Stable foams, factors that produce

The presence of mixed surfactant adsorption seems to be a factor in obtaining films with very viscous surfaces [411]. For example, in some cases the addition of a small amount of non-ionic surfactant to a solution of anionic surfactant can enhance foam stability due to the formation of a viscous surface layer, which is possibly a liquid crystalline surface phase in equilibrium with a bulk isotropic solution phase [25,110], In general, some very stable foams can be formed from systems in which a liquid crystal phase is present at lamella surfaces and in equilibrium with an isotropic interior liquid. If only the liquid crystal phase is present, stable foams are not produced. In this connection foam phase diagrams may be used to delineate compositions that will produce stable foams [25,110],... 

Those factors that were previously mentioned that produce finer-textured foams also produce more stable foams. Factors such as surfactant type, concentration, increasing pressure, and higher inputs of mechanical energy generate more stable foams. For higher temperatures such as those that exist downhole, dynamic foam stability relies upon surfactant type and concentration rather than the addition of thickeners (polymer stabilizers). It is not known what rates are necessary to maintain dynamic stability in fractures, or whether those conditions typically exist. 

There is no simple, direct relationship between elasticity and emulsion or foam stability because additional factors, such as film thickness and adsorption behaviour, are also important [204]. Nevertheless, several researchers have found useful correlations between EM and emulsion or foam stability [131,201,203], The existence of surface elasticity explains why some substances that lower surface tension do not stabilize foams [25]. That is, they do not have the required rate of approach to equilibrium after a surface expansion or contraction as they do not have the necessary surface elasticity. Although greater surface elasticity tends to produce more stable bubbles, if the restoring force contributed by surface elasticity is not of sufficient magnitude, then persistent foams may not be formed due to the overwhelming effects of the gravitational and capillary forces. More stable foams may require additional stabilizing mechanisms. 

It has been found that a combination of surfactants produces a slower drainage and an improved foam stability. For example, mixtures of anionic and nonionic surfactants or anionic surfactant and long-chain alcohol produce much more stable films than the single components, but this could be attributed to several factors. For example, the addition of a nonionic surfactant to an anionic surfactant causes a reduction in the cmc of the anionic. The mixture can also produce a lower... 

Also of significance is that initial instability of a thin film in accordance with the above mechanism does not inevitably lead to film rupture. The analysis, like all others in this chapter, is based on linear stability theory, and hence is valid only for small amplitude perturbations. It has been observed experimentally that at low surfactant concentrations instability of a film some tens of nanometers in thickness does produce rupture. But for many surfactants it is found that above a critical concentration, the instability leads to formation of black films which are only slightly thicker than the total length of two surfactant molecules (She-ludko, 1967). These black films can be very stable and are a major factor in foam stabihty. 

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