Insoluble monolayers of weakly ionising low molar mass materials and their deposition to form Langmuir-Blodgett multilayers

Abstract

The properties of insoluble monolayers of weakly ionising materials e.g. carboxylic acids and amines are sensitive to the pH of the subphase and to the type and concentration of added electrolyte. The effects which these variables have on the π-A isotherms and on the structural parameters deduced from X-ray and neutron reflectivity data are dicussed in terms of the ionisation (and ion incorporation) within the monolayer. Of particular importance for the successful deposition of such monolayers onto solid substrates (as Langmuir-Blodgett films) is the requirement that they be maximally stable. We discuss how the stability of the floating monolayer (with respect to collapse or dissolution) is affected by changes in pH, surface pressure and electrolyte addition and, via analysis of the deposited multilayers, conclude that maximum monolayer stability occurs when the film is roughly half-ionised. The mechanisms of the successive transfer of monolayers from water surfaces to solid substrates remain poorly understood. We review the main findings of such studies to date, focussing in particular on the understanding to be gained from measuring the dynamic contact angle and transfer ratio during deposition. The problem involves, inter alia, understanding the nature of the forces between approaching surfaces and the properties of the thin liquid film between them. Interesting results from force-distance measurements between mica surfaces are discussed in this context.