A remark on the dependence of foam lamella stability on the geometric structure of phosphine oxide surfactants

Generally, a foam represents a highly complex system which still lacks a general theory on foam stability. To find out what parameters under which conditions are decisive for stabilizing a foam system, we are interested in investigating the influence of the geometrical structure of the surfactant molecule. Therefore, we have looked for an appropriate experiment enabling the physical conditions as well as the chemical system to be as simple as possible. The amphiphiles chosen [n-decyldimethyl phosphine oxide (I) and tri-n-butyl phosphine oxide (II)] are advantageous stabilizers in various respects because they are chemically very stable, non-ionic compounds, the solutions of which can easily be brought in the grade of "surface-chemical purity", and they reveal ideal surface behaviour. As the chemical structure of these two amphiphiles consists of an identical polar group and an identical number of carbon atoms, the different foamability of the two structures should mainly be attributable to differences in their molecular geometry. The adsorption parameters reveal, amongst other things, pronounced differences in the amphiphiles' cross-sectional areas. The surface area demand of the species with bulkier head group (II) is 75% larger than that of the other species (I). Foam lamella stability was determined in a rectangular glass frame. Experimentally it was found that the lamella lifetimes of the two systems are very different. The bulkier amphiphile's lifetime is smaller by one order of magnitude. The lamella stability can be described quantitatively in terms of the effective surface elasticity values of the solutions concerned, taking into account their adsorption parameters and the lamellae drainage times. Thus, as amphiphiles with rather large cross-sectional areas can produce only comparatively low surface elasticity, this results in smaller foam lamella stability. Finally, the complete lamella lifetime versus concentration dependence of one species can be calculated quantitatively from the adsorption parameters of the two amphiphiles if the lifetimes of the other surfactant are determined experimentally. (C) 1998 Elsevier Science B.V. All rights reserved.