Influence of Water Structure on Solvolysis in Water-in-Oil Microemulsions

Hydrolysis Reactions of Two Benzoyl Chlorides as a Probe to Investigate Reverse Micelles Formed by the Ionic Liquid-Surfactant bmim-AOT

In this work, two hydrolysis reactions were used as a probe to investigate the properties of reverse micelles (RMs) formed by the ionic liquid-surfactant 1-butyl-3-methylimidazolium 1,4-bis-2-ethylhexylsulfosuccinate (bmim-AOT). The results were compared with those found for RMs generated with sodium 1,4-bis-2-ethylhexylsulfosuccinate (Na-AOT). As external nonpolar solvents, n-heptane (n-Hp), isopropyl myristate (IPM), and methyl laurate (ML) were used. Thus, the effect of changing the Na⁺ cation by bmim⁺ was analyzed, as well as the impact of the replacement of a conventional external nonpolar solvent by biocompatible solvents. The kinetics of the hydrolysis reactions of 4-methoxybenzoyl chloride (OMe) and 4-(trifluoromethyl)benzoyl chloride (CF₃) were studied. The results indicate that the replacement of the Na⁺ counterion by bmim⁺ in AOT RMs alters the rates of reactions carried out in them and produces changes in the reaction mechanism. In bmim-AOT RMs, the bmim⁺ cation is located between the surfactant molecules; this has an important influence on the reaction intermediates' stability and, therefore, in the reaction rates and mechanisms. Also, the results indicate that when IPM is used as an external solvent instead of ML or n-Hp, interfacial water molecules have larger nucleophilicity due to the higher interface penetration of IPM.

Characterization of Reverse Micelles Formulated with the Ionic-Liquid-like Surfactant Bmim-AOT and Comparison with the Traditional Na-AOT: Dynamic Light Scattering, 1H NMR Spectroscopy, and Hydrolysis Reaction of Carbonate as a Probe

The present study investigated how the presence of butylmethylimidazolium cation (bmim⁺) alters the interfacial properties of reverse micelles (RMs) created with the ionic liquid-like surfactant 1-butyl-3-methylimidazolium 1,4-bis-2-ethylhexylsulfosuccinate (bmim-AOT), in comparison to sodium 1,4-bis-2-ethylhexylsulfosuccinate (Na-AOT) RMs, employing dynamic light scattering (DLS) and ¹H NMR techniques. Moreover, through the hydrolysis reaction of bis(4-nitrophenyl)carbonate inside both RMs as reaction probe, interfacial properties changes were explored in more detail. The kinetic solvent isotope effect was also analyzed. Micellar systems were formed using n-heptane as external nonpolar solvent and water as the polar component. According to the DLS studies, water is encapsulated inside the organized media; however, a different tendency is observed depending on the cationic component of the surfactant. For Na-AOT system, the results suggest that the micellar shapes are probably spherical, while in the case of bmim-AOT, a transition from ellipsoidal to spherical micelles could be occurring when water is added. ¹H NMR data show that water is structured differently when Na⁺ cation is replaced by bmim⁺; in bmim-AOT RMs, the interaction of water with the surfactant is weaker and the water hydrogen-bonding network is less disturbed than in Na-AOT RMs. Kinetic studies reveal that the hydrolysis reaction in bmim-AOT RMs was much more favorable in comparison to Na-AOT RMs. In addition, when water content decreases in bmim-AOT RMs, the hydrolysis reaction rate increases and the solvent isotope effect remains constant, while for Na-AOT solutions, both the reaction rate and the solvent isotope effect decrease. Our results indicate that bmim⁺ cation would be located in the surfactant layer in such a way the negative charge density in the interface is less than that in Na-AOT RMs, and the reaction is more favorable. Additionally, as ¹H NMR studies reveal, the interfacial water molecules would be more available in bmim-AOT RMs to participate in the nucleophilic attack. Therefore, the present study evidences how the replacement of Na⁺ counterion by bmim⁺ alters the composition of the interface of AOT RMs.

Does the optimum hydrophilic lipophilic balance condition affect the physical properties of mixed reverse micelles? A spectroscopic investigation

Synergism in several physical properties as realized in many mixed surfactant reverse micellar (RM) systems often manifests optimum hydrophilic-lipophilic balance (HLB), interdroplet interaction, or both. Such synergism is often desired for specific applications of RM systems; however, a proper rationale on the effect of such phenomenon imparted on the structure, dynamics, and activity of water molecules in RM waterpool is strongly demanded. In the present contribution we have investigated how the optimum HLB condition of mixed RM composed two nonionic surfactants (Igepal 210 and Igepal 630) affects the physical properties of entrapped water molecules in the RM waterpool. The studied mixed RM exhibits synergistic water solubilization behavior as a function of the mixing ratio with a maximum in solubilization capacity being reached at X(Ig630) = 0.3. Dynamic light scattering (DLS) studies show a bimodal distribution of droplet size in this region, whereas it is monomodal in terminal compositions. Fourier transform infrared spectroscopy (FTIR) study in the 3000-3800 cm(-1) region identifies a linear trend in which the content of "bound" water increases at the expense of the "network" water as the content of the hydrophilic surfactant Igepal 630 is increased in the mixture. Subnanosecond relaxation dynamics of the entrapped water as revealed by the fluoroprobe coumarin 500 corroborates a similar linear trend as observed in the FTIR measurements as the rotational diffusion gets retarded with the increase of ethylene oxide chain length of Igepal. Reaction kinetics of solvolysis of benzoyl chloride reaction, however, does not offer any linear trend as it gets slower in the optimum HLB region, the nonlinearity being a consequence of the distribution of the substrate in the different phases.

Effect of hydrophobic interaction on structure, dynamics, and reactivity of water

The effect of hydrophobic interaction on water is still controversial and requires more detailed experimental and theoretical investigation. The interaction between organic-water molecular complexes might be indicative of the perturbation of hydrogen-bond network in the tetrahedral structure of bulk waters, due to hydrophobic effect. In this contribution, femto/picosecond-resolved solvation dynamics techniques have been adopted to explore the dynamical modification of water clusters in hydrophobic solvent methyl tert-butyl ether (MTBE). The dynamical evolution of water molecules at the surface of micelle-like MTBE has also been studied. Dynamic light scattering techniques have been employed to determine the size of the molecular clusters being formed in respective solvents. Fourier transform infrared (FTIR) spectroscopy well measures the changes in O-H vibration frequency of water induced by MTBE. We have also monitored temperature dependent picosecond-resolved solvation dynamics in order to explore the energetics associated with water solvation in bulk MTBE. Using detailed ab initio calculations at the MP2 level, our study attempts to predict the possible structures, energies, and thermochemical parameters of corresponding MTBE-water molecular complexes in more detail. The chemical reactivity of water further confirms the effect of the hydrophobic interaction on water molecules. The results impart an understanding on hydrophobic interaction imposed by a biomolecule on the structure and reactivity of water, significant for the in vivo cellular condition.

Alkaline Fading of Triarylmethyl Carbocations in Self-Assembly Microheterogeneous Media

This review reports on the alkaline fading of crystal violet and other related carbocations in the presence of different microheterogeneous media (micelles, microemulsions and vesicles).

A molecular picture of diffusion controlled reaction: role of microviscosity and hydration on hydrolysis of benzoyl chloride at a polymer hydration region

In this study, we have attempted to explore the molecular mechanism associated with a diffusion controlled reaction at a polymer hydration region by monitoring temperature-dependent solvolysis reaction of benzoyl chloride (BzCl) in water-poly(ethylene glycol) mixture at low water concentration. BzCl being highly hydrophobic resides in the vicinity of the PEG surface and the reaction takes place at the interface. Temperature-dependent solvolysis allows one to estimate the overall Arrhenius type activation energy barrier associated with the reaction. To understand the relative contribution of hydration and diffusive motion on the overall activation energy we studied the temperature-dependent picosecond-resolved solvation dynamics using a fluorescence probe Coumarin 500 (C500). The observed acceleration of solvation dynamics with temperature finds its origin in temperature-induced transition of bound to free type interfacial water molecules near the PEG surface. Temperature-dependent acoustic and densimetric studies also support this phenomenon. The temperature-induced enhancement of the local viscosity experienced by the probe, which is calculated from the rotational anisotropy studies, furnishes the activation barrier for microviscosity as applicable to the Kramers model. The activation energy barriers estimated from the temperature-dependent solvation dynamics and microviscosity studies are correlated with that obtained from the solvolysis reaction.

First Kinetic Determination of Partition Coefficients for Organic Compounds between the Three Microenvironments of AOT-Based Microemulsions

Kinetic data for the hydrolysis of N-picolinoylimidazole (I) and 2,4-dinitrophenylpicolinate (II) in AOT [bis(2-ethylhexyl) sodium sulfosuccinate] microemulsions are used to determine for the first time the two partition constants for each substrate (i.e. K(wi) and K(oi), corresponding to the incorporation of substrate molecules from water microdroplets and the continuous medium, respectively, into the interface). Application of the pseudophase formalism to the partition constants allowed the rate constant in each phase to be determined. The rate of hydrolysis of II increased with decreasing polarity of the medium; as a result, the hydrolysis reaction took place largely at the interface. On the other hand, the rate of hydrolysis of acylimidazole I decreased with decreasing polarity, possibly as a result of changes in the resonance structures of the reagent causing the hydrolysis process to occur preferentially in water microdroplets.

Change in the Acid Hydrolysis Mechanism of Esters Enforced by Strongly Acid Microemulsions

A kinetic study was carried out on the acid hydrolysis of 4-nitrophenylacetate and 4-nitrophenyllaurate in water/HOT/isooctane microemulsions. The substitution of Na+ in the sodium salt of bis(2-ethylhexyl)sulfosuccinate by H+ has permitted us to obtain a functionalized surfactant (HOT) and, consequently, strongly acid microemulsions. The use of HOT-based microemulsions allows us to reach concentrations of H+ in the aqueous core corresponding to a Hammett acidity function of H0 = -2. The rate constant at the interface and the distribution constants of the carboxylic esters throughout the different microenvironments of the microemulsion have been quantified by application of the pseudophase formalism. The results obtained show that the hydrolysis rate constant at the interface increases as the water content of the system decreases. The correlation of the rate constants at the interface of the microemulsion with the Hammett acidity function, H0 (on the basis of the Bunnett-Olsen criterion), has allowed us to confirm that the hydrolysis process takes place via an A2 mechanism for high water contents and through an A1 mechanism for values of W <or= 15 (W = [H2O]/[HOT]).

Influence of Changes in Water Properties on Reactivity in Strongly Acidic Microemulsions

Replacing the counterion in sodium bis(2-ethylhexyl)sulfosuccinate (NaOT, usually known as AOT or Aerosol OT) with H+ (HOT) allows strongly acidic microemulsions to be obtained through the effect of a change in the solvation mechanism of the surfactant, where the Na+...OH2 interaction is displaced by a stronger H+...OH2 interaction. This raises the proportion of water bound to the counterion, which is reflected in the FT-IR spectrum for water trapped in the microemulsion and the 1H NMR spectrum for the hydrogen atoms in the water molecules. In NaOT microemulsions, the resonance signal for hydrogen atoms in the water molecules increases from delta approximately 3.9 ppm at W = 2 (with W = [H2O]/[NaOT]) to delta approximately 4.8 ppm at W = 50. In HOT microemulsions, the disparate strength of Na+...OH2 and H+...OH2 interactions results in a decrease in the resonance signal for the hydrogen atoms in the water molecules from delta approximately 8.6 ppm at W = 2 to delta approximately 4.9 ppm at W = 50. These changes in the physical properties of water alter chemical reactivity in a way that is clearly apparent in solvolytic processes in NaOT and HOT microemulsions. Thus, the rate constants of reactions involving an associative mechanism increase with decreasing W in NaOT microemulsions, but decrease with decreasing W in HOT microemulsions. The disparate behavior is a result of a decreased nucleophilicity of interfacial water in HOT microemulsions relative to NaOT microemulsions. For a dissociative process the rate constants are greater in HOT microemulsions than in NaOT ones, and increase with increasing W in both types of microemulsions, which can be ascribed to an increased electrophilicity of interfacial water in HOT microemulsions.

Solvolysis of benzoyl halides in water/NH4DEHP/isooctane microemulsions

A study was carried out on the solvolysis reactions of different benzoyl halides in microemulsions of water/NH4DEHP/isooctane, where NH4DEHP is ammonium bis(2-ethylhexyl) phosphate. Because of the low solubility of benzoyl halides in water, they are distributed between the continuous medium and the interface of the microemulsion, where the reaction takes place. The application of the pseudophase model has allowed us to obtain the distribution constants and the rate constants at the interface for the benzoyl halides. Reaction mechanisms and the changes in these mechanisms in terms of the water content of the microemulsion have been determined on the basis of kinetic data. The influence of the substituent and the leaving group on the reaction rate has been investigated. A comparison of kinetic results with those previously obtained in water/AOT/isooctane microemulsions allows a kinetic evaluation of the change in the microemulsion properties with the surfactant.

Water in Oil Microemulsions as Reaction Media for a Diels−Alder Reaction between N-Ethylmaleimide and Cyclopentadiene

The Diels-Alder reaction between N-ethylmaleimide and cyclopentadiene in water/AOT/isooctane microemulsions, where AOT denotes sodium bis(2-ethylhexyl)sulfosuccinate, was studied. The rate of the reaction was found to be higher than that obtained in pure isooctane, irrespective of the particular microemulsion composition used. The efficiency of this catalytic action ranged from a factor of 3 at low water contents (viz., W = [H2O]/[AOT] = 2) to 15 at W = 35. On the basis of these results, the reaction takes place simultaneously in the continuous medium and at the microemulsion interface. The favorable arrangement of the reactants at the interface results in more than 95% of the reaction occurring in this microenvironment. The kinetic analysis revealed the rate constant at the microemulsion interface to change with the water content. For small W values a bimolecular rate constant at the interface close to that observed in hexane was obtained. This value increases with W and for W > 20, a value close to that obtained in ethanol was found. This can be ascribed to the absence of hydrogen bonding at the microemulsion interface as well as the accelerating effects due to enforced hydrophobic interactions.

First Evidence of Simultaneous Different Kinetic Behaviors at the Interface and the Continuous Medium of w/o Microemulsions

A study was carried out on the butylaminolysis reaction of 4-nitrophenyl caprate in AOT/chlorobenzene/water microemulsions, with the observed rate constant, kobs, showing both first- and second-order dependence on butylamine concentration. The first-order term in [BuNH2] is due to the reaction occurring at the interface of the microemulsion while the second-order term is due to the reaction in the continuous medium. The different kinetic behavior is accounted for by the mechanism by which the reaction proceeds: at the interface of the microemulsion, the rate-determining step is the formation of the addition intermediate, T+/-, whereas in the continuous medium the slow step is the base-catalyzed decomposition of this intermediate. The application of the pseudophase formalism allows the observed kinetic behavior to be explained and to obtain the rate constants at the interface, ki2=0.13 M-1 s-1, and in the continuous medium, ko2KT=2.46x10(-2) M-2 s-1. These values indicate that the reaction rate decreases approximately 23 times upon going from the aqueous medium to the interface of the microemulsion, whereas the rate constant in the continuous medium is consistent with that obtained in pure chlorobenzene, ko2KT=2.09x10(-2) M-2 s-1.

Reactivity of Benzoyl Chlorides in Nonionic Microemulsions: Potential Application as Indicators of System Properties

The solvolysis reactivity of benzoyl chlorides entails a high sensitivity on medium properties. A systematic study of the reaction of a series of these substrates, varying the electron-withdrawing character of the substituent, has been performed in nonionic microemulsions. The kinetic effects due to variation of microemulsion compositions can be assigned to modifications in system properties, to be precise, to modifications in interface properties. Microemulsion properties that are obtained from kinetic analysis of solvolysis show a good agreement with the characterization of the microemulsion that was made via 1H NMR and solvatochromic fluorescence probes. Benzoyl chlorides with electron-donating groups react through a dissociative mechanism, whereas electron-withdrawing groups favor an associative mechanism. A comparative analysis of reactivity between the different substrates at the interface shows a variation in the contributions of both reaction pathways, associative and dissociative, to the whole reaction mechanism. The confined media shift the point where the mechanism changes from an associative to a dissociative pathway, far away from the turning point in water. Furthermore, the change in mechanism can be modulated by modification of the microemulsion composition.

Microheterogeneous Solvation for Aminolysis Reactions in AOT-Based Water-in-Oil Microemulsions

A kinetic study was carried out on the aminolysis of p-nitrophenyl acetate (NPA) by n-decylamine (DEC), piperazine (PIP) and sarcosine (SAR) in AOT/isooctane/water (w/o) microemulsions. By using the pseudophase model both the rate constants at the interface, k2i, and the water microdroplet, k2w, can be obtained. The obtained results show that k2i increases together with the water content of the microemulsion, whereas k2w increases as the water content of the system decreases. In the aqueous microdroplet the predominant interaction Na+...OH2 causes a decrease in the strength of the hydrogen bonds and therefore facilitates the desolvation of the reagents as W decreases. This desolvation of the reagents causes the increase of k2w as W decreases. In the interface of the microemulsion the predominant interaction SO3-...HOH causes an increase in the electronic density on the water molecules and the consequent decrease in their efficiency in the solvation of the partial negative charge, which develops on the carbonyl oxygen atom in the transition state of the reaction. This decrease in the solvation causes k2i to decrease together with the water content of the system.

Reverse Micellar Aggregates: Effect on Ketone Reduction. 1. Substrate Role

The reduction of three aromatic ketones, acetophenone (AF), 4-methoxyacetophenone (MAF), and 3-chloroacetophenone (CAF), by NaBH(4) was followed by UV-vis spectroscopy in reverse micellar systems of water/AOT/isooctane at 25.0 degrees C (AOT is sodium 1,4-bis-2-ethylhexylsulfosuccinate). The first-order rate constants, k(obs), increase with the concentration of surfactant due to the substrate incorporation at the reverse micelle interface, where the reaction occurs. For all the ketones the reactivity is lower at the micellar interface than in water, probably reflecting the low affinity of the anionic interface for BH(4)(-). Kinetic profiles upon water addition show maxima in k(obs) at W(0) approximately 5 probably reflecting a strong interaction between water and the ionic headgroup of AOT; at W(0) < 5 by increasing W(0) BH(4)(-) is repelled from the anionic interface once the water pool forms. The order of reactivity was CAF >> AF > MAF. Application of a kinetic model based on the pseudophase formalism, which considers distribution of the ketones between the continuous medium and the interface, and assumes that reaction take place only at the interface, gives values of the rate constants at the interface of the reverse micellar system. At W(0) = 5, we conclude that NaBH(4) is wholly at the interface, and at W(0) = 10 and 15, where there are free water molecules, the partitioning between the interface and the water pool has to be considered. The results were used to estimate the ketone and borohydride distribution constants between the different pseudophases as well as the second-order reaction rate constant at the micellar interface.

Reverse Micellar Aggregates: Effect on Ketone Reduction. 2. Surfactant Role

Kinetics of the reduction of 3-chloroacetophenone (CAF) with sodium borohydride (NaBH(4)) were followed by UV-vis spectroscopy at 27.0 degrees C in different reverse micellar media, toluene/BHDC/water and toluene/AOT/water, and compared with results in an isooctane/AOT/water reverse micellar system. AOT is sodium 1,4-bis-2-ethylhexylsulfosuccinate, and BHDC is benzyl-n-hexadecyl dimethylammonium chloride. The kinetic profiles were investigated as a function of variables such as surfactant and NaBH(4) concentration and the amount of water dispersed in the reverse micelles, W(0) = [H(2)O]/[surfactant]. In all cases, the first-order rate constant, k(obs), increases with the concentration of surfactant as a consequence of incorporating the substrate into the interface of the reverse micelles where the reaction takes place. The reaction is faster at the cationic interface than at the anionic one probably because the negative ion BH(4)(-) is part of the cationic interface. The effect of the external solvent on the reaction shows that reduction is favored in the isooctane/AOT/water reverse micellar system than that with an aromatic solvent. This is probably due to BH(4)(-) being more in the water pool of the toluene/AOT/water reverse micellar system. The kinetic profile upon water addition depends largely on the type of interface. In the BHDC system, k(obs) increases with W(0) in the whole range studied while in AOT the kinetic profile has a maximum at W(0) approximately 5, probably reflecting the fact that BH(4)(-) is part of the cationic interface while, in the anionic one, there is a strong interaction between water and the polar headgroup of AOT below W(0) = 5 and, above that, BH(4)(-) is repelled from the interface once the water pool has formed. Application of a kinetic model based on the pseudophase formalism, which considers the distribution of the ketone between the continuous medium and the interface and assumes that reaction takes place only at the interface, has enabled us to estimate rate constants at the interface of the reverse micellar systems. At W(0) < 10, it was considered that NaBH(4) is wholly at the interface and, at W(0) >/= 10, where there are free water molecules, also the partitioning between the interface and the water pool was taken into account. The results were used to evaluate CAF and NaBH(4) distribution constants between the different pseudophases as well as the second-order reaction rate constant of the reduction reaction in the micellar interface.

Influence of anionic and cationic reverse micelles on nucleophilic aromatic substitution reaction between 1-fluoro-2,4-dinitrobenzene and piperidine

The nucleophilic aromatic substitution (S(N)Ar) reaction between 1-fluoro-2,4-dinitrobenzene and piperidine (PIP) were studied in two different reverse micellar interfaces: benzene/sodium 1,4-bis(2-ethylhexyl) sulfosuccinate (AOT)/water and benzene/benzyl-n-hexadecyl dimethylammonium chloride (BHDC)/water reverse micellar media. The kinetic profiles of the reactions were investigated as a function of variables such as surfactant and amine concentration and the amount of water dispersed in the reverse micelles, W0 = [H2O]/[surfactant]. In the AOT system at W0 = 0, no micellar effect was observed and the reaction takes place almost entirely in the benzene pseudophase, at every AOT and PIP concentration. At W0 = 10, a slight increment of the reaction rate was observed at low [PIP] with AOT concentration, probably due to the increase of micropolarity of the medium. However, at [PIP] > or = 0.07 M the reaction rates are always higher in pure benzene than in the micellar medium because the catalytic effect of the amine predominates in the organic solvent. In the BHDC system the reaction is faster in the micellar medium than in the pure solvent. Increasing the BHDC concentration accelerates the overall reaction, and the saturation of the micellar interface is never reached. In addition, the reaction is not base-catalyzed in this micellar medium. Thus, despite the partition of the reactants in both pseudophases the reactions effectively take place at the interface of the aggregates. The kinetic behavior can be quantitatively explained taking into account the distribution of the substrate and the nucleophile between the bulk solvent and the micelle interface. The results were used to evaluate the amine distribution constant between the micellar pseudophase and organic solvent and the second-order rate coefficient of S(N)Ar reaction in the interface. A mechanism to rationalize the kinetic results in both interfaces is proposed.