Clouding of a nonionic surfactant: The effect of added surfactants on the cloud point

Abstract

The nonionic surfactant Triton X-100 is soluble in water giving only one isotropic solution phase at about 32°C. The cloud point (CP) of the aqueous system has been measured over the entire concentration range and the lower consolute temperature at about 0.2 wt% of Triton X-100 solution is obtained. The CP, at different concentrations of Triton X-100 solutions, is also determined in the presence of zwitterionic, catanionic, and ionic surfactants. At a low concentration (1 wt%) of nonionic surfactant, lecithin drastically depresses the CP, but the degree of CP depression is found to diminish with increased concentration of Triton X-100, and an increase of the CP in the presence of lecithin is observed with about 24 wt% of Triton X-100. On the other hand, the addition of decanol registers only a decrease of the CP for the concentrations of Triton X-100 employed (1–24 wt%). No significant change in the CP of the nonionic surfactant solution is observed by adding uncharged catanionic surfactants (e.g., dodecylammonium dodecanoate); the same result is obtained in the presence of simple electrolytes. The presence of small amounts of ionic surfactants (N ≈ 0.01–0.03, where N is the molar ratio of ionic surfactant to Triton X-100) in the system leads to a substantial increase in the CP of 1 wt% of the Triton X-100 solution, but the increase is relatively less with cationic (cetyltrimethylammonium with bromide or sulfate counterions) than with anionic (dodecyl sulfate with sodium or magnesium counterions) surfactants. At N = 0.01–0.02 the rise of the CP is almost linear with ionic surfactant concentration while at N ⩾ 0.02 the CP increases slowly by further addition of anionic or cationic surfactants as well as by changing the valency of the counterion. The valency of the counterion affects the CP of the nonionic surfactant system differently for different ionic surfactants. The cloud point rise in the presence of sodium dodecyl sulfate (the degree of counterion binding, β ≈ 0.55) is higher than that with magnesium dodecyl sulfate (β ≈ 0.8), whereas approximately identical rises in the CP are observed with cetyltrimethylammonium bromide (β ≈ 0.7) and cetyltrimethylammonium sulfate (β ≈ 0.8). The clouding phenomenon of the aqueous Triton X-100 system in the presence of various additives is discussed in the light of inter- and intramicellar interactions.