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Solid Surfaces—Adsorption

Adsorption is a physicochemical process whereby ionic and nonionic solutes become concentrated from solution at solid-liquid interfaces.3132 Adsorption and desorption are caused by interactions between and among molecules in solution and those in the structure of solid surfaces. Adsorption is a major mechanism affecting the mobility of heavy metals and toxic organic substances and is thus a major consideration when assessing transport. Because adsorption is usually fully or partly reversible (desorption), only rarely can it be considered a detoxification process for fate-assessment purposes. Although adsorption does not directly affect the toxicity of a substance, the substance may be rendered nontoxic by concurrent transformation processes such as hydrolysis and biodegradation. Many chemical and physical properties of both aqueous and solid phases affect adsorption, and the physical chemistry of the process itself is complex. For example, adsorption of one ion may result in desorption of another ion (known as ion exchange). [Pg.795]

Interface and colloid science has a very wide scope and depends on many branches of the physical sciences, including thermodynamics, kinetics, electrolyte and electrochemistry, and solid state chemistry. Throughout, this book explores one fundamental mechanism, the interaction of solutes with solid surfaces (adsorption and desorption). This interaction is characterized in terms of the chemical and physical properties of water, the solute, and the sorbent. Two basic processes in the reaction of solutes with natural surfaces are 1) the formation of coordinative bonds (surface complexation), and 2) hydrophobic adsorption, driven by the incompatibility of the nonpolar compounds with water (and not by the attraction of the compounds to the particulate surface). Both processes need to be understood to explain many processes in natural systems and to derive rate laws for geochemical processes. [Pg.436]

The significance of the development of photoelectron spectroscopy over the last decade for a better understanding of solid surfaces, adsorption, surface reactivity, and heterogeneous catalysis has been discussed. The review is illustrative rather than exhaustive, but nevertheless it is clear that during this period XPS and UPS have matured into well-accepted experimental methods capable of providing chemical information at the molecular level down to 10% or less of a monolayer. The information in its most rudimentary state provides a qualitative model of the surface at a more sophisticated level quantitative estimates are possible of the concentration of surface species by making use of escape depth and photoionization cross-section data obtained either empirically or by calculation. [Pg.92]

As already mentioned, the first step in any heterogeneous catalytic reaction is the adsorption of a gas molecule onto a solid surface. Adsorption heat measurements can provide information about the adsorption process not available using other surface analytical tools. For example, differential heat measurements can provide valuable insights into sites distribution on the catalyst surface as well as quantitative information on the changes in catalyst particle surface chemistry that result from changes in particle size or catalyst support material [148-150],... [Pg.215]

Absorption and Adsorption are not the sam e Absorption is the incorporation (sometimes even consumption) of a matter into a medium (light is absorbed/consumed by a pigment, gas is ab-sorbed/dissolves into a liquid), whereas Adsorption is the adhesion of matter onto a—usually solid—surface (dust on furniture, steam on windscreen, vapours on any solid surface...) Adsorption is further subdivided in chemisorption, in which the matter is bound to a surface by chemical bonds, and phy si sorption, in which the bonding is only a physical effect. The transition between both is fluent. [Pg.151]

Polanyi s scientific work lay most squarely within a physical chemistry that encompassed thermodynamics, X-ray crystallography, the study of reaction rates, and the application of quantum mechanics to the study of molecular forces and transition states. In two particular areas, the investigation of solid-surface adsorption phenomena and X-ray diffraction studies of the properties of solids, Polanyi helped establish new scientific specialities, at the boundaries of physics and chemistry, for studying the solid state. He also turned his research experiences in these fields into a basis for the formulation of a new philosophy of science centered on scientific practice, rather than scientific ideas. [1] It is these themes that I would like to explore, with remarks in my conclusion on Polanyi s influence in solid-state science. [Pg.246]

The transfer of chemicals between two or more environmental compartments or phases can be described by equilibrium partitioning, and knowledge of this partitioning is essential for understanding and describing chemical fate in the environment. Chemical partitioning takes place between adjacent phases such as between a solid and a liquid (dissolution), a liquid and a gas (volatilisation), a solution and a solid surface (adsorption) or a solution and an immiscible liquid (solvent... [Pg.284]

At this time, only a small number of nanoscale processes are characterized with transport phenomena equations. Therefore, if, for example, a chemical reaction takes place in a nanoscale process, we cannot couple the elementary chemical reaction act with the classical transport phenomena equations. However, researchers have found the keys to attaching the molecular process modelling to the chemical engineering requirements. For example in the liquid-vapor equilibrium, the solid surface adsorption and the properties of very fine porous ceramics computed earlier using molecular modelling have been successfully integrated in modelling based on transport phenomena [4.14]. In the same class of limits we can include the validity limits of the transfer phenomena equations which are based on parameters of the thermodynamic state. It is known [3.15] that the flow equations and, consequently, the heat and mass transport equations, are valid only for the... [Pg.48]

For our purposes, adsorption from solution is of more direct relevance than gas adsorption. Most, if not all, topics in the five volumes of FICS Involve one or more elements of it. In the present chapter, the basic elements will be introduced, restricting ourselves to low molecular weight, uncharged adsorbates and solid surfaces. Adsorption of charged species leads to the formation of electrical double layers, which will be treated in chapter 3. Adsorption at fluld/fluid Interfaces follows in Volume III. Adsorption of macromolecules will be Introduced in chapter 5. Between monomers, short oligomers, longer oligomers and polymers there is no sharp transition in the present chapter we shall go as far as non-ionic surfactants, but omit most of the association and micelle formation features, which will be addressed in a later Volume. There will be some emphasis on aqueous systems. [Pg.152]

S.R. Morrison, The Chemical Physics of Surfaces (2nd ed.). Plenum New York, 1990 Introduction/Space charge effects/Experimental methods/Adsorbate-free surfaces/ Bonding of foreign species at the solid surface/Non-volatile foreign additives on the solid surface/Adsorption and desorption/The solid/liquid interface/Photoelfects at semiconductor surfaces... [Pg.1714]

The kinetics of chain-reaction polymerization is illustrated in Fig. 3.28 for a free radical process. Analogous equations, except for termination, can be written for ionic polymerizations. Coordination reactions are more difficult to describe since they may involve solid surfaces, adsorption, and desorption. Even the crystallization of the macromolecule after polymerization may be able to influence the reaction kinetics. The rate expressions, as given in Appendix 7, Fig. A7.1, are easily written under the assumption that the chemical equations represent the actual reaction path. Most important is to derive an equation for the kinetic chain length, v, which is equal to the ratio of propagation to termination-reaction rates. This equation permits computation of the molar mass distribution (see also Sect. 1.3). The concentration of the active species is very small and usually not known. First one must, thus, ehminate [M ] from the rate expression, as shown in the figure. The boxed equation is the important equation for v. [Pg.212]

The presence of surfactants or wetting agents in textile treatment solutions can also introduce other complications in the understanding of the dynamics of the wetting process. Because surfactants adsorb at the SL interface as well as the LV interface, as the liquid front moves across fresh solid surface, adsorption processes will tend to deplete the concentration of available surfactant and may cause localized changes in both ctlv and 6. In many cases, however, adsorption rates at the SL interface is much slower than that at LV interfaces, so that such effects can be taken into consideration without too much difficulty. [Pg.120]

Heterogeneous gas-solid surface adsorption reaction processes can frequently be treated using the same reaction rate law approach used for homogeneous chemical reactions. In such cases, surface sites are often a key reactant, and their concentration is often represented in terms of a fractional occupancy or availability [e.g., O or (1 - O)]. Using these principles, as an example, the rate at which a Pt surface is poisoned by CO gas adsorption can be modeled as 4> = 1 - (1 - 3>o)c where O is the fraction of the Pt surface that is poisoned... [Pg.81]

SOLID SURFACES ADSORPTION AND DESORPTION (OF DIFFERENT SUBSTANCES)... [Pg.87]

The stripes arise by the influence of the boundary layer for the Nl type of chiral nematics, due to the fact that disk-like micelles prefer a flat orientation on many, if not all, solid surfaces adsorption layers of the surfactant form on the solid. Initially hydrophobic surfaces will be covered by a monolayer, the hydrophilic ones by a bilayer. Therefore no spontaneous formation of fingerprints occurs with N -type chiral nematics instead, Grandjean textures are observed. The periodicity of the fingerprints equals half of the pitch, if the sample is prepared correctly. Especially the sample thickness must be large compared to the pitch otherwise there will be helix unwinding by the action of the solid boundary. [Pg.456]

The partition of molecules between two phases can be based on different sorts of equilibrium. Meaningful are equilibria concerning the processes of ion exchange, partition of substances between immiscible solvents (solvent extraction), and accumulation of substances at solid surfaces (adsorption), hi some cases, real chemical bonds are formed, but sometimes only weak forces control the process. These equilibria generally are reversible, and they are mobile, i.e. they tend to react fast to concentration changes. This is a valuable property for sensor applications. Furthermore, they contribute to the accumulation of traces at surfaces, and they are important in manufacturing ordered structures at surfaces. The following discussions are dedicated to equihbria of particular interest. [Pg.72]


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See also in sourсe #XX -- [ Pg.246 ]




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