pH-Dependent Aggregation Behavior of a Sugar-Amine Gemini Surfactant in Water: Vesicles, Micelles, and Monolayers of Hexane-1,6-bis(hexadecyl-1′-deoxyglucitylamine)
✍ Scribed by Mark Bergsma; Matthew L. Fielden; Jan B.F.N. Engberts
- Publisher
- Elsevier Science
- Year
- 2001
- Tongue
- English
- Weight
- 99 KB
- Volume
- 243
- Category
- Article
- ISSN
- 0021-9797
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✦ Synopsis
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 (A h ) 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 A h of ca. 69 Å2 . 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.