Adsorption at the Solid-Solution Interface

The discussion so far has been confined to systems in which the solute species are dilute, so that adsorption was not accompanied by any significant change in the activity of the solvent. In the case of adsorption from binary liquid mixtures, where the complete range of concentration, from pure liquid A to pure liquid B, is available, a more elaborate analysis is needed. The terms solute and solvent are no longer meaningful, but it is nonetheless convenient to cast the equations around one of the components, arbitrarily designated here as component 2. 

We suppose that the Gibbs dividing surface (see Section III-5) is located at the surface of the solid (with the implication that the solid itself is not soluble). It follows that the surface excess F, according to this definition, is given by (see Problem XI-9) 

denotes the total number of moles associated with the adsorbed layer, and N and are the respective mole fractions in that layer and in solution at equilibrium. As before, it is assumed, for convenience, that mole numbers refer to that amount of system associated with one gram of adsorbent. Equation XI-24 may be written 

It is important to note that the experimentally defined or apparent adsorption no AN 2/, while it gives F, does not give the amount of component 2 in the adsorbed layer Only in dilute solution where N 2 0 and = 1 is this true. The adsorption isotherm, F plotted against N2, is thus a composite isotherm or, as it is sometimes called, the isotherm of composition change. 

Everett and co-workers [141] describe an improved experimental procedure for obtaining FJ quantities. Some of their data are shown in Fig. XI-10. Note the negative region for n at the lower temperatures. More recent but similar data were obtained by Phillips and Wightman [142]. 

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Probing Surfactant Adsorption at the Solid-Solution Interface by Neutron Reflectometry... 

In the last 10-15 years, neutron reflectometry has been developed into a powerful technique for the study of surface and interfacial structure, and has been extensively applied to the study of surfactant and polymer adsorption and to determine the structure of a variety of thin films [14, 16]. Neutron reflectivity is particularly powerful in the study of organic systems, in that hydrogen/deu-terium isotopic substitution can be used to manipulate the refractive index distribution without substantially altering the chemistry. Hence, specific components can be made visible or invisible by refractive index matching. This has, for example, been extensively exploited in studying surfactant adsorption at the air-solution interface [17]. In this chapter, we focus on the application of neutron reflectometry to probe surfactant adsorption at the solid-solution interface. 

Much of the early studies of surfactant adsorption at the solid-solution interface were based on classical experimental techniques, such as solution depletion [1, 32], fluorescence spectroscopy [2], and measurements of the differential enthalpy of adsorption [2], Such methods have provided much of the basic initial understanding. However, they provide no direct structural information and are difficult to apply to mixtures [23, 34], However, when combined with other techniques, such as NMR and flow microcalorimetry, they provide some insight into the behaviour of mixtures. This was demonstrated by Thibaut et al. [33] on SDS/C10E5 mixtures adsorbed onto silica and by Colombie et al. [34] on the adsorption of SLS/Triton X-405 mixtures onto polystyrene particles. 

Atkin et al. [57] have recently produced a comprehensive review of the mechanisms of cationic adsorption at the solid-solution interface, and made a detailed comparison between AFM, neutron reflectivity, fluorescence quenching,... 

Clearly, neutron reflectivity has contributed much to our understanding of the nature of surfactant adsorption at the solid-solution interface. It has already been successfully applied to an extensive range of systems, as illustrated in this chapter. 

The effect of the end groups of polymer chains on adsorption at the solid-solution interface has been studied. The results show that changes in the end group moiety can be sufficient to enable one polymer to displace an otherwise identical polymer from the interface. The preferential adsorption of end groups is also shown by comparing the adsorption isotherms of linear and cyclic polymers. 

Jada, A., Siffert, B. and Riess, G. (1993) Adsorption at the solid-solution interface and miceUe formation in water of a PEO-PS-PEO triblock copolymer. Colloid. Surf A Physicochem. Engng. Aspects, 75, 203-209. 

Mattson, J. S. and Smith, C. A., 1973, Enhanced protein adsorption at the solid-solution interface dependence on surface charge. Science 181(4104) 1055-1057. 

Among the first heterogeneous systems involving polymers to be studied by the spin-label technique were solids suspended in polymer solutions [S, 9, 27-29]. The aim of these studies was to obtain new and complementary information on the nature and mechanism of polymer molecule adsorption at the solid-solution interface. In most of these studies the labelled polymer adsorbed from solution on the surface of adsorbents, such as silica, yield composite ESR spectra compris-... 

Anion adsorption at the solid-solution interface occurs in the same way cation adsorption occurs except that anions adsorb almost exclusively on cationic surface... 

In the adsorption calorimetry experiment, a small amount n" of the stock solution injected during a given injection is diluted in the supernatant liquid inside the cell and some of the resulting species subsequently adsorb onto solid particles. They displace a certain amount of solvent molecules and can exchange with some pre-adsorbed molecules or ions, because of the limited extent of the adsorption space. The effects of desolvation and re-solvation of various compounds taking part in the displacement process contribute to the competitive character of adsorption at the solid-solution interface. The flow chart of the batch displacement experiment is shown in Fig. 6.22. Since the enthalpy effects accompanying dilution of the stock solution inside the cell should be known, both the dilution and adsorption experiments are carried out under the same conditions (cf.. Fig. 6.19). 

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