maintained by circulating water through the outer jacket from a thermostatically controlled water bath. The reproducibility of the conductance measurements was found to be within ±1%.

2.2.3. Dynamic light scattering (DLS) and zeta potential measurements. Hydrodynamic diameter (D_h) and zeta potential (ξ/mV) measurements of the self-assembled systems (single and mixed micelles and w/o mixed microemulsions) were performed using a Nano ZS-90 (Malvern, U.K.) dynamic light scattering spectrometer at 303 K. The solutions were equilibrated for 2–3 hours before measurement. Solutions were filtered carefully through a 0.22 μm Millipore™ membrane filter loaded to a glass round aperture (PCS8501, Malvern, U.K.) cell with a 1.0 cm optical path length for measurements. A He–Ne laser of 632.8 nm was used as the light source, while the scattering angle was set at 90°. Temperature was controlled by an inbuilt Peltier heating–cooling device (±0.1 K). D_h of each sample was estimated from the intensity autocorrelation function of the time-dependent fluctuation in intensity and can be defined as.²⁵

D_h = k_BT/3πηD (1)

where, k_B, T, η and D indicate the Boltzmann constant, temperature, viscosity and diffusion coefficient of the solution, respectively. Zeta potentials were measured using a folded capillary cell (DTS1060, Malvern, U.K.) made of polycarbonate with gold plated beryllium-copper electrodes. One cell was used for a single measurement. To check the reproducibility of the results at least 6 measurements were performed.

2.2.4. Field emission scanning electron microscopy (FESEM) measurements. A field emission scanning electron microscope (FESEM, HITACHI S-4800) was used to study the morphology of single and mixed micelles and w/o mixed microemulsions. A high vacuum (∼10⁻⁷ Torr) field emission setup was applied to deposit a thin film of micelle and microemulsion solutions on glass plates.

2.2.5. Formation of microemulsion. Microemulsion formation was accomplished by adding oil (Hp or Dc) at a constant water and surfactant level to destabilize an otherwise stable w/o microemulsion and then restabilizing it by adding a requisite amount of co-surfactant (Pn) with a constant composition of interface and continuous phase. The experimental procedure with theoretical backgrounds has been reported elsewhere.²⁶

2.2.6. Viscosity measurements. Viscosity measurements were performed using a LVDV-II + PCP cone and plate type Roto-viscometer (Brookfield Eng. Lab, USA). The temperature was kept constant (303 K) for viscosity measurements within ±0.1 K by circulating thermostated water, through a jacketed vessel containing the solution. The reproducibility of the viscosity measurements was found to be within ±1%.

2.2.7. Fluorescence lifetime measurements. The fluorescence lifetime measurements of w/o mixed microemulsions were performed at 303 K using a bench-top spectrofluorimeter from Photon Technology International (PTI), USA (Model: Quantamaster-40). The present PTI lifetime instrument employs the stroboscopic technique (Strobe) for time-resolved fluorescence measurements. In the present experiments, two curves were considered at the measured wavelength (450 nm using a 310 nano LED as a light source) viz. the instrument response function (IRF) and the decay curve of the probe molecule, 7-hydroxycoumarin (10⁻⁵ mol dm⁻³). The IRF was acquired from a non-fluorescing scattering solution (herein, Ludox AM-30 colloidal silica, 30 wt% suspension in water) held in a quartz cell of 1 cm path length. The lifetime values of probe molecules were then obtained by convoluting the IRF with a model function and then comparing the results with experimental decay. Analysis was performed using Felix GX (version 2.0) software. A value of χ² in between 0.99 and 1.22 and a symmetrical distribution of residuals are considered as a good fit.

2.2.8. Fourier transform infrared spectroscopy (FTIR) studies. FTIR spectra of w/o mixed microemulsions were recorded using a Perkin Elmer Spectrum RXI spectrometer (USA) (absorbance mode) using a CaF₂-IR crystal window (Sigma Aldrich) equipped with a Presslock holder with a scan number of 100 and a spectral resolution of 4 cm⁻¹ at 303 K. We focused our attention on the 3000–3800 cm⁻¹ window (mid-infrared region). Deconvolution of spectra was done with the help of a Gaussian curve fitting program (Origin software).

2.2.9. C–C cross coupling (Heck) reaction in micelles and microemulsions. 3 ml of micellar or w/o microemulsion solution and 4.48 mg (0.02 mmol, 4 mol%) of Pd(OAc)₂ were taken in a 25 ml round bottom flask and the mixture placed in a pre-heated oil bath at 303 K for 30 minutes with constant stirring. Thereafter, 109 mg (0.5 mmol) of 4-iodotoluene, 76.8 mg (0.6 mmol) of n-butylacrylate and 101.12 mg (1 mmol) of TEA were introduced into it, and the resulting mixture was further stirred for 60 minutes. The resulting multicomponent solution (in each case) showed excellent stability towards the reagents. More precisely, similar experiments were performed in single (CTAB and C₁₆E₂₀) and mixed micelles (at equimolar composition), and mixed microemulsions (at equimolar composition) at a different molar ratio of water to surfactant (ω) and also, in microemulsions of other constituents (viz. water, Hp, Dc, Hp/Pn, Dc/Pn). Progress of the reaction was monitored by silica gel thin layer chromatography (TLC). After completion, the reaction was quenched with water and the organic part was extracted thrice with diethyl ether. The combined ethereal layer was dried over anhydrous sodium sulphate and concentrated under vacuum. The yield of the Heck coupled product was determined by HPLC (Agilent Technologies, 1260 Infinity). Further, the product was purified by column chromatography using silica gel where a mixture of petroleum ether and ethyl acetate was used as an eluent. Finally, the product was characterized using ¹H-NMR and ¹³C-NMR (Bruker, 300 MHz), which are provided in the ESI.†

3. Results and discussion

Before discussing the outcome of the Heck reaction in a micellar or microemulsion medium, first we focus on the formation of single and mixed micellar and microemulsion systems from a thermodynamic point of view. In order to understand the interactions between individual constituents, we discuss the microstructural, microenvironmental and morphological characteristics of the formulated systems. In view of this, we divide this section into four parts; where the first part of this section contains a summary of the formation and characterization of micellar systems in single and mixed surfactants both experimentally and theoretically. In the second part, reaction site and yields of the Heck reaction are rationalized in terms of the interaction parameters and Gibbs free energy of micellization. Further, the formation and microstructural characteristics of the mixed microemulsions along with the yields and possible locations of the reaction at the oil/water interface are discussed in the last two parts. Finally, a comparison of reaction yields is provided between mixed micellar and microemulsion systems along with individual constituents.

3.1. Formation and characterization of single and mixed micelles

The amphiphilic behaviour in terms of the critical micellar concentration (CMC) of pure CTAB, C₁₆E₂₀ and their mixture at an equimolar composition (1 : 1) in water is determined from the sharp inflection point in the surface tension (γ) against log[surfactant] and specific conductivity against [surfactant] plots at 303 K, as shown in Fig. 1A and B.

Fig. 1 Tensiometric (A) and conductometric (B) determination of the CMC of mixed aqueous CTAB/C₁₆E₂₀ systems at an equimolar composition at 303 K.

Further, a representative illustration of CMC for aqueous C₁₆E₂₀ system is presented in Fig. S1 (ESI†). The CMC values of these systems are also presented in Table S1 (ESI†). The lower CMC for CTAB/C₁₆E₂₀ mixtures compared to pure CTAB is attributed to the formation of a pseudo-double chain complex arising from the strong electrostatic interaction between the quaternary ammonium cationic segment (CTA⁺) and the POE of C₁₆E₂₀. In addition, there also exists an enhanced hydrophobic interaction between the hydrocarbon chains, which favours mixed micelle formation and decreases the CMC.²⁷

For ideal mixing of cationic/non-ionic surfactant systems, CMC values have been calculated using the Clint equation,²⁸

1/CMC_ideal = α₁/CMC₁ + α₂/CMC₂ (2)

where, α₁ and α₂ are the stoichiometric molar fractions of CTAB and C₁₆E₂₀, respectively. For the mixed systems, lower experimental CMC values (CMC₁₂) are obtained than those expected from the Clint equation (Table S1†). This observation indicates non-ideal behaviour in aqueous solution and also, demonstrates favourable synergism between the constituent surfactants in mixed micelles.

The nature and strength of interactions between the CTAB and C₁₆E₂₀ molecules in the mixed micelles are evaluated from the interaction parameter by employing Rubingh’s approach, based on regular solution theory,²⁹

where, X^m₁ is the micellar molar fraction of the CTAB incorporated in the mixed micelle. The interaction parameter, β^m, is an indicator of the degree of interaction between two surfactants in mixed micelles and accounts for the deviation from ideality, which is given by;²⁹

β^m = [ln(CMC₁₂α₁/CMC₁X^m₁)]/(1 − X^m₁)² (4)

For a binary system (1 : 1), β^m values are found to be negative (Table 1), which also indicates a synergistic interaction between the component surfactants. According to Rubingh’s model, the activity coefficient, f^m_i, of individual surfactants within the mixed micelles is related to the interaction parameter through the following equations,^29,30

f^m₁ = exp[β^m(1 − X^m₁)²] (5)

f^m₂ = exp[β^m(X^m₁)²] (6)

where, f^m_i = 1 indicates an ideal mixed system.³⁰ The significance of the other terms was discussed earlier. It is seen from Table 1 that both the values of f^m₁ and f^m₂ are lower than unity, which indicates the formation of mixed micelles and reflects the non-ideality of a multicomponent mixed system. Further, the maximum surface excess concentration at the air–water interface (Γ_max) and the minimum area per surfactant head group adsorbed at the interface (A_min) are calculated from the surface tension data by fitting the Gibbs adsorption isotherm,³¹

A_min = 10¹⁸/Γ_maxN_a (8)

Table 1 Interfacial, thermodynamic and interaction parameters of single and mixed surfactants

(A) Interfacial and thermodynamic^a parameters
System	T/K	ΠCMC × 10^3/N m^−1	10^6 × Γmax/mol m^−2	Amin/nm^2 per molecule	−ΔG^0m/kJ mol^−1	−ΔG^0ads/kJ mol^−1
a The average errors in ΔG^0m and ΔG^0ads are ±3%.b Ref. 27.c For interaction parameters CTAB = 1 & C16E20 = 2.d For interaction parameters C16E20 = 1 & CTAB = 2.
CTAB	303	32.0^b	1.31^b	1.27^b	40.5^b	64.9^b
CTAB/C16E20	303	34.7	0.86	1.93	71.03	99.74
C16E20	303	27.64	2.25	0.7379	39.5	51.78

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(B) Interaction^c parameters for CTAB/C16E20 (1 : 1)
CMC (Cond.)	CMC (ST)	CMC (Avg.)	α and g	β	f1	f2
0.0109	0.0115	0.0112	0.829 and 0.171	−4.58^c	0.0114^c	0.89^c
				−4.47^d	0.2465^d	0.0461^d

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Herein, dγ/dlog C is the slope of the plot of γ versus log C (where C represents the concentration of the surfactant). Here, for mixed micelles of cationic/non-ionic surfactants, the value of n (the number of species produced by an amphiphile and whose concentration at the interface varies with surfactant concentration in the solution) is 3. The values of Γ_max and A_min are presented in Table 1 and follow the order C₁₆E₂₀ > CTAB > CTAB/C₁₆E₂₀ and C₁₆E₂₀ < CTAB < CTAB/C₁₆E_20, respectively. At an equimolar composition of mixed CTAB/C₁₆E₂₀, minimum Γ_max (and thus maximum A_min) values are obtained due to the intercalation of the POE of C₁₆E₂₀ within the positively charged CTAB surfactant molecules which also reduces the repulsion among them.³² The steric hindrance due to the long POE chains of C₁₆E₂₀ also causes higher A_min values for CTAB/C₁₆E₂₀ combinations.³²

Another parameter that directly proves the effectiveness of surface tension reduction is the surface pressure at the CMC, i.e. Π_CMC. It indicates the maximum reduction of surface tension caused by the dissolution of the amphiphilic molecules and is usually defined as;

Π_CMC = γ_sol − γ_CMC (9)

where, γ_sol and γ_CMC represent the surface tension of pure water and surfactant solution at the CMC, respectively. As per Table 1, the aqueous CTAB-C₁₆E₂₀ mixtures display higher values of Π_CMC than their individual counterparts in accordance with their higher surface activity as mentioned earlier, although their efficiency in bringing about a reduction in the surface tension of water varies only slightly with the mixture composition.³³

The standard free energy of micelle formation per mole of monomer unit (ΔG⁰_m) for these systems is evaluated from the equation;³⁴

ΔG⁰_m = (1 + g)RT ln X_CMC (10)

where, X_CMC and g are the CMC expressed in mole fraction unit and the fraction of counter ions bound to the micelle, respectively. The fraction of counter-ion binding is related to;³⁴

g = (1 − S_mic/S_mn) (11)

The lower values of g for mixed CTAB/C₁₆E₂₀ (i.e. 0.171) compared to the pure CTAB micelle (i.e. 0.47) signify a lowering of effective surface charge density in the mixed micelles.³⁵ The standard free energy of interfacial adsorption (ΔG⁰_abs) at the air–water interface of micelles is evaluated from the relation;³⁶

ΔG⁰_ads = ΔG⁰_m − (Π_CMC/Γ_max) (12)

The values of ΔG⁰_m and ΔG⁰_ads are represented in Table 1. Both the ΔG⁰_m and ΔG⁰_ads values are found to be more negative in the binary mixture compared to the pure micelle, which indicates that the micellization as well as adsorption processes are more favourable compared to the individual surfactant. Further, more negative ΔG⁰_ads values compared to ΔG⁰_m values suggest that the adsorption process is more spontaneous than micelle formation. Micelle formation in the bulk is a secondary process and less spontaneous than interfacial adsorption.^35,37 In order to get more insight into the surface charge and structural characteristics of pure and mixed micelles, we performed zeta potential, DLS and FESEM measurements, which are dealt with in subsequent paragraphs.

It is already established that zeta potential (ζ) is a very good indicator of the interaction and stability of colloidal systems.³⁸ In view of this, the effect of amphiphile(s) concentration on the surface charge of aqueous CTAB, C₁₆E₂₀ and CTAB/C₁₆E₂₀ systems was investigated by measuring the ζ parameter, which is presented in Fig. 2A. The positive ζ value for cationic CTAB increases with increasing the concentration of CTAB. However, non-ionic C₁₆E₂₀ shows a negative potential, possibly due to the large number of oxygen atoms in the POE groups.³⁹ Interestingly, the ζ values for CTAB/C₁₆E₂₀ mixtures are found to be higher than that of single CTAB, which also proves the synergistic interaction between them.⁴⁰

Fig. 2 (A) Measurement of the zeta potential of single and binary (equimolar) surfactants in an aqueous medium at 303 K. (B) Hydrodynamic diameter (D_h) of micellar systems at 303 K. FESEM images of (C) CTAB/water and (D) CTAB/C₁₆E₂₀/water at the micellar region, (E) CTAB/C₁₆E₂₀/water at the post-micellar region, and (F) C₁₆E₂₀/water at the micellar region.

The DLS technique is employed to determine the size of the aggregates of CTAB, C₁₆E₂₀, and CTAB/C₁₆E₂₀ binary system at their corresponding CMCs at 303 K and is depicted in Fig. 2B. DLS data shows a monomodal size distribution with a hydrodynamic diameter (D_h) ranging from ∼10–15 nm for all the systems. The micellar size is observed to follow the trend C₁₆E₂₀ < CTAB/C₁₆E₂₀ < CTAB. The smaller size of C₁₆E₂₀ micelles originates from the relatively weaker interactions of C₁₆E₂₀ with water in comparison to CTAB.^5,41 Further, the morphology of the four micellar systems was investigated using an FESEM technique and is illustrated in Fig. 2C–F. A uniformly distributed spherical shaped structure is observed for all these systems at micellar region (Fig. 2C, D and F), which is indicated by circles in the respective figures. Further, Fig. 2E represents some agglomerated structures (shown by hollow arrows), which consist of two or three single spherical micro-droplets for binary CTAB/C₁₆E₂₀ systems at the post-micellar region.

In order to understand the interaction of surfactant molecules with solvent qualitatively, we performed quantum chemical DFT calculations. We executed model calculations by using the B3LYP functional with the 6-31G basis set to understanding the interactions of CTAB and C₁₆E₂₀ with H₂O at single and binary (1 : 1) compositions using the Gaussian 09 program. Frequency calculations were also performed to verify that all the optimized geometries correspond to a local minimum which has no imaginary frequency. At the outset, we optimized the structures of the isolated CTAB, C₁₆E₂₀ and H₂O, binary complex (1 : 1) of CTAB and C₁₆E₂₀ with H₂O, and ternary complex (1 : 1 : 1) of CTAB, C₁₆E₂₀ and H₂O. The most stable optimized structures of binary (1 : 1) and ternary (1 : 1 : 1) complexes are presented in Fig. 3, and their interaction energies (calculated from the difference in stabilization energy between complexes and monomers) are also shown in the respective figure. Also, the optimized structures of isolated CTAB, C₁₆E₂₀ and CTAB/C₁₆E₂₀ are illustrated in the ESI (Fig. S2†). DFT calculations indicate a highly stable ternary complex, CTAB/C₁₆E₂₀/H₂O, with a stabilization energy of −102.09 kJ mol⁻¹. The stability of the proposed complexes follows the order CTAB/C₁₆E₂₀/H₂O > C₁₆E₂₀/H₂O (−59.09 kJ mol⁻¹) > CTAB/H₂O (−41.93 kJ mol⁻¹). This result suggests that the stabilization of the CTAB/C₁₆E₂₀/H₂O complex corroborates well the synergism in the β parameter, ΔG⁰_m and ΔG⁰_ads values (Table 1). A similar observation was also reported earlier.^42,43

Fig. 3 Optimized geometries at the B3LYP/6-31G level: (A) C₁₆E₂₀/H₂O complex, (B) CTAB/H₂O complex, and (C) CTAB/C₁₆E₂₀/H₂O complex. Colour code for atoms: blue, nitrogen; red, oxygen; dark gray, carbon; and light gray, hydrogen.

3.2. C–C cross coupling Heck reaction in single and mixed micellar media

Micellar media actually demonstrate that the compartmentalization of these systems leads to the formation of a nanoreactor environment at the periphery of the micelle/water pseudo-phase and provides a more sustainable approach for synthetic organic chemistry.⁴⁴ In this report, repeated experiments with a model substrate in the characterized single and mixed micelles were performed to articulate the effect of restricted or confined reactor systems in the light of their physicochemical and thermodynamic properties to rationalize the yield of Heck reaction products at ambient temperature (303 K) (vide. Fig. 4A). More precisely, it is expected to underline the variation in degree of performance of the Heck reaction from the viewpoint of the distinctive features of micellar structure. The results are presented in Fig. 4 and Table S2 (ESI†). Various combinations (for example, water, single and mixed micelles of CTAB and C₁₆E₂₀) of the nanoreactor systems (entries 1–4) were explored in order to standardize the best reaction environment in terms of the yield of the desired product of the Heck reaction (Table S2†). It is clearly evident that the yield is very poor in water (7%) compared to microheterogeneous systems such as single CTAB (57%), C₁₆E₂₀ (44%) or mixed micelle (66%). Hence, these results clearly warrant the envisaged need for a confined environment of aggregated systems which plays a significant role in performing the Heck reaction.⁴⁵ It can be seen from Fig. 4 and Table S2† that an equimolar composition of mixed micelle exhibits a synergism in the yield of Heck reaction products and consequently, offers a better reactor than its constituent surfactants. More precisely, the mixed micelle possesses some intrinsic properties of its formation that lead to its special characteristics towards Heck reaction performance. This is justified from the higher negative values of interaction parameter (β^m) and free energy of mixed micelle formation (ΔG⁰_m) than individual surfactants (Table 1). This suggests that the formation of the mixed micelle is most spontaneous as well as functionally operative. Further, it is noted that the cationic CTAB-mediated micelle shows a higher yield than non-ionic C₁₆E₂₀ which corroborates well with the spontaneity of formation of the respective micelle as designated through the corresponding negative values of both ΔG⁰_m and ΔG⁰_ads (Table 1).⁴⁶ Surfactant-mediated self-assemblies or micelles are reported to be compatible with aryl halogens.⁴⁵ Other substrates (excluding the Pd²⁺ catalyst, which might be present in the continuous aqueous phase) are expected to be confined in the micellar core. Hence, the probable location of the micelle-mediated Heck reaction is in the micelle/water pseudo-phase whereas the catalysis is likely to be operative.

Fig. 4 (A) Pictorial representation of the Heck reaction in self-organized media, and (B) yield of Heck reaction in single (CTAB and C₁₆E₂₀) and mixed micelles (CTAB/C₁₆E₂₀, 1 : 1) at 303 K.

So far we conclude that due to synergistic interaction between CTAB and C₁₆E₂₀ along with spontaneous micellization, a mixed surfactant based micelle provides an efficient medium for performing the Heck reaction. Also, the experimentally observed synergistic interaction corroborates well with quantum chemical DFT calculations. Now, questions arise how the synergistic interaction between the aforementioned surfactants is stimulated in a non-polar or non-aqueous medium and also, whether the Heck reaction occurs favourably in this mixed microemulsion system or not. To answer these questions, the formation and characterization of a mixed microemulsion system at different physicochemical conditions and correlation of the estimated parameters with the reaction mechanism of Heck coupled products is required.

3.3. Formation and characterization of w/o mixed microemulsions

CTAB requires the presence of a co-surfactant, typically a medium chain alcohol, in order to form a stable microemulsion.⁴⁷ In this study, Pn and Hp (or Dc) were used as a co-surfactant and oil, respectively. In a w/o microemulsion system, CTAB and C₁₆E₂₀ in an equimolar composition are considered to populate the oil/water interface in partial association with the co-surfactant (Pn). On the other hand, Pn is further distributed between the interface and the bulk oil, because of its negligible solubility in water.²⁶ Thus, at a fixed [surfactant(s)], a critical concentration of Pn is required for the stabilization of the mixed microemulsions. To estimate what amount of Pn (in moles) is distributed at the interface and in the oil phase for the formation of a stable microemulsion, we employed a simple titrimetric technique (known as the dilution method).⁴⁸ The distribution vis-à-vis the transfer process of Pn from the continuous oil phase to the interfacial region is discussed in detail in the ESI, and shown in Fig. S3.† This method is used for the estimation of various parameters concerned with the formation of CTAB/C₁₆E₂₀ mixed microemulsion systems (with X_CTAB or X_C16E20 = 0.5) in Hp and Dc at 303 K and at different molar ratios of water to surfactant (ω = 10, 20, 30, 40 and 50). The estimated parameters, such as number of moles of Pn at the interface (nⁱ_a), in the oil phase (n^o_a), compositional variations of surfactants and Pn (nⁱ_a/n_s) at the interface, the distribution constant of Pn between the continuous oil phase and the interface of the droplet (K_d), and standard Gibbs free energy change of transfer of Pn from oil to the interface (ΔG⁰_t) are presented in Table S3 in the ESI.† It is evident from Fig. 5A that nⁱ_a or nⁱ_a/n_s values decrease with an increase in ω for all these systems. A similar trend was reported earlier by Paul et al.⁴⁹ for water/Brij-35/Pn/Dc or Dd and Kundu et al.⁵⁰ for SDS/Brij-58 or Brij-78/Pn/Hp or Dc microemulsions. At a very low ω, the concentration of Pn at the interface is virtually constant, because long POE chain of C₁₆E₂₀ probably resides at the interface in a twisted form due to strong ion–dipole interactions between the compact quaternary ammonium group in CTAB and EO groups in C₁₆E₂₀. With an increase in droplet size by the addition of more water, helically twisted POE chains unfold and consequently, occupy a larger surface area of the droplet.⁵¹ Therefore, unoccupied surface area at the interface is reduced to a greater extent and hence, nⁱ_a gradually decreases with an increase in ω. However, the spontaneity of the transfer process further decreases with an increase in ω which indicates that the trend of Pn transferring from the oil to the interface is weakened.⁵⁰ Hp stabilized systems also show higher spontaneity of the Pn transfer process than Dc, which is also justifiable from the higher Pn population at the interface (nⁱ_a) in the former system. Similar observations were reported by Kundu et al.,⁵⁰ Zheng et al.,⁵² and Digout et al.⁵³ for w/o microemulsion systems stabilized by single as well as mixed surfactants. After successful formation of a stable mixed microemulsion in conjunction with Pn in Hp or Dc at different ω, it is necessary to understand the microstructural and microenvironmental properties of these systems to reveal the nature of droplet–droplet interactions within confined environments.

Fig. 5 (A) Plot of nⁱ_a/n_s vs. water content (ω) for mixed CTAB/C₁₆E₂₀ microemulsions at equimolar composition comprising 0.5 mmol of mixed surfactant and 14.0 mmol of Hp or Dc stabilized by Pn. (B) Hydrodynamic diameter (D_h) as a function of ω for the systems mentioned above. FESEM images of similar systems at a fixed X_C16E20 = 0.5, ω = 10 and 303 K, where (C) Hp and (D) Dc are the oil. (E) Variation in the Gaussian profiles (area fraction) of the normalized spectra of different water species, and (F) fluorescence lifetime (〈τ〉) of HCM (at λ_ex = 310 nm) as a function of ω in CTAB/C₁₆E₂₀ (1 : 1)/Pn/Hp/water microemulsions at 303 K.

In view of this, size and size distributions of w/o microemulsion droplets were measured by the DLS technique. Fig. 5B depicts the variation in droplet size for the mixed microemulsions in both oils as a function of water content (ω = 10–50) at a fixed composition (X_{C16E20 or CTAB} = 0.5) and surfactant–cosurfactant mass ratio (=1 : 2) at 303 K. The droplet size increases with an increase in ω for both oils, keeping other parameters constant, which clearly indicates the swelling behaviour of w/o microemulsions with the addition of water.⁵⁴ The linear variation of droplet size at a lower range of ω values indicates that the droplets do not interact with each other and are probably spherical. The deviation from linearity at higher ω value is due to several factors, of these, the most relevant ones being enhanced droplet–droplet interaction and shape of the microemulsions. Further, Hp-based systems produce smaller droplets compared to Dc-based systems. This is probably due to the shorter chain of Hp compared to Dc, which easily penetrates the interface to make it rigid. It can be concluded that with an increase in ω, inter-droplet interaction increases which is higher for the Dc-based system compared to Hp. Both conductance and viscosity measurements in these systems also support the observations from the DLS study (Fig. S4A and B† and inset). The morphology of mixed microemulsions in both oils (Hp and Dc) was also investigated at a fixed X_C16E20 = 0.5, ω = 10 and 303 K by employing FESEM and is illustrated in Fig. 5C–D. The micrographs for the Hp based microemulsion (Fig. 5C) reveal smaller particles arranged in rice grain-like patterns, which produce dispersed spheres of nearly homogeneous type morphology. On the other hand, the Dc based microemulsion (Fig. 5D) produces disintegrated isolated bodies of large globular and near globular particles forming spherical entities. All types of spherical morphology (viz. small as well as large) are marked by circles in both figures. For Dc based microemulsion, severe aggregation occurs and single droplets cannot be discerned distinctly.

To understand the dynamics and nature of encapsulated water, we recorded FTIR spectra of encapsulated water in CTAB/C₁₆E₂₀ (1 : 1)/Pn/Hp/water microemulsions at varied ω (10 → 50). We focus our attention on the 3000–3800 cm⁻¹ frequency window as this is the fingerprint region for symmetric and asymmetric vibrational stretching of O–H bonds in water.^55,56 It is to be noted that a small amount of Pn is used as structure forming co-surfactant in the present study. Hence, the spectrum of Pn (at the same concentration) is subtracted from the spectral intensity of the O–H stretching band at the corresponding ω in order to eliminate the supplementary IR intensities due to the O–H stretching vibrations of Pn molecules, and the differential spectra are analysed.^57,58 Water usually coexists in three ‘states’ or ‘layers’ in the w/o microemulsion.^58–62 In view of this, the peaks observed for water O–H were fitted as a sum of Gaussian functions with the help of a Gaussian curve fitting program, and the vibrational characteristics, particularly the peak area corresponding to each peak, were analyzed using a three states model to unravel the nature of water inside the nanopool and the change in water properties as a function of ω. According to the three states model, the solubilized water in microemulsions is identified as free, bound and trapped water molecules and a representative result of deconvolution is depicted in the ESI, Fig. S5.† The free water molecules, occupying the core of the surfactant aggregates, form strong hydrogen bonds among themselves, that is, they possess similar properties to those of bulk water, which shifts the O–H stretching band to a lower frequency at about 3250 cm⁻¹ (vide. Fig. S5†).^58–62 The bound water, i.e. the surfactant head group bound water molecules, resonates in the mid frequency region and an IR peak appears at about 3450 cm⁻¹ (vide. Fig. S5†).^58–62 Apart from these two types of water species, the water molecules dispersed among the long hydrocarbon chains of the surfactant molecules are termed trapped water molecules.^58–62 As the trapped water molecules are matrix-isolated dimers or monomeric in nature, they generally absorb in the high frequency region at about 3550 cm⁻¹. The relative abundance [Gaussian profiles (area fraction)] of different water species in these systems as a function of ω are presented in Fig. 5E. It reveals that the relative abundance of free water increases from 29% to 46% and that of bound water decreases from 44% to 22% with increasing water content (ω = 10 → 50) vis-à-vis the corresponding increase in droplet size (D_h) from 5.24 nm to 11.10 nm in the same ω range. Actually, once water is added to a microemulsion forming system, a portion of the water goes to the interface and hydrates the head groups of surfactants until they become fully hydrated at a certain ω. Further added water goes primarily to the inner core, leading to a continuous increase in the fraction of unbound free water with an increase in ω.⁶¹ In addition, the population corresponding to the trapped water molecules (monomers/dimmers) shows an overall weak increasing tendency with a hike from 27% to 32% with increasing ω (=10 → 50). It is inferred from this investigation that a few water molecules are displaced from the structured water pool shell to the interfacial region with increasing ω.⁶³

A fluorescence lifetime study of HCM (fluorophore) in mixed surfactant microemulsions was also employed to obtain information about the configuration of the altered interfacial region of mixed amphiphiles interface upon hydration (ω = 10 → 50). The choice of HCM is based on the fact that the probes reside at the interface and/or face the polar core upon excitation (at λ_ex = 310 nm).^64,65 In the present context, Fig. 5F depicts that the fluorescence lifetime 〈τ〉 of HCM is found to be influenced by the variation of water content in mixed microemulsions. A regular decreasing trend in 〈τ〉 for the fluorophore (HCM) is observed for both Hp (3.13 to 1.41 ns) and Dc (3.23 to 1.51 ns) derived systems with an increase in ω (10 → 50). The larger droplet size of the microemulsion at ω = 50 compared to ω = 10 results in an increase in the curvature of the surfactant film. Hence a greater fraction of water interacts with the interface, leading to a relatively faster relaxation.⁶⁶ This result is also consistent with findings from FTIR measurements where increase in ω leads to an increase in free water population, which is responsible for the observed faster lifetime for CTAB/C₁₆E₂₀ (1 : 1)/Pn/Hp or Dc/water microemulsions at a higher ω.⁶⁷

3.4. C–C cross coupling Heck reaction in mixed microemulsion

In view of the distinctive role of the mixed micelle at equimolar composition in carrying out the Heck reaction, another set of compartmentalized/microheterogeneous systems, water/oil (Hp or Dc) mixed surfactant microemulsions (MEs) stabilized by equimolar (1 : 1) composition of CTAB/C₁₆E₂₀ and Pn (S : CS = 1 : 2 wt%), were explored as a function of ω (=10 → 50) (entries 9–18, Table S2† and Fig. 6). In addition, the yields of Heck products are also measured in the constituents of these formulations as media (entries 5–8, Table S2†). Both oils (Hp and Dc) and oil/Pn mixtures at 1 : 2 weight ratios (Hp/Pn, and Dc/Pn) show a low yield compared to that of mixed microemulsions, while Hp-derived systems show much higher yield than Dc (Fig. 6A). A profound effect on the overall yield of Heck products is distinctly validated by changing the template from micelle → mixed micelle (1 : 1) → mixed microemulsions (1 : 1) in Hp and Dc (Table S2†). Though, equimolar (1 : 1) surfactant composition (similar to that of mixed micelle) is fixed in all formulations, yields are found to be dependent on water content (ω), oil type and availability of Pn (as co-surfactant) at the oil–water interface for their stabilization (Fig. 6B). The highest yield of the desired Heck product is achieved at ω = 10 in Hp and Dc derived microemulsions. Thereafter, a sharp decrease in yield is observed with increasing ω (up to 30) and subsequently, a mild or sluggish decrease is achieved when ω = 50. It is worthy to mention that yield is much higher at lower ω = 10 (ME_Hp, 79% and ME_Dc, 68%) and 20 (ME_Hp, 70% only) than that of mixed micelle (66%), whereas yields are less than that of mixed micelle at rest of the ω values. However, the overall results are rationalized as follows. It is inferred from the yields of the Heck products in microemulsions [ω range from 10 → 50 in both oils (ME_Hp, 79% → 54% and ME_Dc, 68% → 41%)] in conjugation with the yields in constituent elements viz. water (7%), oils (Hp, 37% and Dc, 15%), and oil/Pn mixtures (Hp/Pn, 40% and Dc/Pn, 19%) (Fig. 6A and Table S2†) that the Heck reaction occurs neither in the water or the oil domain; evidently it occurs in the palisade layer of the oil–water interface of the microemulsions. This is well corroborated by previous reports, in which the authors claim that the most plausible reaction location or site was the interfacial region or within the surfactant palisade layer of the w/o microemulsion.^68,69 In this context, the interaction between two constituents [such as, Pn (cosurfactant) and TEA] of these multicomponent systems plays a decisive role in the performance of the Heck reaction leading to formation of the final product. It is worthy of mentioning that TEA is an essential reactant in the Heck reaction, which requires a stoichiometric amount of base to neutralize the acid (herein, HI) ensuing from the exchange of a hydrogen atom with an aryl group.^16,17 In addition, the requirements of Pn depend upon the composition of microemulsions, size of the polar head group and charge type of surfactant, water content, degree of oil penetration, droplet size/microstructure, etc. and render the overall stability towards multicomponent systems from a physicochemical and thermodynamic point of view.^26,49–53 More precisely, it is revealed from the viewpoints mentioned above that the predominance of dipole–dipole interactions between Pn and TEA in a confined environment enhances the availability of TEA in the vicinity of the interfacial region as well as in confined water with special reference to bound water. Consequently, the formation of OH⁻ base surrounding the interface proceeds in the following way;^68,70

(C₂H₅)₃N + H₂O ↔ (C₂H₅)₃N·H₂O ↔ (C₂H₅)₃NH⁺ + OH⁻

Fig. 6 Yield of Heck products (A) in individual constituents as well as in w/o mixed microemulsions (MEs), CTAB/C₁₆E₂₀ (1 : 1)/Pn/Hp or Dc/water at ω = 10, and (B) variation in yield as a function of ω (=10 → 50) in the aforementioned MEs at 303 K.

Hence, the penetration of OH⁻ in the palisade layer of microemulsion cannot be ruled out, and subsequently, a basic environment in the vicinity of the interface is likely to be formed, which is essentially required for the Heck reaction.^68,69 Therefore, the most satisfactory yield of the product is achieved at a lower ω of 10 or 20 (Fig. 6 and Table S2†), where predominance of Pn in this range of ω is evidenced by characterization of these microemulsions using the dilution method.⁶⁸ It is revealed from Fig. 5A and Table S3† that the interfacial Pn population (nⁱ_a) and Gibbs free energy of the Pn transfer process (−ΔG⁰_t) (which is an indicator of the spontaneity of microemulsion formation) sharply decrease initially and thereafter mildly with an increase in ω. Interestingly, it is worthy of mentioning that decreasing trends reflected in Fig. 5A and 6B bear resemblance to each other, which is strong evidence for decreasing yield with increasing ω. Furthermore, Hp-based systems exhibit higher yields than Dc-based systems. It can be argued that higher values of nⁱ_a and −ΔG⁰_t in Hp continuum than Dc continuum (Fig. 5A and Table S3†) are responsible for the variation in yield in these two sets of systems. Herein, we report a mixed microemulsion mediated Heck coupling reaction which provides comparatively better results than single or mixed micelle, validating the spontaneous nature of the reactor framework with a concomitant rapid Pn transfer process (Pn_oil → Pn_interface) obtained from dilution experiments in varied physicochemical environments. In conclusion, a CTAB and C₁₆E₂₀ mediated micelle and microemulsion (in single and mixed states) Heck coupling procedure (through the generation of new C–C bonds) is developed using ligand-free catalysts, and could be applied in many industrial processes, especially in the synthesis of fine chemicals and active pharmaceutical intermediates.^19,71

Currently, investigations of C–C coupling reactions catalysed by Pd(II) acetate and TEA as a base in w/o microemulsions comprising surfactants of different charge types (both in single and mixed states) under different physicochemical conditions (for example, variations in composition of the system, concentration of both base and catalyst, water content, temperature, etc.) are underway in our laboratory to probe the reaction mechanism of the Heck reaction in these complex microheterogeneous systems.

4. Overall comprehension

In aqueous medium, an equimolar composition of CTAB and C₁₆E₂₀ provides several advantages compared to the individual surfactants in terms of a lower CMC, more attractive interaction between them and spontaneous micellization (i.e., more negative free energy of micellization) as well as adsorption process at the air–water interface. Interestingly, quantum chemical DFT calculations reveal the larger interaction energy of equimolar CTAB and C₁₆E₂₀ with water compared to any other combination. The zeta potential (ζ) value for CTAB/C₁₆E₂₀ mixtures also confirms the synergistic interaction between them. Micellar aggregates produce spherical droplets with size ranges from 10 to 15 nm. To understand the distinctive features of micellar structures as chemical nanoreactors, the Heck reaction is performed between 4-iodotoluene and n-butylacrylate. The reaction yield of the Heck coupled product is found to be higher in a mixed CTAB/C₁₆E₂₀ micellar solution compared to single CTAB or C₁₆E₂₀ micelles and even higher than bulk water. The higher yield at mixed composition correlates with spontaneous micellization and adsorption processes. It is now interesting to study the behaviour of these surfactants in non-polar solvents further. Population of Pn at the oil–water interface of the formulated w/o microemulsions vis-à-vis spontaneity of the transfer process is found to decrease with an increase in ω for both oils. The morphology and size of mixed microemulsion droplets vary with water content as well as oil chain length. The present study also reveals that the relative abundance of free water increases and that of bound water decreases with increasing water content in mixed microemulsions and is well corroborated with the corresponding increase in droplet size under an identical range of ω. The product yield of the Heck reaction significantly increases in mixed microemulsion systems compared to with individual constituents. With increasing water content in a microemulsion, the product yield diminishes and it is found to be lower for Dc-stabilized systems compared to Hp. The spontaneity of the Pn transfer process from oil to the interface for stabilization of the microemulsion (as obtained from dilution experiments) finely correlates with trends in product yield in these systems.

5. Summary and future outlook

The present report focuses on the microstructure and properties of micelles and w/o microemulsion systems using a similar set of surfactants (CTAB and C₁₆E₂₀) at single and mixed states, and also employing these organized assemblies as a chemical nanoreactor for performing the C–C cross coupling Heck reaction. An equimolar composition of CTAB and C₁₆E₂₀ shows non-ideal solution behaviour in aqueous medium with synergistic interaction between them, which further correlates with gas phase quantum chemical calculations. Understanding the microscopic origins of these mixing properties might facilitate a more informed and designed use of such mixed systems in a range of novel formulations from hydrogels to surface coatings.^72,73 The mixed micellar composition provides an efficient medium for carrying out the Heck reaction compared to individual micellar and bulk water media. So far, an operationally simple procedure is developed for carrying out traditional Heck couplings at ambient temperature in a mixed micelle, without resorting to sonication, electrochemistry, or using water-soluble phosphine.⁷⁴ Use of inexpensive ionic and non-ionic amphiphiles allows cross-coupling to take place especially under mild and environmentally attractive conditions. To determine whether the key features responsible for favorable micellization via synergistic interaction reciprocate the interfacial architecture of microemulsions, a multitechnique approach was applied to fully characterize the mixed microemulsion systems at different hydration levels. The estimation of Pn distribution at the oil–water interface and bulk oil phase along with related energetics from a dilution method complements more detailed information obtained from more labour intensive small-angle neutron scattering (SANS) studies of w/o microemulsions.⁷⁵ Although these parameters are obtained through straightforward macroscopic measurements, the Gibbs free energies of transfer of Pn from bulk oil to the interface are a sensitive probe of the microenvironment around various solute moieties, and are amenable for the investigation of relatively complex molecular structures. The droplet size and relaxation dynamics of a fluoroprobe inside the confined environment correlate well with variation in the different states of water. Knowledge of such states of solubilized water in w/o microemulsions is important because has a bearing on the applications of these species, for example, in solubilisation, catalysis of chemical reactions,⁷⁶ and also on the size and polydispersity of nanoparticles synthesized in microemulsion media.⁷⁷ In order to understand the mechanism of the Heck reaction in a non-polar medium instead of an aqueous medium, the formulated w/o microemulsion at different compositions along with individual constituents is used as a template for the aforementioned reaction. A profound effect on the overall yield of the Heck product is distinctly validated by changing the template from micelle → mixed micelle (1 : 1) → mixed microemulsion (1 : 1), which indicates the influence of the nature of surfactant–solvent interactions, and thus affects the reaction yield.⁷⁸ Herein, it is proposed that the Heck reaction occurs in neither the water or the oil domain, and more precisely, occurs at the micelle–water pseudo-phase and palisade layer of the oil–water interface of microemulsions. In summary, each amphiphilic nanoreactor is analogous to the traditional chemist’s flask, with the added advantages of reduced reagent consumption, rapid mixing, automated handling, and continuous processing. Building on advances in continuous flow chemistry,⁷⁹ our study thus provides a new route to regulate and even to enhance reaction yields in micellar and w/o microemulsion media according to the purpose and could be found useful for future applications in various domains such as enzyme activity, and/or organic synthesis.^80,81

Acknowledgements

S. B. and K. K. thank DBT, Govt. of India for Senior Research Fellowship (SRF) and Research Associateship, respectively. G. C. thanks UGC, New Delhi, India for UGC-BSR Research Fellowship. Professor BKP would like to thank ISI, Kolkata. Professor SKS also thanks Science & Engineering Research Board (SERB), DST, Govt. of India (Sanction No. SB/S1/PC-034/2013) for financial assistance. The computer resources of Computer Centre, IIT Madras are gratefully acknowledged.

References

1. P. M. Holland and D. N. Rubingh, Mixed Surfactant Systems: An Overview, ACS Symposium Series, American Chemical Society, Washington, DC, 1992, vol. 501, ch. 1, pp. 2–30.
2. R. Zana, Dynamics of Surfactant Self-Assemblies: Micelles, Microemulsions, Vesicles and Lyotropic Phases, CRC Press, Taylor & Francis Group, Boca Raton, 2005.
3. D. N. Rubingh and M. Bauer, Lipase Catalysis of Reactions in Mixed Micelles, ACS Symposium Series, American Chemical Society, Washington, DC, 1992, vol. 501, ch. 12, pp. 210–226.
4. A. Shome, S. Roy and P. Das, Langmuir, 2007, 23, 4130–4136.
5. A. Das and R. K. Mitra, J. Phys. Chem. B, 2014, 118, 5488–5498.
6. J. Zhang, B. Han, J. Liu, X. Zhang, J. He, Z. Liu, T. Jiang and G. Yang, Chem.–Eur. J., 2002, 8, 3879–3883.
7. J. Hao, Self-Assembled Structures: Properties and Applications in Solution and on Surfaces, CRC Press, Taylor & Francis Group, Boca Raton, 2010.
8. N. Azum, M. A. Rub, A. M. Asiri and H. M. Marwani, J. Mol. Liq., 2014, 197, 339–345.
9. N. Azum, M. A. Rub and A. M. Asiri, J. Mol. Liq., 2016, 216, 94–98.
10. N. Azum, M. A. Rub and A. M. Asiri, Colloids Surf., B, 2014, 121, 158–164.
11. M. A. Rub, F. Khan, N. Azum, A. M. Asiri and H. M. Marwani, J. Taiwan Inst. Chem. Eng., 2016, 60, 32–43.
12. N. Azum, M. A. Rub, A. M. Asiri, K. A. Alamry and H. M. Marwani, J. Dispersion Sci. Technol., 2014, 35, 358–363.
13. Mixed Surfactant Systems, Surfactant Science Series, ed. K. Ogino and M. Abe, Marcel Dekker, New York, 1993, vol. 46.
14. D. Blankschtein and A. Shiloach, Langmuir, 1998, 14, 1618–1636.
15. F. D. Souza, B. S. Souza, D. W. Tondo, E. C. Leopoldino, H. D. Fiedler and F. Nome, Langmuir, 2015, 31, 3587–3595.
16. R. F. Heck, J. Am. Chem. Soc., 1968, 90, 5526–5531.
17. R. F. Heck, J. Am. Chem. Soc., 1968, 90, 5546–5548.
18. X.-F. Wu, P. Anbarasan, H. Neumann and M. Beller, Angew. Chem., Int. Ed., 2010, 49, 9047–9050.
19. J. G. deVries and A. H. M. de Vries, Eur. J. Org. Chem., 2003, 2003, 799–811.
20. T. Dwars, E. Paetzold and G. Oehme, Angew. Chem., Int. Ed., 2005, 44, 7174–7199.
21. J.-Z. Jiang and C. Cai, J. Colloid Interface Sci., 2006, 299, 938–943.
22. Y. Kasaka, B. Bibouche, I. Volovych, M. Schwarze and R. Schomäcker, Colloids Surf., A, 2016, 494, 49–58.
23. D. C. Crans, S. Schoeberl, E. Gaidamauskas, B. Baruah and D. A. Roess, J. Biol. Inorg. Chem., 2011, 16, 961–972.
24. J. Tang, Y. Wang, D. Wang, Y. Wang, Z. Xu, K. Racette and F. Liu, Biomacromolecules, 2013, 14, 424–430.
25. A. L. Kholodenko and J. F. Douglas, Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top., 1995, 51, 1081–1090.
26. S. P. Moulik, L. Digout, W. Aylward and R. Palepu, Langmuir, 2000, 16, 3101–3106.
27. R. K. Mahajan and R. Sharma, J. Colloid Interface Sci., 2011, 363, 275–283.
28. J. H. Clint, J. Chem. Soc., Faraday Trans. 1, 1975, 71, 1327–1334.
29. D. N. Rubingh, in Solution Chemistry of Surfactants, ed. K. L. Mittal, Plenum Press, New York, 1979, vol. 1, p. 337.
30. M. J. Rosen and X. Y. Hua, J. Colloid Interface Sci., 1982, 86, 164–172.
31. M. J. Rosen, Surfactants and Interfacial Phenomena, Wiley-Interscience, New York, 2nd edn, 1989, pp. 65–68.
32. W. H. Ansari, N. Fatma, M. Panda and K. Ud-Din, Soft Matter, 2013, 9, 1478–1487.
33. R. Sanan, R. Kaur and R. K. Mahajan, RSC Adv., 2014, 4, 64877–64889.
34. C. C. Ruiz, Colloid Polym. Sci., 1999, 277, 701–707.
35. T. Chakraborty, S. Ghosh and S. P. Moulik, J. Phys. Chem. B, 2005, 109, 14813–14823.
36. M. J. Rosen and S. Aronson, Colloids Surf., 1981, 3, 201–208.
37. A. B. Pereiro, J. M. M. Araujo, F. S. Teixeira, I. M. Marrucho, M. M. Pineiro and L. P. N. Rebelo, Langmuir, 2015, 31, 1283–1295.
38. A. V. Delgado, F. Gonzalez-Caballero, R. J. Hunter, L. K. Koopal and J. Lyklema, Pure Appl. Chem., 2005, 77, 1753–1805.
39. P. K. Misra and P. Somasundaran, J. Surfactants Deterg., 2004, 7, 373–378.
40. S. K. Mehta and S. Chaudhary, Colloids Surf., B, 2011, 83, 139–147.
41. E. Odella, R. D. Falcone, J. J. Silber and N. M. Correa, Phys. Chem. Chem. Phys., 2014, 16, 15457–15468.
42. A. Pan, S. S. Mati, B. Naskar, S. C. Bhattacharya and S. P. Moulik, J. Phys. Chem. B, 2013, 117, 7578–7592.
43. N. Cheng, P. Yu, T. Wang, X. Sheng, Y. Bi, Y. Gong and L. Yu, J. Phys. Chem. B, 2014, 118, 2758–2768.
44. S. Khamarui, D. Sarkar, P. Pandit and D. K. Maiti, Chem. Commun., 2011, 47, 12667–12669.
45. M. M. Shinde and S. S. Bhagwat, J. Dispersion Sci. Technol., 2012, 33, 117–122.
46. S. Bhattacharya, A. Srivastava and S. Sengupta, Tetrahedron Lett., 2005, 46, 3557–3560.
47. E. Palazzo, L. Carbone, G. Colafemmina, R. Angelico, A. Ceglieb and M. Giustini, Phys. Chem. Chem. Phys., 2004, 6, 1423–1429.
48. J. E. Bowcott and J. H. Schulman, Z. Elektrochem., 1955, 59, 283–290.
49. B. K. Paul and D. Nandy, J. Colloid Interface Sci., 2007, 316, 751–761.
50. K. Kundu and B. K. Paul, Colloid Polym. Sci., 2013, 291, 613–632.
51. H. Preu, A. Zradba, S. Rast, W. Kunz, E. H. Hardy and M. D. Zeidler, Phys. Chem. Chem. Phys., 1999, 1, 3321–3329.
52. O. Zheng, J.-X. Zhao and X.-M. Fu, Langmuir, 2006, 22, 3528–3532.
53. L. Digout, K. Bren, R. Palepu and S. P. Moulik, Colloid Polym. Sci., 2001, 279, 655–663.
54. C. C. Villa, J. J. Silber, N. M. Correa and R. D. Falcone, ChemPhysChem, 2014, 15, 3097–3109.
55. S. A. Corcelli and J. L. Skinner, J. Phys. Chem. A, 2005, 109, 6154–6165.
56. H. Graener and G. Seifert, J. Chem. Phys., 1993, 98, 36–45.
57. Z. S. Nickolov, V. Paruchi, D. O. Shah and J. D. Miller, Colloids Surf., A, 2004, 232, 93–99.
58. S. K. Mehta, G. Kaur, R. Mutneja and K. K. Bhasin, J. Colloid Interface Sci., 2009, 338, 542–549.
59. J.-B. Brubach, A. Mermet, A. Filabozzi, A. Gerschel, D. Lairez, M. P. Krafft and P. Roy, J. Phys. Chem. B, 2001, 105, 430–435.
60. A. Das, A. Patra and R. K. Mitra, J. Phys. Chem. B, 2013, 117, 3593–3602.
61. Q. Zhong, D. A. Steinhurst, E. E. Carpenter and J. C. Owrutsky, Langmuir, 2002, 18, 7401–7408.
62. S. Bardhan, K. Kundu, S. Das, M. Poddar, S. K. Saha and B. K. Paul, J. Colloid Interface Sci., 2014, 430, 129–139.
63. C. G. Blanco, L. J. Rodriguez and M. M. Velazquez, J. Colloid Interface Sci., 1999, 211, 380–386.
64. S. Biswas, S. C. Bhattacharya, B. B. Bhowmik and S. P. Moulik, J. Colloid Interface Sci., 2001, 244, 145–153.
65. S. Chatterjee, R. K. Mitra, B. K. Paul and S. C. Bhattacharya, J. Colloid Interface Sci., 2006, 298, 935–941.
66. D. M. Willard and N. E. Levinger, J. Phys. Chem. B, 2000, 104, 11075–11080.
67. P. Setua, C. Ghatak, V. G. Rao, S. K. Das and N. Sarkar, J. Phys. Chem. B, 2012, 116, 3704–3712.
68. B. Kar, S. Bardhan, K. Kundu, S. K. Saha, B. K. Paul and S. Das, RSC Adv., 2014, 4, 21000–21009.
69. M. Hager, U. Olsson and K. Holmberg, Langmuir, 2004, 20, 6107–6115.
70. N. Li, Q. Cao, Y. Gao, J. Zhang, L. Zheng, X. Bai, B. Dong, Z. Li, M. Zhao and L. Yu, ChemPhysChem, 2007, 8, 2211–2217.
71. H. U. Blaser, A. Indolese, A. Schnyder, H. Steiner and M. Studer, J. Mol. Catal. A: Chem., 2001, 173, 3–18.
72. M. A. Haque, T. Kurokawa and J. P. Gong, Soft Matter, 2012, 8, 8008.
73. A. M. Alkilany, P. K. Nagaria, M. D. Wyatt and C. J. Murphy, Langmuir, 2010, 26, 9328.
74. B. H. Lipshutz and B. R. Taft, Org. Lett., 2008, 10, 1329–1332.
75. A. L. Compere, W. L. Griffith, J. S. Johnson Jr, E. Caponetti, D. Chillura-Martino and R. Triolo, J. Phys. Chem. B, 1997, 101, 7139–7146.
76. P. López-Cornejo, P. Pérez, F. García, R. De la Vega and F. Sánchez, J. Am. Chem. Soc., 2002, 124, 5154–5164.
77. W. Zhang, X. Qiao and J. Chen, Colloids Surf., A, 2007, 299, 22–28.
78. G. N. Smitha, P. Browna, C. James, S. E. Rogers and J. Eastoe, Colloids Surf., A, 2016, 494, 194–200.
79. D. T. McQuade and P. H. Seeberger, J. Org. Chem., 2013, 78, 6384–6389.
80. F. Lopez, G. Cinelli, L. Ambrosone, G. Colafemmina, A. Ceglie and G. Palazzo, Colloids Surf., A, 2004, 237, 49–59.
81. K. Holmberg, Eur. J. Org. Chem., 2007, 2007, 731–742.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra06776j

‡ Present address: Department of Biotechnology, Bhupat and Jyoti Mehta School of Biosciences, Indian Institute of Technology Madras, Chennai 600036, India.

§ S. B. and K. K. have contributed equally to this work.

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