α‑Gel Formation by Amino Acid-Based Gemini\nSurfactants

Abstract

Ternary\nmixtures being composed of surfactant, long-chain alcohol,\nand water sometimes form a highly viscous lamellar gel with a hexagonal\npacking arrangement of their crystalline hydrocarbon chains. This\nmolecular assembly is called “α-crystalline phase”\nor “α-gel”. In this study, we have characterized\nα-gels formed by the ternary mixtures of amino acid-based gemini\nsurfactants, 1-hexadecanol (C16OH), and water. The surfactants used\nin this study were synthesized by reacting dodecanoylglutamic acid\nanhydride with alkyl diamines and abbreviated as 12-G<i>s</i>G-12 (<i>s</i>: the spacer chain length of 2, 5, and 8\nmethylene units). An amino acid-based monomeric surfactant, dodecanoylglutamic\nacid (12-Glu), was also used for comparison. At a fixed water concentration\nthe melting point of the α-gel increased with increasing C16OH\nconcentration, and then attained a saturation level at the critical\nmole ratio of 12-G<i>s</i>G-12/C16OH = 1/2 under the normalization\nby the number of hydrocarbon chains of the surfactants. This indicates\nthat, to obtain the saturated α-gel, a lesser amount of C16OH\nis required for the gemini surfactants than for the monomeric one\n(the critical mole ratio of 12-Glu/C16OH = 1/3). Small- and wide-angle\nX-ray scattering measurements demonstrated an increase in the long-range <i>d</i>-spacing of the saturated α-gels in the order 12-Glu\n<12-G8G-12 < 12-G5G-12 < 12-G2G-12. In the three gemini surfactant\nsystems, the decreased spacer chain length resulted in the increased\nmaximum viscosity and elastic modulus of the saturated α-gels\nat a given water concentration. This is caused by the decreased amount\nof excess water being present outside the α-gel structure (or\nthe increased amount of water incorporated between the surfactant–alcohol\nbilayers). To the best of our knowledge, this is the first report\nfocusing on the formation of α-gel in gemini surfactant systems.