Thermodynamics of Thin Liquid Films

The coalescence of disperse systems, such as foams and emulsions, and the contact of air bubbles with solid particles, e.g. in the process of flotation, takes place in two steps. The first is characterised by a flocculation of the system, the formation of thin liquid films with an equilibrium thickness. In the second step the film becomes thin enough for the interparticular attractions to overcome the film state so the two separated interfaces form a new interface. The situation where a small bubble attaches a liquid interface is shown in Fig. 2D. 1. 

At this point we want to describe briefly the thermodynamic precondition for the formation of thin liquid films and some of their physical properties which differ from the related liquid bulk phase. Further we report on the present state of knowledge on the rate of thinning of liquid films dependent on the related surface rheological properties. 

The situation of an interlayer between two liquid phases is given by Derjaguin (1993) (Fig. 2D.2). This makes obvious what happens when the peculiarities of thin liquid films are neglected. 

The topics of the early scientific work of Derjaguin and his collaborators was the evaluation of the term disjoining pressure as basic property of a thin liquid film. Derjaguin Obuchov (1936) and Derjaguin Kussakov (1939) have detected the growth of repulsive forces in such films as the film becomes thitmer. The classic thermodynamics of Gibbs was extended by the thermodynamic formulation of the disjoining pressure concept. 

Staring from the Gouy-Chapman theory the observed repulsion force in thin liquid films must be of electrostatic nature caused by the overlap of the corresponding diffuse electrical double 

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The thermodynamics of thin liquid films and adhesion is well documented in the literature [15,17,100-103]. The first theoretical approaches were developed mainly for thin soap films. Most of the results of the thermodynamics of soap... 

Membrane and Detailed Models of a Thin Liquid Film Thermodynamics of Thin Liquid Films The Transition Zone between Thin Film and Plateau Border... 

Defay, R., Prigogine, I. and Sanfeld, A., J. Colloid Interface Sci., 58(1977)498 de Feijter, J.A., "Thermodynamics of Thin Liquid Films" in "Thin Liquid Films", Ivanov, LB., (Ed.), Marcel Dekker, Inc. New York, Basel (Surfactant Science Series), 29(1988)1... 

In order to generate foam, surfaces of thin liquid films always have to be stabilised by layers of surfactants, polymers or particles. This is why pure liquids never foam. Foaming is always accompanied by an increase in the interfacial area and, hence, its free energy. Thus, in a thermodynamic sense foams are basically unstable and are, therefore, sooner or later destroyed. The lifetime of a foam can span a remarkable range from milliseconds to very long duration. 

The thermodynamic state of the thin liquid film is described by the isotherm of disjoining pressure FI(/i). The opportunity the achieve a direct experimental measurement of the disjoining pressure is one of the main achievements in the study of thin liquid films. 

The special properties of thin liquid films, in particular of foam films, involve studying various colloid-chemical aspects, such as kinetics of thinning and rupture of films, transition from CBF to NBF, isotherms of disjoining pressure, thermodynamic (equilibrium) properties, determination of the electrical parameters of surfactant adsorption layer at the liquid/gas... 

Study of processes leading to rupture of foam films can serve to establish the reasons for their stability. The nature of the unstable state of thin liquid films is a theoretical problem of major importance (it has been under discussion for the past half a century), since film instability causes the instability of some disperse systems. On the other hand, the rupture of unstable films can be used as a model in the study of various flotation processes. The unstable state of thin liquid films is a topic of contemporary interest and is often considered along with the processes of spreading of thin liquid films on a solid substrate (wetting films). Thermodynamic and kinetic mechanisms of instability should be clearly distinguished so that the reasons for instability of thin liquid films could be found. Instability of bilayer films requires a special treatment, presented in Section 3.4.4. 

For the transport of heavy ions to a solid surface coated with an adherent water film, like aluminium oxide, the visco-elastic properties of electric field forces and the concentration of heavy ions may be important for the rate of adsorption. For this reason we need information not only on relaxations restricted to a surface of an extended liquid, but also on the adherent water layer at the adsorbents. The last issue may be a bridge to the thermodynamics and flow properties of thin liquid films have been studied by some excellent research groups. 

Instabilities of thin liquid films have been investigated for many decades. The general contours of the mechanics and thermodynamics of thin-film instability have emerged during the last 50 years. " " On the theoretical side, Sheludko and Vrij " ... 

PA Kralchevsky. Effect of Film Curvature on the Thermodynamic Properties of Thin Liquid Films. PhD thesis, Sofia University, Sofia, Bulgaria, 1984 [in Bulgarian]. 

The theory of seaweed formation does not only apply to solidification processes but in fact to the completely different phenomenon of a wettingdewetting transition. To be precise, this applies to the so-called partial wetting scenario, where a thin liquid film may coexist with a dry surface on the same substrate. These equations are equivalent to the one-sided model of diffusional growth with an effective diffusion coefficient which depends on the viscosity and on the thermodynamical properties of the thin film. 

The thin liquid films bounded by gas on one side and by oil on the other, denoted air/water/oil are referred to as pseudoemulsion films [301], They are important because the pseudoemulsion film can be metastable in a dynamic system even when the thermodynamic entering coefficient is greater than zero. Several groups [301,331,342] have interpreted foam destabilization by oils in terms of pseudoemulsion film stabilities [114]. This is done based on disjoining pressures in the films, which may be measured experimentally [330] or calculated from electrostatic and dispersion forces [331], The pseudoemulsion model has been applied to both bulk foams and to foams flowing in porous media. 

The development of the thermodynamics of thin films is related to the problem of stability of disperse systems. An important contribution to its solving are the works of the Russian scientists Derjaguin and Landau [1] and the Dutch scientists Verwey and Overbeek [2], known today as the DVLO theory. According to their concept the particular state of the thin liquid films is due to the change in the potential energy of molecular interaction in the film and the deformation of the diffuse electric layers. The thermodynamic characteristic of a state of the liquid in the thin film, as shown in Section 3.1, appears to be the dependence of disjoining pressure on film thickness, the n(/t) isotherm. The thermodynamic properties of... 

In order to understand the nature of surface forces which characterise the thermodynamic state of black foam films as well as to establish the CBF/NBF transition, their direct experimental determination is of major importance. This has been first accomplished by Exerowa et al. [e.g. 171,172] with the especially developed Thin Liquid Film-Pressure Balance Technique, employing a porous plate measuring cell (see Section 2.1.8). This technique has been applied successfully by other authors for plotting 11(A) isotherms of foam films from various surfactants solutions [e.g. 235,260,261]. As mentioned in Chapter 2, Section 2.1.2, the Pressure Balance Technique employing the porous ring measuring cell has been first developed by Mysels and Jones [262] for foam films and a FI(A) isotherm was... 

We owe Borislav Toshev deep indebtedness for writing Section 3.1, entitled Elements of the Thermodynamics of Foam Films, where he advances a new approach to handling thin liquid film thermodynamics. 

Ivanov, LB. and Kralchevsky, P.A., Mechanics and thermodynamics of curved thin liquid films, in Thin Liquid Films, Ivanov, I.B., Ed., Marcel Dekker, New York, 1988, p. 49. 

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