Solid-liquid interface, soluble

Fig. 19. The extent of the liquid - solid interface, as long as this is plane, has no effect on solubility. When the small solid 1, in a conical container, dissolves, its interface with the solvent increases. When a material is deposited on the large solid 2, the surface area of the latter decreases...

As a technique for selective surface illumination at liquid/solid interfaces, TIRF was first introduced by Hirschfeld(1) in 1965. Other important early applications were pioneered by Harrick and Loeb(2) in 1973 for detecting fluorescence from a surface coated with dansyl-labeled bovine serum allbumin, by Kronick and Little(3) in 1975 for measuring the equilibrium constant between soluble fluorescent-labeled antibodies and surface-immobilized antigens, and by Watkins and Robertson(4) in 1977 for measuring kinetics of protein adsorption following a concentration jump. Previous rcvicws(5 7) contain additional references to some important early work. Section 7.5 presents a literature review of recent work. 

The solubilities of gases in solid metals are much lower than liquid metals. Figure 10.18 shows the solubility of hydrogen in copper and copper-aluminum alloys. Because of the lower solubility in the solid, gas bubbles are released at the liquid-solid interface as the metal freezes. With long dendrites the gas bubbles are trapped and the result is gas porosity. 

Solute particles in a liquid solvent are not normally soluble in the solid phase of that solvent. When solvent crystals freeze, they typically align themselves with each other at first and keep the solute out. This means that only a fraction of the molecules in the liquid at the liquid-solid interface are capable of freezing while the solid phase consists of essentially pure solvent that is able to melt freely. 

The adsorbent can be a solid or a liquid. The adsorbate is dissolved in a liquid or is (present in) a gas. Adsorption on an A-S interface concerns, for instance, adsorption of water from moist air on a solid see Section 8.2. Also other volatiles can adsorb from air, e.g., flavor compounds. At the liquid-solid interface, the solid generally is in contact with solvent as well as adsorbate, i.e., solute molecules. For a liquid-liquid interface (generally O-W) the adsorbate may be soluble in both liquids. The molecules adsorbed... 

In addition to molecule-molecule and molecule-substrate interactions, also the solvent plays an important role. This is actually an aspect which to a large extent has been neglected. Insight in the role of solvents is still in its infancy. The choice of solvents is mainly motivated by practical considerations solubility of the molecules, low vapor pressure, chemical inertness, and low affinity for self-adsorption. However, it is our belief that the choice of solvent will become more and more important to direct the self-assembly of molecules at the liquid-solid interface. From time to time, it is reported that in a given solvent pattern x is formed and in another solvent pattern y, without going into the details of the effect of the solvent. Recently, a number of studies appeared with a clear focus on the effect of solvent on the self-assembly process. Basically, two different phenomena are observed. 

Figure 13 Representation of the solute and solvent concentrations n" and n" by shaded areas at a liquid-solid interface (the solid is not soluble in the solvent and the solute does not diffuse in the solid).

The presence of a liquid phase and a liquid-solid interface in multiphase reactors results in added transport resistances. For instance, the effective diffusivity in liquid-filled pores (of the order of 10 to 10 cmVsec) is much smaller than that in gas-filled pores (of the order of 10 cmVsec). The solubility of the gaseous reactant is an important factor since the gaseous reactant has to be dissolved into the liquid reactant for the reaction to take place on the catalyst surface. As emphasized in Chapter 4, the Biot number for heat is much larger than the Biot number for mass for liquid-solid systems the opposite is true for gas-solid systems. Therefore, the major external resistance lies in the mass transport, and the pellet is not necessarily isothermal. In many cases, however, the equilibrium gas concentration in the liquid is quite low and, thus, the heat evolved is small in spite of high heats of reaction. The pellet can be considered isothermal in such a case,... 

Microbes were frequently found to synthesise surface-active molecules in order to mobilise hydrophobic organic substrates. These biosurfactants, which are either excreted by the producing organisms or remain bound to their cell surfaces, are composed of a hydrophilic part making them soluble in water and a lipophilic part making them accumulate at interfaces. With respect to their physical effects, one can distinguish two types of biosurfactants firstly, molecules that drastically reduce the surface and interfacial tensions of gas-liquid, liquid-liquid and liquid-solid systems, and, secondly, compounds that stabilise emulsions of nonaqueous phase liquids in water, often also referred to as bioemulsifiers. The former molecules are typically low-molar-mass... 

In industrial PET synthesis, two or three phases are involved in every reaction step and mass transport within and between the phases plays a dominant role. The solubility of TPA in the complex mixture within the esterification reactor is critical. Esterification and melt-phase polycondensation take place in the liquid phase and volatile by-products have to be transferred to the gas phase. The effective removal of the volatile by-products from the reaction zone is essential to ensure high reaction rates and low concentrations of undesirable side products. This process includes diffusion of molecules through the bulk phase, as well as mass transfer through the liquid/gas interface. In solid-state polycondensation (SSP), the volatile by-products diffuse through the solid and traverse the solid/gas interface. The situation is further complicated by the co-existence of amorphous and crystalline phases within the solid particles. 

Gas-liquid relationships, in the geochemical sense, should be considered liquid-solid-gas interactions in the subsurface. The subsurface gas phase is composed of a mixture of gases with various properties, usually found in the free pore spaces of the solid phase. Processes involved in the gas-liquid and gas-solid interface interactions are controlled by factors such as vapor pressure-volatilization, adsorption, solubility, pressure, and temperature. The solubility of a pure gas in a closed system containing water reaches an equilibrium concentration at a constant pressure and temperature. A gas-liquid equilibrium may be described by a partition coefficient, relative volatilization and Henry s law. 

The melting points of the components of a reaction couple are most frequently different. Therefore, there is a certain range of temperature, in which one of the components is in the solid state, while the other in the liquid state. If soluble, the solid substance will dissolve in the liquid phase. The dissolution process should clearly affect the growth kinetics of a chemical compound layer at the solid-liquid interface. 

For a compound to be qualified as a surfactant, it should also exhibit surface activity. It means that when the compound is added to a liquid at low concentration, it should be able to adsorb on the surface or interface of the system and reduce the surface or interfacial excess free energy. The surface is a boundary between air and liquid and the interface is a boundary between two immiscible phases (liquid-liquid, liquid-solid and solid-solid). Surface activity is achieved when the number of carbon atoms in the hydrophobic tail is higher than 8 [3]. Surfactant activities are at a maximum if the carbon atoms are between 10 and 18 at which level a surfactant has good but limited solubility in water. If the carbon number is less than 8 or more than 18, surfactant properties become minimal. Below 8, a surfactant is very soluble and above 18, it is insoluble. Thus, the solubility and practical surfactant properties are somewhat related [1]. 

The second type is a stable dispersion, or foam. Separation can be extreme difficult in some cases. A pure two-component system of gas and liquid cannot produce dispersions of the second type. Stable foams can be produced only when an additional substance is adsorbed at the liquid-surface interface. The substance adsorbed may be in true solution but with a chemical tendency to concentrate in the interface such as that of a surface-active agent, or it may be a finely divided solid which concentrates in the interface because it is only poorly wetted by the liquid. Surfactants and proteins are examples of soluble materials, while dust particles and extraneous dirt including traces of nonmisci-ble liquids can be examples of poorly wetted materials. 

In this chapter we will see how the surface activity of a molecule is related to its molecular structure and look at the properties of some surfactants which are commonly used in pharmacy. We will examine the nature and properties of films formed when water-soluble surfactants accumulate spontaneously at liquid/air interfaces and when insoluble surfactants are spread over the surface of a liquid to form a monolayer. We will look at some of the factors that influence adsorption onto solid surfaces and how experimental data from adsorption experiments may be analysed to gain information on the process of adsorption. An interesting and useful property of surfactants is that they may form aggregates or micelles in aqueous solutions when their concentration exceeds a critical concentration. We will examine why this should be so and some of the factors that influence micelle formation. The ability of micelles to solubilise water-insoluble drugs has obvious pharmaceutical importance and the process of solubilisation and its applications will be examined in some detail. 

A series of studies of surface activity of soluble and insoluble compounds at organic liquid-air interfaces has been reported by Zis-man, Ellison, Bernett, and Jarvis [4,10,11,16,17,18]. The most surface active compounds were foxmd to be various fluorocarbon derivatives having the proper organophobic-organophilic balance. If one considers a plastic solid to be either a supercooled liquid or a liquid of very high viscosity, one would e3q)ect many of these partially fluorinated compoimds also to manifest great surface activity when dissolved in... 

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