A. Introduction

Surface tension is a property of surfaces and refers usually to liquid-gas interfaces whereas interfacial tension is reserved for liquid-liquid interfaces.

In a liquid-gas interface surface tension arises because the molecules on the liquid surface experience a different environment than those of the bulk liquid. A molecule on the surface is attracted equally by molecules below it and on its sides. The intermolecular attraction with gas molecules above it is usually weaker. This effect puts the surface layer under tension, with the liquid adopting a minimum surface area in order to minimise the Gibbs energy of surface formation. Typical values of surface tension vary from 20 dyn/cm for oils, 70 dyn/cm for water and 500 dyn/cm for mercury.

Knowledge of surface tension is important in understanding foaming, wetting, emulsification and drop formation. The property plays an important role in two-phase flow, hydrocarbon reservoir analysis, and the design of absorbers and fractionation columns.

Extensive accounts of estimation methods are given by Hirschfelder et al. (1954) and Poling et al. Poling (2001). The SI unit of surface tension is N/m. In practice, the dyn/cm is widely used and is equivalent to 1 mN/m.