Analytical Sciences

Adsorption Behavior of Lauric Acid at Heptane/Water Interface as Studied by Second Harmonic Generation Spectroscopy and Interfacial Tensiometry

Akira YAMAGUCHI,* Tatsuya UCHIDA,** Kimihisa NOCHI,* Tomohisa YAMASHITA,* and Norio TERAMAE*

*Department of Chemistry, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578, Japan
**School of Life Science, Tokyo University of Pharmacy and Life Science, Hachioji 190-0392, Japan

Interfacial tensiometry and second harmonic generation (SHG) spectroscopy were applied to examine the adsorption behavior of lauric acid (LA) at a heptane/water interface. From interfacial tensiometry measurements, the adsorption kinetics of LA was revealed to be diffusion-controlled, and the adsorption constant of LA was estimated to be 9.6 × 10⁴ M^-1. The adsorption isotherms obtained by SHG measurements were analyzed by taking account of both the molecular orientation of LA at the interface and a surface electric field generated by the adsorbed LA layer. It was confirmed that the carboxylic groups of adsorbed LA molecules were well ordered at the heptane/water interface and the orientation of the carboxylate group was invariant during the adsorption process.

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