A discussion of oil foam stability fundamentals is most meaningful for liquid foams; as mentioned the surface properties of solid plastic foams do not vary to a useful degree for an analysis and the final properties of the foamed product are mostly dependent on the time history of the solidification during the preparation. Metal foams, conversely, have a sizable surface free energy and the relation to stability has recently been shown amenable to analysis by Kaptay [34••].
For liquid foams the primary distinction of properties is between two related but different terms; foamability and foam stability [35]. The former term is defined by the maximum volume of foam created in the foaming process, while the latter epitomizes the rate of reduced foam height after formation. Since these aspects both are amenable to direct observation, the experimental methods in the determinations of foam stability are not in need of review and it is sufficient to draw attention to one method, which has not been applied to the extent merited. Oyo and Minagawa [36•] as well as Pacho and Davies [37•] early demonstrated electric capacitance measurements as a simple and exact method to identify air/liquid and air/foam interfaces. The authors used it to monitor the 2-dimensional distribution of the liquid content of the foam versus time and also to observe the processes taking place during foam collapse. The method certainly deserves more attention.
The early attempts to treat the stability of oil foams [22], [23] and [24] were obviously encumbered by the weight of the traditional aqueous foam approach and, in spite of the fact that some of these efforts were of high quality; the progress was not satisfactory to provide an understanding of the behavior of commercial non-aqueous foams. The key reason for the lack of fundamental advancement is found in the fact that hydrocarbon foams lacked the key element of surface modification [6], which meant that the only constituents available for the analysis were the properties of the bulk liquid. For this reason the analysis was limited to factors affecting the rheology of the liquid phase in question; ultimately leading to drainage considerations as the only relevant factor. It is certainly undisputable that this is the valid – and in fact the sole – line of attack to comprehend the stability of oil foams from a one-phase liquid [23] and [38]. However, the fact that the actual problem of oil foams occurs in more complex media renders the approach insufficient; albeit not completely irrelevant.
As a consequence, the restriction of the approach to one-phase condensed systems had to be abandoned in order to attain an understanding of the fundamentals of the stability of oil foams in the highly varied applications. Accordingly, the incentive to make progress in both foamability and foam stability in non-aqueous systems arose directly from the articulation of industrial problems. Of these the first one; the foaming in extraction towers of different kinds, was obviously a multi-phase problem and led the outstanding colloid scientist Sidney Ross to investigate foamability in systems of two organic solvents with limited mutual solubility. In an elegant and simple investigation, Ross [39•] demonstrated the decisive effect of the relative surface tension of the two liquids involved. If the lower tension liquid was the minor component in the two-phase combination, the system showed foamability.
A similar situation led to an initial solution to the problem of finding a basis of the stability of foams from non-aqueous liquids. The industrial predicament was urgent; the well established formulations for personal care and pharmaceutical aerosols giving rich foams during extrusion were suddenly useless, when environmental concerns forced chlorofluorocarbons as propellant gas to be replaced by nitrogen or carbon dioxide. As is easily realized, the development of new formulations is costly as well as time consuming and the situation was serious, because at the time the fundamental literature was not geared to provide an explanation as to why a change of propellant from one inert gas to another should affect the foaming at aerosol extrusion. The problem was solved by Jederstroem, a chemist at Pharmacia Co, Uppsala Sweden, who had observed an obscure note about foam stability and phase diagrams [40•]. His use of the concept [40•] solved not only the actual problem, but introduced a logical and systematic method [41] for formulating foaming aerosols with any desired foaming behavior.
The phase diagram for the entire formulation, including the propellant, was determined; Fig. 1, at the ambient pressure of the package and the results at first gave ample evidence that the propellant was far from an inert gas under the package conditions. It did in fact form a liquid with the remaining components of the formulation except at the lowest propellant contents. Secondly, the results offered a simple a priori method for formulations with any degree of foamability and foam stability of the extruded material according to Fig. 1. The evaporation path in the system, Fig. 1, is closely approximated by a straight line from the propellant corner of the phase diagram to the line for no propellant left, A to B. Extrusions of compositions with an evaporation path ending within the liquid crystal range at the line, gave extremely stable foams lasting more than 10 min with no reduction in foam height, while paths ending in the multi-phase range gave foams of intermediate stability. A path ending in the one-phase liquid span resulted in extremely unstable foams with complete collapse within seconds.