PROPERTIES OF MONOLAYERS AT AIR-WATER INTERFACE .I. EFFECT OF SPREADING SOLVENT ON SURFACE PRESSURE OF OCTADECYLTRIMETHYLAMMONIUM BROMIDE

This is the first of a series of papers on a set of experiments designed to inquire into both the structure of the air-water interface and the mode of interaction of molecules which can reside there. The interplay of ionic, dipolar, and dispersion forces in two dimensions can be determined if sets of precise surface potentials or surface pressures are available, and here we describe the apparatus and technique required to obtain accurate surface pressure. A single valid surface pressure-area per molecule (π-A) curve is given for an insoluble cationic film, namely octadecyltrimethylammonium bromide (C18TAB), spread from pure water onto a substrate of 0.1 M sodium chloride. With this curve and results obtained from a crystal spreading method we show that old criteria for assessing the reliability of π-A results for an expanded film are invalid and that spreading solvents are a major source of error.