The effect of added salts (NaCl, Na2SO4, and NaSCN) or polyols (glycerin, 1,3-butanediol, ethylene glycol, and poly(ethylene glycol) 400) on liquid crystalline structures of polyoxyethylene-type nonionic surfactants was investigated by means of small-angle X-ray scattering (SAXS). The effective cross-sectional areas of the lipophilic parts of aggregates, a s, in both hexagonal and lamellar phases decreases upon addition of salts, which lower a cloud point in a dilute aqueous nonionic surfactant solutions. On the other hand, if added salt raises the cloud point, the a s increases. The similar results were obtained in the case of adding polyols. Since the a s mainly depends on the EO-chain length, the above results are the direct evidence that the hydration or dehydration of the EO-chain is affected by these additives which causes the change in the a s in surfactant self-organizing structures. The effect of polyols on the three-phase behavior in water/heptaethylene glycol dodecyl ether (C12EO7)/heptane system was also investigated. Since 1,3-butanediol largely affects the HLB temperature, a considerable amount of the 1,3-butanediol is incorporated in the surfactant aggregates whereas the three-phase temperature is almost unchanged in ethylene glycol and poly(ethylene glycol) 400 systems. Hence, it is considered that the a s value in 1,3-butanediol system is less accurate than those in ethylene glycol and poly(ethylene glycol) 400 systems.

Phase behavior of diglycerol fatty acid esters (Qn-D, where n represents the carbon number in the alkyl chain length of amphiphile, n = 10-16) were investigated in different nonpolar oils, liquid paraffin (LP70), squalane, and squalene. There is surfactant solid at lower temperature, and the surfactant solid does not swell in oil, and the melting temperature is almost constant in a wide range of compositions. In all of the systems, a lamellar liquid crystal (L(alpha)) is formed in a concentrated region at a temperature between the solid melting temperature and the isotropic two- or single-phase regions. In the dilute regions, reverse vesicles are formed in L(alpha) + O regions. There are two liquid-phase regions above the L(alpha) present region. This two-phase boundary corresponds to the cloud-point curve of nonionic surfactant aqueous solutions. However, instead of being less soluble in water at high temperature for the cloud point, the surfactant becomes more soluble in the organic solvents at high temperature. Namely, the effect of temperature on the solubility is opposite to the clouding phenomenon. When the hydrocarbon chain of the diglycerol surfactant decreases, the two-phase region becomes wider. In the case of a fixed surfactant, the surfactant is most miscible with squalene (narrowest two-phase regions) and the order of dissolutions tendency is squalene > LP70 > squalane. These results show that the hydrophilic moiety (diglycerol group) is more insoluble in oil compared with that of a conventional poly(oxyethylene)-type nonionic surfactant. Formation of reversed rodlike micelles was confirmed by SAXS scattering curve. When the hydrocarbon chain of surfactant is short, the micellar size becomes larger. In a fixed surfactant system, the reverse micellar size increases by changing oil from squalene to LP70. A small amount of water induces a dramatic elongation of reverse micelles.

We have studied nonaqueous phase behavior and self-assemblies of monoglycerol fatty acid esters having different alkyl chain lengths in different nonpolar oils, namely, liquid paraffin (LP 70), squalane, and squalene. At lower temperatures, oil and solid surfactants do not mix at all compositions of mixing. Upon an increase in the temperature of the surfactant system, the solid melts to give isotropic single or two-liquid phases, depending on the nature of the oil and the surfactant. All monolaurin/oil systems form an isotropic single-phase liquid, but with a decreasing alkyl chain length of surfactant, they become less lipophilic and immiscible in oils. As a result, a two-phase domain is observed in the oil rich region of all monocaprylin/oil systems over a wide range of concentrations. Judging from the phase diagrams, the surfactants are the least miscible with squalane, and the order of miscibility tendency is squalene > LP 70 > squalane. With a further increase of temperature, the solubility of the surfactant in the oil increases, and the two-liquid phase transforms to an isotropic single phase. This phase transformation corresponds to the reverse of the cloud-point phenomenon observed in aqueous nonionic surfactant systems. Small-angle X-ray scattering (SAXS) measurements show the presence of reversed rodlike micelles in the isotropic single phase, and the length of the aggregates decreases with increasing temperature and increasing alkyl chain length of the surfactant. These results indicate a rod-sphere transformation with increasing lipophilicity of the surfactant and confirms the validity of Ninham's penetration model in the reversed system. An addition of a small amount of water dramatically enhances the elongation of the reverse micelles. Increasing the surfactant concentration or changing the oil from squalene to LP 70 also increases the length of the rodlike aggregates.