Liquids surface adsorption

The relation between the extent of separation and the extent of reaction is briefly considered in Section 5.1. How chemical reactions alter the separation equilibria in gcts-liquid, vapor-liquid, liquid-liquid, solid-liquid, surface adsorption equilibria, etc., is described in Section 5.2. The role of chemical reactions in aitering the separation in... 

A zero or near-zero contact angle is necessary otherwise results will be low. This was found to be the case with surfactant solutions where adsorption on the ring changed its wetting characteristics, and where liquid-liquid interfacial tensions were measured. In such cases a Teflon or polyethylene ring may be used [47]. When used to study monolayers, it may be necessary to know the increase in area at detachment, and some calculations of this are available [48]. Finally, an alternative method obtains y from the slope of the plot of W versus z, the elevation of the ring above the liquid surface [49]. 

The case of a vapor adsorbing on its own liquid surface should certainly correspond to mobile adsorption. Here, 6 is unity and P = the vapor pressure. The energy of adsorption is now that of condensation Qu, and it will be convenient to define the Langmuir constant for this case as thus, from Eq. xvn-39. 

Liquid-phase adsorption methods are widely used for quaUty control and specification purposes. The adsorption of iodine from potassium iodide solution is the standard ASTM method D1510-83 (2). The surface area is expressed as the iodine number whose units are milligrams of iodine adsorbed per gram of carbon. It is quite fortuitous that the values of iodine numbers turn out to be about the same as the values for surface areas in square meters per gram by nitrogen adsorption for nonporous carbon blacks. 

Ionic species present in liquids undergo adsorption at interfaces such that predominantly one sign of charge is more strongly bound at the contacted surface than the other. This results in a bound layer close to the surface farther from which is a diffuse layer having a net countercharge. This two-layer... 

It is worth noting in Figures 7. lb and 7.2b that the zero energy level choice (point C) is not only, by definition, a point in vacuum close to the surface of the solution (Fig. 7.1a, 7.2a), but also, as clearly shown by Trasatti,16 a point in vacuum close to the surface of the emersed (liquid or adsorption covered) electrode. 

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). 

One way to control gaseous pollutants like SO2 and SO3 is to remove the gases from fuel exhaust systems by absorption into a liquid solution or by adsorption onto a solid material. Absorption involves dissolving the gas in a liquid while adsorption is a surface phenomenon. In each case, a subsequent chemical reaction can occur to further trap the pollutant. Lime and limestone are two solid materials that effectively attract sulfur dioxide gas to their surfaces. The ensuing chemical reaction converts the gaseous pollutant to a solid nontoxic substance that can be collected and disposed or used in another industry. 

In both situations the interaction of the medium inside the pore with the pore wall (1) is increased (2) or changed which affect the transport and separation properties (surface diffusion, multilayer adsorption) and/or help overcome equilibrium constraints in membrane reactors. Membrane modifications can be performed by depositing material in the internal pore structure from liquids (impregnation, adsorption) or gases. Several modification possibilities are schematically shown in Figure 2.3. Some results obtained by Burggraaf, Keizer and coworkers are summarized in Table 2.7. Composite structures on a scale of 1-5 nm were obtained. 

The nature of the surface migration provides the key to the differences between adsorption on liquid and solid surfaces. For liquid surfaces, it is, to a good approximation, possible to assume that the heat of adsorption (AH) is the same at all points on the surface. In other words, such a surface... 

In a large variety of applications, the surface of a solid plays an important role (e.g., active charcoal, talc, cement, sand, catalysis). Solids are rigid structures and resist any stress effects. Many such considerations in the case of solid surfaces will be somewhat different for liquids. The surface chemistry of solids is extensively described in the literature (Adamson and Gast, 1997 Birdi, 2002). Mirror-polished surfaces are widely applied with metals, where the adsorption at the surface is much more important. Further, the corrosion of metals initiates at the surfaces, thus requiring treatments based on surface properties. As described in the case of liquid surfaces, analogous analyses of solid surfaces can be carried out. The molecules at the solid surfaces are not under the same force field as in the bulk phase (Figure 5.1). 

Adsorption at liquid surfaces can be monitored using the Gibbs adsorption isotherm since the surface energy, y, of a solution can be readily measured. However, for solid substrates, this is not the case, and the adsorption density has to be measured in some other manner. In the present case, the concentration of adsorbate in solution will be monitored. In place of the Gibbs equation, we can use a simple adsorption model based on the mass action approach. 

Very little accurate information is at present available on the adsorption of vapours and gases on liquid surfaces. Iredale Phil. Mag. XLV. 1094, 1923 XLVlii. 175, 1924), however, has studied the adsorption of a number of vapours at a mercury surface. The experimental data have led to a number of interesting conclusions. 

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