Solutes and displacers

The solutes and displacers (Aldrich, Milwaukee, W1 and Sigma, St. Louis, M3) were used as received, without further purification. HPDC-grade methanol and acetonitrile (Mallinckrodt, Paris, ICY), as well as water produced by a Millipore Q unit (Millipore, Bedford, MA) were used to make all solutions. 

It can be seen from this equation that the greater the difference between the standard potentials of the two metals Mi and M2, the larger will be the equilibrium ratio of activities (or concentrations) of the respective ions. The greater the difference between the standard potentials, therefore, the more completely will one metal displace another from a solution of its ions. The metal with the more positive (oxidation) potential, as recorded in Table XLIX, will, in general, pass into solution and displace the metal with the less positive potential. The series of standard potentials, or electromotive series, as it is sometimes called, thus gives the order in which metals are able to displace each other from solution the further apart the metals are in the series the more completely will the higher one displace the lower one. It is not true, however, to say that a metal lower in the series will not displace one higher in the series some displacement must always occur until the required equilibrium is established, and the equilibrium amounts of both ions are present in the solution. 

The choice of displacer concentration is critical for successful displacement. If the displacer concentration is increased, then the solute concentrations in the isotachic train also increase. If the displacer concentration is decreased, then displacement does not occur and the solutes elute as overloaded peaks in the elution mode. Rhee and Amundson have shown that there is a critical displacer concentration below which displacement cannot occur. This concentration depends primarily on the saturation capacities of the solutes and displacer. 

Adsorption at the Phase Interface.—Solutes that lower surface tension tend to accumulate on the free surfaces of colloidal particles. This surface condensation is termed adsorption, and is of special significance in colloidal systems because of the enormous area presented by the disperse phase. Colloids often display preferential absorption for a particular class of solutes, and one solute may compete with another solute and displace it from the adsorbing surface. 

In a solution of molecules of uniform molecular weight, all particles settle with the same value of v. If diffusion is ignored, a sharp boundary forms between the top portion of the cell, which has been swept free of solute, and the bottom, which still contains solute. Figure 9.13a shows schematically how the concentration profile varies with time under these conditions. It is apparent that the Schlieren optical system described in the last section is ideally suited for measuring the displacement of this boundary with time. Since the velocity of the boundary and that of the particles are the same, the sedimentation coefficient is readily measured. 

Cementation. A metal can be removed from solution by displacing it with a mote active metal. This simple, inexpensive method has been commonly used to recover copper from dilute (1—3 kg/m ) solution using shredded iron and de-tinned iron cans as reducing agent. 

The most significant chemical property of zinc is its high reduction potential. Zinc, which is above iron in the electromotive series, displaces iron ions from solution and prevents dissolution of the iron. For this reason, zinc is used extensively in coating steel, eg, by galvanizing and in zinc dust paints, and as a sacrificial anode in protecting pipelines, ship hulls, etc. 

The capability of zinc to reduce the ions of many metals to theh metallic state is the basis of important appHcations. However, metals are removed from zinc solutions by displacement with finely divided zinc before winning by electrolysis. Gold and silver are displaced from cyanide leach solutions with zinc and the following metals are similarly recovered from various solutions platinum group, cadmium, indium, thallium, and sometimes copper. 

Displacement Equilibria. Species in solution are generally in formation—dissociation equiUbrium, and displacement reactions of any given metal or ligand by another are possible. Thus,... 

There are two ways a solute can interact with a stationary phase surface. The solute molecule can interact with the adsorbed solvent layer and rest on the top of it. This is called sorption interaction and occurs when the molecular forces between the solute and the stationary phase are relatively weak compared with the forces between the solvent molecules and the stationary phase. The second type is where the solute molecules displace the solvent molecules from the surface and interact directly with the stationary phase itself. This is called displacement interaction and occurs when the interactive forces between the solute molecules and the stationary phase surface are much stronger than those between the solvent molecules and the stationary phase surface. An example of sorption interaction is shown in Figure 9. 

The second type of interaction, displacement interaction, is depicted in Figure 10. This type of interaction occurs when a strongly polar solute, such as an alcohol, can interact directly with the strongly polar silanol group and displaces the adsorbed solvent layer. Depending on the strength of the interaction between the solute molecules and the silica gel, it may displace the more weakly adsorbed solvent and interact directly with the silica gel but interact with the other solvent layer by sorption. Alternatively, if solute-stationary phase interactions are sufficiently strong, then the solute may displace both solvents and interact directly with the stationary phase surface. 

It is clear that such a surface offers a wide range of sorption and displacement processes that can take place between the solute and the stationary phase surface. Due to the bi-layer formation there are three different surfaces on which a molecule can interact by sorption and three different surfaces from which molecules of solvent can be displaced and allow the solute molecule to penetrate to the next layer. During a chromatographic separation under these circumstances, all the alternatives are possible. Nevertheless, depending on the magnitude of the forces between the solute molecule and the molecules in each layer, it is likely that one particular type of interaction will dominate. The various types of interaction are included in Figure 11. 

Where there are multi-layers of solvent, the most polar is the solvent that interacts directly with the silica surface and, consequently, constitutes part of the first layer the second solvent covering the remainder of the surface. Depending on the concentration of the polar solvent, the next layer may be a second layer of the same polar solvent as in the case of ethyl acetate. If, however, the quantity of polar solvent is limited, then the second layer might consist of the less polar component of the solvent mixture. If the mobile phase consists of a ternary mixture of solvents, then the nature of the surface and the solute interactions with the surface can become very complex indeed. In general, the stronger the forces between the solute and the stationary phase itself, the more likely it is to interact by displacement even to the extent of displacing both layers of solvent (one of the alternative processes that is not depicted in Figure 11). Solutes that exhibit weaker forces with the stationary phase are more likely to interact with the surface by sorption. 

Scott and Kucera [4] carried out some experiments that were designed to confirm that the two types of solute/stationary phase interaction, sorption and displacement, did, in fact, occur in chromatographic systems. They dispersed about 10 g of silica gel in a solvent mixture made up of 0.35 %w/v of ethyl acetate in n-heptane. It is seen from the adsorption isotherms shown in Figure 8 that at an ethyl acetate concentration of 0.35%w/v more than 95% of the first layer of ethyl acetate has been formed on the silica gel. In addition, at this solvent composition, very little of the second layer was formed. Consequently, this concentration was chosen to ensure that if significant amounts of ethyl acetate were displaced by the solute, it would be derived from the first layer on the silica and not the less strongly held second layer. 

Pagano s exact solution for the stresses and displacements is too complex to present here. The corresponding classical lamination theory result stems from the equilibrium equations, Equations (5.6) to (5.8), which simplify to... 

The nonuniform displacement can occur in the injection of the polymer solution and in the injection of the solvent. The former will be manifested in early fractions. The latter will be seen in the broadening of the distribution in late fractions and in the prolonged time necessary to wash the column. 

A longer column is preferred because of a greater processing capacity nd an increased number of plates, as long as the back pressure does not exceed the upper limit and the nonuniform displacement of the solution and the solvent is not serious. The theoretical plate in HOPC is defined as a section in the column in which equivalently full exchange of all of the polymer components... 

Goi. As noted previously, an a-chlorine atom renders a ring-nitrogen atom very weakly basic. Cyanuric chloride (5) is a very weak base both because s-triazines are of low basicity and because each of the ring-nitrogen atoms is alpha to two chlorine atoms. Hence, this compound should be insensitive to acid catalysis or acid autocatalysis and this has been observed for the displacement of the first chlorine atom with alcohols in alcohol-acetone solution and with water (see, however. Section II,D,2,6). 

When an electric field is imposed perpendicular to the flow, differential interaction between the various solutes and the electric field produce a lateral displacement of individual analyte streams between the two electrodes (Fig. 11-5). Thus, the separations are accomplished in free solution. Individual fractions are collected through an array of closely spaced ports evenly placed across the other end of the chamber. 

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