Interfacial composition and structural parameters of water/C12-s-C12 x 2Br/n-hexanol/n-heptane microemulsions studied by the dilution method

The interfacial composition of the stable water/C12-s-C12 x 2Br/n-hexanol/n-heptane microemulsions has been studied in detail by dilution method. The results showed a marked maximum amount of the n-hexanol populating on the surfaces of droplets (represented as a = n(a)i/n(s), where n(a)i and n(s) are respectively the moles of n-hexanol and gemini surfactant on the surface of droplets) with increasing water content. At a constant level of water addition (the molar ratio of water to surfactant W0 = 20), a decreased with increasing the spacer length in the C12-s-C12 x 2Br molecule. The structural parameters of a w/o microemulsion were also estimated by analyzing the data of dilution experiments, and we found that the radius of the water pool was very sensitive to the increment of water content. The radius of the water pool varied from 0.74 to 5.35 nm with increasing W0 from 10 to 50. The variation extent reached 4.61 nm. In the cases of water/CPC/n-butanol/isopropyl myristate and water/CTAB/n-butanol/isopropyl myristate, however, the corresponding variation extents were only 1.22 and 1.68 nm, respectively, when increasing comparable water content. The ratio of N(a)/N(2C), where N(a) and N(2C) are respectively the average numbers of n-hexanol and the total average numbers of alkyl chains of gemini surfactant populating on per droplet surface, decreased obviously with increasing water content at W0 > 15. This indicated that C12-2-C12 x 2Br favored to form large droplets that were suitable to solubilize more water.     

Phase behavior and microstructure of C12E5 nonionic microemulsions with chlorinated oils

New non-ionic microemulsions consisting of pentaethyleneglycol dodecyl ether, water, and 1-chloroalkanes were prepared, and their phase behavior was studied. A homologous series of five different 1-chloroalkanes from 1-chlorooctane to 1-chlorohexadecane was studied. The phase behavior of the microemulsions was determined by vertical sections through the Gibbs' phase prism ("fish" plots), from which valuable information such as the microemulsion balance temperature (T(0)), efficiency of the surfactant (phi*), temperature extension of the three-body phase (DeltaT), mean temperature (T(m)), and the monomeric solubility in oil (phi(mon,oil)) was obtained. The chlorinated alkanes in the microemulsions shift the balance temperature to about 14 degrees C lower compared with their n-alkane counterparts. This indicates the polar nature of the chlorinated oils and their ability to penetrate the surfactant film. The chlorinated alkanes thus behave as short n-alkane molecules and lower the spontaneous curvature of the microemulsion droplets. The efficiency of the surfactant and the monomeric solubility in oil systematically depend on the alkyl chain length of the oil, with the efficiency and solubility decreasing with increasing alkyl chain length of 1-chloroalkane. The size and shape of the microemulsion droplets in the microemulsion phase were studied by small-angle X-ray scattering (SAXS). For a surfactant-to-oil volume fraction ratio of 0.80, the droplets can be described by ellipsoidal shapes, and the size of the droplets increased with increasing alkyl chain length.     

Effects of nonionic surfactant C12E5 on the cooperative dynamics of water

A dielectric relaxation study of binary mixtures of nonionic surfactant C12E5 + water has been made as a function of temperature in the isotropic micellar, lamellar, and hexagonal regions of the phase diagram. Two dielectric dispersion steps were found and could be assigned to the intermolecular cooperative dynamics of water at the micellar interface and in the bulk water domains. A quantitative analysis is given. The relaxation amplitudes were used to determine effective hydration numbers. The activation energies of water relaxation were calculated from the relaxation times. The data indicate weaker surfactant-water and water-water interactions near the micellar interface compared to those of bulk liquid water. Further analysis revealed the presence of water clusters large enough to show a cooperative relaxation mode even at high surfactant concentrations. However, the relaxation time of this mode is larger compared to that of pure water. This points out the importance of confinement effects on water dynamics.     

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.     

Phase equilibrium measurements for systems containing propanenitrile with tert-butyl ethyl ether and C4-hydrocarbons

A static total pressure apparatus was used to measure isothermal VLE for systems containing propanenitrile +n-butane (322.02 K), +1-butene (312.55 K), +2-methylpropane (307.85 K) and +2-methylpropene (312.59 K). Vapour liquid equilibrium (VLE) for propanenitrile + tert-butyl ethyl ether (ETBE) was measured both with the static total pressure apparatus at 312.85 K and 358.32 K and with a glass circulation still apparatus at 102.6 kPa and 65.2 kPa. The system of propanenitrile + ETBE evidenced azeotropic behaviour.     

Odd-even variations in the wettability of n-alkanethiolate monolayers on gold by water and hexadecane: a molecular dynamics simulation study

Molecular dynamics (MD) simulations were performed to investigate odd-even chain length dependencies in the wetting properties of self-assembled monolayers (SAMs) of n-alkanethiols [CH3(CH2)n-1SH] on gold by water and hexadecane. Experimentally, the contact angle of hexadecane on the SAMs depends on whether n is odd or even, while contact angles for water show no odd-even dependence. Our MD simulations of this system included a microscopic droplet of either 256 water molecules or 60 hexadecane molecules localized on an n-alkanethiolate SAM on gold with either an even or odd chain length. Contact angles calculated for these nanoscopic droplets were consistent with experimentally observed macroscopic trends in wettability, namely, that hexadecane is sensitive to structural differences between odd- and even-chained SAMs while water is not. Structural properties for the SAMs (including features such as chain tilt, chain twist, and terminal methyl group tilt) were calculated during the MD simulations and used to generate IR spectra of these films that compared favorably with experimental spectra. MD simulations of SAMs in contact with slabs of water and hexadecane revealed that the effects of these solvents on the structure of the SAM was restricted to the chain terminus and had no effect on the inner structure of the SAM. The density profiles for water and hexadecane on the SAMs were different in that water displayed a significant depletion in its density at the liquid/SAM interface from its bulk value, while no such depletion occurred for hexadecane. This difference in contact may explain the lack of an odd-even variation in the wetting characteristics of water on these surfaces, because the water molecules are positioned further away from the surface and, therefore, are not sensitive to the structural differences in the average orientations for the terminal methyl groups in odd- and even-chained SAMs. In contrast, the differences in the wetting properties of hexadecane on the odd- and even-chained SAMs may reflect the closer proximity of these molecules to the SAM surface and a resulting greater sensitivity to the differences in the terminal methyl group orientations in the SAMs. SAM-solvent interaction energies were calculated during the MD simulations, yielding interaction energies that differed on the even- and odd-chained surfaces by approximately 10% for hexadecane and negligibly for water, in accord with estimates using experimental wetting results.     

Preparation and characterization of water-in-oil decane/AOT microemulsions containing silver and gold nanoparticles and large amounts of water

Stable microemulsions with water contents as high as 10 vol % have been obtained, including those additionally containing silver and gold nanoparticles. Especial attention has been focused on the influence of water and stabilizer contents on the structure of adsorption layers on nanoparticles. The properties of nanoparticles obtained via the traditional microemulsion synthesis have been compared with the properties of nanoparticles that have preliminarily been concentrated with the help of electrophoresis and dried. The electrophoretic concentration and drying of nanoparticles have been shown to improve the stability of their microemulsions. Microemulsions with the highest content of water have been studied to determine the occurrence of percolation and the influence of nanoparticles on their percolation temperature and electrical conductivity.     

Solvation dynamics of Hoechst 33258 in water: an equilibrium and nonequilibrium molecular dynamics study

Integrated within an appropriate theoretical framework, molecular dynamics (MD) simulations are a powerful tool to complement experimental studies of solvation dynamics. Together, experiment, theory, and simulation have provided substantial insight into the dynamic behavior of polar solvents. MD investigations of solvation dynamics are especially valuable when applied to the heterogeneous environments found in biological systems, where the calculated response of the environment to the electrostatic perturbation of the probe molecule can easily be decomposed by component (e.g., aqueous solvent, biomolecule, ions), greatly aiding the molecular-level interpretation of experiments. A comprehensive equilibrium and nonequilibrium MD study of the solvation dynamics of the fluorescent dye Hoechst 33258 (H33258) in aqueous solution is presented. Many fluorescent probes employed in experimental studies of solvation dynamics in biological systems, such as the DNA minor groove binder H33258, have inherently more conformational flexibility than prototypical fused-ring chromophores. The role of solute flexibility was investigated by developing a fully flexible force-field for the H33258 molecule and by simulating its solvation response. While the timescales for the total solvation response calculated using both rigid (0.16 and 1.3 ps) and flexible (0.17 and 1.4 ps) models of the probe closely matched the experimentally measured solvation response (0.2 and 1.2 ps), there were subtle differences in the response profiles, including the presence of significant oscillations for the flexible probe. A decomposition of the total response of the flexible probe revealed that the aqueous solvent was responsible for the overall decay, while the oscillations result from fluctuations in the electrostatic terms in the solute intramolecular potential energy. A comparison of equilibrium and nonequilibrium approaches for the calculation of the solvation response confirmed that the solvation dynamics of H33258 in water is well-described by linear response theory for both rigid and flexible models of the probe.     

Aggregation behavior of tetrakis(4-sulfonatophenyl)porphyrin in AOT/water/decane microemulsions

AOT/water/decane microemulsions have been used to entrap the water-soluble 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS4). Quasi-elastic light scattering technique has confirmed the confinement of the porphyrin and its various aggregates into the inner water pool. Various species have been detected as function of the size of the microemulsions, concentration of the porphyrin, pH, and aging of the solutions by using a combination of UV-vis absorption, steady fluorescence emission, fluorescence lifetime measurements, and time-resolved fluorescence anisotropy. Under neutral pH conditions, the porphyrin is present as the free base monomer (S414) in the inner water compartment, and it is free to rotate when the size of the droplet is large enough and the porphyrin concentration is low. On increasing the concentration and/or decreasing the microemulsion size, a H-dimer of the free base (S406) is prevalently formed. Aging both the S414 and S406 species leads to the formation of a new species (S424), which has been postulated as a H-type dimer of the diacid porphyrin. On decreasing the pH, the species S414 and S406 almost instantaneously convert into the diacid porphyrin, which is monomeric (S434). This latter is an intermediate in the eventual formation of J-aggregated TPPS4 (S490). A marked stability has been observed for the S424 species, which do not interconvert on changing the pH of the bulk aqueous phase.     

Dilution method study on the interfacial composition and structural parameters of water/C12-EOx-C12.2Br/n-hexanol/n-heptane microemulsions: effect of the oxyethylene groups in the spacer

The interfacial composition and the structure of the water/C(12)-EO(x)-C(12).2Br/n-hexanol/n-heptane microemulsion have been investigated by the dilution method. The results showed that C(12)-EO(x)-C(12).2Br formed a stable water/oil microemulsion with the assistance of n-hexanol. Owing to the relatively large size of the head group, more n-hexanol molecules are populated on the droplet surface than in the C(12)-2-C(12).2Br system. The radius of the water pool of the C(12)-EO(3)-C(12).2Br system is not as sensitive to W(0) as in the C(12)-2-C(12).2Br system. Another feature of the present system is that its droplet size is considerably smaller than that of the C(12)-2-C(12).2Br system, and also those of the CPC and CTAB systems at relative high water content. This provides a potential application for the synthesis of nanoparticles with small size.     

Molecular dynamics study of temperature dehydration of a C12E6 spherical micelle

Hydration of a spherical micelles of C12E6 in solution is studied by molecular dynamics simulation. The interface is found to be separated in an inner part composed of water and hydrophobic and hydrophilic moieties and an outer part with hydrophilic moiety and water only. Hydration numbers in the inner and in the outer parts are in excellent agreement with experimental data from various different methods. Temperature dehydration occurs in the inner region only and is related to the presence of water molecules directly in contact with the hydrophobic core at low temperature.     

Structure and dynamics of hydrogen bonds in the interface of a C12E6 spherical micelle in water solution: a MD study at various temperatures

The temperature dehydration of a C(12)E(6) spherical micelle is characterized through the study of the structure and dynamics of the hydrogen bonds formed by water within the micellar interface. Water molecules in proximity of the hydrophilic fragment of the C(12)E(6) surfactants form strong H-bonds with the oxyethilene units E and with the polar alcoholic heads. The activation energies of such H-bonds fall in the range 2-3 Kcal mol(-1). On the exposed oil core, the number of water-water H-bonds decreases as an effect of dehydration. The dynamics of such bonds exhibits a slow relaxation with respect to the bulk, and two time scales can be discerned: the first one, tau approximately 3-6 ps, is typical of water-water H-bonds around small hydrophobic molecules, whereas the second one, tau approximately 40-80 ps, is probably due to the confining effect of the long hydrophilic fragments which reduces the probability of a water molecule to leave the hydration layer of the exposed oil core. Water molecules around the core form H-bond clusters whose size and distribution change with temperature. From a cluster analysis, the system appears to be below the percolation threshold, suggesting that the exposed oily surface is formed by disconnected patches of size around 1 nm(2), close to the estimate of the solvated hydrophobic patches on protein surfaces. The network connectivity is also considered for concentric hydration shells along the interface: it turns out that near the oil core, the cluster size is larger than elsewhere in the interface demonstrating a strong structural effect induced by the exposed hydrocarbon tails. Temperature affects the cluster size only in the innermost shell.     

Heat capacity and phase behavior of {C6E4+water} solutions by DSC

The specific heat capacities of {2-(hexyloxytriethoxy)ethanol (C₆E₄)+water} system have been measured from 280 to 333 K within the whole composition range by DSC. Changes of specific, apparent and partial molar heat capacities of investigated aqueous solution vs. composition and temperature, considered as an effect of structural transformations were analyzed in order to draw boundary between region where amphiphile molecules occur as monomers and small aggregates and the area in which the first micelles appear. For each solution, the temperature dependences of the differential heat flow were analyzed in order to find the curve of phase coexistence, i.e. the boundary between one- and two-phase areas for the examined system.     

Nonequilibrium molecular dynamics simulations of the thermal conductivity of water: A systematic investigation of the SPC/E and TIP4P/2005 models

We report an extensive nonequilibrium molecular dynamics investigation of the thermal conductivity of water using two of the most accurate rigid nonpolarizable empirical models available, SPC/E and TIP4P/2005. Our study covers liquid and supercritical states. Both models predict the anomalous increase of the thermal conductivity with temperature and the thermal conductivity maximum, hence confirming their ability to reproduce the complex anomalous behaviour of water. The performance of the models strongly depends on the thermodynamic state investigated, and best agreement with experiment is obtained for states close to the liquid coexistence line and at high densities and temperatures. Considering the simplicity of these two models the overall agreement with experiments is remarkable. Our results show that explicit polarizability and molecular flexibility are not needed to reproduce the anomalous heat conduction of water.     

Modelization of the release from a tetradecane/water/hexadecane multiple emulsion: evidence of significant micellar diffusion

The release of tetradecane from a multiple emulsion of the type tetradecane/water/hexadecane was studied experimentally using the differential scanning calorimetry technique. The kinetics of the tetradecane release was measured for three formulations containing different concentrations of hydrophilic surfactant (2%, 4%, and 7%). A new mass transfer model derived from the shrinking core model was developed. The values of the model parameters deduced from the least-squares fittings led to the determination of the tetradecane diffusivity. Thus, the preponderant mechanism of mass transfer was proved to be micellar diffusion and not molecular diffusion. This conclusion was confirmed by considering the effect of the change in the hydrophilic surfactant concentration.     

Properties of binary surfactant systems of nonionic surfactants C12E10, C12E23, and C12E42 with a cationic gemini surfactant in aqueous solutions

Properties of binary surfactant systems of nonionic surfactants poly(ethylene oxide) (PEO) lauryl ethers (C(12)E(10), C(12)E(23), C(12)E(42)) with a cationic gemini surfactant, butanediyl-α,ω-bis(tetradecyldimethylammonium bromide) (14-4-14), have been investigated by Steady-state Fluorescence (FL), zeta potential, Dynamic Light Scattering (DLS), Transmission Electron Microscopy (TEM), Cryogenic Transmission Electron Microscopy (CryoTEM), and X-ray Diffraction (XRD). Through FL measurements, critical micelle concentration (CMC) of the three binary systems for different mixing mole fractions is determined and the values fall between those of pure constituent surfactants. Ideal CMC (CMC(ideal)), mole fraction in aggregates (X), interaction parameter (β), activity coefficients (f(1) and f(2)), and excess free energy of mixing (ΔG(ex)) have been calculated. All these parameters indicate nonideal behavior and synergistic interactions between the constituent surfactants, which is explained in terms of electrostatic attraction between headgroups of constituent surfactants and reduction of electrostatic repulsion between headgroups of 14-4-14 due to the presence of nonionic surfactants. DLS, TEM and CryoTEM results show that nonionic surfactants facilitate the formation of larger aggregates. Micelles and vesicles in larger size compared with those of 14-4-14 coexist in the mixed solutions. Both surfactant composition and PEO chain length are found to play a strong effect on the properties of the binary systems.     

Temperature effect on the surface phase transitions of monolayer films of C12E1 at air/water interface

The temperature-dependent conformational states of a monolayer film of ethylene glycol monododecyl ether (C₁₂E₁) at the air/water interface have been investigated using ellipsometry, surface tension, external reflection–absorption FTIR spectroscopy and two-dimensional infrared (2DIR) correlation analysis. The ellipticity coefficients and the entropy associated with C₁₂E₁ adsorption changed almost discontinuously at certain temperatures, which manifested the interfacial phase transitions. The phase transition and coexistence of two phases were further clarified using 2DIR correlation analysis with temperature perturbation. The asynchronous correlation maps revealed that both bands of asymmetric and symmetric C–H stretching vibration in one-dimensional IR were split into two components, which confirmed the coexistence of two phases at the interface.     

微乳液相行物为结构的研究:氯化钠/水/十二烷基硫酸钠/正丁醇/正辛烷系统

本文报道NaCl/H2O/SDS/n-C4H90H/n-C8H18系统在WC4H9OH/WSDS=4时, 不同盐浓度下的相图, 结果表明, 卤水中NaCl浓度的增大导致这个系统相分裂区域扩在, 并在一定范围内产生三相共存和出现中相微乳液。中相微乳液的密度, 粘度, 电导率和折光率与最佳盐浓度以及微观物型转变有对应关系, 红外光谱测定表明, 油相中氢键的缔合强度比中相微乳液显著增强。     

Shear and dilatational relaxation mechanisms of globular and flexible proteins at the hexadecane/water interface

Proteins adsorbed at fluid/fluid interfaces influence many phenomena: food emulsion and foam stability (Murray et al. Langmuir 2002, 18, 9476 and Borbas et al. Colloids Surf., A 2003, 213, 93), two-phase enzyme catalysis (Cascao-Pereira et al. Biotechnol. Bioeng. 2003, 83, 498; 2002, 78, 595), human lung function (Lunkenheimer et al. Colloids Surf., A 1996, 114, 199; Wustneck et al.; and Banerjee et al. 2000, 15, 14), and cell membrane mechanical properties (Mohandas et al. 1994, 23, 787). Time scales important to these phenomena are broad, necessitating an understanding of the dynamics of biological macromolecules at interfaces. We utilize interfacial shear and dilatational deformations to study the rheology of a globular protein, lysozyme, and a disordered protein, beta-casein, at the hexadecane/water interface. Linear viscoelastic properties are measured using small amplitude oscillatory flow, stress relaxation after a sudden dilatational displacement, and shear creep response to probe the rheological response over broad experimental time scales. Our studies of lysozyme and beta-casein reveal that the interfacial dissipation mechanisms are strongly coupled to changes in the protein structure upon and after adsorption. For beta-casein, the interfacial response is fluidlike in shear deformation and is dominated by interfacial viscous dissipation, particularly at low frequencies. Conversely, the dilatational response of beta-casein is dominated by diffusion dissipation at low frequencies and viscous dissipation at higher frequencies (i.e., when the experimental time scale is faster than the characteristic time for diffusion). For lysozyme in shear deformation, the adsorbed protein layer is primarily elastic with only a weak frequency dependence. Similarly, the interfacial dilatational moduli change very little with frequency. In comparison to beta-casein, the frequency response of lysozyme does not change substantially after washing the protein from the bulk solution. Apparently, it is the irreversibly adsorbed fraction that dominates the dynamic rheological response for lysozyme. Using stress relaxation after a sudden dilatational displacement and shear creep response, the characteristic time of relaxation was found to be 1000 s in both modes of deformation. The very long relaxation time for lysozyme likely results from the formation of a glassy interfacial network. This network develops at high interfacial concentrations where the molecules are highly constrained because of conformation changes that prevent desorption.     

CTAB/C4H9OH/C7H16/H2O体系中w/o型微乳液的导电活化能

通过测定不同温度时CTAB/C4H9OH/C7H16/H2O体系中w/o型微乳液的电导率计算了体系的导电活化能。研究了活化能随含水量的变化规律。发现活化能随含水量的变化与微乳液出现渗滤现象有关。讨论了表面活性助剂与表面活性剂的质量比km及起始含油量R对峰值活化能Ea,max的影响。