Interaction forces between mica surfaces at small separations in polar and non-polar liquids

Abstract

This paper describes experiments undertaken to measure directly the forces of interaction between sheets of mica in a variety of environments and to determine the interfacial energies resulting from these interactions. In mica, the interactions are due to dispersion forces and the electrostatic attraction arising from the ions in the cleavage plane. The method used in this study consists of measuring the work necessary to cleave the crystal and determining the equilibrium shape of the cleaved sheets near the bifurcation line. The intermolecular attraction acting in the gap causes the thin sheets to be drawn towards each other. The sheets may be treated as elastic beams carrying a distributed load. By comparing their observed shape with the theoretical shape without a load, it is possible to test theoretical expressions for the intermolecular forces. Observations have been made in UHV and in polar and non-polar liquids. Hamaker coefficients have been evaluated for the systems used and the Madelung coefficients determined in order to investigate the electrostatic interaction energy resulting from charge domains of various sizes on the mica surfaces. The experiments and calculations show that the electrostatic forces contribute about 90% of the total interaction energy.