Physicochemical studies of water-in-oil nonionic microemulsion in presence of benzimidazole-based ionic liquid and probing of microenvironment using model C–C cross coupling (Heck) reaction

Abstract

The present report focuses on the evaluation of the interfacial composition and the thermodynamics of the transfer of 1-pentanol (Pn) from a continuous oil phase to the interface of w/o nonionic microemulsion [Tween-20/Pn/cyclohexane(Cy)/water] in the absence and the presence of an ionic liquid (IL) (1-butyl-3-propylbenzimidazolium bromide) under different physicochemical conditions [viz. variation in concentrations of IL (0.0 → 0.20 mol dm⁻³) and temperature (293 → 323 K] at a fixed molar ratio of water to surfactant (ω) by the Schulman's method of cosurfactant titration at the oil/water interface. The overall transfer process has been found to be spontaneous, exothermic and organized in the absence or the presence of IL, but shown to be influenced by [IL]. The microstructure and state of water organization inside a pool of these systems have been characterized by different experimental techniques, e.g., conductivity, DLS and FTIR in the absence or the presence of IL. In addition, a C–C cross coupling reaction (Heck reaction) has been employed to explore the properties of IL (additive) in the confined environment of the microemulsion vis-à-vis its interaction with the constituents of the interface. The reaction progress has been monitored using the above techniques. The reaction ended with the highest yield (75%) in the presence of 0.05 mol dm⁻³ of IL, wherein the microemulsion forms spontaneously with the highest stability.

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Introduction

Ionic liquids (ILs) are organic salts/solvents constituted of distinct cations and anions and have low vapor pressure and high thermal and chemical stability. They are non-inflammable and have good catalytic and solvation power.¹ The importance and the application of ILs are reflected by the contribution of different researchers.² A number of physicochemical investigations of imidazole-based ILs are available in literature,³ whereas the study of benzimidazole-based ILs is relatively new and rare.⁴ Benzimidazole is an important motif found in both naturally occurring and biologically active compounds. It is an important pharmacophore as well.⁵

In the field of colloid and interface science, the investigation of surfactant molecular assemblies in a room temperature ionic liquid (RTIL) is of great interest because the former systems can solubilize substances that are essentially insoluble in the latter, and thus, the solubilizing power of surfactant assemblies may widen the application of RTILs. From this point of view, the elucidation of the self-assembling phenomena of surfactant molecules in ILs and their applications have become an interesting area of research.⁶ Furthermore, IL based microemulsions are used as reaction media in various organic transformations, viz. aminolysis of esters,⁷ Diels–Alder reaction,⁸ and Matsuda–Heck reaction.⁹ Water-in-oil (w/o) microemulsions or reverse micelles (RMs) are macroscopically homogeneous mixtures of oil, water, surfactant and/or cosurfactant, whereas at the microscopic level, they consist of individual domains of oil and water separated by a monolayer of the surfactant and/or the cosurfactant. The stability, flexibility and microheterogeneity of microemulsions make them convenient for biological and technological applications.¹⁰ The microstructure of microemulsions critically depends on the system composition, temperature, and additives.¹⁰ Very recently, we have reported the characteristic roles of the surfactant(s), the cosurfactant and oils for the formation and stabilization of w/o microemulsions in the absence or presence of an additive by employing Schulman's method of cosurfactant titration of the oil/water interface (or, the dilution method).^11–13 Wang et al.¹⁴ employed the dilution method for the first time to investigate the physicochemical parameters of [bmim][BF₄]/Brij-35/1-butanol/toluene-based IL/O microemulsion.

In view of these studies, we have undertaken the study of the formation of a water-in-oil (w/o) nonionic surfactant microemulsion, water/Tween-20/Pn/Cy, with special reference to its characteristic features of the oil/water interface under different physicochemical conditions in the absence or presence of IL, 1-butyl-3-propylbenzimidazolium bromide ([bpBzim]Br), as an additive. Thermodynamics of formation, microstructure, transport properties and dynamics of H-bonding of this system, water–IL/Tween-20/Pn/Cy w/o microemulsion, have been investigated by employing the dilution method, conductivity, DLS, and FTIR. Further, an in-depth characterization of the microenvironment of the system in the absence or presence of IL has been made by performing a model C–C cross coupling reaction (Heck reaction). The famous reaction, Heck, is one of the finely studied responses in the meadow of organic synthesis.¹⁵ A report on the study of the Heck reaction in IL-microemulsion is available in literature.¹⁶ The yield of the Heck coupled product depends on the type of surfactant used as well as the nature of confinement of these systems.^9,16 An attempt has been made to monitor the effective physicochemical changes in the microemulsion, leading to microenvironmental changes during the course of reaction. Finally, a correlation of the results in terms of the evaluated physicochemical parameters vis-à-vis microstructural features during the Heck reaction has been drawn and provides a new horizon to understand the most plausible location/site as well as to determine the actual reaction parameter required for an effective reaction in the studied micro-heterogeneous system. To the best of our knowledge, such a comprehensive study on w/o microemulsion in the presence of benzimidazole-based IL has not been reported earlier.

Results and discussions

Schulman's cosurfactant titration at the oil/water interface (the dilution method)

In order to underline the influence of IL and its content on the interfacial composition of Tween-20-based w/o microemulsion stabilized in Pn and Cy under various physicochemical conditions (i.e., at different temperatures and a fixed molar ratio of water to the surfactant, ω), n_aⁱ/n_s values [i.e. compositional variations of amphiphiles (both Tween-20 and Pn) at the interface] are plotted against [IL] (0.0 → 0.20 mol dm⁻³) and the respective plots are depicted in Fig. 1 (inset A). The mathematical evaluation and the results of interfacial composition have been discussed elaborately indicating all possible interactions among the constituents in the microenvironment of this compartmentalized system (depicted in Scheme 1) and are presented in ESI (Sec. A and B) and Fig. S1.†

Fig. 1 Plots of ΔG⁰_0→i as a function of [IL] for water/Tween-20/Pn/Cy microemulsion system at ω (= 30) with varying temperatures. Inset A: plots of interfacial composition (n_aⁱ/n_s) as a function of [IL] for water/Tween-20/Pn/Cy microemulsion system at ω (= 30) with varying temperatures. Inset B: same plots for K_d.

Scheme 1

Thermodynamics of transfer of Pn (oil → interface) (in absence or presence of IL)

In this section, an analysis of the transfer of Pn from the oil phase to the interface (Pn_oil → Pn_interface) of the water (IL)/Tween-20/Pn/Cy microemulsion system by employing the dilution method is presented from a thermodynamic point of view, which is rarely reported.^14,17 The details of the estimation have been given in ESI (Sec. A, eqn (S1)–(S16)).†

Effect of [IL] and temperature on ΔG⁰_0→i

ΔG⁰_0→i values are negative in the absence or presence of IL, and hence, spontaneous formation of w/o microemulsions is suggested. Both similar and dissimilar values of this energetic parameter under comparable physicochemical conditions are reported.^14,17–19 Both K_d and ΔG⁰_0→i show maxima in the presence of 0.05 mol dm⁻³ of IL at each temperature (Fig. 1 and inset B).

Although the variations of K_d and ΔG⁰_0→i values between 0.05 mol dm⁻³ and 0.10 mol dm⁻³ may be small at higher temperatures (viz. 313 and 323 K), both the values are still higher at 0.05 mol dm⁻³ as compared to the other concentrations of IL (Fig. 1 and inset B, Table S1†). This indicates that the transfer process of Pn from the oil phase to the interface is much favoured at 0.05 mol dm⁻³ of IL irrespective of the temperature. Further, a low line secondary maximum in K_d or ΔG⁰_0→i values appears at 0.15 mol dm⁻³ of [IL] (Fig. 1, including the error bars). A plausible explanation for this type of trend can be explained from the view of the physicochemical (molecular) interactions among the constituents involved in the transfer process of Pn (oil → interface).^11,12,14,17 In this system, the interaction between the nonionic surfactant (Tween-20) and the alkanol (Pn) is of the dipole–dipole or dipole-induced dipole or London dispersion type, because of the presence of uncharged or neutral hydroxyl groups and POE chains (Tween-20); in contrast, the interaction between [bpBzim]⁺ and Pn is possibly of the ion–dipole type. An ion–dipole interaction is much stronger than dipole–dipole or dipole-induced dipole or London dispersion interactions.²⁰ Therefore, the association between Pn and Tween-20 becomes more favorable in the presence of 0.05 mol dm⁻³ of [bpBzim][Br]. Consequently, the transfer process of Pn from the oil phase to the interface is much favoured at 0.05 mol dm⁻³ of [IL] irrespective of the temperature. However, at a higher IL concentration (i.e., 0.10 → 0.20 mol dm⁻³), the addition of IL diminishes the interfacial area per polar head group of surfactant molecules by screening the steric repulsion between nonionic surfactants (herein, Tween-20), and this makes the interfacial layer more rigid and favors a greater curvature of the interface.^14,17 As a result, the attractive interaction among the droplets decreases, and subsequently, the transfer process of Pn (oil → interface) is diminished. However, the appearance of a low line secondary maximum at [IL] that is equal to 0.15 mol dm⁻³ is likely to be originated from the non-ideality of the systems. Usually, different types of forces (viz. London dispersion forces, dipole–dipole forces, and dipole–dipole induced forces) act on real mixtures, making it difficult to predict the properties of such solutions. Non-ideal mixtures are identified by determining the strength and types (specifics) of intermolecular forces (viz. intermolecular forces between similar molecules and intermolecular forces between dissimilar molecules) in that particular system.²¹ However, non-ideal behavior is not uncommon in multicomponent derived microemulsion systems.^22–26

Further, it has been observed that K_d or ΔG⁰_0→i gradually increases with an increase in temperature (Fig. 1 and inset B); this suggests that the transfer of Pn (oil → interface) is much favored at higher temperatures. The higher absolute values of ΔG⁰_0→i at higher temperatures indicate a comparatively stronger interaction between Tween-20 and Pn at the interface, which corroborates well with the interfacial composition (Fig. 1, inset A). This trend corroborates well with the reports of Zhang et al.¹⁸ and Wang et al.¹⁴ for [bmim][BF₄]/CTAB/alkanol/toluene and [bmim][BF₄]/Brij-35/1-butanol/toluene w/o microemulsion systems, respectively. However, the overall scenario indicates a tendency towards the formation of a more spontaneous w/o microemulsion accompanied with 0.05 mol dm⁻³ of IL, as evident from K_d and ΔG⁰_0→i values at each temperature (Table S1,† Fig. 1 and inset B).

Further, a significant difference in the K_d or ΔG⁰_0→i values has been observed for 303 K and 313 K (Fig. 1 and inset B). This may be due to the influence of temperature on the constituents (particularly, on Tween-20 consisting of POE chains as polar head groups and IL as an organic electrolyte, [bpBzim][Br]), and hence, two different mechanisms might be operative for the formation of microemulsions in the lower temperature region (i.e., 293 and 303 K) as well as in the higher temperature region (i.e., 313 and 323 K). The ordered structure formed from IL molecules around the hydrophilic moiety (POE chains) of Tween-20, produced by an ionic interaction instead of a hydrogen bonding interaction, is considered to be an origin for the surfactant self-assembly²⁷ as represented in Scheme 1. It is obvious that Tween-20 molecules assemble to form reverse micelles in order to avoid the entropy loss due to the ordered arrangement of IL molecules in the low temperature region. On the other hand, in the higher temperature region, the IL molecules participating in the ionic interaction with POE chains of Tween-20 may be less ordered (more disordered) due to their enhanced dehydration.^27,28 Hence, it is imperative that the enthalpy loss for the release of solvated IL molecules be overcome by the enthalpy gain for the contact of POE chains in the higher temperature region. The reversal of entropy and enthalpy parameters might occur during the transition from the low temperature region (293 and 303 K) to the high temperature region (313 and 323 K). This is why, a significant difference in ΔG⁰_0→i values has been observed for two sets of temperatures.

Effect of [IL] and temperature on ΔH⁰_0→i and ΔS⁰_0→i

Due to the nonlinear dependence of (ΔG⁰_0→i/T) on (1/T) (the result fits a two degree polynomial equation (Fig. S2†)), at each composition of the surfactant (at [IL] = 0.0 → 0.20 mol dm⁻³), four values of the standard enthalpy change, ΔH⁰_0→i and the standard entropy change (ΔS⁰_0→i) of the Pn transfer process (oil → interface) at four different temperatures have been evaluated according to eqn (S12) and (S15)† and are presented in Table S1† and Fig. 2. For a pure Tween-20 system (i.e., in the absence of IL), the overall transfer process is exothermic at all experimental temperatures with a negative entropy change (more ordered) in Cy (Table S1† and Fig. 2). Therefore, Pn causes heat release during the transfer process. Consequently, the negative entropy change may be due to more organization of the interface and its surroundings. Therefore, it may be argued that in the presence of Pn and Tween-20, the interface to some extent becomes ordered. A similar observation was also reported by Bardhan et al.¹² for water/Brij-35/Pn/IPM microemulsion systems. However, De et al.²⁹ and Kundu et al.¹¹ reported an opposite behavior (i.e., positive enthalpy and entropy changes) for Tween-20/Bu or Pn/Hp/water and Brij-58 or Brij-78/Pn/Hp or Dc/water microemulsion systems, respectively. Hence, the difference in the trends of ΔH⁰_0→i and ΔS⁰_0→i may be attributed to the differences in the hydrophobic configuration and/or the size of the polar head group of the nonionic surfactants vis-à-vis the type of oil.

Fig. 2 Plots of ΔH⁰_0→i as a function of [IL] for water/Tween-20/Pn/Cy microemulsion system at ω (= 30) and varying temperatures. Inset A: same plot for ΔS⁰_0→i.

Further, it can be observed from Table S1† and Fig. 2 that, the overall transfer process is exothermic at all the experimental temperatures with negative entropy change (order) in Tween-20/Pn system in the presence of IL ([IL] = 0.05 → 0.20 mol dm⁻³). Such a negative enthalpy or entropy change was observed by Mukherjee et al.¹⁹ for triisobutyl (methyl) phosphoniumtosylate/IPM/trihexyl (tetradecyl) phosphoniumbis 2,4,4-(trimethylpentyl) phosphinate–isopropanol or butanol systems. It is quite likely that the negative contribution towards ΔH⁰_0→i is identified with the transfer of the surfactant tail from water to liquid hydrocarbon state in the interfacial layer and for restoring the hydrogen bonding structure of the water around the surfactant head group.³⁰ Further, it can be argued for negative entropy, – ΔS⁰_0→i as follows: during formation of nano-droplets in w/o microemulsions, the penetration of water into the oil (Cy)/Tween-20/Pn (amphiphiles) continuum forming cavity and subsequently, organization of the amphiphiles at the droplet interface results in an increase in overall order with negative entropy of the multicomponent system.³¹ Further, – ΔS⁰_0→i suggests that both entropy as well as enthalpy are involved in the transfer process.³² Consequently, IL dependent microemulsion compositions with larger – ΔH⁰_0→i values (more exothermic) results in larger – ΔS⁰_0→i values (ordered) i.e., greater interaction between the constituents at the interface, leading to more ordered interface due to transfer of Pn (oil → interface).³³

The variation of ΔH⁰_0→i and ΔS⁰_0→i values with temperatures from Table S1† and Fig. 2 indicates that the presence of IL influences the degree of exothermicity with orderliness of the transfer process (oil → interface). Interestingly, the transfer process approaches towards less exothermic as well as less ordered with increase in temperature up to 0.10 mol dm⁻³ of IL, and thereafter, the degree of exothermicity and orderliness reverses in the vicinity of 0.136 mol dm⁻³. Subsequently, the process is more exothermic as well as more ordered with increase in temperature at [IL] equals to 0.15 and 0.20 mol dm⁻³ (Fig. 2). This trend can be explained in the following way: the addition of IL to Tween-20 based microemulsions might have at least two effects. First, the partial reduction of the hydration of water around the polar head group of the non-associated surfactant molecule occurs with increase in temperature. As a result, a decrease in the energy needed for breaking down this structure during the process of microemulsion formation and also for a decrease in the exothermic contribution i.e., decrease in the value of ΔH⁰_0→i. The second consequence of the presence of higher IL concentrations appears to be related to the influence of counter ion binding on the structure of the microemulsions.³⁴ The reduction in disorder may be attributed to the effect of the liberation of surfactant hydration water molecules on microemulsification to be less important than the loss of freedom when monomers join each other to form reverse micelles.³⁵

Also, ΔS⁰_0→i values have been found to increase up to 0.10 mol dm⁻³ of IL with temperature, and thereafter, a reversal of trend has been observed in the vicinity of 0.136 mol dm⁻³, as in the case of ΔH⁰_0→i (Fig. 2, inset A). This result is indeed interesting. It is quite likely that molecular interactions, arising from the tendency of the water molecules to regain their normal tetrahedral structure, and the attractive dispersion forces between hydrocarbon chains act cooperatively to remove the hydrocarbon chains from the water cages, leading to the disorder of water, and subsequently, increase in the entropy values.^36,37 On the other hand, the decrease in entropy may be due to the breakdown of the normal hydrogen-bonded structure of water accompanied by the formation of water with different structures.^36,37 The presence of a higher concentration of hydrophilic species (herein, IL) promotes an ordering of water molecules in the vicinity of the hydrophilic head group of Tween-20.³⁶ However, no significant change in both thermodynamic parameters (i.e., ΔH⁰_0→i and ΔS⁰_0→i) with temperature has been observed at certain IL concentrations (i.e., at 0.05 and ∼0.136 mol dm⁻³), leading to the formation of isoenthalpic and isoentropic microemulsion systems (Fig. 2 and inset A). A similar observation was also reported earlier for mixed surfactant microemulsion systems consisting of cationic or anionic–nonionic structures.^11,12,38

Effect of [IL] on (ΔC⁰_p)_0→i

Since ΔH⁰_0→i is a function of temperature, the (ΔC⁰_p)_0→i values have been evaluated for these systems (in the absence or presence of IL) from the slope (= δΔH⁰_0→i/δT) of the plots of ΔH⁰_0→i vs. T (not illustrated) according to eqn (S11†).¹² All these values are presented in Table S1.† It can be observed from Table S1† that, at [IL] = 0.0 → 0.10 mol dm⁻³, (ΔC⁰_p)_0→i > 0; whereas, (ΔC⁰_p)_0→i < 0 at [IL] = 0.15 and 0.20 mol dm⁻³. Negative values are usually observed for the self-association of amphiphiles and can be attributed to the removal of large areas of non-polar surface from contact with water on the formation of reverse micelles.³⁹ However, (ΔC⁰_p)_0→i tends to almost zero at [IL] = 0.05 and ∼0.136 mol dm⁻³, which corroborates well with the profile of ΔH⁰_0→i vs. [IL] at different temperatures (Fig. 2). This indicates the formation of temperature-insensitive microemulsions at [IL] of 0.05 and ∼0.136 mol dm⁻³. Similar observation [i.e., reversal of trend of (ΔC⁰_p)_0→i with concentration of methanol] was reported earlier by Perez-Casas et al.⁴⁰ for water/AOT/methanol/decane reverse micelles. Kunz et al.⁴¹ reported that the formation of temperature-insensitive microemulsions is important for some practical purposes (e.g. for formulation of a product).

Electrical conductivity and dynamic light scattering measurement

The electrical conductivity, size and size distribution of w/o microemulsion systems have been measured by conductometric and DLS, respectively, for a water/Tween-20/Pn/Cy microemulsion system with the variation of [IL] (= 0.0 → 0.20 mol dm⁻³) at a constant ω (= 30) and fixed temperature (303 K). The results are depicted in Fig. S3.† It can be observed from Fig. S3† that the electrical conductivity gradually increases with an increase in [IL] in the microemulsion. This trend may be attributed to the high conducting nature of IL, as the blank experiment (similar concentration of IL in water) shows an identical trend with the variation of [IL] (Fig. S3,† inset A). It can be observed that the conductance values in the water–IL media range from 0.0016 to 8.2 mS cm¹, whereas low values (17.84–231.0 μS cm¹) are obtained for IL in microemulsions at similar concentrations. The low value in the microemulsion system can be justified as follows: in the present system, the electrostatic interaction between the imidazolium cation of IL and the electronegative oxygen atoms of the POE units of Tween-20 is quite likely to occur along with the water–IL hydrogen bonding interaction (Scheme 1). However, the latter part, i.e., a water–IL hydrogen bonding interaction is present in the bulk IL or water–IL media. The electrostatic interaction makes the palisade layer comprising Tween-20/Pn/IL more rigid⁴² and subsequently, decreases the conductance values. Hence, it can be concluded that the physicochemical properties of water molecules localized in the interior of the microemulsions are different from those of the water–IL media.

Further, the hydrodynamic diameter (D_h) of a microemulsion droplet decreases from 3.90 nm to 2.31 nm with an increase in [IL] (0.0 → 0.20 mol dm⁻³) wherein about a 3 fold increase of the droplet count rate has been observed under the prevailing condition (Fig. S3,† inset B and C). The droplet count rate is directly proportional to the droplet number of the microemulsion system. Hence, the addition of IL shrinks the droplet size and thereby, increases the droplet number. It was reported earlier that the curvature of the oil/water interface of a microemulsion can be adjusted by adding IL (a class of organic electrolyte)⁴³ or NaCl (inorganic electrolyte)⁴⁴ at different concentrations. The addition of electrolyte gives rise to a decrease in the repulsive interaction between the head groups of the nonionic surfactant, Tween-20, which further increases the packing parameter of the surfactant molecules (P = v/al, where ‘v’ and ‘l’ are the volume and the length of a hydrophobic chain, respectively, and ‘a’ is the area of the polar head group of the surfactant) and decreases the droplet diameter. In addition, the presence of IL within the water pool weakens the hydrogen-bonding between water and the POE chains of Tween-20, thereby reducing the hydration of POE chains, which results in the formation of smaller droplets due to a decrease in the swelling of the POE chains.⁴⁵ In other words, when IL is solubilized in microemulsions, it decreases the average area occupied by each head group of surfactant (Tween-20) and subsequently enhances the packing density and rigidity of the surfactant monolayer of the droplet, which may reduce the size of the droplets.⁴⁶ Typical values of the polydispersity index (PDI) obtained here are in the range between 0.1–0.2, which indicates the monodispersity of the sample.^12,13,38

FTIR spectroscopy

Reports on the properties of the encapsulated water in the range of the size and type of w/o microemulsions by studying the states of water organization using an FTIR measurement, are available in literature.^13,37,41,47 The characteristics of the water molecules in a confined environment of w/o microemulsion depend strongly on the water content or the droplet size and the nature of the interface as well.^47,48 As discussed in the previous section, the values of the hydrodynamic diameter of a droplet (D_h) varies from 3.90 nm to 2.31 nm as a function of [IL] (0.0 → 0.20 mol dm⁻³) at fixed ω (= 30) and temperature (= 303 K) from DLS measurements. Further, the values of the hydrodynamic diameter of a droplet (D_h) are well comparable with those of microemulsion systems, wherein the existence of different types/states of water species is reported in a confined environment by using an FTIR measurement.^47,49 However, the influence of Pn (cosurfactant) on the O–H stretching vibration of the confined water needs to be underlined for the present system. To eliminate the effect of Pn on the O–H vibration of water, the spectra of Pn at the same concentration are subtracted from the spectral intensity of the O–H stretching band, and the differential spectra have been analyzed. Different types of hydrogen bonded water molecules exist in reverse micelles, which can be broadly classified into two major classes, namely, bound and bulk-like water molecules.^13,37,41,50 The differential spectra obtained in the present study have been deconvoluted into two peaks at ∼3500 and ∼3300 cm¹, corresponding to the O–H stretching frequency of the bound and bulk-like water molecules, respectively (Fig. S4†), and a representative result of deconvolution (relative abundance of different water species) is depicted in Fig. S5.† It is observed from Fig. S5† that the bound water proportion is the least in the absence of IL. In contrast, the proportion of bound water increases with an increase in [IL] in the microemulsion. These results indicate the significant role of IL in determining the proportion of different water species (bound and bulk) in the confined environment of w/o microemulsion. However, this result is not very surprising if one considers the possibility of interaction between IL and water molecules. With an increase in the IL content, more bulk water molecules hydrate IL molecules and subsequently, increase the proportion of bound water molecules in the microemulsion core.⁴² Further, it is evident from the DLS measurements that the droplet size decreases with an increase in the IL content. It is reported that as the droplet size decreases, the bound water proportion increases in the microemulsion and vice versa.^47,50 Hence, DLS and FTIR results corroborate each other.

Study of C–C cross coupling (Heck) reaction in microemulsion

A model Heck reaction (Scheme 2) has been performed in the well characterized system (presented herein), to explore the properties of IL (additive) within the restricted geometry provided by microemulsions and the nature of interaction of the IL with the constituents of the interface and the subsequent changes in the microenvironment during the reaction and the detection of the most probable reaction site and yield of the Heck reaction. The reaction progress was monitored by employing various instrumental techniques such as, conductance, FTIR, and UV-visible spectroscopy.

Scheme 2

Standardization of Heck coupling reaction

The reaction has been performed in the presence of different bases (inorganic and organic) to optimize the reaction at 323 K. It has been found that water soluble bases (potassium carbonate and tetramethyl ethelene diamine) produce an almost comparable yield (30% and 40%, respectively) of the desired product, whereas the highest yield (75%) has been achieved using a sparingly water soluble base, viz., triethyl amine (TEA) (Table S2†). Hence, a subsequent study on the Heck reaction in microemulsion media at varying [IL] (0.0 → 0.2 mol dm⁻³) was performed in the presence of TEA as the base, and the standard mechanism of the Heck coupling reaction is presented in Scheme S1.†

In order to underline the effect of encapsulation or compartmentalization (provided with microemulsion), the Heck reaction has also been performed in bulk IL (water–IL media) containing [IL] similar to that available in microemulsions, and the results are presented in Table 1. The study showed (Table 1) that the yield of the desired product in water–IL media was very low in comparison with that in the microemulsion systems. The strong effect of the confined environment of the microemulsion, therefore, seems to play a vital role in studying this reaction.⁹ However, the yield of the final product in the microemulsion is not a direct function of the IL concentration (Table 1). The reaction has also been critically monitored in the case of two different temperature-insensitive formulations at [IL] = 0.05 and 0.136 mol dm⁻³ (discussed in the previous section, “Effect of [IL] on (ΔC⁰_p)_0→i”, Fig. 2). The yield of the final product has been found to be the highest in the lower range of [IL] (i.e., at 0.05 mol dm⁻³), at which the highest spontaneity of the Pn transfer process and temperature-insensitivity were exhibited (Fig. 1 and 2). However, in the higher range of [IL] (i.e., at ∼0.136 mol dm⁻³), the yield is the least, where another temperature-insensitive formulation is obtained. Hence, it can be suggested that the temperature insensitive composition is not the sole factor affecting the product yield; rather, the highest spontaneity of the Pn transfer (oil → interface) leads to the highest yield of the product.

Table 1 Heck coupling reaction in both water and microemulsion media (water/Tween-20/Pn/Cy) as a function of [IL] in the presence of TEA at a fixed ω (= 30) and 323 K

Ionic liquid (mol dm^−3)	Yield (%) in water–IL	Yield (%) in microemulsion
0.00	07.0	17
0.05	04.5	75
0.10	06.8	25
0.136	08.3	13
0.15	10.0	59
0.20	12.0	22

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Conductance study

Electrical conductance of the w/o microemulsion system has been measured at regular intervals during the course of reaction both in the absence and in the presence of IL, and the results are displayed in Fig. 3. In both cases, the trend of the conductance curve is almost identical with smaller values in the absence of IL. In the absence of IL, the addition of palladium acetate [Pd(OAc)₂] results in a small increase in conductance due to the presence of a charge carrier (viz. Pd(OAc)₂) and is followed by a decrease in conductivity, which may be due to the formation of the aqua palladium complex in the mixture.⁵¹

Fig. 3 The conductivity of microemulsion in absence and presence of IL (= 0.05 mol dm⁻³) at regular intervals during Heck reaction at 323 K.

On the contrary, in the presence of IL, the addition of Pd(OAc)₂ decreases the conductivity indicating the formation of a palladium complex (Pd–NHC) with IL, the precursor of N-heterocyclic carbene (NHC).⁵² However, the process of Pd–NHC complexation is much faster in comparison with the palladium–aqua complexation which subsides the conducting properties of Pd(OAc)₂. However, a sharp increase in conductivity has been observed in both cases after the addition of reagents (4-iodotoluene, triethylamine and butylacrylate), implying the progress of the Heck reaction.

FTIR measurement

The FTIR observation studies during the reaction (Fig. 4) are very supportive of the conductivity experiment. The addition of palladium acetate reduces the amount of bound water and indicates the formation of a Pd–NHC complex which decreases the interaction with water molecules. Further, the optimal decrease of bound water implies the end point of complexation. Thereafter, a regular enhancement of the amount of bound water has been observed with the addition of reagents (4-iodotoluene, triethylamine and butylacrylate), indicating the formation of the Heck product and the other possible side products (such as halogen acid and the corresponding amine salt). This trend continues until the completion of reaction.

Fig. 4 The variation of Gaussian profiles (area fraction) of the normalized spectra of different water species (bound water and bulk water) of IL containing a (0.05 mol dm⁻³) w/o microemulsion system with the reaction time.

UV-visible spectroscopy

Comparing the UV-visible spectra of individual components and the spectra recorded during the course of reaction at regular intervals of time, a single absorption peak at 271 nm has been observed after 10 minutes of the reaction (Fig. S6†). The λ_max value of the product is reported to be 274 nm.⁵³ The little decrease in λ_max of the product from that of the literature value may be due to the H-bonding between the ester group of the product and the reaction medium. The absorption became more intense after 40 minutes of reaction. The intensity of the spectral band increases with the progress of the reaction, indicating an increase in the concentration of the product with the progress of time.

Comprehension of the results

The Heck reaction was carried out in the w/o microemulsion with a varying amount of IL (0.0 → 0.20 mol dm⁻³), and the corresponding results are presented in Table 1. The optimal concentration of IL required for an effective Heck reaction is 0.05 mol dm⁻³ in the present system. Further, it is evident from the dilution method that the spontaneity of formation with maximization in the stability of the microemulsion system can be achieved with 0.05 mol dm⁻³ of IL (Table S1† and Fig. 1). It is obvious that 0.05 mol dm⁻³ of IL facilitates the Pn transfer process (oil → interface) due to the favorable organization of the constituents at the interface and thereby, results in achieving more stability, which leads to better performance of the Heck reaction in a microemulsion medium. The decrease in the conductance with the addition of palladium acetate [Pd(OAc)₂] indicates the formation of a palladium complex (Pd–NHC) with IL (Fig. 3). The FTIR study also supported the formation of the Pd–NHC complex which reduces the number of IL molecules inside the water pool and thereby decreases the amount of bound water (Fig. 4). The ratio between the amounts of bound and bulk water indicates that no interaction persists between water and the newly formed Pd–NHC complex. However, it can be concluded that the availability of the Pd–NHC complex inside the water pool is the least. In bulk IL, i.e., water–IL media, the possibility of the Pd–NHC complex formation is negligible because of the instability of N-heterocyclic carbene in water; this subsequently reduces the rate of the forward reaction. It can be inferred from the low yield of the desired product in the water–IL media indicates that the formation of the stable Pd–NHC complex during the performance of Heck reaction in compartmentalized systems plays a pivotal role, which leads to higher yields as compared to the corresponding bulk [IL] (Table 1). In addition, the use of water soluble bases (viz., K₂CO₃ and TMEDA) which are quite likely to be located in the water pool of microemulsion, results in a low yield of the desired product. In contrast, TEA is preferentially soluble in Pn, as revealed by the solubility analysis, and ends up with the best results (75% yield of the product) (entry 2, Table S2†). A plausible explanation emerges from the increase in the population of Pn at the interfacial region vis-à-vis its interaction with TEA, which rationalizes the availability of all constituents involved in this reaction. It can be inferred that apart from the molecular interactions of the constituents involved in the formation of the microemulsion as a template (discussed in ESI (Sec. B†)), two types of interactions are likely to occur at the oil/water interface or in the palisade layer of the microemulsion. For example, (i) the dipole–dipole interaction between Pn and TEA, which enhances the availability of TEA in the interfacial region with an increasing population of Pn at the interface, and (ii) water binding to TEA to form a surrounding OH⁻ base in the following fashion [eqn (1)]:⁵⁴

(C₂H₅)₃N + H₂O ↔ (C₂H₅)₃N·H₂O ↔ (C₂H₅)₃NH⁺ + OH⁻ (1)

Hence, small amounts of OH⁻ are likely to have entered the palisade layer of the microemulsion, and subsequently, a basic environment develops in the peripheral region of the interface, which is essentially required for the Heck reaction (Scheme 2 and S1†) and a good yield of the product is achieved. However, the correlation between [IL] and the reaction yield is not straightforward. After getting the utmost yield of 75% at [IL] (= 0.05 mol dm⁻³), the reaction yield decreases to 25% at [IL] (= 0.10 mol dm⁻³) and shows a further increase (59%) at [IL] (= 0.15 mol dm⁻³). As stated earlier, a low line secondary maxima in K_d and ΔG⁰_0→i values appear at 0.15 mol dm⁻³ of [IL] after achieving the highest values at 0.05 mol dm⁻³ of [IL] (Fig. 1) and the second highest yield of the Heck product has been found at the same IL concentration (= 0.15 mol dm⁻³) (Table 1). K_d and ΔG⁰_0→i values actually signify the spontaneity of the Pn transfer process from the bulk to the interface. Hence, it is probable that the accumulation of Pn at the interface governs the availability of TEA and OH in the vicinity of the interface as well as tunes the interfacial characteristics to different degrees by an interaction with IL (of different contents), which influences the yield of the desired Heck product. Gayet et al.⁹ reported that the IL content affects the yields of the Matsuda–Heck reaction in reverse microemulsions. All these observations together imply that the most plausible location or site of the Heck reaction in the studied microheterogeneous system is the interfacial region. However, the present report is not comprehensive from the viewpoint of the direct correlation between the IL content and the reaction yield (herein, the Heck couple product). However, this is trivial as a maximum in both K_d and ΔG⁰_0→i values was obtained at 0.05 mol dm⁻³ and we compare the maximum values of physicochemical parameters (K_d and ΔG⁰_0→i) with those of the highest yield of the Heck product at the same concentration of [IL]. Several factors, such as changes in molecular interactions between the constituents at the interface, microstructure, and polarity due to the presence of phenyl group in IL with the variation in [IL], might be responsible for the overall yield of the final product. Further studies in this direction by employing SANS, ¹H-NMR along with two-dimensional rotating frame Nuclear Overhauser Effect (NOE) experiments (ROESY) are warranted.

Experimental

Materials and methods

Polyoxyethylenesorbitanmonolaurate (Tween-20, ≥99%), palladium acetate [Pd(OAc)₂, ≥99.98%] and 4-iodo-toluene (CH₃C₆H₄I, ≥99%) were purchased from Sigma Aldrich, USA. 1-Pentanol (Pn, ≥99%) and cyclohexane (Cy, ≥98%) were the products of Fluka, Switzerland. Triethyl amine (TEA, ≥99.5%) and n-butyl acrylate were purchased from Merck, Germany. All these chemicals were used without further purification. The IL, 1-butyl-3-propylbenzimidazolium bromide ([bpBzim]Br), was synthesized in accordance with our reported method.⁴ Doubly distilled water of conductivity less than 3 μS cm¹ was used in the experiments.

The dilution experiment was performed to investigate the interfacial composition of the Tween-20 based microemulsion under different physicochemical conditions, as described earlier^11–14,17 by using the spectrophotometric technique⁵⁵ to measure the change in the sample turbidity produced by Pn addition (Fig. S7†). The details of the spectrophotometric technique are provided in our previous report.¹² Basics of the dilution method and thermodynamics of the transfer of the cosurfactant from oil to the interface have been dealt in ESI (Sec. A†).

Conductivity measurements were performed using Mettler Toledo (Switzerland) Conductivity Bridge. The instrument was calibrated with a standard KCl solution. The uncertainty in the conductance measurement was within ±1%.

DLS measurements were carried out using Zetasizer Nano ZS90 (ZEN3690, Malvern Instruments Ltd, U.K.). He–Ne laser having a wavelength of 632.8 nm was used, and the measurements were made at a scattering angle of 90°. Details of the measurement have been provided in our previous report.^12,13

FTIR absorption spectra were recorded in the range of 400–4000 cm¹ with a Shimadzu 83000 spectrometer (Japan) by using a CaF₂-IR crystal window (Sigma-Aldrich) equipped with a press lock holder with 100 number scans and a spectral resolution of 4 cm¹. The deconvolution of spectra was performed using the Origin software.

Three milliliters of microemulsion (water/Tween-20/Pn/Cy) containing IL (0–0.20 mol dm⁻³) at ω (= 30) and 4.48 mg (0.02 mmol, 4 mol%) Pd(OAc)₂ were taken in a 25 ml round bottom flask and the mixture was placed in a preheated oil bath at 323 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 triethylamine (TEA) were introduced into it, and the resulting mixture was heated at 323 K for 45 minutes. The resulting multicomponent solution shows no instability towards temperature or otherwise. The progress of the reaction was monitored by silica gel thin layer chromatography (TLC). In addition, conductance, FTIR, and UV-Vis spectroscopy were employed to characterize the microenvironment of the microemulsion with the progress of the reaction. The yield of the Heck coupled product was determined by HPLC. Finally, the product was characterized by ¹H-NMR and ¹³C-NMR (ESI, Sec. C†). A similar experiment was performed in the water–IL media (bulk IL) at the corresponding [IL] as used in the microemulsion system.

Conclusions

The present study is focused on the characterization of a quaternary water-in-oil microemulsion comprising of Tween-20, Pn and Cy in the absence and presence of IL, 1-butyl-3-propylbenzimidazolium bromide ([bpBzim]Br), with a detailed description of the interfacial composition as a function of the transfer of Pn from a bulk oil phase to the interface on the system composition. Synergism in the distribution constant (K_d) and −ΔG⁰_0→i has been observed in the vicinity of 0.05 mol dm⁻³ of IL at each temperature (293, 303, 313 and 323 K). This indicates that the transfer process of Pn from the oil phase to the interface is more favoured at 0.05 mol dm⁻³ of IL irrespective of the temperature. The standard enthalpy change (ΔH⁰_0→i) and the standard entropy change (ΔS⁰_0→i) of the transfer process have been found to be negative [i.e., exothermic process with less disorder (organized)] in the absence or presence of IL at all the experimental temperatures. Further, a temperature-insensitive microemulsion has been formed at [IL] of 0.05 and ∼0.136 mol dm⁻³. FTIR study reveals an increase in the proportion of bound water molecules with increasing [IL]. This shows the significant role of IL in determining the states of different water species (bound and bulk) in the confined environment of the w/o microemulsion.

Additionally, an in-depth characterization of the microenvironment of the w/o microemulsion in the presence of IL has been made during the performance of the model C–C cross coupling (Heck) reaction. The reaction ends up with the highest yield in the presence of 0.05 mol dm⁻³ of IL, wherein the Pn transfer process is reported to be the most spontaneous as evident from the physicochemical and thermodynamic parameters obtained by the dilution method. All findings of the present investigation, starting from the simple titrimetric method to sophisticated instrumentations, lead to the conclusion that the most plausible reaction location/site is the interfacial region of the w/o microemulsion, where the population of all the active ingredients of both the template and the Heck reaction impart stability to the system. The confinement of IL (as additive) improved the reactivity of the Heck reaction, which can be used in various domains, such as biocatalysts or nanomaterial synthesis.⁹ The understanding of physicochemical parameters and interactions during the progress of the organic reaction in the w/o microemulsion has implications for the design of suitable reaction media for organic synthesis.

Acknowledgements

We thank the Council of Scientific & Industrial Research, New Delhi for financial support (02(0022)/11/EMR-II). BK and SB are thankful to UGC for their fellowship. The financial support in the form of an operating research grant to BKP and Senior Research Fellowship to KK from the authority of Indian Statistical Institute, Kolkata, India are thankfully acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Basics of the dilution method, interfacial composition, thermodynamic parameters, plots of conductivity and hydrodynamic diameter. FTIR spectra of O–H band with deconvoluted curves, mechanism of Heck reaction, UV-visible spectra, ¹H-NMR and ¹³C-NMR analysis. See DOI: 10.1039/c3ra46632a

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