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 (γ) 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 1×10−3 M. This value is high, as γ is at the cmc (57 mN m−1). The area per headgroup (Ah) extracted from the slope of the curve of γ vs concentration below the cmc is 109 Å2. 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 Ah of ca. 69 Å2. The dependence of Ah 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.