Fluid-solid interface, adsorption studies

Adsorption at the fluid-solid interface for interaction site systems has been studied in two contexts adsorption from dense fluids, and adsorption from the dilute gas phase. Work in the latter context has been focused particularly on submonolayer adsorption on graphite surfaces. 

Fundamental studies on the adsorption of supercritical fluids at the gas-solid interface are rarely cited in the supercritical fluid extraction literature. This is most unfortunate since equilibrium shifts induced by gas phase non-ideality in multiphase systems can rarely be totally attributed to solute solubility in the supercritical fluid phase. The partitioning of an adsorbed specie between the interface and gaseous phase can be governed by a complex array of molecular interactions which depend on the relative intensity of the adsorbate-adsorbent interactions, adsorbate-adsorbate association, the sorption of the supercritical fluid at the solid interface, and the solubility of the sorbate in the critical fluid. As we shall demonstrate, competitive adsorption between the sorbate and the supercritical fluid at the gas-solid interface is a significant mechanism which should be considered in the proper design of adsorption/desorption methods which incorporate dense gases as one of the active phases. 

The aim of this investigation has been to set-up a real model for precipitate flotation which would comprise adsorption processes at both the aicdiquid and liquid-solid interfaces, as well as some elements of fluid mechanics. All the parameters in the model have been determined experimentally. The initial concentrations of both DBS and Cu(0H)2 in the experiment have been varied in parallel. Recoveries of the Rotated species have been determined in order to calculate their real and maximal (theoretical) values for the surface concentrations. Formation of the particle-bubble aggregates has been monitored by determining the number and diameter of both the particles and air bubbles. The collector adsorption in the flotation systems has been studied via recovery of DBS itself on air bubbles, as well as in the presence of Cu(0H)2. 

The discussion in this chapter has focused on the properties of liquids at interfaces. A related area of contemporary research is the study of solid gas interface. The solid surface is quite different in that atomic or molecular components of a solid are relatively motionless compared to those of liquid. For this reason it is easier to define a plane associated with a well-defined solid surface. The approach to studying adsorption on solids has been more molecular with the development of sophisticated statistical mechanical models. On the other hand, the study of liquid I gas and liquid liquid interfaces has been much more macroscopic in approach with a firm connection to classical thermodynamics. As the understanding of liquids has improved at the molecular level using contemporary statistical mechanical tools, these methods are being applied now to fluids at interfaces. 

Flow adsorption calorimetry is recognized as an important method to study adsorption/desorption phenomena at solid-fluid interfaces. According to a review by Groszek [37], flow adsorption microcalorimetiy is now developed to a point where it provides accurate and reliable adsorption and desorption data for events occurring at the solid-liquid or gas-solid interface over a wide range of temperatures, pressures and solution concentrations. 

An interesting question that arises is what happens when a thick adsorbed film (such as reported at for various liquids on glass [144] and for water on pyrolytic carbon [135]) is layered over with bulk liquid. That is, if the solid is immersed in the liquid adsorbate, is the same distinct and relatively thick interfacial film still present, forming some kind of discontinuity or interface with bulk liquid, or is there now a smooth gradation in properties from the surface to the bulk region This type of question seems not to have been studied, although the answer should be of importance in fluid flow problems and in formulating better models for adsorption phenomena from solution (see Section XI-1). 

The characterization of surface activity of fillers is obtained by use of several analytical techniques [1]. Examples of them are inverse gas chromatography [1, 2], the adsorption of a low molecular weight analog of elastomers [3], the adsorption of elastomer chains fi om dilute solutions [4], the wettability, viscosity of PDMS fluids in the boundary layer at the surface of solids [5], the determination of the specific surface area, and the analysis of surface groups [1]. It should, however, be mentioned that the results obtained by these methods do not provide direct information on the elastomer behavior at the interface, due to the use of small probe molecules or the presence of a solvent in the systems studied. 

The use of an evanescent wave to excite fluorophores selectively near a solid-fluid interface is the basis of the technique total internal reflection fluorescence (TIRF). It can be used to study theadsorption kinetics of fluorophores onto a solid surface, and for the determination of orientational order and dynamics in adsorption layers and Langmuir-Blodgett films. TIRF microscopy (TIRFM) may be combined with FRAP ind FCS measurements to yield information about surface diffusion rates and the formation of surface aggregates. 

This book deals mainly with dynamic properties of amphiphiles at liquid/air and liquid/liquid interfaces rather than at solid/liquid interfaces. However static and dynamic contact angles are discussed in Appendix 3B as these phenomena are determined by the kinetics of adsorption of surfactants also at the fluid interface. Some specific aspects of lateral transport phenomena studied by many authors are briefly review in Appendix 3D. 

Work in the area of simultaneous heat and mass transfer has centered on the solution of equations such as 1—18 for cases where the structure and properties of a solid phase must also be considered, as in drying (qv) or adsorption (qv), or where a chemical reaction takes place. Drying simulation (45—47) and drying of foods (48,49) have been particulady active subjects. In the adsorption area the separation of multicomponent fluid mixtures is influenced by comparative rates of diffusion and by interface temperatures (50,51). In the area of reactor studies there has been much interest in monolithic and honeycomb catalytic reactions (52,53) (see Exhaust CONTROL, industrial). For these kinds of applications psychrometric charts for systems other than air—water would be useful. The construction of such has been considered (54). 

Adsorption layers of the same kind as at fluid interfaces are also formed at low-energy solid -water surfaces, as it was established on PE, polystyrene, paraffin, carbon black, and other related materials. The classical Langmuir or Frumkin adsorption isotherm is often applicable to describe this behaviour. Studies on surfactant adsorption at various solid surfaces have been summarised in a great number of reviews [2, 7, 8, 54, 98, 101, 111, 121, 126, 141, 144, 145, 177, 186, 190, 194-198]. The adsorption at the solid/liquid interfaces is governed by a number of factors ... 

It follows from the above considerations, that at present and in the near future theoretical descriptions requiring simple but realistic models of the adsorption process will be of great importance in the studies of adsorption at the solid/fluid interface. In the generally accepted model of the adsorption system, the real concentration profile is replaced by a step function which divides the fluid phase between the surface and bulk phases. These phases are at the thermodynamic equilibrium with the thermodynamically inert adsorbent which creates a potential energy field above the surface. The inertness of the solid is believed to be true in the case of physical adsorption, but there are several instances when it can be questioned [54]. 

During the second half of this century, the study of adsorption on solid surfaces has steadily gained considerable interest. Adsorption at the solid-fluid interface plays a signiflcant role in various disciplines of the natural science and underlies a number of technological processes. 

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