Surfactant adsorption layers

For the solid-liquid system changes of the state of interface on formation of surfactant adsorption layers are of special importance with respect to application aspects. When a liquid is in contact with a solid and surfactant is added, the solid-liquid interface tension will be reduced by the formation of a new solid-liquid interface created by adsorption of surfactant. This influences the wetting as demonstrated by the change of the contact angle between the liquid and the solid surface. The equilibrium at the three-phase contact solid-liquid-air or oil is described by the Young equation ... 

Miller, R., Alahverdjieva, V.S., Fainerman, V.B. (2008). Thermodynamics and rheology of mixed protein-surfactant adsorption layers. Soft Matter, 4, 1141-1146. 

Miller, R., Fainerman, V.B., Makievski, A.V., Kragel, J., Grigoriev, D.O., Kazakov, V.N., Sinyachenko, O.V. (2000a). Dynamics of protein and mixed protein + surfactant adsorption layers at the water-fluid interface. Advances in Colloid and Interface Science, 86, 39-82. 

Investigations of the adsorption kinetics showed that a surfactant adsorption layer is formed. In the beginning, the adsorption ran very fast. After one minute, already 40% of the equilibrium amount was adsorbed. Then the adsorption became slower until after 10 to 30 min the adsorption equilibrium is reached. The fast adsorption gives... 

It can be summarized that ellipsometric measurements proved the formation of a surfactant adsorption layer on the photoresist surface. At ceg- it is assumed to form a monolayer. To get more information about the adsorption layer and its influence on the surface properties of the photoresist, an electrokinetic characterization of unexposed and processed photoresist in solutions of the cationic surfactant was carried out. The zeta potential of the photoresist layers is given in Fig. 8 as a function of the surfactant concentration. The measurement was performed at pH = 6 in a background electrolyte (KC1) concentration of 10-5 M to ensure the minimum conductivity of the solution necessary for the measurement. 

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... 

A weak influence on Ylvw has also the change (of reasonable values) in the parameters of the surfactant adsorption layers in the film [263]. A certain decrease in its value can be attributed to the screening of van der Waals interactions by the change in the double electric layer in the presence of electrolyte [257,258], At idi2 > 4 this effect can be accounted for if in Eq. (3.89) b = 0 [258]. If all correction are introduced in the calculation of Ylvw, the accuracy of the theoretical fl(Ii) isotherm increases from 1 to 15% in the thickness interval studied. 

The kinetic factors of foam stability as well as of the stability of any disperse system, are mainly determined by the stabilising ability of the surfactant adsorption layers. This action is spread over all structural elements of the foam. 

As already mentioned (see Chapter 3), at the instant of foam formation the films and borders are in non-equilibrium state. The films thin mainly due to the capillary pressure, while the borders thin due to gravity or a pressure drop (when the foam is dried by the Foam Pressure Drop Technique [21-23]). The surfactant adsorption layers decrease the flow rate through the borders and films and the process of thinning becomes similar to the flow in thin gaps with solid surfaces. As indicated in Sections 3.2.1 and 5.3 the degree of retardation of the flow depends on the surfactant type and concentration as well as on the film type. A complete immobility at the film and border surfaces usually is not reached. 

The stabilising ability of the surfactant adsorption layers are due mainly to the fact that they ensure immobility of the surface layers of films and borders and do not allow the origination of a convective transport. They create also a definite dependence of the border profile on the surfactant kind and pressure gradient. Along with that, the stabilising role of the... 

Murray, B.S. Interfacial rheology of mixed food protein and surfactant adsorption layers with respect to emulsion and foam stability. Proteins at Liquid Interfaces, D. Mobius and R. Miller, eds., Elsevier, Amsterdam, 1998. 

TABLE 5.4 Free Energy and Chemical Potential for Surfactant Adsorption Layers ... 

In general, the total adsorption F of an ionic species include contributions from both the adsorption layer (surfactant adsorption layer + adsorbed counterions in the Stem layer), F , and the diffuse layer,... 

Fig. 11-23. A schematic structure of the surfactant adsorption layer in the region of two-dimensional condensation (a schematic representation of Frumkin s experiments)...

In neutron reflection the surfactant adsorption layer can be viewed as a layer of uniform refractive index over the underlying solution with a different refractive index. The refractive index for neutrons (not to be confused with the refractive index for light) is so close to unity that it is much more convenient to describe reflection in terms of the scattering length density, p, that is related to the refractive index, n, as... 

The above expressions yield no information on the structure of the surfactant adsorption layers at different regions of the isotherm, and thus assumptions regarding the latter have to be made. These equations can only be used for qualitative explanation of the experimental results, rather than for theoretical predictions. 

There are two major means by which one can introduce surfactants into the system in order to gain control over the properties of solids. First, direct addition of surfactant into the liquid (or one of the liquids) which is then brought in contact with the solid surface. Second, prior firm immobilization of the surfactant adsorption layer at the solid surface. Such a layer modifies the solid surface. Below we will discuss each of these processes in some detail. 

Surface modification is broadly used in controlling the surface properties of fillers in rubber, synthetic polymers and other materials (see Chapter IX). The surfactant adsorption layers that make surfaces hydrophobic are used to prevent caking in hygroscopic powders (fertilizers), as anticorrosive agents and in numerous other processes. 

The effective elasticity of films with surfactant adsorption layers. An increase in film size related, for instance, to film deformation (flexure, stretching) due to the action of external force, leads to changes in equilibrium between adsorption layer and surfactant solution in the volume of film. If deformation occurs slowly, and the film thickness is small, the stretching causes some of surfactant molecules in the film to move from the depth onto the surface. As a result, the surfactant concentration in the bulk of film decreases, leading to a decrease in equilibrium adsorption. Consequently, the surface tension increases (the Gibbs effect) [6]. The dependence of surface... 

In the case of highly mobile interface between dispersed phase and dispersion medium (as in foams and emulsions) the condition of zero fluid flow velocity at interface (non-slip condition), determining the validity of Reynolds equation, may not be obeyed. In this case the decrease in the film thickness occurs at a greater rate. However, in foam and emulsion films stabilized by surfactant adsorption layers the conditions of fluid outflow from an interlayer are close to those of outflow from a gap between solid surfaces even in cases when surfactant molecules do not form a continuous solid-like film. This is the case because at surfactant adsorption below Tmax the motion of fluid surface leads to the transfer of some portions of surfactant adsorption layer from central regions of film to peripherical ones, adjacent to the Gibbs-Plateau channels. As a result, the value of adsorption decreases in the center of film, but increases at the periphery, which stipulates the appearance of the surface... 

Stabilization of emulsions by powders can be viewed as a simple example of structural- mechanical barrier, which is a strong factor of stabilization of colloid dispersions (see Chapter VIII, 5). The stabilization of relatively large droplets by microemulsions, which can be formed upon the transfer of surfactant molecules through the interface with low a (Fig. VII-10), is a phenomenon of similar nature. The surfactant adsorption layers, especially those of surface active polymers, are also capable of generating strong structural mechanical barrier at interfaces in emulsions. Many natural polymers, such as gelatin, proteins, saccharides and their derivatives, are all effective emulsifiers for direct emulsions. It was shown by Izmailova et al [49-52]. that the gel-alike structured layer that is formed by these substances at the surface of droplets may completely prevent coalescence of emulsion drops. 

Fig. 1.1. Schematic of the coupling between hydrodynamic flow with compression and dilatation of the surfactant adsorption layer the adsorption/desorption processes are marked by arrows...

Since this book is dedicated to the dynamic properties of surfactant adsorption layers it would be useful to give a overview of their typical properties. Subsequent chapters will give a more detailed description of the structure of a surfactant adsorption layer and its formation, models and experiments of adsorption kinetics, the composition of the electrical double layer, and the effect of dynamic adsorption layers on different flow processes. We will show that the kinetics of adsorption/desorption is not only determined by the diffusion law, but in selected cases also by other mechanisms, electrostatic repulsion for example. This mechanism has been studied intensively by Dukhin (1980). Moreover, electrostatic retardation can effect hydrodynamic retardation of systems with moving bubbles and droplets carrying adsorption layers (Dukhin 1993). Before starting with the theoretical foundation of the complicated relationships of nonequilibrium adsorption layers, this introduction presents only the basic principles of the chemistry of surfactants and their actions on the properties of adsorption layers. 

According to Langevin (1986) the surfactant adsorption layers between oil and water domains in microemulsions can be considered as lyotropic liquid crystals. In contrast to typical lyotropic... 

Narrowly defined, the main contributions to film pressure or interfacial tension decrease come from the osmotic term and the repulsion of the electrical double layers of ionic surfactants including the effects of counterions. Interactions in mixed adsorption layers are of broad interest for the description of the state of surfactant adsorption layers. For the clarification of the adsorption mechanism at liquid interfaces the replacement of solvent molecules, mainly water, has been intensively studied by Lucassen-Reynders(1981). 

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. 

The structure of a surfactant adsorption layer get more complicated as its bulk concentration increases. This book is devoted to dynamic adsorption properties of liquid interfaces. The dynamic problems at interfaces are so complicated that their solution is only possible for the simplest cases of adsorption layers formed by dilute surfactant solutions. For this reason we have not considered any special problems relating to surfactant adsorption layer structures. 

As is well known, a lot of effects of surfactants, like damping of surface waves, the rate of thinning of liquid films, foaming and stabilisation of foams and emulsions, cannot just be described by a decrease in interfacial tension or by van der Waals and electrostatic interaction forces between two interfaces. The hydrodynamic shear stress at an interface covered by a surfactant adsorption layer is a typical example for the stimulation of an important surface effect. This effect, shown schematically in Fig. 3.9., is called the Marangoni effect. 

For soluble surfactant adsorption layers the vertical mass transfer occurs under two different conditions, after the formation of a fresh surface of a surfactant solution and during periodic or aperiodic changes of the surface area. From the thermodynamic point of view the "surface phase" is an open system. The theoretical and practical aspects of this issues have been outlined in many classical papers, published by Milner (1907), Doss (1939), Addison (1944, 1945), Ward Tordai (1946), Hansen (1960, 1961), Lange (1965). New technique for measuring the time dependence of surface tension and a lot of theoretical work on surfactant adsorption kinetics under modem aspects have recently been published by Kretzschmar Miller (1991), Loglio et al. (1991), Fainerman (1992), Joos Van Uffelen (1993), MacLeod Radke (1993), Miller et al. (1994). This topic will be discussed intensively in Chapters 4 and 5. The relevance of normal mass exchange as a surface relaxation process is discussed in Chapter 6. 

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