ADHESION AND SURFACE-ENERGY OF MICA IN AIR AND WATER

The adhesion between mica surfaces in nitrogen, dry air and water has been studied with a surface force apparatus. The adhesion (pull-off fo rce) measured between air-cleaved mica surfaces in dry air or nitrogen is large, with initial values as high as 1.2-1.6 Nm(-1) This correspo nds to energies per surface of 130-170 mJ m(-2), in agreement with mea surements of the energy required to cleave mismatched and resealed mic a sheets. Exposure to the atmosphere, whether laboratory air or dry ni trogen, slowly reduces the adhesion with time. Details of the experime ntal procedure are important for obtaining reproducibly high values of the adhesion. In particular, the use of volatile compounds (such as s ym-diphenylcarbazide, used in early surface force measurements) to glu e the mica to the silica disks may lead to significantly reduced adhes ion. The adhesion measured between hydrogen mica surfaces (where the s urface potassiums ions have been exchanged for hydrogen ions by dippin g in acidic solution) is similar, but the ion-exchange procedure appea rs often to clean the surface of adsorbed contaminants, leading to val ues that are more consistently high. The magnitude of the adhesion can not be explained by dispersion interactions alone and indicates that p olar contributions are important. The contact adhesion in water is muc h smaller than in nonpolar liquids, due to shielding of ionic contribu tions. Surface energies from the adhesion measured in water and in dry nitrogen together with the equilibrium spreading pressures of water o n mica calculated from recently measured adsorption isotherms of water agree with the Young equation. It is suggested that the adsorbed laye r on mica surfaces consists chiefly of water and potassium carbonate, formed by reaction with carbon dioxide on initial cleavage in air. Thi s layer affects the properties of water condensates on (potassium) mic a, but no evidence for the presence of ''water-soluble organics'' is f ound. The time dependence of the adhesion, however, may be related to the slow adsorption of contaminants, possibly of organic origin.