Surfactant Adsorption and Gibbs Monolayers

A large body of literature has grown up around the question of how one can relate the chemical structure of a surfactant to its surface activity in various situations. At the present time, most such structure-property relationships are semiquantitative at best, but they can serve one well as a guide to the choice of the best surfactant for a given situation. Several works reviewing the hterature in that area are cited in the Bibliography, and some specific examples will be noted in the appropriate contexts of later chapters. 

The basic concepts behind the factors governing the adsorption of surface-active molecules at interfaces has already been mentioned several times in 

For now, the discussion will be limited to some general concepts related to adsorption at liquid-fluid interfaces, such as some general relationships between surfactant structure and the rate and effect of adsorption. 

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As mentioned in the Sec. 1, an important thermo-dynamic parameter of a surfactant adsorption monolayer is its Gibbs (surface) elasticity. The physical concept of surface elasticity is the most transparent for monolayers of insoluble surfactants, for which it was initially introduced by Gibbs (18, 19). The increments A a and AT in the definition of Gibbs elasticity ... 

The Gibbs elasticity characterizes the lateral fluidity of the surfactant adsorption monolayer. For high values of the Gibbs elasticity the adsorption monolayer at a fluid interface behaves as tangentially immobile. Then, if two oil drops approach each other, the hydro-dynamic flow pattern, and the hydrodynamic interaction as well, is the same as if the drops were solid particles, with the only differenee that under some conditions they could deform in the zone of contact. For lower values of the Gibbs elastieity the... 

Gibbs and Insoluble Monolayers The adsorption of surfactant molecules at the surface of a liquid can be so strong that a monomolecular film (Gibbs monolayer) of unidirectionally ordered surfactants is formed (Fig. 5). Since the decrease in surface tension is directly related to the surface excess adsorption of the surfactant by the Gibbs adsorption equation (Eq. 6), the formation of the Gibbs mono-layer can be monitored by decrease of the surface tension. The maximum number of molecules filling a given area depends upon the area occupied by each molecule. 

A schematic comparison of the two situations of the film balance is illustrated in Fig. 6. The trough is filled with pure water and the left and right side of the surface is separated. At the beginning, the surface of the subphase is completely clean and the surface tension of each side is that of pure water (yo)- In case (a), a soluble surfactant is placed on one side of the trough. A Gibbs monolayer will be immediately formed by adsorption of the surfactant molecules to the surface on this side, while the residual molecules will be dissolved in the subphase. The surfactant molecule in the aqueous phase can diffuse to the opposite side of the barrier therefore, after the system reaches equilibrium, the formation of the saturated film accompanying the same decrease of the surface tension will be achieved on both the sides. 

Surfactant molecules consist of two blocks, one with affinity to water and the other to oil. When added to the mixture of oil and water, they self-assemble at the oil-water interface, so that the hydrophilic block stays in water and the hydrophobic one remains in oil. As the surfactant concentration increases, the monolayer at the oil-water interface becomes more densely populated and the interfacial tension decreases, as predicted by the Gibbs adsorption equation ... 

When a surfactant is injected into the liquid beneath an insoluble monolayer, surfactant molecules may adsorb at the surface, penetrating between the monolayer molecules. However it is difficult to determine the extent of this penetration. In principle, equilibrium penetration is described by the Gibbs equation, but the practical application of this equation is complicated by the need to evaluate the dependence of the activity of monolayer substance on surface pressure. There have been several approaches to this problem. In this paper, previously published surface pressure-area Isotherms for cholesterol monolayers on solutions of hexadecy1-trimethyl-ammonium bromide have been analysed by three different methods and the results compared. For this system there is no significant difference between the adsorption calculated by the equation of Pethica and that from the procedure of Alexander and Barnes, but analysis by the method of Motomura, et al. gives results which differ considerably. These differences indicate that an independent experimental measurement of the adsorption should be capable of discriminating between the Motomura method and the other two. 

In principle, the penetration or adsorption of surfactant, Tg is given by the Gibbs equation. For a non-ionic monolayer and an ionised surfactant (as in the system examined), this equation is ... 

Another dynamic factor affecting the rate of diffusion transfer, mentioned long ago by Gibbs [9], is the elasticity of the surfactant monolayers which decreases the capillary pressure in small bubbles during their compression and increases it in large bubbles during their expansion. This effect is most pronounced in bubbles whose adsorption layers contain insoluble surfactants. Analysis of the influence of this factor on diffusion transfer has been reported in [486], However, no experimental verification has been performed so far. 

The main problem in the thermodynamic theory of penetration is to determine the dependence of the adsorption of a soluble surfactant on its bulk concentration for any given (constant) adsorption of the insoluble surfactant (surface concentration), and the onset of the surface pressure jump in mixed monolayers, caused by the adsorption of a soluble surfactant in the presence of the insoluble component. There exist several main theoretical approaches to the description of the penetration thermodynamics. One is based on the Gibbs adsorption equation for multicomponent monolayers [143-146], Another approach, initially proposed by Pethica... 

The definition of Gibbs elasticity given by Eq. (19) corresponds to an instantaneous (Aft t ) dilatation of the adsorption layer (that contributes to o ) without affecting the diffuse layer and o. The dependence of o on Ty for nonionic surfactants is the same as the dependence of o on Ty for ionic surfactants, cf Eqs (7) and (19). Equations (8) and (20) then show that the expressions for Eq in Table 2 are valid for both nonionic and ionic siufactants. The effect of the surface electric potential on the Gibbs elasticity Eq of an ionic adsorption monolayer is implicit, through the equilibrium siufactant adsorption T y which depends on the electric properties of the interface. To illustrate this let us consider the case of Langmuir adsorption isotherm for an ionic surfactant (17) ... 

Here, is the so called foam parameter, and t is the viscosity m the surfactant-containing phase (Liquid 1 in Fig. 15) the influence of the transition zone film - bulk liquid is not accounted for in Eq. (76). Note that the bulk and surface diffusion fluxes (see the terms with and Z) in the latter equation), which tend to damp the surface tension gradients and to restore the uniformity of the adsorption monolayers, accelerate the film thinning (Fig. 1). Moreover, since Din Eq. (76) is divided by the film thickness h, the effect of surface diflhsion dominates that of bulk diffusion for small values of the film thickness. On the other hand, the Gibbs elasticity Eq (the Marangoni effect) decelerates the thinning. Equation (76) predicts that the rate of... 

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