Solid Interface

Here the problem is very much simpler, mainly because the surface areas of particulate or porous solids are much higher than the available areas of liquid/vapour interfaces. If no component of the solution penetrates into the solid (or), then rf = 0 and equation (A111.5) can be written, for a binary solution, in the form 

The meaning of the numerator of this expression is seen by considering the difference between the amount of component 2 actually present (rc°xf ) and the amount that would have been present if the equilibrium concentration had been uniform in the liquid up to the solid surface (nnr.(). This is the surface excess corresponding to the choice of the solid surface as the dividing surface  

In dilute solution (x — 1) the relative and reduced surface excesses become equal. 

Kxpcrimcntal data for adsorption from solution are usually expressed as specific reduced surface excess isotherms, sometimes called composite isotherms, in which n()Ax, /m is plotted as a function 

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A belief that solid interfaces are easier to understand than liquid ones shifted emphasis to the former but the subjects are not really separable, and the advances in the one are giving impetus to the other. There is increasing interest in films of biological and of liquid crystalline materials because of the importance of thin films in microcircuitry (computer chips ), there has been in recent years a surge of activity in the study of deposited mono- and multilayers. These Langmuir-Blodgett films are discussed in Section XV-7. 

In the context of the structural perturbations at fluid-solid interfaces, it is interesting to investigate the viscosity of thin liquid films. Eaily work on thin-film viscosity by Deijaguin and co-workers used a blow off technique to cause a liquid film to thin. This work showed elevated viscosities for some materials [98] and thin film viscosities lower than the bulk for others [99, 100]. Some controversial issues were raised particularly regarding surface roughness and contact angles in the experiments [101-103]. Entirely different types of data on clays caused Low [104] to conclude that the viscosity of interlayer water in clays is greater than that of bulk water. 

The illustrative data presented in Table VII-3 indicate that the total surface energy may amount to a few tenths of a calorie per gram for particles on the order of 1 /xm in size. When the solid interface is destroyed, as by dissolving, the surface energy appears as an extra heat of solution, and with accurate calorimetry it is possible to measure the small difference between the heat of solution of coarse and of finely crystalline material. 

D. Dependence of Other Physical Properties on Surface Energy Changes at a Solid Interface... 

The usual situation, true for the first three cases, is that in which the reactant and product solids are mutually insoluble. Langmuir [146] pointed out that such reactions undoubtedly occur at the linear interface between the two solid phases. The rate of reaction will thus be small when either solid phase is practically absent. Moreover, since both forward and reverse rates will depend on the amount of this common solid-solid interface, its extent cancels out at equilibrium, in harmony with the thermodynamic conclusion that for the reactions such as Eqs. VII-24 to VII-27 the equilibrium constant is given simply by the gas pressure and does not involve the amounts of the two solid phases. 

This chapter and the two that follow are introduced at this time to illustrate some of the many extensive areas in which there are important applications of surface chemistry. Friction and lubrication as topics properly deserve mention in a textbook on surface chemistiy, partly because these subjects do involve surfaces directly and partly because many aspects of lubrication depend on the properties of surface films. The subject of adhesion is treated briefly in this chapter mainly because it, too, depends greatly on the behavior of surface films at a solid interface and also because friction and adhesion have some interrelations. Studies of the interaction between two solid surfaces, with or without an intervening liquid phase, have been stimulated in recent years by the development of equipment capable of the direct measurement of the forces between macroscopic bodies. 

There has been a general updating of the material in all the chapters the treatment of films at the liquid-air and liquid-solid interfaces has been expanded, particularly in the area of contemporary techniques and that of macromolecular films. The scanning microscopies (tunneling and atomic force) now contribute more prominently. The topic of heterogeneous catalysis has been expanded to include the well-studied case of oxidation of carbon monoxide on metals, and there is now more emphasis on the flexible surface, that is, the restructuring of surfaces when adsorption occurs. New calculational methods are discussed. 

The liquid-solid interface, which is the interface that is involved in many chemical and enviromnental applications, is described m section A 1.7.6. This interface is more complex than the solid-vacuum interface, and can only be probed by a limited number of experimental techniques. Thus, obtaining a fiindamental understanding of its properties represents a challenging frontier for surface science. 

One of tlie less explored frontiers in atomic-scale surface science is the study of the liquid-solid interface. 

This interface is critically important in many applications, as well as in biological systems. For example, the movement of pollutants tln-ough the enviromnent involves a series of chemical reactions of aqueous groundwater solutions with mineral surfaces. Although the liquid-solid interface has been studied for many years, it is only recently that the tools have been developed for interrogating this interface at the atomic level. This interface is particularly complex, as the interactions of ions dissolved in solution with a surface are affected not only by the surface structure, but also by the solution chemistry and by the effects of the electrical double layer [31]. It has been found, for example, that some surface reconstructions present in UHV persist under solution, while others do not. 

Studies of the liquid-solid interface can be divided into those that are perfonned ex situ and those perfomied in situ. In an ex situ experiment, a surface is first reacted in solution, and then removed from the solution and transferred into a UFIV spectrometer for measurement. There has recently been, however, much work aimed at interrogating the liquid-solid interface in situ, i.e. while chemistry is occurring rather than after the fact. 

Figure Al.7.14. 3.4 mn x 3.4 mn STM images of 1-docosanol physisorbed onto a graphite surface in solution. This image reveals the hydrogen-bonding alcohol molecules assembled in lamellar fashion at the liquid-solid interface. Each bright circular region is attributed to the location of an individual hydrogen...

Cyr D M, Venkataraman B and Flynn G W 1996 STM investigations of organic molecules physisorbed at the liquid-solid interface Chem. Mater. 8 1600... 

Drake B, Sonnenfeld R, Schneir J and Hansma P K 1987 Scanning tunneling microscopy of process at liquid-solid interfaces Surf. Sc/. 181 92... 

The characterization of surfaces undergoing corrosion phenomena at liquid-solid and gas-solid interfaces remains a challenging task. The use of STM for in situ studies of corrosion reactions will continue to shape the atomic-level understanding of such surface reactions. 

Yackoboski K, Yeo Y H, McGonigal G C and Thomson D J 1992 Molecular position at the liquid/solid interface measured by voltage-dependent imaging with the STM Ultramicroscopy 42-44 963... 

The power of optical spectroscopies is that they are often much better developed than their electron-, ion- and atom-based counterparts, and therefore provide results that are easier to interpret. Furtlienuore, photon-based teclmiques are uniquely poised to help in the characterization of liquid-liquid, liquid-solid and even solid-solid interfaces generally inaccessible by other means. There has certainly been a renewed interest in the use of optical spectroscopies for the study of more realistic systems such as catalysts, adsorbates, emulsions, surfactants, self-assembled layers, etc. 

Figure Bl.22.8. Sum-frequency generation (SFG) spectra in the C N stretching region from the air/aqueous acetonitrile interfaces of two solutions with different concentrations. The solid curve is the IR transmission spectrum of neat bulk CH CN, provided here for reference. The polar acetonitrile molecules adopt a specific orientation in the air/water interface with a tilt angle that changes with changing concentration, from 40° from the surface nonnal in dilute solutions (molar fractions less than 0.07) to 70° at higher concentrations. This change is manifested here by the shift in the C N stretching frequency seen by SFG [ ]. SFG is one of the very few teclnhques capable of probing liquid/gas, liquid/liquid, and even liquid/solid interfaces.

One of the most important advances in electrochemistry in the last decade was tlie application of STM and AFM to structural problems at the electrified solid/liquid interface [108. 109]. Sonnenfield and Hansma [110] were the first to use STM to study a surface innnersed in a liquid, thus extending STM beyond the gas/solid interfaces without a significant loss in resolution. In situ local-probe investigations at solid/liquid interfaces can be perfomied under electrochemical conditions if both phases are electronic and ionic conducting and this... 

Figure C2.3.14. Isolated surfactant modes of adsorjDtion at liquid-solid interfaces for a surfactant having a distinct headgroup and hydrophobic portion (dodecyltrimetlrylammonium cation) (a), (b) headgroup specific interaction (c), (d) hydrophobic tail interaction, (e),(f) headgroup and tail interactions.

Because densification occurs via tire shrinkage of tliennodynamically unstable pores, densification and microstmcture development can be assessed on tire basis of tire dihedral angle, 0, fonned as a result of tire surface energy balance between tire two solid-vapour and one solid-solid interface at tire pore-grain boundary intersection [, 78, 79 and 80],... 

By relating the work required to force a volume dp" of mercury into the pore of a solid to the work required to form an element d/4 of mercury-solid interface, and making use of the Young-Dupre equation (3.70) one arrives at the expression... 

Microjet Formation during Cavitation at Liquid—Solid Interfaces... 

Conduction with Change of Phase A special type of transient problem (the Stefan problem) involves conduction of heat in a material when freezing or melting occurs. The liquid-solid interface moves with time, and in addition to conduction, latent heat is either generated or absorbed at the interface. Various problems of this type are discussed by Bankoff [in Drew et al. (eds.). Advances in Chemical Engineering, vol. 5, Academic, New York, 1964]. 

The definition of the heat-transfer coefficient is arbitrary, depending on whether bulk-fluid temperature, centerline temperature, or some other reference temperature is used for ti or t-. Equation (5-24) is an expression of Newtons law of cooling and incorporates all the complexities involved in the solution of Eq. (5-23). The temperature gradients in both the fluid and the adjacent solid at the fluid-solid interface may also be related to the heat-transfer coefficient ... 

Heat transfer by nucleate boiling is an important mechanism in the vaporization of liqmds. It occurs in the vaporization of liquids in kettle-type and natural-circulation reboilers commonly usea in the process industries. High rates of heat transfer per unit of area (heat flux) are obtained as a result of bubble formation at the liquid-solid interface rather than from mechanical devices external to the heat exchanger. There are available several expressions from which reasonable values of the film coefficients may be obtained. 

The reaction kinetics approximation is mechanistically correct for systems where the reaction step at pore surfaces or other fluid-solid interfaces is controlling. This may occur in the case of chemisorption on porous catalysts and in affinity adsorbents that involve veiy slow binding steps. In these cases, the mass-transfer parameter k is replaced by a second-order reaction rate constant k. The driving force is written for a constant separation fac tor isotherm (column 4 in Table 16-12). When diffusion steps control the process, it is still possible to describe the system hy its apparent second-order kinetic behavior, since it usually provides a good approximation to a more complex exact form for single transition systems (see Fixed Bed Transitions ). 

Detention Test This test utihzes a 1- to 4-L beaker or similar vessel. The sample is placed in the container, flocculated by suitable means if required, and allowed to settle. Smah samples for suspended-sohds analysis are withdrawn from a point approximately midway between liquid surface and settled solids interface, taken with sufficient care that settled solids are not resuspended. Samphng times may be at consecutively longer intei vals, such as 5, 10, 20, 40, and 80 min. 

Thermodynamics of Wetting. The fundamental objective of flotation is to contact solid particles suspended in water with air bubbles (Fig. 19-65 ) and cause a stable bubble-particle attachment (Fig. 19-65Z ). It is seen that attachment of the particle to an air bubble destroys the solid-water and air-water interfaces and creates air-solid interface. The free energy change, on a unit area basis, is given by... 

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