Effect of the surfactant concentration on the kinetic stability of thin foam and emulsion films

Abstract

The thinning and the lifetime of foam and emulsion films formed in a model experimental cell have been investigated. The foam films were stabilised by either sodium dodecyl sulfate or sodium dodecyl polyoxyethylene-2 sulfate. The emulsion films contained either Tween 20 or Span 20. The time of hydrodynamic drainage of the films increased linearly as the logarithm of the surfactant concentration. This linear dependence was valid whatever the type of film or surfactant and not only below the critical micelle concentration (c.m.c.) but also much above this concentration threshold. The experimental results are relevant to the hydrodynamic basis of foam and emulsion stabilisation. They are compared with the earlier hydrodynamic theories of film drainage. A reasonable, but not excellent, agreement between the experimental data and the theory could be achieved in the region below the c.m.c. of the surfactant. The data about the complex system above the c.m.c. still remain unexplained by an adequate theory. The investigation provides some guidelines for choosing the optimal type and concentration of surfactant in colloid systems of practical importance.