Calorimetric study of the solubilization of cholesterol,retinol and retinal by reversed AOT micelles

Distribution constants and standard enthalpies of transfer of cholesterol, retinol and retinal partitioned between n-heptane and water containing reversed sodium bis(2-ethylhexyl) sulfosuccinate (AOT) micelles as a function of the molar concentration ratio (R=[water]/[AOT]) were evaluated by a calorimetric method. The results indicate that, in spite of the bulky hydrocarbon radical, these solubilizates behave like alcohols with a short alkyl chain. Moreover, cholesterol is always solubilized in the palisade layer of the reversed micelles whereas retinol and retinal are preferentially solubilized in the aqueous pseudophase. The influence of the enthalpic and the entropic contributions to the transfer of the solubilizates from n-heptane to reversed AOT micelles are also considered.     

Calorimetric study of the solubilization of ethylenediamine,N,N-dimethylaminoethylamine and N,N,N′,N′-tetramethylethylenediamine by reversed AOT micelles

Distribution constants and standard enthalpies of transfer of ethylenediamine (en), N,N-dimethylaminoethylamine (dmen) and N,N,N,N-tetramethylethylenediamine (tmen) partitioned between n-heptane and water containing reversed sodium bis(2-ethylhexyl) sulfosuccinate (AOT) micelles as a function of the molar concentration ratio R (R=[water]/[AOT]) were evaluated by a calorimetric method. The results show that en, dmen and tmen molecules, solubilized in the reversed micelles, are distributed between the micellar aqueous core and the micellar palisade layer. An analysis of the thermodynamic parameters of the partition process demonstrates the peculiar solvent properties of the water containing reversed AOT micelles.     

Volumetric and transport properties of aerosol-OT reversed micelles containing light and heavy water

Densities and viscosities of sodium bis(2-ethylhexyl) sulfosuccinate (AOT in-heptane system containing light and heavy water, as a function of the molar ratio R (R=[H₂O or D₂O]/[AOT]) were measured at 0, 5, 25 and 40°C. At low R values, the apparent molar volume of deuterium oxide is smaller than that of light water. The difference is related to the strength of the hydrogen bonding H₂O and D₂O. The viscosities of both H₂O-AOT-n-heptane and D₂O-AOT-n-heptane systems were explained in terms of intermicellar interactions mainly governed by hydration of the head groups of AOT.     

Kinetics of the Oxidation of Phenylhydrazine by [Fe(CN)6]3− in Water‐in‐Oil Microemulsions

The oxidation reaction of phenyl hydrazine (Phh) by hexacyanoferrate ([Fe(CN)₆]^3?) has been studied in water‐in‐oil (w/o) microemulsion media. The kinetic profile of the reaction was investigated as a function of [Phh], droplet size, and droplet concentration. Comparison of the kinetic profiles of the reaction in microemulsion, water‐urea, and water‐AOT‐urea media indicates that the kinetic profile of the reaction in microemulsion shows a behavior similar to that of the reaction in water‐AOT‐urea medium at 4 M urea. An initial increase and then a decrease in k_obs is observed with increasing molar ratio, W_o(=[H₂O]/[AOT]) at constant [AOT] (=0.4 M), whereas k_obs decreases upon increasing the AOT concentration at constant molar ratio.     

Calorimetric investigation of reaction Ca(NO3)2 + Na3PO4 in water and microemulsions saturated with CO2

The reactions of Ca(NO₃)₂ + Na₃PO₄ in water and water/sodium bis(2‐ethylhexyl) sulfosuccinate (AOT)/hydrocarbon microemulsions saturated CO₂ with various molar ratios of water to surfactant R, oil phases, and surfactant concentrations were investigated by isothermal titration calorimetry. The product of the reaction was confirmed to be sodium‐and‐carbonate‐substituted hydroxyapatite (NaCO₃HAP) by Fourier transform infrared spectra (FTIR), energy dispersive spectrometry (EDS), and X‐ray diffraction (XRD). From calorimetric measurements, the molar enthalpies of solution of water in the AOT/n‐dodecane system, and the molar enthalpies, the rate constants, and the activation energies of the reactions were determined. It was found that the enthalpy of solution of water in AOT/n‐dodecane micells and the molar enthalpy of the reaction in the microemulsions increased with the decreases of R until R = 7; below that they kept almost constant. It may be attributed to the increase of the ratio of the bound water to the free water with the decrease of R until there was no free water when R < 7. However, the reaction rate constant k₁ was affected by the ionic strength of the medium and log k₁ showed a linear dependence on 1/R in the whole range of R we investigated. It was also observed that the rate constant and the enthalpy of the reaction remained almost unchanged when the surfactant concentration and the nature of oil phase varied. © 2011 Wiley Peiodicals, Inc. Int J Chem Kinet 43: 322–330, 2011     

Excess enthalpies for n-alkane/ -carotene+n-alkane/AOT/water systems

The experimental data of excess enthalpies for β-carotene/n-alkane+n-alkane/AOT/water systems at 298.15 K are reported. The H_E dependence on AOT (sodium bis(2-ethylhexyl) sulfosuccinate) concentration and hydrocarbon chain length was investigated. The excess enthalpy was measured using the flow microcalorimeter UNIPAN type 600.     

Oxidative refolding of reduced, denatured lysozyme in AOT reverse micelles

The refolding kinetics of the reduced, denatured hen egg white lysozyme in sodium bis(2-ethylhexyl)sulfosuccinate (AOT)-isooctane-water reverse micelles at different water-to-surfactant molar ratios has been investigated by fluorescence spectroscopy and UV spectroscopy. The oxidative refolding of the confined lysozyme is biphasic in AOT reverse micelles. When the water-to-surfactant molar ratio (omega 0) is 12.6, the relative activity of encapsulated lysozyme after refolding for 24 h in AOT reverse micelles increases 46% compared with that in bulk water. Furthermore, aggregation of lysozyme at a higher concentration (0.2 mM) in AOT reverse micelles at omega 0 of 6.3 or 12.6 is not observed; in contrast, the oxidative refolding of lysozyme in bulk water must be at a lower protein concentration (5 microM) in order to avoid a serious aggregation of the protein. For comparison, we have also investigated the effect of AOT on lysozyme activity and found that the residual activity of lysozyme decreases with increasing the concentration of AOT from 1 to 5 mM. When AOT concentration is larger than 2 mM, lysozyme is almost completely inactivated by AOT and most of lysozyme activity is lost. Together, our data demonstrate that AOT reverse micelles with suitable water-to-surfactant molar ratios are favorable to the oxidative refolding of reduced, denatured lysozyme at a higher concentration, compared with bulk water.     

Solution behavior of aerosol ot/water/oil system

The phase diagrams of water-cyclohexane containing 5, 10, and 20 wt% sodium 1,2-bis(2-ethylhexyloxycarbonyl)-1-ethanesulfonate (Aerosol OT, AOT) as a function of temperature were studied. There is the water phase in which AOT is dissolved and a large amount of oil is solubilized at higher temperature, and the oil phase in which AOT is dissolved and a large amount of water is solubilized at lower temperature. It is evident from the phase behavior that the hydrophile-lipophile property of AOT is well balanced. Accordingly, the phase diagram and properties of AOT solution are affected rather sensitively by the addition of a small amount of hydrophilic or lipophilic additive or by temperature. Careful elimination of inorganic salts also influences markedly the solution properties of AOT. Thus, the addition or removal of known amounts of inorganic salts, such as Na₂SO₄ is also very important to control the solution properties related to practical applications. In this context phase diagrams of both carefully purified and commercial AOT with or without additives have been determined.     

Kinetics for the Reaction between Potassium Ferricyanide and Cobalt Chloride in Aqueous Solution and Microemulsion

The kinetics for the reaction between potassium ferricyanide (K₃Fe(CN)₆) and cobalt chloride (CoCl₂) in aqueous solution and water/bis(2-ethylhexyl) sodium sulfosuccianate (AOT)/isooctane microemulsions were studied by three-wavelength spectrophotometry at 298.2 K. The second-order rate constants (k₂) were calculated from the time dependence of the concentration of reactant K₃Fe(CN)₆. The result showed that the reaction rates in water/AOT/isooctane microemulsions were slower than that in the aqueous solution, and k₂ decreased with molar ratio (ω) of water to AOT in microemulsions, which was interpreted by the transition state theory and confirmed that the reaction took place at the interfaces of the microemulsion water pools.     

298.15 K时斯蒂芬酸钾盐和铯盐在水和DMF中的溶解热

在298.15 K时, 采用微热量热仪测定斯蒂芬酸钾盐[K₂(TNR)(H₂O)]_n和斯蒂芬酸铯盐[Cs₂(TNR)(H₂O)₂]_n两种含能配合物在水和N,N-二甲基甲酰胺(DMF)溶剂中的溶解热, 研究其溶解过程和溶解热化学性质. 结果表明, 两种配合物溶解于水是吸热过程, 而溶解于DMF则为放热过程, 这主要是由于溶质和溶剂的分子结构及其极性不同而导致的. 通过对实验数据计算拟合, 求得这两种配合物的溶解焓(Δ_solH)、相对表观摩尔焓(ФL_i)、相对偏摩尔焓(L_i)及稀释焓(Δ_dilH_1,2)的经验公式和标准溶解焓值(Δ_solH_m^θ).     

Volumetric properties of water–poly(acrylic acid) salt systems from the pure solid to highly concentrated solutions

Volumetric properties of poly(acrylic acid) alkali-metal salts (Li, Na, K, Rb, and Cs) with different degrees of neutralization and water contents were studied in the range from pure solid to highly concentrated solutions. The apparent partial molar volume ?₂ of the polymer and the partial molar volume V₁ of water were calculated from density data. The value of ?₂ decreased with decreasing polymer concentration and eventually leveled off. Values of V₁, which at low water contents were much smaller than that of free water, increased with increasing water content and eventually reached a constant value equivalent to that of free water, thus indicating the appearance of free water. Water contents corresponding to the appearance of free water increased in the order of Li < Na < K < Rb < Cs, differing from the usual trend of hydration numbers observed in dilute solutions. The change of the slope of the plots of V₁ versus composition suggested a change in the hydration mechanism. For Li, Na, and K salts, the limiting values of V₁ at very low water content is considerably smaller than the 18 cm³/mol of free water. In contrast, for Rb and Cs salts, these values were relatively large, indicating the relatively weak electrostriction effects of these ions.     

Solvation of small hydrophobic molecules in formamide: A calorimetric study

The solubility and enthalpy of solution of benzene, cyclohexane, hexane, and heptane in formanide have been determined from titration microcalorimetric experiments at 25°C. The solution enthalpies are significantly more endothermic than in water but still the solubility is much higher. The entropy changes in formamide are small and positive and do not vary significantly with size. The enthalpies of solution of some 1-alkanols, 1-chloro- and 1,5-dichloropentane and pentane-1,5-diol were measured as functions of concentration. The solution enthalpies for 1-alkanols from methanol to decanol increase linearly with chain length. The enthalpic interaction coefficients h_xx are small and negative in formamide while they are large and positive in water. The partial molar heat capacities C _p,2 ^o for 1-propanol, 1-pentanol, benzene and cyclohexane in formamide were determined at 25°C from drop heat capacity measurements. Values of C _p,2 ^o are only slightly larger than the molar heat capacities of the liquid solutes.     

Refractive index of water-AOT-n-heptane microemulsions

Refractive index measurements on water/AOT/n-heptane microemulsions as a function of the volume fraction of the dispersed phase (water plus AOT) and of the water/AOT molar ratio R have been performed at 25°C. The refractive index was found to vary monotonically with without any change in rate during the crossover of the percolation threshold. Such a behavior suggested that, well above the percolation threshold, the water-AOT-n-heptane microemulsions are still formed by water-containing AOT reversed micelles dispersed in the oil phase. The analysis of the experimental data allowed an evaluation the fraction of the water molecules bonded to the AOT head group as a function of R.     

Thermodynamic investigation of salting effect of ovalbumin in various salts solutions

Calorimetric and volumetric investigations of ovalbumin salting by sodium chloride, lithium chloride, potassium chloride, lithium sulfate and ammonium sulfate buffered solutions in the range of concentrations of about 0.2–1.2 M of salts are reported. The evolution of enthalpies and apparent molar volumes vs. salts concentration was investigated using ITC MicroCal titration microcalorimeter and Anton Paar DMA 60/602 digital densimeter. It was found that the changes in enthalpies of salting and apparent molar volumes follow a similar trend. Conclusions about the ability of salts: to precipitate ovalbumin solution are presented.     

Structural and dynamical investigation of gelation containing water-in-oil microemulsions

The gelatin (Bloom 300)/water/AOT/n-heptane system has been investigated at fixed water/AOT molar ratioR (R=31.1) as a function of the gelatin content. Several experimental techniques (densitometry, refractometry, conductometry, rheology, dielectrometry, ultrasonics, hypersonics) have been used to investigate the role played by the gelatin molecule in the observed sol-gel transition above a critical gelatin content. The results appear consistent with the hypothesis of a rigid network of gelatin-water rods coated by surfactant molecules coexisting with gelatin-free AOT reversed micelles at the gelation point.     

Influence of Confined Water on the Photophysics of Dissolved Solutes in Reverse Micelles

The photophysical parameters of two probes with largely different hydrophobic character, namely, coumarin 1 and coumarin 343, are investigated in sodium bis‐(2‐ethylhexyl)sulfosuccinate (AOT)/hexane/water reverse micelles at various water/AOT molar ratio w₀. Correlation of photophysical parameters such as fluorescence quantum yield, fluorescence lifetime, and emission maxima with w₀ indicate distinctly different trends below and above w₀≈7 for both probes. The variation of the average rotational correlation times obtained from fluorescence anisotropy decays for both probes in reverse micelles further corroborate the above observation. Similar studies were also performed in nonaqueous reverse micelles with acetonitrile as polar solvent. Similar to aqueous reverse micelles, breaks in the photophysical parameters with increasing acetonitrile/AOT molar ratios w′₀ were also observed in these cases, although at a much lower w′₀ value of 3. The present results indicate that around w₀≈7 for aqueous reverse micelles (and around w′₀≈3 for nonaqueous reverse micelles) a distinct change occurs in the probe microenvironment, which is rationalized on the basis of the relative populations of interfacial and core water. We propose that until the ionic head groups and counterions are fully solvated by polar solvents, that is, up to w₀≈7 (or w′₀≈3), the interfacial water population dominates. Above these molar ratios coalescence of excess water molecules with each other to form truncated H‐bonded water clusters leads to a sizable population of core water. This is further substantiated by changes in the IR absorption spectra for the O? D stretching mode of diluted D₂O in reverse micelles with varying w₀. Critical comparison of the present results with relevant literature reports provide clear support for the proposals made on water structure in reverse micelles. The role of relative size of the probe and the reverse micelles for differences in polar solvent to AOT ratios (w₀=7 and w′₀=3) in the observed breaks in the two types of reverse micelles is also discussed.     

Thermodynamic and19F NMR studies of antimony trifluoride in water

Densities, specific heat capacities per unit volume and enthalpies of dilution at 25°C and osmotic coefficients at 37°C were measured for antimony trifluoride in water as functions of concentration. From the first three properties the apparent and partial molar volumes, heat capacities and relative enthalpies were derived. As well, pH measurements in water at 25°C and¹⁹F NMR spectra in water and methanol at 33°C were also carried out. All the thermodynamic properties together with the chemical shifts abruptly change in the very dilute concentration region (<0.1m) and, then, tend to a constant value. These trends have been rationalized through a simple model based on an equilibrium of dissociation of SbF₃ into two ionic species. From the simulation of all the data it is derived that two concomitant equilibria are present in solution: the hydrolysis process of SbF₃ which explains the pH values and the ionic dissociation of SbF₃ which accounts for the¹⁹F NMR data.     

Temperature and salt addition effects on the solubility behaviour of some phenolic compounds in water

Solubility-temperature dependence data for six phenolic compounds (PhC), contained in olive mill wastewater (OMWW), in water and in some chloride salts (KCl, NaCl, and LiCl) aqueous solutions have been presented and solution standard molar enthalpies (Δ_solH^∘) were determined using Van’t Hoff plots. The temperature was varied from 293.15 K to 318.15 K. Solubility data were estimated using a thermostated reactor and HPLC analysis. It has been observed that solubility, in pure water and in aqueous chloride solutions, increases with increasing temperature. The salting-out LiCl > NaCl > KCl order obtained at 298.15 K is confirmed. Results were interpreted in terms of the salt hydration shells and the ability of the solute to form hydrogen-bond with water. The standard molar Gibbs free energies of transfer of PhC (Δ_trG^∘) from pure water to aqueous solutions of the chloride salts have been calculated from the solubility data. In order to estimate the contribution of enthalpic and entropic terms, standard molar enthalpies (Δ_trH^∘) and entropies (Δ_trS^∘) of transfer have also been calculated. The decrease in solubility is correlated to the positive Δ_trG^∘ value which is mainly of enthalpic origin.     

Thermochemical Study of Solutions of Copper(II) Chloride in Methanol-Rich Water + Methanol Mixtures

The calorimetric enthalpies of dilution of water + methanol solutions of CuCl₂ (m < 3.3 mol/kg; methanol mole fractions of 0.5–1.0) with corresponding water + alcohol solutions at 298. 15 K are studied. These data are used to calculate the enthalpies of dilution and the relative partial molar enthalpies for the entire solution and its components (solvent and solute). The data are considered in terms of the structural evolution of the solution and copper(II) chloride complexing; these factors influence the dissolution energy of the electrolyte and its state in solution.     

Synthesis and Characterization of [Zn‐Al] Layered Double Hydroxides: Effect of the Operating Parameters

In this study, [Zn‐Al] layered double hydroxides (LDHs ) were prepared using the coprecipitation method at constant pH . The synthesis parameters (including the aging time, synthesis pH , nature of the alkali, concentration of metallic salts, and the cationic molar ratio R = Zn/Al) were varied in order to elucidate their effect on the properties of the obtained materials. Different characterization techniques, namely X‐ray diffraction, Fourier transform infrared, thermogravimetric analysis‐differential thermogravimetry, transmission electron microscopy, energy‐dispersive X‐ray, inductively coupled plasma, and photoluminescence, were used in this study. It was found that obtaining well‐crystallized LDHs requires (i) an aging time of 24 h and (ii) a synthesis pH value in the range 8–12. Our results also show that the concentration of the cationic metallic salts has no influence on the structural properties of the LDHs . The use of NH₄OH as alkali for adjusting the pH value during the synthesis favors the formation of nitrated LDH phases while NaOH gives rise to carbonated ones. Moreover, it was found that irrespective of the molar cationic ratio used (between 1 and 5), [Zn‐Al] LDHs could be obtained. The sample synthesized at R = 2 exhibited the best crystallinity.