A review with 58 refs. discusses formation and stability of double emulsions employing polymeric and macromol. surfactants. Controlled release and transport mechanisms are discussed. [on SciFinder(R)]

A review with 75 refs. Research work and progress on several major issues related to double emulsions have taken place recently. Such developments include the intrinsic thermodn. instability derived from the droplets size of the double emulsion; how the release of active ingredients entrapped within the inner phase can be prolonged and controlled; new advanced methods for studying the structure and nature of double emulsion droplets and their interfaces; new possible applications of double emulsions for sustained and prolonged release of active ingredients in cosmetics, food, agriculture and pharmaceuticals; double emulsions as intermediate systems in the prepn. of microspheres or nanospheres; and the scope and limitations of possible new applications of multicompartment microspheres prepd. from double emulsions. [on SciFinder(R)]

A system of coating comprises applying a primary coating having a surface energy 22-28 dynes/cm2 of polymeric material having nonstick and hardness properties and applying a top coating having surface energy ∼18-21 dynes/cm2 of a silicon polymer, and in between a bi- or polyfunctional org. compd. to wet the surface of the primary coating and chem. or phys. bond and couple the primary coating and top coating together. The coated substrates are resistant to marking by graffiti, to marine fouling organisms, and adhesion by ice. Steel substrates were coated with a primary coating of epoxy-siloxane emulsion compn., and top coated with poly(di-Me siloxane) contg. Tween 80 coupling agent. [on SciFinder(R)]

The adsorption of diisooctyl sulfosuccinate (AOT) from high ionic strength soln. onto well-defined calcium oxalate monohydrate (COM) and dihydrate (COD) crystals was studied. Adsorption at the COM/soln. interface is characterized by a two-step (LS type) isotherm, starting at low equil. concns. (5 mg dm-3). At ceq = 15-30 mg dm-3 it reaches a plateau which is followed by a relatively steep inflection (ceq = 30-50 mg dm-3) and a further slow increase of the adsorption as a function of increasing AOT concn. In suspensions without surfactant the particles were neg. charged. Upon adsorption an initial slight decrease of the neg. $\zeta$-potential, coinciding with the first plateau, occurred which was followed by a sharp increase in concordance with the increasing surface concn. of the surfactant. In contrast, adsorption onto COD is characterized by a sigmoid isotherm. It commenced at 14 mg dm-3 of AOT and increased abruptly up to a plateau. The max. adsorbed amt. was about half the max. amt. adsorbed on COM, the corresponding adsorption densities in mols. per square nanometer being 7.45 for COD and 14.22 for COM, resp. COD crystals suspended in electrolyte soln. without surfactant were almost uncharged, and the neg. $\zeta$-potential increased in concordance with AOT adsorption. The results are discussed in accordance with literature data and by considering the ionic structure of the different crystal faces. It is assumed that the first adsorption step in the COM/surfactant system is due to electrostatic interactions causing head-on adsorption of the surfactant mols. at high-energy sites, while in the second step a bilayer is formed. In the COD/surfactant system the hydration layers covering the COD crystal faces are shielding them from electrostatic interactions. Consequently AOT adsorption at the COD/soln. interface proceeds only through surface aggregation, resulting in a bilayer of intertwined surfactant mols. [on SciFinder(R)]

Time domain dielec. spectroscopy of reverse water/acrylamide/Aerosol-OT(AOT)/toluene microemulsions shows that percolation induced by increasing cosurfactant concn. (increasing cosurfactant chem. potential) obeys scaling above and below a percolation threshold. This scaling anal. suggests that the obsd. percolation is close to static percolation limits. Self-diffusion measurements derived from NMR pulsed-gradient spin-echo expts. reveal an increase in water proton diffusion above the percolation threshold. This increase is assigned to water transport through fractally chained assemblies of microemulsion droplets. The diffusion of water, cosurfactant, and surfactant (AOT) below threshold is modeled quant. taking into account the chem. partitioning equil. between the microemulsion droplets and the toluene continuous pseudophase. Above threshold, the apparent increasing water and cosurfactant partitioning into the toluene (continuous) pseudophase suggests facilitated transport through fractal aggregates. Ad dynamic partitioning model is used to est. the vol. of percolating fractal cluster, and yields an order parameter for water-in-oil to percolating cluster microstructural transitions. This same order parameter is also illustrated to derive from self-diffusion data wherein percolation and transformation to sponge phase microstructure are driven by increases in temp. and in disperse phase vol. fraction. For microstructural transitions driven by three different field variables, chem. potential, temp., and disperse phase vol. fraction, this order parameter shows that the onset of percolation corresponds to the onset of increasing water proton self-diffusion, and that the onset of increasing surfactant self-diffusion corresponds to the formation of bicontinuous microstructures and the onset of transformation to middle phase microemulsion. [on SciFinder(R)]

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)]

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)]

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)]

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)]

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)]