W/O microemulsions of styrene monomer and dimer

The reason for the lower water solubility in W/O microemulsions of the styrene dimer, trans, trans-1,4-diphenyl-1,3-butadiene, in comparison with the monomer was investigated by determining the phase equilibria in the microemulsion system with pentanol as the cosurfactant and sodium dodecyl sulfate as the surfactant. The main factor for the low tolerance for water in the system was the incompatibility of the dimer with the cosurfactant and surfactant.     

The structure of O/W microemulsions formulated with alkylglucosides

By following a method proposed by Kahlweit, an equilibrium diagram was determined in O/W quaternary microemulsions. These systems were formed by octanol (cosurfactant)/water/octane (oil)/alkylglucoside (surfactant). The experiment study was performed at two different temperatures (25°C and 50°C). The objective of this study work was to determine the structure of these lower microemulsions and to study the influence of the cosurfactant. Hence, different experimental techniques were employed: light scattering (static and dynamic), Kerr effect (static and dynamic), viscosity and refractometric measurements. It was concluded that the surfactant volume fraction in equilibrium with micelles is 0.044 and the micelle shapes are revolution ellipsoids.     

Determination of water states and the structural parameters of W/O microemulsions

The states of water in sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/isooctane/water reverse(W/O) microemulsions system have been investigated by using Fourier transform infrared spectroscopy(FT-IR) technique. The broad peak obtained for hydroxy(O-H) of water has been resolved by least square curve-fitting. It has been observed that the water solubilized in microemul-sion droplets has four states, i.e. bound water with sulfo-group, free water, bound water with sodium counterion in the water pool of microemulsion droplets and a small amount of trapped water in the palisade layer of microemulsion droplets. The following have also been determined: the aggregation number (n), the radius of the water pool in the microemulsions(rw), the thickness of the bound water with sulfo-group(d1), the thickness of the bound water with sodium counterion(d2), the total thickness of the bound water (d) and the effective area of head groups of AOT(AAOT).     

Blockcopolymers in microemulsions differential heats of solution and shift of the percolation temperature in W/O microemulsions

Solutions of blockcopolymers (POE-b-PI-b-POE) in fluids of interacting aqueous nanodroplets (W/O microemulsions) are studied. The interaction strength between the (pseudo) two components is measured by the shift of the percolation temperature relative to that of the pure microemulsion. A quantitative measure of the interaction, the differential heat of solution, is thermodynamically related to the slopes of the equilibrium temperature of the system with varying monomeric nanodroplet concentration and the experimental percolation line in the presence of copolymer.     

Relationship between rheological properties and one-step W/O/W multiple emulsion formation

Formation of a normal (not temporary) W/O/W multiple emulsion via the one-step method as a result of the simultaneous occurrence of catastrophic and transitional phase inversion processes has been recently reported. Critical features of this process include the emulsification temperature (corresponding to the ultralow surface tension point), the use of a specific nonionic surfactant blend and the surfactant blend/oil phase ratio, and the addition of the surfactant blend to the oil phase. The purpose of this study was to investigate physicochemical properties in an effort to gain a mechanistic understanding of the formation of these emulsions. Bulk, surface, and interfacial rheological properties of adsorbed nonionic surfactant (CremophorRH40 and Span80) films were investigated under conditions known to affect W/O/W emulsion formation. Bulk viscosity results demonstrated that CremophorRH40 has a higher mobility in oil compared than in water, explaining the significance of the solvent phase. In addition, the bulk viscosity profile of aqueous solutions containing CremophorRH40 indicated a phase transition at around 78 ± 2 °C, which is in agreement with cubic phase formation in the Winsor III region. The similarity in the interfacial elasticity values of CremophorRH40 and Span80 indicated that canola oil has a major effect on surface activity, showing the significance of vegetable oil. The highest interfacial shear elasticity and viscosity were observed when both surfactants were added to the oil phase, indicating the importance of the microstructural arrangement. CremophorRH40/Span80 complexes tended to desorb from the solution/solution interface with increasing temperature, indicating surfactant phase formation as is theoretically predicted in the Winsor III region. Together these interfacial and bulk rheology data demonstrate that one-step W/O/W emulsions form as a result of the simultaneous occurrence of phase-transition processes in the Winsor III region and explain the critical formulation and processing parameters necessary to achieve the formation of these normal W/O/W emulsions.     

Probing the relationship between interfacial concentrations and lipase activity in cationic W/O microemulsions: a quantitative study by chemical trapping

Interfacial concentrations and/or space: which one is the predominant factor in regulating lipase activity at the water-oil interface? This work is an endeavor toward probing the relationship between lipase activity and interfacial concentrations in cationic water-in-oil (W/O) microemulsions through quantitative study by a chemical trapping method. The interfacial concentrations of water ([H2Oi]), bromide ([Bri-]), and n-hexanol ([HexOHi]) were estimated in the W/O microemulsions of six surfactants with varying headgroup architecture and hydrophilicity across a wide W0 ([H2O]/[surfactant]) range. The surfactants were prepared by the replacement of methyl groups of cetyltrimethylammonium bromide (1) by n-propyl (2-4), one hydroxyethyl (5), and one methoxyethyl (6) group. The estimated [H2Oi] was found not to change much (30.0-36.7 M) with the variation in headgroup hydrophilicity or size from 1-5. However, [Bri-] was found to increase with a decrease in the degree of dissociation (alpha), being higher for 1 and 5 (2.4-3.3 M) and relatively lower (0.9-1.9 M) for others depending on W0. Interestingly, [H2Oi] was found to be little higher (41.5-42.2 M) in the case of 6. The present study elucidates the importance of interfacial water and counterion concentrations in modulating the lipase activity in reverse micelles. In our previous report, the lipase activity was found to increase from 1-4 and in 6, whereas that observed in 5 was comparable with 1, being largely regulated by the surfactant head group size (Das, D.; Roy, S.; Mitra, R. N.; Dasgupta, A.; Das, P. K. Chem.-Eur. J. 2005, 11, 4881). The only other parameter that increased distinctly with lipase activity is the headgroup size, not [H2Oi]. Thus, the role of [H2Oi] in comparison to the surfactant's headgroup size is not found to be that significant. Moreover, the lower [Bri-] in 2-4 and 6 perhaps enhances the probability of enzyme and substrate localization at the interface, leading to higher lipase activity.     

Formation of zinc sulfide nanorods and nanoparticles in ternary W/O microemulsions

A simple, easy approach to the synthesis of semiconductor ZnS nanorods and nanoparticles exhibiting versatile morphology-formation ability is reported. Water-insoluble zinc sulfide nanocrystals were synthesized in ternary water-in-oil (w/o) microemulsion systems stabilized by either nonionic or, in contrast, cationic surfactant. Products were visualized by transmission electron microscopy (TEM) and identified by energy-dispersive X-ray spectroscopy (EDAX); electron diffraction (ED) was also performed for individual nanorods. With varying molar ratios of water to surfactant (omega0) in solution, hence changing droplet sizes of water pool of microemulsions consequently, several morphologies with different size spans were encountered in the formation of ZnS, such as nanorods and spherical or ellipsoidal particles. Meanwhile, product morphology was also found to be sensitive to the absolute reactant concentration and concentration ratio of [Zn2+] to [S2-], the incubation time, and the ambient temperature. A schematic mechanism for the formation of ZnS nanocrystals and their morphological diversity is described. It is feasible to extend this method to the synthesis of one-dimensional nanocrystals of other semiconductors, given suitable formulae of microemulsions and other appropriate reaction conditions.     

On the incorporation of the non-steroidal anti-inflammatory naproxen into cationic O/W microemulsions

Microemulsions (ME) containing hexadecyltrimethylammonium bromide (HTAB)/ethanol as surfactant, isopropylmyristate (IM) or butylstearate (BS) as oil phase and aqueous buffer were studied. Pseudo-ternary phase diagrams of the investigated systems were obtained at constant surfactant/cosurfactant molar ratio (1:5) by titration in order to characterize the proportions between the components to obtain clear systems. Oil in water microemulsions were prepared in a wide range of phase volume (phi). UV-vis absorption spectra of naproxen at pH 5.5 showed that the solubility of Np increases significantly in the presence of O/W ME in high phase volumes. For both, IM and BS microemulsions, the dynamic light scattering experiments showed that the size of the oil droplets remains constant in low values of phi, increasing abruptly in high phi values. Phase solubility study revealed that for both IM and BS microemulsions, the drug incorporation followed a straight-line profile in all range of phi. The data could be analyzed through the phase-separation model and the association constants (K) calculated varied from 27 to 90 M(-1), depending on the pH and on the microemulsion oil phase.     

Styrene and polystyrene microemulsions. I. Solubility in the W/O system

The solubility of styrene, its oligomers, and different molecular weight polymers was determined in W/O microemulsions stabilized by sodium dodecyl sulfate and pentanol. The results showed a complex pattern due to the interactions with pentanol, surfactant, and associated structures.     

W/O与O/W，W/O与双连续结构微乳液的界面电性质

无探针伏安法和电容法测定结果表明,在质量比H_2O/n-C_5H_(11)OH = 50/50条件下,SDS总含量<3%时,上相呈W/O结构,下相呈O/W结构,随着体系SDS 总含量的增大,界面上SDS量增加,界面电势ΔE、界面电容C、界面电荷传递电流 i_p均增加,而界面电阻率ρ_i则减小;当体系 SDS总含量≥3%时,上相呈W/O结构 ,下相呈双边续结构,各界面电性质变化幅度均变缓。     

Diffusion behavior of pharmaceutical O/W microemulsions studied by dynamic light scattering

Dynamic light scattering experiments have been performed at various concentrations, of pharmaceutical oil-in-water microemulsions consisting of Eutanol G as oil, a blend of a high (Tagat O2) and a low (Poloxamer 331) hydrophilic–lipophilic balance surfactant, and a hydrophilic phase (propylene glycol/water). We probe the dynamics of these microemulsions by dynamic light scattering. In the measured concentration range, two modes of relaxation were observed. The faster decaying mode is ascribed classically to the collective diffusion D _c (total droplet number density fluctuation). We show that the slow mode is also diffusive and suggest that its possible origin is the relaxation of polydispersity fluctuations. The diffusion coefficient associated with this mode is then the self-diffusion D _s of the droplets. It was found that D _c and D _s had opposite volume fractions of oil plus surfactants (ϕ) dependence and a common limiting value D ₀ for ϕ=0. Average hydrodynamic radius (R _h=10.5 nm) of droplets was calculated from D ₀. R _h is supposed to compose the inner core, a surfactant film including possible solvent molecules, which migrate with the droplet. The concentration dependence of diffusion coefficients reflects the effect of hard sphere and the supplementary repulsive interactions which arises due to loss of entropy, when absorbed chains of surfactant intermingle on the close approach of the two droplets. This mechanism could also explain the observed stability of our systems. The estimated extent of polydispersity is 0.22 from the amplitude of slower decaying mode. The polydispersity in microemulsion systems is dynamic in origin. Results indicate that the time scale for local polydispersity fluctuations is at least three orders of magnitude longer than the estimated time between droplet collisions.     

Dilution method study on the interfacial composition, thermodynamic properties and structural parameters of W/O microemulsions stabilized by 1-pentanol and surfactants in absence and presence of sodium chloride

The phase behaviors, interfacial composition, thermodynamic properties and structural characteristics of water-in-oil microemulsions under varied molar ratio of water to surfactant (omega) at 303 K and also by varying temperatures at a fixed omega(=40) by mixing with 1-pentanol and decane or dodecane in absence and presence of sodium chloride have been studied by the method of dilution. The surfactants used were cetyl pyridinium chloride (CPC), sodium dodecyl sulfate (SDS) and polyoxyethylene (23) lauryl ether (Brij-35). The compositions of 1-pentanol and the surfactant at the interfacial region, the distribution of 1-pentanol between the interfacial region and the continuous oil phase, and the effective packing parameter (P(eff)) at the threshold level of stability have been estimated. The thermodynamics of transfer of 1-pentanol from the continuous oil phase to the interface have been evaluated. The structural parameters viz. radii of the droplet (R(e)) and the waterpool (R(w)), effective thickness of the interfacial layer (d(I)), average aggregation numbers of surfactants (N (s)) and the cosurfactant (1-pentanol) (N (a)) and the number of droplets (N(d)) have also been estimated. The prospect of using these w/o microemulsions for the synthesis of nanoparticles with small size, have been discussed in the light of the radii of the droplet, and waterpool, the extent of variation of effective thickness of the droplet under varied molar ratio of water to surfactant and temperature. An attempt has been made to rationalize the results in a comprehensive manner.     

Nonionic surfactants with linear and branched hydrocarbon tails: compositional analysis, phase behavior, and film properties in bicontinuous microemulsions

Nonionic alcohol ethoxylates are widely used as surfactants in many different applications. They are available in a large number of structural varieties as technical grade products. This variety is mainly based on the use of different alcohols, which can be linear or branched and contain primary, secondary, or tertiary OH groups. Technical grade products are poorly defined as they are composed of alcohol mixtures being different in chain length and structure. On the other hand, monodisperse alcohol ethoxylates are commercially available; however, these surfactants exist only with primary and linear alcohols. In the field of microemulsion research the monodisperse alcohol ethoxylates are widely used. The phase behavior and film properties of these surfactants were studied intensively with respect to the size of the hydrophilic and hydrophobic moieties. Due to the lack of appropriate model surfactants until now, there is little information on how the structure of the hydrocarbon tail influences the microemulsion behavior. To examine structural influences, we synthesized a series of surfactants with the composition C10E5 and having different linear and branched hydrocarbon tails. The surfactants were monodisperse with respect to the hydrocarbon tail but polydisperse with respect to the ethoxylation degree. However, a detailed characterization showed that they were similar concerning the average ethoxylation degree and EO chain length distribution. The phase behavior was investigated for bicontinuous microemulsions, and the film properties were analyzed by small-angle neutron scattering (SANS). Our results show that the structure of the hydrocarbon tail strongly influences the microemulsion behavior. The most efficient surfactant is obtained if the hydrocarbon tail is linear and the hydrophilic group is attached in the C-1 position. Surfactants having the hydrophilic group bound to the C-2 or C-4 position or which contain a branched hydrocarbon tail are less efficient and exhibit in most cases visibly lower phase inversion temperatures. Both the efficiency and temperature behavior mainly can be explained on the basis of increased bulkiness of the branched structures compared to the fully linear version. The phase behavior results are largely confirmed by the SANS investigations. Those results show that the fully linear surfactant exhibits the most rigid interfacial film. In additional experiments this surfactant was compared with its monodisperse analogue. According to the phase diagrams, the surfactant having the polydisperse hydrophilic moiety is drastically more efficient although the film stiffnesses are almost identical.     

FT-IR studies on the conformation and effective head-group area of AOT molecules in W/O microemulsions

Using Fourier transform infrared(FT-IR) spectroscopy technique, the carbonyl stretching vibration bands of AOT in sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/isooctane/water reverse (W/O) microemulsions system have been investigated by least square curve fitting. The results indicate that an asymmetric adsorbed peak of carbonyl stretching vibration of AOT molecule is situated in (1739 ± 1) and (1725 ± 2) cm-1. The two peaks correspond to different carbonyls in gauche conformation and trans conformation of AOT molecules, respectively. With different water contents (W0), the variations of peak intensity ratio (/= l1739/l1725) reflect the change of the ratio for the two conformation populations and the variations of the effective head-group area of AOT molecule have relations to the ratio of two conformation populations.     

Synthesis,acid-base properties,and interphase distribution of new aminoalkylphosphates and -phosphonates

Using the Todd-Atherton reaction, we synthesized new ω-aminoalkylphosphates differing by location of phosphate amino group in the molecule. For these compounds and for α-, β-, and γ-amino-phosphonates of similar structure ionization constants of phosphate and phosphonate groups were determined which turned out to be similar. In all the cases the increase in basicity of amine centers was shown with the growing distance between them and the phosphorus-containing substituent. By the method of two-phase potentiometric titration the constants of partition between water and a series of organic solvents were measured. The data obtained are interpreted in the framework of additive Leo-Hansch model that allows estimation of the increments of phosphate and amidophosphate groups. These data can be applied to the study of biological activity and extraction properties of the compound of such class.     

Variation of surface properties and textural features of spinel ZnAl2O4 and perovskite LaMnO3 nanoparticles prepared via CTAB-butanol-octane-nitrate salt microemulsions in the reverse and bicontinuous states

Two binary oxides, a spinel, ZnAl2O4, and a typical perovskite, LaMnO3, have been prepared via CTAB-1-butanol-n-octane-nitrate salt microemulsion in the reverse and bicontinuous states. The exact point of the reverse and bicontinuous states of the microemulsion used in the synthesis was determined by conductivity experiments. The materials obtained after heating at 800 degrees C were characterized by XRD analysis for their crystal structure, N2 porosimetry for their surface area and porosity, and SEM and TEM photography for their texture. The ZnAl2O4 spinel obtained via the reverse microemulsion appears in SEM in a more fragmented form and with a higher specific surface area (143.7 m(2)g(-1)), compared to the corresponding solid prepared via the bicontinuous microemulsion, which appears more robust with lower surface area (126.7 m(2)g(-1)). Nevertheless both materials reveal in TEM a sponge-like structure. The perovskite materials LaMnO3 prepared via the reverse microemulsion showed in SEM a peculiar doughnut-like texture, each doughnut-like secondary particle having a diameter of 2 microm. The corresponding sample developed via the bicontinuous microemulsion showed in SEM uniform secondary particles of size approximately 0.2 microm. Both perovskite samples LaMnO3 appear well crystallized with relative low surface areas, 23.7 m(2)g(-1) for the reverse sample and 10.9 m(2)g(-1) for the bicontinuous one. The TEM photographs reveal that both of them, of reversed and bicontinuous origin, are made up of primary nanoparticles in the size range 40-100 nm. In SEM those materials showed a different secondary structure.     

ESR study on the stability of W/O gel-emulsions

W/O gel-emulsions (high-internal-phase-volume-ratio emulsions) form in water (or brine) /tetraethyleneglycol dodecyl ether/heptane system above the HLB (hydrophile-lipophile balance) temperature of the system. A salt, which largely decreases cloud temperature in a water-nonionic surfactant system, makes the surfactant film rigid and the gel-emulsions hence become very stable. The effect of aded salt on the apparent order parameter S, and the isotropic hyperfine splitting constant a _N in gel-emulsions was determined by the ESR spin probe method using 5-doxyl stearic acid as the spin probe. The apparent order parameter S, and the isotropic hyperfine splitting constant a _N increase with increasing salinity in Na₂SO₄, CaCl₂, and NaCl systems. It is considered that the surfactant molecules are tightly packed in these systems and this tendency is highly related to the stability of gel-emulsions. The salt dehydrates the hydrophilic moiety of surfactant and hence the lateral interactions of surfactant molecular layer at the water-oil interface increases.The observed difference in the apparent order parameter between the ordinary emulsions and the gel-emulsions suggests that most of the surfactant molecules are adsorbed at the oil-water interface (the surface of the water droplet) in gel-emulsions.