pH-dependent aggregation behavior of a sugar-amine gemini surfactant in water:: Vesicles, micelles, and monolayers of hexane-1,6-bis(hexadecyl-1′-deoxyglucitylamine)

The pH-dependent aggregation behavior of one representative of a recently described series of sugar-amine gemini surfactants has been investigated. The surface tension (gamma) and turbidity of hexane-1,6-bis(hexadecyl-1'-deoxyglucitylamine) drop steeply between pH 5.5 and 4.0, consistent with a vesicle-to-micelle transition. The critical micelle concentration (cmc) at low pH (3.0) was determined by surface tension measurements to be 1 x 10(-3) M. This value is high, as gamma is at the cmc (57 mN m(-1)). The area per headgroup (A(h)) extracted from the slope of the curve of gamma vs concentration below the cmc is 109 Angstrom (2). In an attempt to obtain a reasonable estimate of the headgroup area at higher pH, surface pressure vs area measurements were performed on a monolayer supported on pure water (pH 6), providing an A(h) of ca. 69 Angstrom (2). The dependence of A(h) on pH is consistent with the proposed vesicle-to-micelle transition. Measurements of the gel-to-liquid crystalline phase transition using differential scanning calorimetry at a range of pHs revealed a drop in both the phase transition temperature and the transition enthalpy with decreasing pH. The pH dependence of the aggregation behavior can thus be summarized as follows: (1) pH 7.5-5.5, bilayer vesicles; (2) pH 5.5-4.0, a "drop region" where aggregate morphology is sensitive to small changes in pH; and (3) pH < 4.0, micelles. (C) 2001 Academic Press.