Aspects of aqueous foam stability in the presence of hydrocarbon oils and solid particles

Abstract

The control of foaming industrially is an important process, and in the case of aqueous foams is often achieved by addition of a dispersion of small hydrophobic solid particles in a mineral oil. Our work is concerned with the acquisition of reliable data for well-defined, chemically pure systems in order to evaluate in a convincing way some of the possible mechanisms of foam breakdown which have been presented in the literature. We present here some of our findings on the destabilisation of foams and single films by dispersed hydrocarbon oil droplets, by spherical hydrophobic particles and (in the case of single films) cylindrical hydrophobic rods, and by solids and oils in combination. In our work on oils we have focused on the process of oil drop entry into the air-aqueous solution interface, which is a prerequisite for foam and film breaking. The feasibility of entry is related to the various interfacial tensions in the system, usually combined in the form of the entry coefficient. We discuss some of the problems in the determination and use of entry coefficients in predicting whether entry should or should not occur. Where entry is possible, the rate of drop entry is an important factor in the consideration of foam breakdown. We report on the residence times of single oil drops placed under the air-solution interface and relate these to the lifetimes of films and foams in the presence of dispersed entering oil drops. In the description of the effects of solid particles on foam breakdown the parameter of central importance is the contact angle which the air-solution interface makes with the solid surface. The effectiveness of cylindrical hydrophobic rods in rupturing single soap films rises dramatically when the contact angle approaches 90°. On the other hand, when spherical hydrophobic particles are added to a foam we find that foams can be stabilised, and indeed maximum stability has been observed for a contact angle of 90°. For slightly higher angles however, foam stability falls drastically. In connection with planned work on the effects of solids and oils together on foam stability we discuss ways of modifying contact angles of oil-solution interfaces with solid surfaces.