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.     

NMR and SANS studies of aggregation and microemulsion formation by phosphorus fluorosurfactants in liquid and supercritical carbon dioxide

(1)H NMR relaxation and diffusion studies were performed on water-in-CO(2) (W/C) microemulsion systems formed with phosphorus fluorosurfactants of bis[2-(F-hexyl)ethyl] phosphate salts (DiF(8)), having different counterions (Na(+), NH(4)(+), N(CH(3))(4)(+)) by means of high-pressure in situ NMR. Water has a low solubility in CO(2) and is mainly solubilized by the microemulsion droplets formed with surfactants added to CO(2) and water mixtures. There is rapid exchange of water between the bulk CO(2) and the microemulsion droplets; however, NMR relaxation measurements show that the entrapped water has restricted motion, and there is little "free" water in the core. Counterions entrapped by the droplets are mostly associated with the surfactant headgroups: diffusion measurements show that counterions and the surfactant molecules move together with a diffusion coefficient that is associated with the droplet. The outer shell of the microemulsion droplets consists of the surfactant tails with some associated CO(2). For W/C microemulsions formed with the phosphate-based surfactant having the ammonia counterion (A-DiF(8)), the (1)H NMR signal for NH(4)(+) shows a much larger diffusion coefficient than that of the surfactant tails. This apparent paradox is explained on the basis of proton exchange between water and the ammonium ion. The observed dependence of the relaxation time (T(2)) on W(0) (mole ratio of water to surfactant in the droplets) for water and NH(4)(+) can also be explained by this exchange model. The average hydrodynamic radius of A-DiF(8) microemulsion droplets estimated from NMR diffusion measurements (25 degrees C, 206 bar, W(0) = 5) was R(h) = 2.0 nm. Assuming the theoretical ratio of R(g)/R(h) = 0.775 for a solid sphere, where R(g) is the radius of gyration, the equivalent hydrodynamic radius from SANS is R(h) = 1.87 nm. The radii measured by the two techniques are in reasonable agreement, as the two techniques are weighted to measure somewhat different parts of the micelle structure.     

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 相似文献   

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.     

A nucleophilic substitution reaction in microemulsions based on either an alcohol ethoxylate or a sugar surfactant

A nucleophilic substitution reaction between 4-tert-butylbenzyl bromide and a series of iodide salts has been performed in oil-in-water microemulsions based on either a fatty alcohol ethoxylate or a sugar surfactant. The reaction kinetics was compared with the kinetics of the same reaction performed in a microhomogeneous reaction medium, d-MeOH. Previous results showing a particularly high reactivity in the microemulsion based on the fatty alcohol ethoxylate was confirmed. It was shown that in both microemulsions the reaction rate was almost independent of the choice of counterion to iodide. This indicates that complexation of the cation with the surfactant headgroup, which, in particular, could have taken place with surfactants containing oligooxyethylene chains (a “crown ether effect”), seems not to be of importance.

¹²⁷I NMR studies, as well as quadrupole splitting experiments performed by ²H NMR, indicate that there is a certain accumulation of iodide at the oil–water interface of the microemulsions. It is difficult to draw any quantitative conclusions in this respect, however.

The results obtained in this study, combined with results from previous investigations of the same reaction, indicate that the unexpectedly high reactivity obtained in the microemulsion based on a surfactant containing an oligooxyethylene headgroup is most probably due to the nucleophile being poorly solvated when present in the headgroup layer of such a microemulsion. Poorly solvated anions are known to be highly reactive nucleophiles.     

Study of nano-emulsion formation by dilution of microemulsions

The influence of different dilution procedures on the properties of oil-in-water (O/W) nano-emulsions obtained by dilution of oil-in-water (O/W) and water-in-oil (W/O) microemulsions has been studied. The system water/SDS/cosurfactant/dodecane with either hexanol or pentanol as cosurfactant was chosen as model system. The dilution procedures consisted of adding water (or microemulsion) stepwise or at once over a microemulsion (or water). Starting emulsification from O/W microemulsions, nano-emulsions with droplet diameters of 20 nm are obtained, independently on the microemulsion composition and the dilution procedure used. In contrast, starting emulsification from W/O microemulsions, nano-emulsions are only obtained if the emulsification conditions allow reaching the equilibrium in an O/W microemulsion domain during the process. These conditions are achieved by stepwise addition of water over W/O microemulsions with O/S ratios at which a direct microemulsion domain is crossed during emulsification. The nature of the alcohol used as cosurfactant has been found to play a key role on the properties of the nano-emulsions obtained: nano-emulsions in the system using hexanol as cosurfactant are smaller in size, lower in polydispersity, and have a higher stability than those with pentanol.     

A nucleophilic substitution reaction performed in different types of self-assembly structures

A nucleophilic substitution reaction between 4-tert-butylbenzyl bromide and potassium iodide has been performed in oil-in-water microemulsions based on various C12Em surfactants, i.e., dodecyl ethoxylate with m number of oxyethylene units. The reaction kinetics was compared with the kinetics of reactions performed in other self-assembly structures based on very similar surfactants and in homogeneous liquids. The reaction was fastest in the micellar system, intermediate in rate in the microemulsions, and most sluggish in the liquid crystalline phase. Reaction in a Winsor I system, i.e., a two-phase system comprising an oil-in-water microemulsion in equilibrium with excess oil, was equally fast as reaction in a one-phase microemulsion. The reactions in microemulsion were surprisingly fast compared to reaction in homogeneous, protic liquids such as methanol and ethanol. The rate was independent of the microstructure of the microemulsion; however, the rate was very dependent on the type of surfactant used. When the C12Em surfactant was replaced by a sugar-based surfactant, octyl glucoside, the reaction was much more sluggish. The high reactivity in microemulsions based on C12Em surfactants is belived to be due to a favorable microenvironment in the reaction zone. The reaction is likely to occur within the surfactant palisade layer, where the water activity is relatively low and where the attacking species, the iodide ion, is poorly hydrated and, hence, more nucleophlic than in a protic solvent such as water or methanol. Sugar surfactants become more hydrated than alcohol ethoxylates and the lower reactivity in the microemulsion based on the sugar surfactant is probably due to a higher water activity in the reaction zone.     

Effect of ionic surfactants on the phase behavior and structure of sucrose ester/water/oil systems

The phase behavior and structure of sucrose ester/water/oil systems in the presence of long-chain cosurfactant (monolaurin) and small amounts of ionic surfactants was investigated by phase study and small angle X-ray scattering. In a water/sucrose ester/monolaurin/decane system at 27 degrees C, instead of a three-phase microemulsion, lamellar liquid crystals are formed in the dilute region. Unlike other systems in the presence of alcohol as cosurfactant, the HLB composition does not change with dilution, since monolaurin adsorbs almost completely in the interface. The addition of small amounts of ionic surfactant, regardless of the counterion, increases the solubilization of water in W/O microemulsions. The solubilization on oil in O/W microemulsions is not much affected, but structuring is induced and a viscous isotropic phase is formed. At high ionic surfactant concentrations, the single-phase microemulsion disappears and liquid crystals are favored.     

States of water located in the continuous organic phase of 1-butyl-3-methylimidazolium tetrafluoroborate/Triton X-100/triethylamine reverse microemulsions.

We demonstrate a novel ionic liquid (IL) microemulsion, consisting of 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4) and nonionic surfactant Triton X-100 prepared in triethylamine which is used either as an organic solvent or a Lewis base. The effects of small amounts of added water on the microstructure of the IL microemulsion are investigated by various techniques. UV/Vis spectroscopic analysis and FTIR spectra indicate that these water molecules are not solubilized into the IL pools of the microemulsions. 1H NMR spectra further show that the added water binds with triethylamine to form a surrounding OH- base environment. Some of OH- ions enter the palisade layers of the IL microemulsions and a continuous base interface is created. The unique solubilization behavior of water reveals that it is possible to use the triethylamine microemulsions as a template to prepare metal hydroxides as well as metal oxides in the microemulsions, which is not possible when using traditional microemulsions.     

The effect of water on the microstructure of 1-butyl-3-methylimidazolium tetrafluoroborate/TX-100/benzene ionic liquid microemulsions

The ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) forms nonaqueous microemulsions with benzene with the aid of nonionic surfactant TX-100. The phase diagram of the ternary system was prepared, and the microstructures of the microemulsion were recognized. On the basis of the phase diagram, a series of ionic liquid-in-oil (IL/O) microemulsions were chosen and characterized by dynamic light scattering (DLS), which shows a similar swelling behavior to typical water-in-oil (W/O) microemulsions. The existence of IL pools in the IL/O microemulsion was confirmed by UV/Vis spectroscopic analysis with CoCl2 and methylene blue (MB) as the absorption probes. A constant polarity of the IL pool is observed, even if small amounts of water are added to the microemulsion, thus suggesting that the water molecules are solubilized in the polar outer shell of the microemulsion, as confirmed by FTIR spectra. 1H NMR spectroscopic analysis shows that these water molecules interact with the electronegative oxygen atoms of the oxyethylene (OE) units of TX-100 through hydrogen-bonding interactions, and the electronegative oxygen atoms of the water molecules attract the electropositive imidazolium rings of [bmim][BF4]. Hence, the water molecules are like a glue that stick the IL and OE units more tightly together and thus make the microemulsion system more stable. Considering the unique solubilization behavior of added water molecules, the IL/O microemulsion system may be used as a medium to prepare porous or hollow nanomaterials by hydrolysis reactions.     

Heavy metals adsorption by novel EDTA-modified chitosan-silica hybrid materials

Ionic liquid based microemulsions were characterized by absorption solvatochromic shifts, (1)H NMR and kinetic measurements in order to investigate the properties of the ionic liquid within the restricted geometry provided by microemulsions and the interactions of the ionic liquid with the interface. Experimental results show a significant difference between the interfaces of normal water and the new ionic liquid microemulsions. Absorption solvatochromic shift experiments and kinetic studies on the aminolysis of 4-nitrophenyl laurate by n-decylamine show that the polarity at the interface of the ionic liquid in oil microemulsions (IL/O) is higher than at the interface of water in oil microemulsions (W/O) despite the fact that the polarity of [bmim][BF(4)(-)] is lower than the polarity of water. (1)H NMR experiments showed that an increase in the ionic liquid content of the microemulsion led to an increase in the interaction between [bmim][BF(4)(-)] and TX-100. The reason for the higher polarity of the microemulsions with the ionic liquid can be explained in terms of the incorporation of higher levels of the ionic liquid at the interface of the microemulsions, as compared to water in the traditional systems.     

Microstructure and structural transition in microemulsions stabilized by aldonamide-type surfactants

Significant efforts were undertaken to characterize the microstructure and structural properties of water-in-oil (w/o), oil-in-water (o/w), and bicontinuous (bc) microemulsions composed of N-alkyl-N-methylgluconamides (n-alkyl = n-C(12)H(25), n-C(14)H(29), n-C(16)H(33)) and n-alcohols (ethanol, n-propanol, n-butanol) or iso-alcohols (iso-propanol, iso-butanol) as cosurfactants, as well as iso-octane and water. The internal structure of so created four-component system was elucidated by means of an analysis of isotropic area magnitudes in phase diagrams and conductivity measurements. Dynamic light scattering (DLS) measurements provided the microemulsion size and polydispersity. Polarity and viscosity of microemulsion microenvironment were acquired by means of electron paramagnetic resonance (EPR), UV-vis absorption spectroscopy (in the case of w/o droplets), and steady-state fluorescence (SSF) (in the case of o/w droplets). The results show that both the surfactant and the cosurfactant types affect the shape and extent of microemulsions. The size of droplets depends strongly on the type of examined microemulsion and the type of cosurfactant (linear or brunched) but is almost independent of the length of the surfactant alkyl chain. The size of microemulsion droplets ranges from 8.1 to 22.6 nm and from 3.7 to 14.3 nm respectively, for o/w and o/w microemulsions, making them good candidates for both template-based reactions and household components solubilizing media.     

AOT/异辛烷/水微乳液中结晶紫与AOT相互作用的热力学研究

用紫外-可见分光光度法在不同温度下测定了结晶紫(CV)在双-2-乙基己基硫代琥珀酸钠(气溶胶OT或AOT)为表面活性剂的W/O微乳液中的吸光度. 根据结晶紫和AOT在微乳液水滴界面缔合的模型对实验数据进行处理, 结果表明, 随着微乳液中水与AOT的摩尔比w的减小和温度的升高, 结晶紫缔合度增大, 根据不同温度下的缔合平衡常数计算了反应的热力学函数, Δ_rG_m, Δ_rH_m和Δ_rS_m     

Layered structure of room-temperature ionic liquids in microemulsions by multinuclear NMR spectroscopic studies

Microemulsions form in mixtures of polar, nonpolar, and amphiphilic molecules. Typical microemulsions employ water as the polar phase. However, microemulsions can form with a polar phase other than water, which hold promise to diversify the range of properties, and hence utility, of microemulsions. Here microemulsions formed by using a room‐temperature ionic liquid (RTIL) as the polar phase were created and characterized by using multinuclear NMR spectroscopy. ¹H, ¹¹B, and ¹⁹F NMR spectroscopy was applied to explore differences between microemulsions formed by using 1‐butyl‐3‐methylimidazolium tetrafluoroborate ([bmim][BF₄]) as the polar phase with a cationic surfactant, benzylhexadecyldimethylammonium chloride (BHDC), and a nonionic surfactant, Triton X‐100 (TX‐100). NMR spectroscopy showed distinct differences in the behavior of the RTIL as the charge of the surfactant head group varies in the different microemulsion environments. Minor changes in the chemical shifts were observed for [bmim]⁺ and [BF₄]^? in the presence of TX‐100 suggesting that the surfactant and the ionic liquid are separated in the microemulsion. The large changes in spectroscopic parameters observed are consistent with microstructure formation with layering of [bmim]⁺ and [BF₄]^? and migration of Cl^? within the BHDC microemulsions. Comparisons with NMR results for related ionic compounds in organic and aqueous environments as well as literature studies assisted the development of a simple organizational model for these microstructures.     

Sucrose ester-based biocompatible microemulsions as vehicles for aceclofenac as a model drug: formulation approach using D-optimal mixture design

We assessed the functionality of sucrose esters (sucrose laurate, myristate, palmitate, and stearate), relatively innocuous nonionic surfactants, in formulation of biocompatible microemulsions. The putative influence of surfactant structure on the extension of microemulsion region was explored through the construction of the pseudo-ternary phase diagrams for the isopropyl myristate/sucrose ester-isopropyl alcohol/water system, using the titration method and mixture experimental approach. Minor changes in surfactant tail length strongly affected the microemulsion area boundaries. D-optimal mixture design proved to be highly applicable in detecting the microemulsion regions. Examination of conductivity, rheology, and thermal behavior of the selected sucrose laurate and sucrose myristate-based microemulsions, upon dilution with water, indicated existence of percolation threshold and suggested the phase inversion from water-in-oil to oil-in-water via a bicontinuous structure. Atomic force micrographs confirmed the suggested type of microemulsions and were valuable in further exploring their inner structure. The solubilization capacity of aceclofenac as a model drug has decreased as the water volume fraction in microemulsion increased. High surfactant concentration and the measured solubility of aceclofenac in microemulsion components suggested that the interfacial film may mostly contribute to aceclofenac solubilization.     

The use of novel water-in-oil microemulsions in microemulsion electrokinetic chromatography

We describe the novel use of water-in-oil (W/O) microemulsions to achieve unique separations in microemulsion electrokinetic chromatography (MEEKC). The choice and concentration of the buffer type, surfactant and co-surfactant were all examined and optimized. Separations of a range of neutral and acidic analytes was shown to be markedly different to that obtained by (oil-in-water) O/W MEEKC. Neutral solutes are separated by virtue of their solubility (log P) values in O/W MEEKC with the more water-insoluble solutes migrating last. This separation process does not occur in W/O, as neutral solutes are not separated in order of log P.     

Role of solubilized water in the reverse ionic liquid microemulsion of 1-butyl-3-methylimidazolium tetrafluoroborate/TX-100/benzene

The ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4) can form nonaqueous microemulsions with benzene by the aid of nonionic surfactant TX-100. The effect of water on ionic liquid-in-oil (IL/O) microemulsions was studied, and it was shown that the addition of small amount of water to the IL microemulsion contributed to the stability of microemulsion and thus increased the amount of solubilized bmimBF4 in the microemulsion. The conductivity measurements also showed that the attractive interactions between IL microdroplets were weakened, that is, the IL/O microemulsion becomes more stable in the present of some water. Fourier transform IR was carried out to analyze the states of the added water, and the result showed that these water molecules mainly behaved as bound water and trapped water, indicating that the water molecules are located in the palisade layers of the IL/O microemulsion. Furthermore, 1H NMR and 19F NMR spectra suggested that the added water molecules built the hydrogen binding network of imidazolium cations and H2O, BF4- anion and H2O, and at the same time the electronegative oxygen atoms of the oxyethylene units of TX-100 and water in the palisade layers, which made the palisade layers more firm and thus increased the stability of the microemulsion. The study can help in further understanding the formation mechanism of microemulsions. In addition, the characteristic solubilization behavior of the added water can provide an aqueous interface film for hydrolysis reactions and therefore may be used as an ideal medium to prepare porous or hollow nanomaterials.     

Lipase-catalyzed enantioselective esterifications using different microemulsion-based gels

Chiral esters with high optical purity have been synthesized at 298.2 K from racemic 2-octanol and alkanoic acids using the commerical lipases fromChromobacterium viscosum (CV) orCandida sp. (SP 525) immobilized in microemulsion-based gelatin gels. The microemulsions consisted of water and alkanes stabilized by the anionic surfactant sodium 1,4-bis(2-ethylhexyl) sulfosuccinate (AOT) and the naturally occurring zwitterionic surfactant soybean lecithin, respectively. The enzymes were solubilized both in water-in-oil (W/O) microemulsions and in microemulsions with a bicontinuous structure. Different microstructures of the gels were chosen since the enzyme may undergo conformational changes in different environments resulting in different catalytic efficiencies toward competing substrates. Therefore, it is of great fundamental interest to know the phase behaviour and the microstructures of the used microemulsion systems. Phase diagrams were determined at 298.2 K for the systems water-hexane-AOT and ethanol/water (11)-hexadecane-soybean lecithin. The former system exhibited a large one-phase W/O microemulsion region, while in the latter a small one-phase region with bicontinuous structure was present. The kinetic enantiomeric ratios (E-values), as determined from enantiomeric excess (e.e.) values at a conversion below 0.5, were higher both in the W/O microemulsion as well as in the bicontinuous microemulsion using the SP 525 lipase, than using the CV lipase. On the other hand, the conversions were higher using gels based on W/O microemulsions (AOT stabilized) than using gels based on microemulsions with a bicontinuous structure (lecithin stabilized).     

Probing the microstructure of nonionic microemulsions with ethyl oleate by viscosity, ROESY, DLS, SANS, and cyclic voltammetry

Microemulsions are important formulations in cosmetics and pharmaceutics and one peculiarity lies in the so-called "phase inversion" that takes place at a given water-to-oil concentration ratio and where the average curvature of the surfactant film is zero. In that context, we investigated the structural transitions occurring in Brij 96-based microemulsions with the cosmetic oil ethyl oleate and studied the influence of the short chain alcohol butanol on their structure and properties as a function of water addition. The characterization has been carried out by means of transport properties, spectroscopy, DLS, SANS, and electrochemical methods. The results confirm that the nonionic Brij 96 in combination with butanol as cosurfactant forms a U-type microemulsion that upon addition of water undergoes a continuous transition from swollen reverse micelles to oil-in-water (O/W) microemulsion via a bicontinuous region. After determining the structural transition through viscosity and surface tension, the 2D-ROESY studies give an insight into the microstructure, i.e., the oil component ethyl oleate mainly is located at the hydrophobic tails of surfactant while butanol molecules reside preferentially in the interface. SANS experiments show a continuous increase of the size of the structural units with increasing water content. The DLS results are more complex and show the presence of two relaxation modes in these microemulsions for low water content and a single diffusive mode only for the O/W microemulsion droplets. The fast relaxation reflects the size of the structural units while the slower one is attributed to the formation of a network of percolated microemulsion aggregates. Electrochemical studies using ferrocene have been carried out and successfully elucidated the structural transformations with the help of diffusion coefficients. An unusual behavior of ferrocene has been observed in the present microheterogeneous medium, giving a deeper insight into ferrocene electrochemistry. NMR-ROESY experiments give information regarding the internal organization of the microemulsion droplets. In general, one finds a continuous structural transition from a W/O over a bicontinuous to an O/W microemulsion, however with a peculiar network formation over an extended concentration range, which is attributed to the somewhat amphiphilic oil ethyl oleate. The detailed knowledge of the structural behavior of this type of system might be important for their future applications.     

Decrease of droplet size of the reverse microemulsion 1-butyl-3-methylimidazolium tetrafluoroborate/Triton X-100/cyclohexane by addition of water

In the present contribution, results concerning the role of small amounts of water in the 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4)-in-cyclohexane ionic liquid (IL) reverse microemulsions are reported. Dynamic light scattering (DLS) revealed that the size of microemulsion droplets decreased remarkably with increasing water content although water is often used as a polar component to swell reverse microemulsions. It was thus deduced that the number of microemulsion droplets was increased which was confirmed by conductivity measurements. The states of dissolved water were investigated by Fourier transform IR (FTIR) spectroscopic analysis showing that water molecules mainly act as bound water. 1H NMR along with two-dimensional rotating frame nuclear Overhauser effect (NOE) experiments (ROESY) further revealed that water molecules were mainly located in the periphery of the polar core of the microemulsion droplets and behave like a chock being inserted in the palisade layer of the droplet. This increased the curvature of the surfactant film at the IL/cyclohexane interface and thus led to the decrease of the microemulsion droplet size. The order of surfactant molecules arranged in the interface film was increased and thus induced a loss of entropy. Isothermal titration calorimetry (ITC) indicated that an enthalpy increase compensates for the loss of entropy during the process of microstructural transition.