Masahiko Ishitobi

Oil‐Induced Structural Change of Wormlike Micelles in Sugar Surfactant Systems

Sucrose monofatty acid esters form spherical or short‐rod micelles in water. By combining the sugar surfactant with lipophilic monoglyceride or short‐EO‐chain, poly (oxyethylene) alkyl ether (CmEOn) one‐dimensional growth of micelles takes place, and the viscosity of the aqueous mixed nonionic surfactant solution dramatically increases due to the entanglement of elongated wormlike micelles. Upon addition of oil (decane or m‐xylene), the viscosity changes in the opposite way. With a slightly polar m‐xylene, the viscosity increases, but the viscosity is sharply decreased upon addition of the saturated hydrocarbon decane. It was confirmed by SAXS results that long wormlike micelles are more extended by m‐xylene because the m‐xylene molecules are solubilized in the surfactant palisade layer, whereas decane is solubilized in the core of micelles and makes an oil pool inside. Consequently, upon addition of decane, wormlike micelles become shortened and change to short fat micelles. The oil‐induced structural change in the micellar shape is highly related to the solubilized site of oil in the aggregates.

Effect of ionic surfactants on the phase behavior and structure of sucrose ester/water/oil systems.

The phase behavior and structure of sucrose ester/water/oil systems in the presence of long-chain cosurfactant (monolaurin) and small amounts of ionic surfactants was investigated by phase study and small angle X-ray scattering. In a water/sucrose ester/monolaurin/decane system at 27 degrees C, instead of a three-phase microemulsion, lamellar liquid crystals are formed in the dilute region. Unlike other systems in the presence of alcohol as cosurfactant, the HLB composition does not change with dilution, since monolaurin adsorbs almost completely in the interface. The addition of small amounts of ionic surfactant, regardless of the counterion, increases the solubilization of water in W/O microemulsions. The solubilization on oil in O/W microemulsions is not much affected, but structuring is induced and a viscous isotropic phase is formed. At high ionic surfactant concentrations, the single-phase microemulsion disappears and liquid crystals are favored.

Formation of cubic-phase microemulsions in sucrose alkanoate systems

Abstract The self-organized structure is changed from hexagonal to lamellar liquid crystals by mixing sucrose lipophilic multi-dodecanoate (L-595) with hydrophilic sucrose monododecanoate (L-1695) in water. Although no cubic phases are found in water, a discontinuous cubic phase (I 1 ) with oil-swollen micelles or a cubic-phase microemulsion is produced upon addition of squalane, n-hexadecane, n-decane, n-octane and n-heptane at the weight ratio of L-1695/L-595=1/1. The small-angle X-ray scattering spectrum and the phase behavior show that the micelles are packed in a face-centered cubic array in the I 1 phase. The single cubic-phase region shifts to higher oil content with the decrease in the molecular size of oil and finally, no cubic phases are found with m -xylene and cyclohexane as oil. Furthermore, from the analysis of compositions of the I 1 phases, it is suggested that the shape of the micelles becomes non-spherical in the short-chain alkane systems.