Foam and Dynamic Surface Properties of Micellar Alkyl Benzene Sulphonates

Abstract

The amount of foam formed when micellar solutions of a homologous series of anionic surfactants are subjected to a fixed agitation in the presence of antifoam often exhibits a maximum at a certain chain length. Here we consider the behavior of an homologous series of alkyl benzene sulphonates in the presence of a synergistic hydrophobic particle/hydrophobic oil antifoam. The equilibrium surface tensions of micellar solutions of homologous anionic surfactant solutions decrease with increase in chain length. However the gradients of dynamic surface tensions against reciprocal time increase with increasing chain length. This implies progressive decreases in the efficiency of transport of surfactant to air-water surfaces as the chain length increases. Measurements of micellar diffusion coefficients imply that this is not due to decreasing rates of diffusion. We tentatively ascribe it to decreasing rates of micelle breakdown as the chain length increases and the CMC decreases. The combination of decreasing equilibrium surface tension and increasing gradients of dynamic surface tension with reciprocal time means that plots of dynamic surface tension (at sufficiently high reciprocal times) against chain length will reveal a minimum. Here we correlate that minimum with the maximum in foam formed in the presence of antifoam. That maximum then arises as a consequence of a compromise between increasing surface activity and decreasing efficiency of transport of surfactant to air-water surfaces during foam generation. Foam and surface tension measurements are compared with theoretical considerations concerning the mode of action of the antifoam.