The dependence of the surface tension of surfactant solutions on drop size

It is shown that when the surface area of a surfactant solution is greatly extended by creating a dispersion of small particles (e.g., drops), the surface tension of the solution of which the particles consist depends on the particle size. This stems from the fact that the amount of surfactant in each particle is finite. Consequently, the distribution of surfactant between the bulk and the interface of a small particle is different from its distribution in the original solution. Calculations for SDS solutions demonstrate that the surface tension of drops appreciably deviates from that of the original solution for drops smaller than about 10 μm in radius. This size range coincides with that typical of aerosols and emulsions. For surfactants of higher surface activity, the effect of size on the surface tension is appreciable even for much larger drops.