Mixing Effect of Polyoxyethylene-Type Nonionic Surfactants on the Liquid Crystalline Structures

Abstract

An effective cross-sectional area per surfactant molecule at hydrophobic interfaces of aggregates, aS, in hexagonal (H1) and lamellar (Lα) liquid crystals was calculated in homogeneous and mixed polyoxyethylene dodecyl ether systems as a function of polyoxyethylene (EO) chain length by means of small-angle X-ray scattering. The aS increases with increasing the EO chain length. The aS in the mixed surfactant system is considerably smaller than that in the single surfactant system, even if the average EO chain length is the same. The reduction of aS is larger than that predicted by ideal mixing of the surfactants. Moreover, if the EO chain lengths of the surfactants are more separated, the aS is smaller. The shapes of surfactant self-organizing structures may be governed by the balance of the attractive and the repulsive forces acting at the hydrophobic interfaces of the aggregates. According to this consideration, the mixing effect of surfactants with the different EO chain lengths on the aS in the Lα phase was discussed. It is considered that the surfactant molecules are tightly packed in the aggregates since the reduction in repulsion force takes place in the excess EO chain part of the hydrophilic surfactant longer than the short EO chain of the lipophilic one. The lower surface tensions and the better stability of macroemulsions and the large solubilizing capacity of microemulsions result from the mixing effect.