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Many organic substances in natural waters are amphipathic; i.e., they contain both a hydrophobic and a hydrophilic moiety. To assess the adsorptive behavior of such substances and to appreciate the factors that influence the distribution between particles and water, we need to understand how these substances interact with polar or nonpolar interfaces. To gain such an understanding, the adsorption of fatty acids of various chain lengths has been investigated on two model surfaces: (1) on a Hg electrode and (2) on 6-A1203 particles. While hydrophobic expulsion dominates the adsorption on the nonpolar Hg surface, the adsorption of short-chain fatty acids (<8 carbon atoms) on the surface of hydrous A1203 is characterized by specific coordinative interaction. Phase-selective polarography is a convenient tool to measure the extent and kinetics of adsorption of those amphipathic substances that become adsorbed primarily because of hydrophobic expulsion.

The electrochemical method based on the measurement of capacitance (out of phase signal) at the hanging mercury drop electrode by using alternating-current voltammetry has been employed for the analysis of surface active substances in seawater. Such determination is of a collective parameter, i.e., all present adsorbable substances take part in adsorption on the electrode in a competitive manner. Calibration problems, sample treatment and comparison of naturally occurring surface active substances with different model substances are discussed.

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