Equilibrium at the Solid–Liquid Interface

Ward and Brooks1 in 1952 provided the only experimental justification for the much used assumption of concentration equilibrium at the interface during solid dissolution investigations. Using both Lamm scale and Rayleigh interferometer measurements of concentration gradients they calculated interfacial concentrations equal to the saturation values for the dissolution of salicylic acid and of n-butyl acetate in water. They also reported probable saturation for the benzoic acid–water system although the calculated concentration at the interface varied somewhat with the experimental method. It appears that a more appropriate instrument for such measurements would be the Mach–Zender interferometer, which permits the direct measurement of concentrations in the vicinity of a solid surface2. For the present work an interferometer was constructed similar to that described by Caldwell for diffusion studies3. The continuous light source was replaced, however, by a high-intensity, short-duration spark discharge4 to provide better photographic resolution of the rapidly changing concentrations in the vicinity of the interface. This instrument was then used to investigate the dissolution in water of benzoic acid and of acetanilide.