Role of Oil Spreading for the Efficiency of Mixed Oil−Solid Antifoams

Abstract

The role of oil spreading in the process of foam destruction by oil-based antifoams (oils or mixed oil−solid compounds) has been the subject of a long debate in the literature. To clarify some aspects of this problem, we compare the entry barriers of antifoam globules in the presence and in the absence of a prespread layer of oil on the surface of the surfactant solution. The film trapping technique is employed to measure precisely the critical capillary pressure, at which the entry of the antifoam globules on the solution surface occurs. The experimental results show that the prespread oil layer reduces by several times the entry barrier for mixed oil−silica antifoams, as compared to the barrier in the absence of spread oil. Thus, the oil spreading facilitates the entry of mixed antifoam globules and the subsequent bridging and rupture of the foam films. A simple mechanistic explanation of this effect is given, taking into account the main role of the solid particles in mixed antifoams, namely, to pierce the asymmetric oil−water−air film, formed when an antifoam globule approaches the solution surface. This explanation is expressed in terms of the three-phase contact angles solid−water−oil and solid−water−air, when spherical solid particles are considered.