Contact angles during monolayer deposition

Abstract

Preliminary results are reported in a study of the contact angles and adhesional energy changes during the transfer of monolayers from the air-water interface to solid (hydrophobic glass) supports. The technique used is the vertical-plate method of Neumann, as modified for this application by Clint and Walker. In building up cadmium arachidate layers, it is found that the hydrophobic (immersion) adhesion energy depends slightly on pH and concomitant extent of soap formation in the film, suggesting that polar group interactions, while small, are not negligible. (The narrow pH range of condensation for cadmium arachidate requires the use of a stable buffer system such as acetate, rather than bicarbonate, for adequate pH control.) The acute contact angles observed during the withdrawal step in layer buildup change slowly after motion is stopped. While no quantitative interpretation is yet possible, it is suggested that this may reflect water and ion redistribution in the interfacial region; such redistribution would also contribute previously ignored surface excess terms to the interfacial energies involved.