Foams steric repulsions

In a simple foam film the thickness of the interface is similar to the length of a surfactant molecule. The thickness of the so-called common black film (CBF) is determined by the DLVO forces, and the thinner Newton black film (NBF) is stabilized by steric repulsion and does not contain any free solvent molecules. A transition from a CBF to a NBF can be induced by the addition of salt leading to a screening of the surface potential. This confirms the electrostatic nature of the repulsive force stabilizing the CBF. The transition from a CBF to a NBF corresponds to an oscillation of the disjoining pressure because of the attractive van-der-Waals forces. This attractive part of the isotherm is mechanically unstable, and it cannot be measured by a TFPB. But a step in film thickness from the thicker CBF to a thinner NBF is detected. 

The stability of a foam is determined through the interplay of a number of factors that involve bulk solution, interfacial properties, and also external forces. We have summarized some of the effects on foam stability of gravity drainage, capillary suction, surface elasticity, viscosity, electric doublelayer repulsion, dispersion force attraction, and steric repulsion. Foams are such complex systems that Lucassen (56) has stated that any attempt to understand their properties in terms of a simple all-embracing theory is doomed to failure. Nevertheless, we have attempted to provide an introduction to the occurrence, properties, and importance of foams as they relate to the petroleum industry. More detailed aspects are taken up in the subsequent chapters of this book. 

Direct quantitative measurements of steric repulsion were made with the surface forces apparatus [1353-1360] and the atomic force microscope [1361-1364]. Although we focused on the interaction between solid surfaces, steric forces also act between fluid interfaces. The first force versus distance curves of steric repulsion were recorded across a liquid foam lamellae with a thin film balance by Lyklema and van Vhet [1365]. Another example is the force measurement between vesicles using the osmotic stress method by Kenworthy et cd. [1366]. Experimentally, the Milner, Witten, and Cates and the de Gennes model both fit force curves measured between polymer bmshes in good solvents reasonably well. 

Because of the extremely strong character of the vdW forces, we could not make paints, inks, pharmaceuticals, cosmetics, many food products, emulsions, foams, bilayer and membranes if these were the only forces in place. The opposite equally strong electric repulsive forces (and other repulsive forces, e.g. steric or hydration) can keep colloidal systems (meta)stable. 

The disjoining pressure isotherms for polymer stabilized foam films can be fitted with theory by considering all interaction forces, namely van der Waals attraction (n ), double layer repulsion (Ilei) and steric interaction (fist). At Cel < Cel. CD Hel predominates over Fist, at least at large film thickness, and in this case the disjoining pressure isotherms can be fitted using the classical DLVO theory, that is, n = riel + IIto In contrast, at Cei> Cei.cn Fist predominates over Ilei, and in this case n=+Fist. 

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