Scaling Properties of the Surface Tension of Isobutyric-Acid-Water Mixtures

Surface tension data of isobutyric-acid-water mixtures, published by Nagarajan, Webb and Widom ( J. Chem. Phys. , 77 (1982) 5771), are reanalysed. Contrary to the conclusion drawn by these authors, it is shown that the critical part of the surface tension has the scaling properties of the surface free energy in the limit of a small surface field. It is suggested that the same behaviour is found at the non-critical interfaces of many other liquid binary mixtures which have a wetting transition close to the critical endpoint. In these systems the asymptotic scaling behaviour of the surface tension is not observable experimentally. Instead, the surface critical exponent Δ 1 can be obtained from surface tension data.