Time-Dependence of Surface Tension of Solutions

The slow rate of change of surface tension observed by Addison with aqueous solutions of fatty alcohols cannot be explained by diffusion alone. It is assumed that the adsorption of non-electrolyte solutes in the surface film is opposed by a constant potential energy barrier, and, on this basis, a relation between spreading pressure and surface age is developed. With the resulting equation, the experimental data of Addison are employed to evaluate the energy term. Nearly constant values for the assumed barrier are obtained, the actual calculated values, in calories per mole at 20°C, being about 1900 for isoamyl alcohol, 1300 for n-hexyl alcohol, and 700 for n-heptyl alcohol. A second relation between spreading pressure and surface age is derived on the assumption that the energy barrier is a linear function of spreading pressure. The use of Addison's data for isoamyl alcohol in this relation indicates that the spreading pressure-dependent portion of the barrier is small and of the same order of magnitude as the energy required to compress the surface film sufficiently to admit the entering solute molecule.