Article pubs.acs.org/Langmuir
α‑Gel Formation by Amino Acid-Based Gemini Surfactants Kenichi Sakai,*,† Kiyomi Ohno,† Kazuyuki Nomura,† Takeshi Endo,† Kazutami Sakamoto,†,‡ Hideki Sakai,† and Masahiko Abe*,† †
Department of Pure and Applied Chemistry in Faculty of Science and Technology and Research Institute for Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan ‡ Faculty of Pharmacy, Chiba Institute of Science, 15-8 Shiomi, Choshi, Chiba 288-0025, Japan S Supporting Information *
ABSTRACT: Ternary mixtures being composed of surfactant, long-chain alcohol, and water sometimes form a highly viscous lamellar gel with a hexagonal packing arrangement of their crystalline hydrocarbon chains. This molecular assembly is called “α-crystalline phase” or “α-gel”. In this study, we have characterized α-gels formed by the ternary mixtures of amino acid-based gemini surfactants, 1-hexadecanol (C16OH), and water. The surfactants used in this study were synthesized by reacting dodecanoylglutamic acid anhydride with alkyl diamines and abbreviated as 12-GsG-12 (s: the spacer chain length of 2, 5, and 8 methylene units). An amino acid-based monomeric surfactant, dodecanoylglutamic acid (12-Glu), was also used for comparison. At a fixed water concentration the melting point of the α-gel increased with increasing C16OH concentration, and then attained a saturation level at the critical mole ratio of 12-GsG-12/C16OH = 1/2 under the normalization by the number of hydrocarbon chains of the surfactants. This indicates that, to obtain the saturated α-gel, a lesser amount of C16OH is required for the gemini surfactants than for the monomeric one (the critical mole ratio of 12-Glu/C16OH = 1/3). Small- and wide-angle X-ray scattering measurements demonstrated an increase in the long-range d-spacing of the saturated α-gels in the order 12-Glu 12-G5G-12 > 12-G8G-12. The yield stress of each α-gel phase also decreases in the same order. These results are consistent with the static viscosity data shown in Figure 7 and confirm the experimental findings that highly viscous α-gels tend to be formed for 12-Glu and 12-GsG-12 with the short spacer unit.
4. CONCLUSIONS We have characterized the α-gels formed by the amino acidbased gemini surfactants (12-GsG-12) with the long-chain alcohol (C16OH) and water. The melting point of the α-gel increases with increasing C16OH content, and then attains a saturation level at the critical mole ratio of 12-GsG-12/C16OH = 1/2 under the normalization by the number of hydrocarbon chains of the surfactants. This indicates that a lesser amount of 7658
dx.doi.org/10.1021/la501186h | Langmuir 2014, 30, 7654−7659
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dx.doi.org/10.1021/la501186h | Langmuir 2014, 30, 7654−7659
