1. Introduction
To call the presence of foams in the daily environment ubiquitous is an understatement; they are encountered in countless situations in ordinary life. Their omnipresence and general importance would as such justify a scientific interest in their fundamentals, but their extreme structure per se has attracted a significant interest from basic science. This is especially the case for foams in which the non-gas part of the structure is liquid, i.e. liquid foams. In fact the intriguing problem of explaining the surprising stability of these dispersions of a fragile array of gas bubbles separated by thin liquid films has attracted a number of outstanding researchers over the years from the early treatments [1], [2], [3] and [4] via later analysis [5] and [6] to the recent treatment of fundamentals [7]. However, of all this vast knowledge gathered of the properties of liquid foams, the overwhelming majority is concerned with structures, in which the liquid part is aqueous, while foams from non-aqueous liquids have been more sparsely treated. The analysis of stability and general properties of solid foams, on the other hand, has gained a separate place in the scientific literature against the fact that their properties are as much dependent on details of their preparation and manufacturing as on basic colloid phenomena and the latter have first recently been adequately examined [8••].
This asymmetry towards fundamental colloidal analysis of the aqueous variety of wet foams is obviously not a reflection on the volume or the commercial importance of the two classes of foams; in point of fact oil based foams; especially the solid varieties, occupy the overwhelming majority of the commercial products. The reason for the partiality in scientific interest is rather to be found in a fundamental difference of the stabilizing mechanisms. In the aqueous foams the added surfactant strongly adsorbs at the interface and the changed surface properties provide a vital part of the basis for the analysis of foam stability [6]. In an aqueous solution increased concentration of a surfactant below the cmc value leads to enhanced adsorption and reduced surface tension, which offers a simple and useful experimental tool to obtain information about the surface properties. In addition the diffuse electric double layer in aqueous solutions has been extensively analyzed for several decennia and has become a convenient tool for colloid chemists.
Surface tension as an analytical tool is not available for oil foams; the value of the inherent surface tension of most oils is at such a low level that there is little or no adsorption to the surface of hydrocarbon based surfactants. Hence, in a non-polar solvent increased concentration of added surfactant does not lead to reduced surface tension; and when exceeding the solubility limit of the surfactant in monomeric form the association to inverse structures in the bulk of the liquid are governed by attractive intermolecular interactions between the dissolved molecules; not between the solvent molecules as is the case in aqueous solutions. As a result the surface properties are only insignificantly – or not at all – changed by an increase of the surfactant concentration and have no significant effect on the stability. Even for non-aqueous liquids with a substantial surface tension, e.g. glycerol, the reduction per se of the surface tension does not provide much assistance in the efforts to clarify the factors behind the stability of their foams, because the relation between surface tension and stability is ambiguous. Instead, surface rheology [6] has a more direct action to affect the stability, but little or no information is available about such measurements for surface layers on glycerol or polar non-aqueous liquids in general.