Thin liquid film drainage: Ionic vs. non-ionic surfactants

Abstract

This paper compares the rate of drainage of thin liquid films (TLF) containing ionic surfactants to that of TLF containing non-ionic surfactants. In essence, the theory of drainage has been developed for films containing non-ionic surfactants, while the validation of the models, based on the theory in the literature, has been performed with experiments on TLF with both, non-ionic and ionic surfactants, usually in the presence of significant background concentrations of electrolyte. Due to the complexity of problem, the dynamic effects on the electrical double layer (EDL) during the film drainage have been ignored for many years in the literature. These effects were finally treated theoretically and the problem solved numerically in a recent work. The new theoretical development however has not yet been validated. In addition, the differences in the kinetics of thinning of TLF with ionic and non-ionic surfactants have not been exposed in the literature until present. This paper is dedicated to revealing these differences. Experiments on kinetics of thinning were conducted with microscopic planar TLF, containing two non-ionic surfactants (tetraethyleneglycol mono-octylether C 8E 4 and n-dodecyl-- d-maltoside C 12G 2) and two ionic surfactants (sodium dodecylsulfate SDS and tetrapentylammonium bromide TPeAB). The TLF with non-ionic surfactants drain according to the well-known theories of Scheludko or Radoev-Manev–Ivanov, which confirms their validity. On the contrary, TLFs with ionic surfactants drain in general at significantly slower rate, as compared to the TLF with non-ionic surfactants, when far from equilibrium. When they are close to the equilibrium conditions, the former drain according to the theory developed for TLF with non-ionic surfactants. An analysis of the experimental results, involving the latest achievements in the field is performed, indicating the complex behaviour of the electrical double layer under dynamic conditions.