Stability of thin liquid films on Langmuir-Blodgett layers on silica surfaces

Abstract

The stability of thin aqueous liquid (wetting) films on modified silica surfaces was investigated by interferometry (measurement of the equilibrium thickness, h eq, and the critical thickness, h crit, of the thin film) and by measurement of the advancing contact angle, θ A. Even and odd numbers (the polar head group and the apolar chain, respectively, toward the aqueous film phase) of Langmuir-Blodgett (LB) layers of octadecylamine and arachic acid were used to modify the solid surface. The silanol groups (OH groups) of the silica surface were methylated or aminated in any case. Among other things it was found that 5 LB layers have about the same hydrophobizing effect as the chemical reaction of the OH groups, that electrostatic interaction forces may occur besides dispersion interactions and that a power law that is of practical use correlates h crit and σ LG (1-cos θ A. The structure of LB layers (penetration of solvent molecules) and possible magnitudes of the Hamaker constant and the surface potential of the polar head groups are discussed.