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Current technologies to modify surfaces and control the surface properties are reviewed, with particular emphasis on alterations for biomedical and bioanalytical applications. A brief tutorial on the mechanisms of biomolecular adsorption provides background for understanding the relationship between surface wettability and biomolecular (particularly protein) adsorption. Methods for modifying surface wettability, materials that minimize or prevent fouling, and techniques for assessing protein fouling and the effectiveness of surface modifications are reviewed. Examples of “designer surfaces” are given, ranging from dynamic coatings in which small molecules or polymers are adsorbed on device surfaces, to covalent modifications such as PEGylation, hydrogel assembly, and the use of functionalized alkyl silanes. New device materials, including hydrogels made from PEGylated monomers, biodegradable polyesters, and photocurable perfluoropolyethers, are also discussed. The chapter closes with approaches that lead to the direct and indirect capture of proteins and peptides and the integration of live cells with microfluidic devices.

Polanyi served as a medical officer in the Austro-Hungarian Army. As time allowed, he worked on the Nernst Heat Theorem, a novel theory about the adsorption of gases and the paradoxes associated with isotopes. His experience of war left him with a life-long concern about understanding the strength of the liberal tradition that had enriched European civilization from 1870 to 1914 as well as the loss of faith in the tradition that had led Europe down the path of self-destruction.

In preparation for a scientific career in Germany, Polanyi chose Australian citizenship and was baptized as a Roman Catholic. During his doctoral studies at Karlsruhe, Polanyi completed his thesis on adsorption and began to work on reaction kinetics. He became engaged to Magda Kemeny, who was also working on a doctorate at the University.

Fritz Haber hired Polanyi to work in the Fiber Chemistry Group of the Kaiser Wilhelm Institute in Berlin. Polanyi helped develop the rotating-crystal method of X-ray crystallography, made solid contributions to understanding the structure of cellulose, pressed forward with his work on adsorption catalysis and electrostatic dipoles, laid the foundation for transition rate theory in reaction kinetics, and investigated the bond strength of crystals; he was also forced to give up a cherished theory about quantum jumps in reaction kinetics, which taught him an important lesson about how scientists work together to distinguish real discoveries from mistaken surmises. Polanyi married Magda Kemeny on February 21, 1921, in a civil ceremony; their first child, George Michael Polanyi, was born on October 1, 1922.

After establishing his credentials as a scientist, Polanyi was transferred to the Kaiser Wilhelm Institute for Physical Chemistry and Electrochemistry and was allowed to focus on reaction rates and transition state theory; the group employed gas-flame experiments to measure reaction rates and calculate the activation energies in them. Polanyi's interest in economics was stimulated by inflation, unemployment and social upheavals in Germany, debates with his brother, Karl Polanyi, who advocated a form of Christian socialism, economic conditions in the Soviet Union, and the rise of nihilism. Polanyi's second son, John Charles Polanyi, was born on January 23, 1929. Developments in quantum theory and dipole-dipole interactions confirmed Polanyi's theory of adsorption potential.

This chapter discusses dispersed matter. Topics covered include disperse phases, stabilization of disperse systems, the Gibbs relation, adsorption isotherms, surface films of sparingly soluble substances, and colloid chemistry.

Adsorption experiments involving soil particles typically are performed in a sequence of three steps: (1) reactio of an adsorptive (ion or molecule) with a soil contacting an aqueous solution of known composition under controlled temperature and applied pressure for a prescribed period of time, (2) separationof the wet soil slurry from the supernatant aqueous solution, and (3) quantitationof the ion or molecule of interest, both in the aqueous solution and in the separated soil slurry along with its entrained soil solution. The reaction step can be performed in either a closed system (batch reactor) or an open system (flow-through reactor), and it can proceed over a time period that is either relatively short (to investigate adsorption kinetics) or very long (to investigate adsorption equilibration). The separation step is similarly open to choice, with centrifugation, filtration, or gravitational settling being conventional methods to achieve separation. The quantitation step, in principle, should be designed not only to determine the moles of adsorbate and unreacted adsorptive, but also to verify whether unwanted side reactions, such as precipitation of the adsorptive or dissolution of the adsorbent, have influenced the experiment. After reaction between an adsorptive i and a soil adsorbent, the moles of i adsorbed per kilogram of dry soil is calculated with the standard equation ni ≡ niT − Mwmi where niT is the total moles of species i per kilogram dry soil in a slurry (batch process) or a soil column (flow-through process), Mw is the gravimetric water content of the slurry or soil column (measured in kilograms water per kilogram dry soil), and mi is the molality (moles per kilogram water) of species i in the supernatant solution (batch process) or effluent solution (flow-through process). Equation 8.1 defines the surface exces, ni, of an ion or molecule adsorptive that has become an adsorbate. Formally, ni is the excess number of moles of i per kilogram soil relative to its molality in the supernatant solution. As mentioned in Section 7.2, this surface excess may be a positive, zero, or negative quantity.

The variation of interfacial tension of a solution with surfactant concentration in bulk can be used, in conjunction with the Gibbs adsorption equation, to probe the behaviour of adsorbed surfactant monolayers. An adsorption isotherm equation expresses the relationship between bulk and surface concentrations of surfactant, and is used to determine thermodynamic quantities of surfactant adsorption. The variation of the surface pressure of a surfactant monolayer with the surface concentration is described by a surface equation of state, which reflects something of the nature of a monolayer. The way in which inorganic electrolytes modify the adsorption and monolayer behaviour of ionic surfactants is explained, and adsorption from surfactant mixtures is also introduced. In the Appendix, the discussion is extended to the treatment of adsorbed monolayers as two-dimensional solutions of surfactant with solvent molecules, rather than as two-dimensional gases.

Small particles can adsorb strongly at fluid interfaces and form monolayers which can be studied using a Langmuir trough. For sufficiently large particles the monolayers can be viewed microscopically. The driving force for particle adsorption is the concomitant removal of fluid/fluid interface. For very small adsorbed particles, the free energy of forming the three-phase contact line around particles (hence the line tension) may also contribute significantly to the free energy of adsorption. Adsorption can be enhanced by having areas of particle surface with different wettability (Janus particles). Monolayers have structures dependent on lateral interactions between particles; for particles at the oil/water interface, electrical repulsion through oil is often the dominant interaction, which can give rise to highly ordered monolayers. Adsorbed particles can either inhibit or facilitate the formation of stable thin liquid films, depending on particle wettability.

The external surface of a material has an atomic or molecular structure that is different from the bulk material. So does any internal interface within a material. Because of this, the energy of a material or any grain or particle within it increases with the curvature of its bounding surface, as described by the Gibbs-Thomson equation. This chapter explains how surfaces control the nucleation of new phases during reactions such as solidification and precipitation, the coarsening and growth of particles during heat treatment, the equilibrium shape of crystals, and the surface adsorption and segregation of solutes and impurities. The Gibbs-Thomson was predated by a number of related equations; it is not clear whether it is named after J. J. Thomson or William Thomson (Lord Kelvin); and it was not put into its current usual form until after Gibbs’, Thomson’s and Kelvin’s time. J. J. Thomson was the third Cavendish Professor of Physics at Cambridge University. He discovered the electron, which had a profound impact on the world, notably via Thomas Edison’s invention of the light bulb, and subsequent building of the world’s first electricity distribution network. William Thomson was Professor of Natural Philosophy at Glasgow University. He made major scientific developments, notably in thermodynamics, and he helped build the first trans-Atlantic undersea telegraph. Because of his scientific pre-eminence, the absolute unit of temperature, the degree Kelvin, is named after him.

Structural charge arises on the surfaces of soil mineral particles in which either cation vacancies or isomorphic substitutions of cations by cations of lower valence occur. The principal minerals bearing structural charge are therefore the micas (Section 2.2), the 2:1 clay minerals (Section 2.3), or the Mn(IV) oxide, birnessite (Section 2.4). These three classes of mineral are all layer type and the cleavage surface on which their structural charge is manifest is a plane of O ions. The plane of O ions on the cleavage surface of a layer-type aluminosilicate is called a siloxane surface.This plane is characterized by hexagonal symmetry in the configuration of its constituent O ions, as shown at the top of Fig. 2.3 and, more explicitly, on the right side of Fig. 2.4, where a portion of the siloxane surface of the micas is depicted. Reactive molecular units on the surfaces of soil particles are termed surface functional groups. The functional group associated with the siloxane surface is the roughly hexagonal (strictly speaking, ditrigonalbecause the hexagonal symmetry is distorted when the tetrahedral sheet is fused to an octahedral sheet to form a layer) cavity formed by six corner-sharing silica tetrahedra. This cavity has a diameter of about 0.26 nm. The reactivity of the siloxane cavity depends on the nature of the electronic charge distribution in the layer structure. If there are no nearby isomorphic cations substitutions to create a negative charge, the O ions bordering the siloxane cavity function as an electron cloud donor that can bind molecules weakly through the van der Waals interaction. These interactions are akin to those underlying the hydrophobic interaction, discussed in Section 3.5, because the O in the siloxane surface can form only very weak hydrogen bonds with water molecules. Therefore, uncharged patches on siloxane surfaces may be considered hydrophobic regions to a certain degree, with, accordingly, an attraction for hydrophobic organic molecules. However, if isomorphic substitution of Al3+ by either Fe2+ or Mg2+ occurs in the octahedral sheet, the resulting structural charge is manifest on the siloxane cavities, as discussed in Section 2.3.

Electrostatic adsorption of anions is one of the important characteristics of variable charge soils. This is caused by the fundamental feature that these soils carry a large quantity of positive surface charge. However, because these soils carry positive as well as negative surface charges, they may exert both attractive and repulsive forces on anions. Therefore, the situation in the adsorption of anions by these soils may be quite complex. There may also be the occurrence of negative adsorption of anions. Besides, for some anion species both electrostatic force and specific force may be involved during their interactions with variable charge soils. As shall be seen in this chapter, such specific force may be operative even for some anion species such as chloride that are generally considered as solely electrostatic in nature during adsorption. Because of historical reasons, the literature on electrostatic adsorption of anions by soils is very limited. Nevertheless, as shall be seen in this chapter, the topic is of interest in both theory and practice. In the present chapter, adsorption of anions shall be discussed mainly from the viewpoint of electrostatic adsorption. The other type of adsorption, specific adsorption or coordination adsorption, shall be dealt with in Chapter 6. The radius of anions is generally much larger than that of cations. Thus, the charge density on anions would be low. When hydrated, because of the smaller ion-dipole force exerted on water molecules, anions are less hydrated than cations. This can be seen in Table 4.1. The rH/rc ratio for cations ranges from 2.22 to 6.37, while that for anions is smaller than 2 except for F-. The orientation of water molecules around anions, especially in the primary hydration region, is also different from that around cations (Conway, 1981). Because of the small rH/rc ratio, hydration does not induce the change in order of size when anions of the same valency are compared. For example, the crystal radii of Cl-, NO3-, and ClO4- are 0.181, 0.264, and 0.292 nm, respectively, while the hydrated radii of these ions are 0.332, 0.335, and 0.338, respectively.

The fate and transport of a variety of chemicals migrating from industrial and municipal waste disposal sites, or applied to agricultural lands, is increasingly becoming a concern. Once released into the subsurface, these chemicals arc subject to a large number of simultaneous physical, chemical, and biological processes, including sorption-desorption, volatilization, and degradation. Depending upon the type of organic chemical involved, transport may also be subject to multiphase flow that involves partitioning of the chemical between different fluid phases. Many models of varying degree of complexity and dimensionality have been developed during the past several decades to quantify the basic physicochemical processes affecting transport in the unsaturated zone. Models for variably saturated water flow, solute transport, aqueous chemistry, and cation exchange were initially developed mostly independently of each other, and only recently has there been a significant effort to couple the different processes involved. Also, most solute transport models in the past considered only one solute. For example, the processes of adsorption-desorption and cation exchange were often accounted for by using relatively simple linear or nonlinear Freundlich isotherms such that all reactions between the solid and liquid phases were forced to be lumped into a single distribution coefficient, and possibly a nonlinear exponent. Other processes such as precipitation-dissolution, biodegradation, volatilization, or radioactive decay were generally simulated by means of simple first- and/or zero-order rate processes. These simplifying approaches were needed to keep the mathematics relatively simple in view of the limitations of previously available computers. The problem of coupling models for water flow and solute transport with multicomponent chemical equilibrium and nonequilibrium models is now increasingly being addressed, facilitated by the introduction of more powerful computers, development of more advanced numerical techniques, and improved understanding of the underlying transport processes. One major frustrating issue facing soil scientists and hydrologists in dealing with the unsaturated zone, both in terms of modeling and experimentation, is the overwhelming heterogeneity of the subsurface environment.

The thickness and hence material content of a surface is generally unknown, and there are two common definitions of a surface/interface. In one the surface is treated as a phase distinct from the surrounding bulk phases, and in the other, due to Gibbs, the Gibbs dividing surface is supposed to be a plane, parallel to the physical interface. The former model gives rise to the surface concentrationΓ‎^s of a surfactant, and the Gibbs model introduces the surface excess concentration, Γ‎^σ‎. Some thermodynamic quantities for surfaces (e.g. surface chemical potential and Gibbs free energy for surfaces) are defined. Adsorption lowers interfacial tension by an amount termed the surface pressure, and the Gibbs adsorption equation allows the calculation of Γ‎^s or Γ‎^σ‎ for a surfactant from the variation of interfacial tension of a liquid/fluid interface with surfactant concentration in bulk solution.

Following the rapid formation of the surface of a surfactant so′′lution, the dynamic interfacial tension falls with time as a result of the finite time needed for surfactant adsorption. Surfaces can either be sheared (involving shape change) or dilated (area is changed), and both these processes can give a viscous and/or elastic response. Usually, surfaces of surfactant solutions exhibit a combination of the two and are viscoelastic. If small sinusoidal area changes are imposed on the surface, changes in tension and area are out of phase because surfactant adsorption is relatively slow. The responses to area change are frequency dependent. The complex dilational viscoelastic modulus, ε‎*, has real (elastic) and imaginary (viscous) parts, ε‎′ and ε‎′′, respectively, whose variation with frequency provides insights into relaxation processes occurring at the surface. The way in which dynamic tensions can give insights into the kinetics of surfactant adsorption is explained.

The physical properties of solid/liquid interfaces are more diverse than those of liquid/fluid interfaces, and consequently the interactions giving rise to adsorption of surfactant or polymeric surfactant are more varied. Solid surfaces can be either hydrophilic or hydrophobic, the former being water-wetted and containing polar or ionogenic sites. Electrical charge at the solid surface is neutralized by ions in the inner and outer Helmholtz planes and in the diffuse part of the electrical double layer. Surface charge has a strong influence on adsorption of ionic surfactants. Standard free energies of surfactant adsorption are obtained by use of an appropriate adsorption isotherm such as the Stern–Langmuir equation. Micellar aggregates of various shapes and sizes can also form at solid/liquid interfaces.

Surfaces, or more generally, interfaces, are important in soft matter by two reasons. The weak forces associated with surfaces, such as the surface tension and inter-surface forces, are important in the flow and deformation of soft matter. Many soft matter systems, especially colloidal dispersions, have large internal interfaces, and the properties of interfaces are crucial for bulk properties. This chapter first discusses the capillary phenomena in usual liquids, and provides thermodynamic definitions for surface tensions and surface adsorptions. Second, it describes surfactants, the materials which change the surface properties. Theory for the micelle formation of surfactants and its effect on surface tension are discussed. Third, the chapter explains the inter-surface force – the force acting between surfaces in close proximity – with reference to the stability of colloidal dispersions.

Fly ash is a by-product from the combustion of coal. The 1985 annual US production was estimated to be about 1 x 108 metric tons. The utilization of fly ash during the 1980s remained stable at about 25% per year. Because of its pozzolanic properties, nearly 50% of the utilized fly ash is consumed in the production of cement and concrete. The vast quantity of fly ash that is not being used and its availability throughout the country and worldwide have motivated research for new uses in commerce and industry. Little is known of the organic adsorbent properties of fly ash. However, if they are found to be favorable, the potential commercial applications of the adsorptive characteristics of fly ash could include its use as an adsorbent sandwich for organics in combination with landfill or other dump-site liners, in traps for organics in waste waters, in filters for organics in process air streams, and as a stabilizer for organic wastes in drums. Variables that may affect the adsorbability of the fly ash towards organics in water include temperature; solution pH; and interactions between solute molecules and fly ash, and between solvent molecules and fly ash. Thus, there is an essential need to characterize each coal fly ash type to enable potential correlation between coal fly ash structural properties and the effectiveness of the adsorption characteristics of coal fly ash for immobilizing organic hazardous waste compounds. The composition and properties of pulverized fly ash depend on the type of coal burned and the nature of the combustion process. Thus, fly ashes from different origins may have significantly different sorption properties towards organic compounds of environmental interest. Eastern and western coal fly ashes differ significantly in their physical and chemical properties. The major minerals found in coal fly ash are α-quartz (SiO2), mullite (3A12O3 ·2SiO2), hematite (Fe2O3), magnetite (Fe3O4), lime (CaO), and gypsum (CaSO4·2H2O). Little is known of the coordination state and distribution of siliceous and aluminous material in coal fly ashes. Most siliceous and aluminous materials in fly ash are amorphous and thus are not detected or quantified by X-ray techniques.