Viscosity stability of O/W emulsion containing α-gel through an ionic-complex system.

Although many active ingredients are used in cosmetic products for moisturizing and whitening the skin, they are often electrolytes, and the stabilities of oil in water (O/W) type emulsion formulae containing electrolytes are generally difficult to control. To solve this problem, formulae containing an α-crystalline phase (α-gel) consisting of water, higher alcohols, and anionic surfactants such as sodium N-stearoyl-N-methyl-taurate (SMT) have been developed. However, in spite of their excellent salt tolerance, these formulae have poor viscosity stability under non-electrolyte conditions, and the viscosity decreases over time. To obtain adequate viscosity stability, the required electrolyte concentration is approximately 1wt%, which is somewhat high for cosmetic applications. To replace the salts, distearyl dimethyl ammonium chloride (DSAC), a cationic surfactant, with an opposite electric charge to SMT, was used in O/W emulsion formulae, resulting in improved viscosity stability at a lower concentration than that of salts. The stabilization mechanism with DSAC was found to be different from that of salts.

α-gel prepared in sodium methyl stearoyl taurate/behenyl alcohol/water system-characterization of structural changes with water concentration.

In this study, the changes in the structural and physicochemical properties of an α-crystalline phase (often called an "α-gel") were assessed in a sodium methyl stearoyl taurate (SMT)/behenyl alcohol/water system. The α-gels were characterized focusing on the effects of the alcohol/surfactant ratio and water concentration. Water molecules solubilized in the interlayer of the α-crystalline phase resulting in expanded interlayer spacing. Beyond the solubilization limit of 85 %, water molecules were trapped in the matrix of the α-crystalline phase in non-equilibrium (i.e., two phases). Accordingly, different self-diffusion coefficients for the solubilized and trapped water molecules were measured using a Fourier transform pulsed gradient spin echo technique to monitor the ¹H NMR spectra. It was concluded that the two self-diffusion coefficients correspond to the water solubilized in the interlayer, i.e., "slow water," and trapped in the matrix of the α-crystalline phase, i.e., "fast water."

Dynamic properties of microemulsions in the single-phase channels.

We have studied the dynamic and rheological properties in the single-phase channels of a microemulsion system with a mixed anionic/nonionic surfactant system and decane from the aqueous to the oil phase. One isotropic channel, called the "upper" channel, begins at the L(3) phase (sponge-like phase) of the binary surfactant mixture on the water side and passes with a shallow minimum for the surfactant composition to the oil side. The other "lower" single-phase channel begins at the micellar L(1) phase and ends in the middle of the phase diagram. Both isotropic channels are separated by a huge anisotropic single phase L(α) channel that reaches from the water side to 90% of oil in the solvent mixture. The structural relaxation time of the viscous fluids could be measured with electric birefringence (EB) measurements, where a signal is caused by the deformation of the internal nanostructure of the fluids by an electric field. For the L(3) phase, the EB signal can be fitted with a single time constant. With increasing oil in the upper channel, the main structural relaxation time passes over a maximum and correlates with the viscosity. Obviously, this time constant controls the viscosity of the fluid (η(o) = G'·τ). It is remarkable that the longest structural relaxation time increases three decades, and the viscosity increases two decades when 10% of oil is solubilized into the L(3) phase. Conductivity data imply that the fluid in the upper channel has a bicontinuous structure from the L(3) phase to the microemulsion with only 10% oil. In this oil range, the conductivity decreases three decades, and the electric birefringence signals are complicated because of a superposition of up to three processes. For higher oil ratios, the structure obviously changes to a HIPE (high internal phase emulsion) structure with water droplets in the oil matrix.

Development of novel cosmetic base using sterol surfactant. II. Solubilizing of sparingly soluble ultraviolet ray absorbers.

Previous studies have reported that O/W emulsion prepared using a surfactant with phytosterol as the hydrophobic moiety exhibited unique morphology; a lamellar structure was present on the surface of the emulsified particles. It is suggested that such a unique self-organized structure was due to the large and bulky planar structure of the sterol. On the other hand, sparingly soluble compounds including ultraviolet ray absorbers and medicines (e.g., indomethacine and finasteride) have been used after they are dissolved mainly in polar oils. However, it is very difficult to dissolve them in bases that contain small amounts of oil components such as lotions. Moreover, many of these sparingly soluble compounds have planar structures such as aromatic rings and are easy to crystallize in polar oil. In this study, sterol surfactants were considered suitable for solubilizing sparingly soluble compounds, since they have a bulky planar structure in their molecules. On this basis, the solubilization of ultraviolet ray absorbers using sterol surfactants was investigated. Methods to solubilize ultraviolet ray absorbers stably and effectively by using a surfactant that had a phytosterol structure have been clarified. Further, the following features were also suggested: (1) the microemulsion of phytosterol surfactant is different from that of other surfactants and (2) a rigid core that has solubilized compounds between the hydrophobic moieties was considered; further, the core was surrounded by a polyoxyethylene chain that prevented the self-aggregation. Analysis using NMR measurements suggested that (1) the polyoxyethylene/polyoxypropylene random copolymer dimethyl ether squeezed in a narrow gap between the hydrophobic moieties of the surfactant, and (2) this eventually increased the solubilized amount of an ultraviolet ray absorber.

Development of novel cosmetic base using sterol surfactant. I. Preparation of novel emulsified particles with sterol surfactant+.

We have developed various kinds of ultrafine emulsifying methods using random copolymer of polyoxyethylene (POE)/polyoxypropylene (POP) dimethyl ether [EPDME]. Among ultrafine emulsions made by these methods, it was revealed that an O/W type emulsion, which had prepared with EPDME, sterol surfactant, and polar oils, had a unique structure that had a lamellar structure on the surface of emulsified particles. To clarify the character of the particles and the mechanism of the emulsification, investigation using small angle X-ray scattering (SAXS) measurement and the phase diagram of the emulsion system was done. FF-TEM observation indicated that a few lamellar layers were deposited from the oily ingredients on the surface of the emulsified particle. It was also presumed that the lamellar structure was formed with sterol surfactant and polar oil. The phase diagram analysis suggested that EPDME could form emulsified particles having lamellar structure on the surface of the particle with hydrophilic sterol surfactant in polar oils.