Effect of added salts or polyols on the liquid crystalline structures of polyoxyethylene-type nonionic surfactants

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.