Colloidal Pd prepd. in a water-org. microemulsion in the presence of the cationic surfactant aliquat 336 (trioctylmethylammonium chloride) is useful for the transfer hydrogenolysis of bromotoluene with sodium formate. The Pd catalytic activity is strongly dependent on the particle dimension, being highest for the smallest particles. The size of the particles is detd. by the prepn. conditions: coarser particles are formed at higher water contents and lower temps. The reaction obeys the mechanism of micellar catalysis. [on SciFinder(R)]

The Winsor IV microemulsion system composed of octaethylene glycol monododecyl ether [C12(EO)8]/dodecane + pentanol (1:1 by wt.)/water has been investigated at const. temp. using small-angle x-ray scattering and elec. cond. measurements. The results are interpreted in terms of structural evolution of the mol. aggregates as a function of the stepwise addn. of water or (oil + alc.). The size and shape of a variety of microstructures are described: small, spherical micelles near the water corner, and hexagonal and lamellar mesophases, which are oil- and alc.-poor. Simple multishell models provide some insight into how diln. with water or swelling with (oil + alc.) influences the overall symmetry of the aggregates, pentanol and dodecane partitioning, surfactant headgroup conformation, and the contribution of pentanol to oil solubilization. The previously identified "local" lamellar structure, which is a surfactant-rich and (oil + alc.)-rich intermediate state between the W/O and O/W regions, is characterized here as a type of ordered, but highly obstructed, bicontinuous microemulsion. [on SciFinder(R)]

The dielec. spectroscopy method (DS) has become a useful technique for the investigation of the structural and dynamic features of the components of both microemulsions and microdroplets over a wide temp. and frequency range. The dielec. parameters obtained by DS det. the geometry of the assocn. structures as well as the overall picture of the dynamics of the different polar groups. aggregates, and assocn. structures representing dipole modes. In this paper the results of a comprehensive study of the different types of microemulsions (ionic and nonionic) DS are presented. The static and dynamic dielec. properties of microemulsions a a function of temp., frequency and concn. of water, oil, surfactant and cosurfactant are considered. The dielec. properties have been investigated in the frequency range 105-1010 Hz using time domain dielec. spectroscopy (TDDS) and over a broad temp. interval enabling us to cover all the man dynamic processes occurring in such systems. The data treatment for the dynamic behavior of the microemulsions was carried out in the time domain in terms of dipole correlation functions and in the frequency domain in terms of complex dielec. permittivity. The correlation functions of the investigated systems exhibit complex nonexponential relaxation behavior, which must be deconvolute into normal modes and represented as a sum of the simple exponential, exp(-t/$\tau$), and nonexponential terms, exp[-(t/$\tau$)v]. The parameter $ν$ characterizes the shape of the relaxation function and the cross-correlation effects, and describes the morphol. of the system. The mol. mechanisms responsible for dielec. polarization in microemulsion of different nature at discussed. Knowledge of the amt. of hydrate water and co-surfactant in the interface can be obtained for nonionic microemulsions. In the case of ionic microemulsions, TDDS is a powerful technique for monitoring the organization of clusters and for investigation of relaxation processes involving rearrangement and movement of the droplets forming the clusters. [on SciFinder(R)]

Self-diffusion of all components in two different microemulsions has been studied by the Fourier-transform pulsed-gradient spin echo 1H NMR technique. The anal. of hydrodynamic and direct droplet-droplet interactions in microemulsions allows the simultaneous application of the Stokes-Einstein equation for surfactant and oil. The result of this anal. is a simple relation connecting self-diffusion coeffs. of components and the size of droplets. It is shown that because of partial solubilization of water in oil the droplet radius can be significantly different from the expected radius. [on SciFinder(R)]

Phospholipase A2 (PLA2) was used to hydrolyze phosphatidylcholine (PC) in microemulsions. Phase diagrams were constructed for mixt. of alcs. (C2-C6), medium chain triglycerides (tricaprylin (TC) or vegetable oils), PC and water, and areas corresponding to isotropic systems were identified. The PC hydrolysis was carried out with high yields of lysophosphatidylcholine at various PC/(TC+alc.) compns. within the areas of isotropic systems at increasing amts. of solubilized water. The initial reaction rates depended on the aggregation state of the PC (size and nature of microemulsion structures), as well as on temp. and mode of calcium addn. At low enzyme concns., hydrolysis was preceded by a lag phase followed by an abrupt increase in rate. By contrast, no latency was obsd. at higher PLA2/PC ratios and the rate was significantly higher. The easy access of the enzyme to the substrate in the curved phospholipid-contg. microemulsion particles facilitates activation of the enzyme and pushes the reaction to completion. Hence, these microemulsions can serve as microreactors for high yield enzymic hydrolysis. [on SciFinder(R)]

Fenugreek gum was extd. from fenugreek seeds and evaluated for its surface activity. This unique galactomannan has a mannose backbone grafted with galactose units at an av. ratio of one. The purified fenugreek gum was found to reduce surface tension to values lower than guar gum (42 and 55 mN/m, resp.). The interfacial activity was surprisingly better than other galactomannans (interfacial tension was reduced to 2 mN/m in vegetable oils) which led to the formation of oil-in-water emulsions with with small droplet size (2-3 $μ$m) and long-term stability. The fenugreek gum was found to adsorb (or 'ppt.') on the oil interface forming a relatively thick interfacial film. The emulsions are more stable than any equiv. emulsions stabilized by other galactomannan gums. No flocculation was obsd. in emulsions stabilized with fenugreek concns. sufficient for a good coverage of the oil interface as expressed by the coverage index (Rc) correlated to the gum/oil wt. ratio (Rc\textgreater12). [on SciFinder(R)]

A review with 57 refs. Multiple emulsions are complex systems of emulsion within an emulsion, where the droplets of the dispersed phase themselves contain even smaller dispersed droplets. The two major types are water-in-oil-in-water (W/O/W) and oil-in-water-in-oil (O/W/O) double emulsions. Double emulsions are unstable thermodn. systems, which tend to cream, flocculate and coalesce. Much effort has been made to develop methods of 'better sealing' in the inner and outer interfaces and to reduce the droplet size of the emulsions. Dissolved or dispersed compds. in the inner phase of double emulsions tend to be released to the outer phase primarily via a reverse micellar transport (which is diffusion-controlled) or through the thin lamellae of a surfactant phase migrating to the outer phase via the outer interface. It is difficult to control the release of the solutes unless macromols. are used to improve lamellar stabilization (steric stabilization) forming a rigid surfactant-polymer complex at the interfaces. The review describes recent achievements in the use of BSA, casein, gelatin and other naturally occurring macromols. together with monomeric-amphiphilic mols. for possible future applications in food systems. [on SciFinder(R)]

Multiple emulsions are complex systems, termed "emulsions of emulsions", i.e. the droplets of the dispersed phase contain even smaller dispersed droplets themselves. Each dispersed globule in the double emulsion is sepd. from the aq. phase by a layer of oil-phase compartments. Double emulsions have significant potential in many applications since, at least in theory, they can serve as an entrapping reservoir for active ingredients that can be released from the inner phase to the outer phase by a controlled and a sustained mechanism. Many of the potential applications are in pharmaceuticals. In practice, double emulsions are thermodynamically unstable systems with a strong tendency for coalescence, flocculation and creaming. Most double emulsions consist of relatively large droplets, cannot withstand storage regimes and have a strong tendency to release the entrapped matter in an uncontrolled manner. Much work has been devoted in the last decade in order to the improve the stability of the multiple emulsions and the control of the release rates of the addenda. The most recent achievements are: Use of polymeric emulsifiers to improve interface coverage and to better anchor into the dispersed phases; Reduce droplets size by improving methods of formation: improved understanding of the release mechanisms and use of various additives to control the release via the reverse micellar mechanism. 35 Refs. [on SciFinder(R)]

Glutamic acid can crystallize in two polymorphic structures depending on the crystn. regime. The study demonstrates an efficient method to preferentially crystallize the nonstable polymorphic structure (the $\alpha$-form) in the presence of surface active agents. The rate of transformation depends on the rate of growth of $\beta$ and not on the rate of dissoln. of $\alpha$. The growth rate of $\beta$ was a function of the supersatn. of the solute in soln. The transformation could be inhibited by the addn. of surfactants. The surfactants are capable of adsorbing preferentially to the $\alpha$-growing crystals and soln. mediating (retarding) the transformation of the $\alpha$- to the $\beta$-form. Probably the surfactant nature and steric considerations were important for the inhibition of both nucleation and growth of the $\beta$-polymorph. A Langmuir approach indicated that the kinetic parameter was related to the vol. of surfactant adsorbed at the crystal surface. No changes in crystal morphol. were obsd., indicating that adsorption was not specific to any crystal face. Different mechanisms of surfactant adsorption were suggested: adsorption of single mols. at low concns. of surfactant and formation of hemimicelles at higher concns. [on SciFinder(R)]

The effects of free and bound water on the dielec. properties of microemulsions prepd. with the non-ionic surfactant Brij 97 were investigated. For the calcn. of the relative amts. of free and bound water, a simple multi-layered dielec. model was applied. The model considers the microemulsion as a sum of three sep. layers consisting of a "dry" microemulsion, free water and bound water, resp. The method is based on dielec. measurements of microemulsions at a frequency for which the dielec. permittivity and the dielec. losses for bound and free water have significantly different magnitudes. The dielec. contribution of free water to the relaxation spectrum of the microemulsion can be obtained, allowing both fractions to be calcd. In contrast to the traditional procedures used for calcn. of bound water from the relaxation spectrum, this method does not involve analyses of overlapping relaxation processes. [on SciFinder(R)]

Proton NMR measurements were conducted on a U-type model quaternary nonionic microemulsion contg. varying amts. of water and having a fixed wt. ratio of surfactant [octaethylene glycol mono-n-dodecyl ether; C12(EO)8], cosurfactant (pentanol), and oil (dodecane). Chem. shift and T1 relaxation time data were used to obtain information about the microemulsion properties and structure. A slow exchange was obsd. between the hydroxyls of water, C12(EO)8, and pentanol in microemulsions contg. up to 10 wt% water. This is precedence of the hydroxyl-water interaction over hydration of the ethylene oxide (EO) groups of the surfactant. The fraction of bound water was calcd. from the chem. shift and T1 data. For the model system, chem. shift measurements indicated an inversion from W/O to O/W microemulsion at about 55-60 wt% water, in agreement with the microviscosity values evaluated from T1 data: 1.1 cP (55 wt% water) compared to 4.7 cP (30 wt% water) and 12.7 cP (10.7 wt% water). The irregularities in the monotonic dependence of the chem. shifts on water content in the microemulsion at 18 and 55 wt% water were interpreted to possibly reflect structural changes that took place in soln. [on SciFinder(R)]