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The first part of this chapter presents experimental data on vibrational relaxation times and vibrational populations obtained in shock tubes and shock tunnels. The corresponding methods of measurement such as interferometry, Raman diffusion spectroscopy, infra-red absorption, and emission are described. The TV and VV relaxation times and non-equilibrium vibrational populations deduced from these measurements are then analysed, and measurements of accommodation and exchange coefficients are described. In the second part, values of dissociation rate constants for various species are proposed. Time-resolved spectra of radiating species (CN, C₂) behind strong shock waves are presented; they allow for the analysis of the kinetics of mixtures characteristic of planetary atmospheres. Experimental results of shock stand-off distances over blunt bodies obtained in shock and gun tunnels are compared to computed results. Experimental data on chemical catalycity are discussed. Finally, in the appendices, kinetic models for relaxing and/or reactive mixtures, simulation methods of emission spectra, and precursor radiation measurement in shock tube are discussed.

Many reactions in solution involve acids and bases, and so this chapter examines these important reactions in detail. Topics covered include the ionisation of water, pH, pOH, acids and bases, conjugate acids and conjugate bases, acid and base dissociation constants, the Henderson-Hasselbalch equation, the Henderson-Hasselbalch approximation, buffer solutions and buffer capacity. A unique feature of this chapter is a ‘first principles’ analysis of how a reaction buffered at a particular pH achieves an equilibrium composition different from that of the same reaction taking place in an unbuffered solution. This introduces some concepts which are important in understanding the biochemical standard state, as required for Chapter 23.

Quantitative treatment of acid–base behaviour is presented, including the definition of acids, bases, and dissociation constants, and the important relationships between solution pH, acid strength, acid–base ratios, and species distribution. Molecular structural features of acids, which influence both the acid strengths and their dependence upon solvent, are summarized. They include the bond strength, the ability to stabilize anions and cations by charge dispersion, and the nature of the atom to which the proton is bonded. The acidity of carbon acids, which are widely used in synthetic procedures, is reviewed. Structural rearrangements on ionization of ketones, esters, and nitroalkanes, which allow the negative charge generated on ionization of the C-H bond to reside on oxygen, leads to greatly enhanced acidity. The inductive influence of strongly electron-withdrawing groups ? to the ionizing C-H bond is important for nitriles and sulphones.

Tables of dissociation constants are given for a wide range of substrates in the most extensively studied non-aqueous solvents, including methanol, dimethylsulphoxide, dimethylformamide, acetonitrile, and tetrahydrofuran. Equations representing correlations among the solvents and with water are also included.

In multicellular organisms, thin lipid membranes serve as semipermeable barriers between aqueous compartments. The plasma membrane of the cell separates the cytoplasm from the extracellular space; endothelial cell membranes separate the blood within the vascular space from the rest of the tissue. Properties of the lipid membrane are critically important in regulating the movement of molecules between these aqueous spaces. While certain barrier properties of membranes can be attributed to the lipid components, accessory molecules within the cell membrane—particularly transport proteins and ion channels—control the rate of permeation of many solutes. Transport proteins permit the cell to regulate the composition of its intracellular environment in response to extracellular conditions. The relationship between membrane structure, membrane function, and cell physiology is an area of active, ongoing study. Our interest here is practical: what are the basic mechanisms of drug movement through membranes and how can one best predict the rate of permeation of an agent through a membrane barrier? To answer that question, this section presents rates of permeation measured in some common experimental systems and models of membrane permeation that can be used for prediction. The external surface of the plasma membrane carries a carbohydrate-rich coat called the glycocalyx; charged groups in the glycocalyx, which are provided principally by carbohydrates containing sialic acid, cause the surface to be negatively charged. On average, the plasma membrane of human cells contains, by mass, 50% protein, 45% lipid, and 5% carbohydrate. Given the mass ratio of protein to lipid is ~ 1 : 1, and assuming reasonable values for the average molecular weight and cross-sectional area for each type of molecule (50 × Mw,lipid = Mw,protein; Alipid = 50 Å2 and Aprotein = 1,000 Å2), the area fraction of protein on a typical membrane is ~ 33%. The lipid composition varies in membranes from different cells depending on the type of cell and its function. In addition, the outermost monolayer of lipids, called the outer leaflet, has a different lipid composition from the inner leaflet.

Most biological processes occur in an environment that is predominantly water: a typical cell contains 70-85% water and the extracellular space of most tissues is 99%. Even the brain, with its complex arrangement of cells and myelinated processes, is ≈ 80% water. Drug molecules can be introduced into the body in a variety of ways; the effectiveness of drug therapy depends on the rate and extent to which drug molecules can move through tissue structures to reach their site of action. Since water serves as the primary milieu for life processes, it is essential to understand the factors that determine rates of molecular movement in aqueous environments. As we will see, rates of diffusive transport of molecules vary among biological tissues within an organism, even though the bulk composition of the tissues (i.e., their water content) may be similar. The section begins with the random walk, a useful model from statistical physics that provides insight into the kinetics of molecular diffusion. From this starting point, the fundamental relationship between diffusive flux and solute concentration, Fick’s law, is described and used to develop general mass-conservation equations. These conservation equations are essential for analysis of rates of solute transport in tissues. Molecules that are initially localized within an unstirred vessel will spread throughout the vessel, eventually becoming uniformly dispersed. This process, called diffusion, occurs by the random movement of individual molecules; molecular motion is generated by thermal energy.

Most chromatographic separations are based on chemical interaction between the solute of interest or impurities to be removed and the separation medium. The exception is separations based upon physical properties such as size (e.g. size exclusion chromatography) or transport in a force field (e.g. electrochromatography). The chemical interaction may be weak (e.g. employing van der Waals forces) or very strong (e.g. involving formation of chemical bonds as in covalent chromatography). Whenever separation is based upon attractive forces between the solute and the separation medium, we talk about adsorption chromatography (also when the solute is merely retarded). The chemical interaction between the solute and the adsorbent (the chromatography medium) is governed by the surface properties of the solute and the adsorbent and is in most cases mediated by the mobile phase or additives to the mobile phase. Macromolecules such as proteins display a variety of properties and, ideally, a selected set of properties is utilized for obtaining the required selectivity (i.e. relative separation from other solutes) using a separation medium of complementary properties. This chapter briefly reviews the different types of forces of interaction between solutes and surfaces commonly employed for chromatographic purifications, important properties of solvents, and some basic surface chemical properties of proteins. This, together with a description of some common types of chromatography modes provides a basis for a rational selection of separation mechanism for the purification of proteins and the choice of mobile phase composition to regulate the relative influence of different interaction mechanisms. The separation mechanisms are focused to adsorptive modes with the exception of affinity chromatography which is discussed in Chapter 9. The different attractive forces acting between molecular and particle surfaces include (1): • dispersion forces • electrostatic dipole interactions • electron donor-acceptor forces • formation of covalent bonds All these forces are due to interactions between electric charges (permanent or induced). Dispersion, or London forces, are caused by induced dipole-induced dipole interactions and are thus classified as a non-specific interaction. This type of non-polar interaction is the dominant force promoting dissolution of non-polar solutes in organic solvents.