Adsorption thin-liquid films

Although many factors, such as film thickness and adsorption behaviour, have to be taken into account, the ability of a surfactant to reduce surface tension and contribute to surface elasticity are among the most important features of foam stabilization (see Section 5.4.2). The relation between Marangoni surface elasticity and foam stability [201,204,305,443] partially explains why some surfactants will act to promote foaming while others reduce foam stability (foam breakers or defoamers), and still others prevent foam formation in the first place (foam preventatives, foam inhibitors). Continued research into the dynamic physical properties of thin-liquid films and bubble surfaces is necessary to more fully understand foaming behaviour. Schramm et al. [306] discuss some of the factors that must be considered in the selection of practical foam-forming surfactants for industrial processes. 

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... 

The comparison of the results for foam films with those for emulsion films has proved to be very useful, especially with respect to emulsion films of the O/W type. Reason for such a comparison provides the fact that in both cases the thin liquid film is in contact with two hydrophobic phases. It is anticipated that the effects related to adsorption and orientation of surfactant molecules at the film/hydrophobic phase interface are very similar, and there are examples illustrating it. Hence, some regularities established for foam films can be applied to emulsion films and vice versa. 

Between the two there is the maximum, which acts as a barrier, inhibiting the free a P tremsition. This barrier can be crossed only when the required activation (Gibbs) energy is available. Whether a thin liquid film is stuck in the a-state or the P-state, or partly in one and partly in the other, depends not only on the activation Gibbs energy but also on the direction from which the equilibrium situation was approached. In this respect there is a difference between liquid films created from gas adsorption and those obtained from thick liquid layers subjected to drainage. We shall now consider the former case, treating the latter in sec. 5.3d. 

Covering a supported metal catalyst with a thin liquid film that differs from the bulk solvent affects both the reaction rate and selectivityTreating a Pd/C catalyst with 2M KOH before using it in a phenol hydrogenation with a heptane solvent gave cyclohexanone in 97% yield. Apparently, the distribution of phenol and phenolate ions between the thin aqueous film around the catalyst and the bulk hydrocarbon solvent enhanced the adsorption of the phenolate ion on the catalyst and the facile transfer of the neutral cyclohexanone to the heptane after the selective hydrogenation was completed (Figure 17.1). 

For foam separation processes, adsorption takes place in solution, the essential basis exists for solute separation by foaming. Foam consists of gas bubbles separated by thin liquid films. The liquid films are often formed by the mutual approach of two already existing liquid surfaces (e.g., two bubbles below the surface). Foam structures may vary between two extreme situations. The first is wet foam, which consists of nearly spherical bubbles separated by rather thick liquid films. The second is dry foam, which may develop from the first type as a result of drainage (i.e., foam drainage). 

In fact, Equation 5.281 describes an interface as a two-dimensional Newtonian fluid. On the other hand, a number of non-Newtonian interfacial rheological models have been described in the literature. Tambe and Sharma modeled the hydrodynamics of thin liquid films bounded by viscoelastic interfaces, which obey a generalized Maxwell model for the interfacial stress tensor. These authors also presented a constitutive equation to describe the rheological properties of fluid interfaces containing colloidal particles. A new constitutive equation for the total stress was proposed by Horozov et al. ° and Danov et al. who applied a local approach to the interfacial dilatation of adsorption layers. 

Kretzschmar Voigt (1989) have recently examined the contribution of interacting forces in surfactant adsorption layers to the film pressure. A detailed knowledge of the geometry of the electrical double layer with respect to the plane of the interface is an essential item in the theoretical description of charged monolayers, thin liquid films and membranes. Fig. 2.12. shows an illustration of structural and energetic aspects of the surfactant monolayer formation. 

Surface Rheology and Adsorption Dynamics in Drainage Processes of Thin Liquid Films... 

On the basis of a differential equation Ivanov (1977) described all stages of thin liquid film evolution. He distinguished the effects of Marangoni-Gibbs and of surface viscosity. Additionally, the substantial effects of surface diffusion and slow adsorption (barrier or kinetic controlled mechanisms) are taken into consideration. A selection of basic equations can be find in Chapter 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. 

The important aspect of adsorption processes at a liquid interface is lateral mobility which can lead to lateral excess transport of adsorbed molecules. Lateral transport disturbs the equilibrium state of an adsorption layer. In many important systems, such as emulsions, foams, and bubbly liquids, the properties of a non-equilibrium adsorption layer can be essential. This has been demonstrated in the systematic work of the Russian and Bulgarian schools summarised in monographs like "Thin Liquid Films" by Ivanov, "Coagulation and Dynamics of Thin Films" by Dukhin, Rulyov and Dimitrov, and "Foams and Foam Films" by Krugljakov and Exerowa. These books pay most attention to thick film drainage and stabilisation/destabilisation of thin liquid films. This book is focused on other dynamic processes at liquid interfaces in general or connected with phenomena of emulsions and foams. 

The situation is still more complex in the presence of surfactants. Recently, a self-consistent electrostatic theory has been presented to predict disjoining pressure isotherms of aqueous thin-liquid films, surface tension, and potentials of air bubbles immersed in electrolyte solutions with nonionic surfactants [53], The proposed model combines specific adsorption of hydroxide ions at the interface with image charge and dispersion forces on ions in the diffuse double layer. These two additional ion interaction free energies are incorporated into the Boltzmann equation, and a simple model for the specific adsorption of the hydroxide ions is used for achieving the description of the ion distribution. Then, by combining this distribution with the Poisson equation for the electrostatic potential, an MPB nonlinear differential equation appears. 

Alahverdjieva, V. S. Khristov Khr. Exerowab, D. Miller R. Correlation between adsorption isotherms, thin liquid films and foam properties of protein/surfactant mix-... 

Foams and emulsions are achieved due to adsorption of foam stabilizing agents like surfactants at the interface between the dispersed and continuous phases. The foam stability is often related to the stability of thin liquid films formed between two air bubbles. All considered foam films are stabilized by ionic surfactant. 

The interactions across a thin film, called the surface forces, to a great extent are determined by the surfactant adsorption at the film surfaces. From a physical viewpoint, a liquid film formed between two phases is termed thin when the interaction of the phases across the film is not negligible. Thin films appear between the bubbles in foams, between the droplets in emulsions, as well as between the particles in suspensions. Furthermore, the properties of the thin liquid film determine the stability of various colloids. 

Having reviewed the properties of single adsorption monolayers, we proceed with the couples of interacting monolayers the thin liquid films. First, we present the thermodynamics of thin films, and then we describe the molecular theory of the surface forces acting in the thin films. We do not restrict ourselves to the conventional DLVO (Deijaguin, Landau, Verwey, Overbeek) forces [2,3], but consider also the variety of the more recently discovered non-DLVO surface forces [4]. The importance of the micelle-micelle interaction for the mechanism of micelle growth is also discussed. 

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