THE EFFECT OF ADDED ELECTROLYTE ON SURFACE PRESSURE AREA PER MOLECULE ISOTHERMS

Surface pressure-area isotherms of two nonionic surfactants, Synperonic A2 and B246, a block copolymer, were determined at the water-air interface. The effects of high concentrations of electrolyte in the subphase were assessed. It is shown that comparison of the pressure-area isotherms leads to a misinterpretation of the effect of the electrolyte on the surfactant monolayer. It is suggested that the surface tension-area isotherms give a better indication of the true effect of the electrolyte. A qualitative discussion of this point is put on a quantitative basis using thermodynamic equations based on the Gibbs surface phase. Using this new approach, the close-packed molecular areas of the surfactants were shown to be independent of electrolyte concentration. This result contrasts with an apparent increase in the surfactant molecular area in the presence of added electrolyte, indicated by the surface pressure-area isotherms. © 1991.