Liquid/liquid/solid contact angles measured by interference microscopy

Abstract

Interference microscopy has been used to measure contact angles in aqueous/organic/graphite systems. For contact angles < 15° this technique is of high precision. The solid substrate, highly oriented pyrolytic graphite (HOPG), was prepared for contact-angle measurements by cleaving under various media and by high-temperature outgassing. Differences in the measured contact angles depending on the surface pretreatment are reported. For the system n-heptane/water/HOPG in which the liquids were outgassed and the graphite cleaved under heptane the mean receding contact angle was 7.25 ± 1°. A similar result is obtained by high-temperature outgassing of the solid. Preliminary experiments show that n-decane and n-undecane show finite (though smaller) contact angles, while n-hexadecane, n-octanol, benzene and toluene displace water from the surface, which is then completely wetted by the organic phase.

The overall mean receding contact angle for the heptane system is 7.25 ± 1.5°.