Surface potential studies of the thermal behaviour of insoluble carboxylic acid monolayers on solid substrates

Abstract

Surface potential changes of monolayers of several fatty acids stabilized by barium ions were studied as a function of sample temperature. Spread monolayers of stearic, arachidic and behenic acids on aqueous subphases containing 0.5 mM BaCl 2 were transferred to silicon wafers. Upon heating, the mean dipole orientation of the monolayer molecules remains nearly unchanged up to a temperature near the melting point of the free carboxylic acid, followed by an abrupt change of dipole orientation in a narrow temperature range up to the melting point. The continuous change of the mean dipole orientation during recooling indicates that the initially ordered monolayer structure cannot be recovered. This result and the coincidence of the drastic loss of monolayer orientation with the melting temperature of the adequate free carboxylic acid suggest a main melting transition. This indicates that the monolayer deposited on the silicon wafer consists of free fatty acid molecules and not, as expected, bivalent fatty acid salt molecules.