Measurement of contact angles of spherical monodisperse particles with surfactant solutions

Abstract

Small hydrophobic particles are often used as antifoam agents, and in connection with the mechanism by which particles rupture foam lamellae, it is necessary to have a knowledge of the contact angle between particles and the foaming solution. In the bridging—dewetting process of film rupture, the minimum contact angle requirement is dependent on particle geometry. There is thus an interest in using particles with a well-defined geometry in the study of foam breakdown, and monodisperse spheres are favoured candidates. It is usual to assume that the contact angle between particles and solution is the same as that between the solution and a flat plate with a coating similar to that of the particle. Tests have been made in the past to confirm such equivalence using packed beds of large particles. This method requires relatively large samples, and is not convenient therefore for the study of the much smaller particles of interest in foam breaking. In this paper we describe a method for the determination of contact angles of surfactant solutions with monodisperse spheres (diameters of approximately 6 nm and 2.5 μm have been used) employing a Langmuir trough technique. The particles are spread as insoluble monolayers on the aqueous surfactant solutions and compressed to monolayer collapse. Using an approach devised by Clint and Taylor (J.H. Clint and S.E. Taylor, Colloids Surfaces, 65 (1992) 61), it is then possible to calculate contact angles from the collapse pressures and a knowledge of the surface tensions of the substrates.