Solvents demixing

Studies described in earlier chapters used cellular automata dynamics to model the hydrophobic effect and other solution phenomena such as dissolution, diffusion, micelle formation, and immiscible solvent demixing. In this section we describe several cellular automata models of the influence of the hydropathic state of a surface on water and on solute concentration in an aqueous solution. We first examine the effect of the surface hydropathic state on the accumulation of water near the surface. A second example models the effect of surface hydropathic state on the rate and accumulation of water flowing through a tube. A final example shows the effect of the surface on the concentration of solute molecules within an aqueous solution. 

Solvent degassing (LC) 553 Solvent demixing (TLC) 660 Solvent effects, splitless injection (GC) 250 Solvent extraction 753 applications 766 hoBogenizer 761 i plitgers 756 micTMethods 764 microwave 761 optimization 753 shake flask 761 solvent reduction 763 Soxhlet 762 Solvent (LC)... 

If solvent B is too strong, then the middle or later peaks may be poorly resolved. In extreme cases of this, the solvent B may itself be strongly held at the top of the column (the column is now separating the solvent mixture, the effect being called solvent demixing). If this occurs then pure A will pass through the column until the column is saturated with solvent B. 

The kinetics of polymer/polymer demixing are many orders of magnitude slower than those for polymer/solvent demixing. In a typical instrument used to study blend demixing a polymer film supported on a thin glass cover slip is placed in a ther-mostatted pressure cell (Fig. 7.18c). The sample is annealed for some hours well... 

Since the mobile phase is moving on a dry bed, several other undesirable effects occur. The adsorption of the first liquid (at the front) on the stationary phase is exothermic, causing the front to have a higher temperature than the rest of the system. Since the temperature of the system is not usually controlled but is allowed to assume the ambient value, some evaporation may occur at the solvent front. If the solvent is composed of a mixture of liquids, preferential evaporation of the most volatile one will cause a slight change in the solvent composition. In fact, the adsorption of a mixed mobile phase will probably also cause some changes in composition because the most polar component will be preferentially sorbed. The situation can become so severe that solvent demixing can occur. At best, a mixed solvent mobile phase is probably not uniform across the planar bed, and some temperature differentials probably exist as well. 

For thicker layers, the attainment of equilibrium in the gas-mobile phase-adsorbent system to avoid complicating effects (solvent demixing, preadsorption) is more difficult. The solutes migrate in a nonequili-brated layer with differentiated velocity — more rapidly in the surface layer (because of the evaporation of solvent) and less rapidly closer to the carrier plate. 

Figure 9.19 CCD profiles for PMMA-g-PDMS copolymers of low PDMS content (17wt% PDMS) prepared anionically using 20,000 molecular weight PDMS grafts (top) supercritical chlo-rodifluoromethane fractionation (bottom) solvent demixing fractionation (DeSimone et al., 1988b).

If binary or ternary solvent mixtures are used in film development or elution from a dry column, Eqs. (2-1), (2-3), and (2-6) may be rendered completely inapplicable by the phenomenon of solvent demixing. Solvent demixing refers to the selective adsorption of one solvent component as the solvent moves through the adsorbent bed, with a resulting change in solvent composition along the bed. The values of K for individual sample components are likewise different at different positions on the bed. Solvent demixing is discussed in Section 8-2. 

In principle the adsorption equilibria for a multicomponent solvent system plus sample can be enormously complex, and no practical understanding of such systems can be hoped for without certain simplifying assumptions. Throughout the present treatment we will assume that no solvent demixing occurs as a result of selective adsorption of one or more solvent components. This is a reasonable assumption for most chromatographic systems, and separations in which solvent demixing is likely to occur can be anticipated, as discussed below. Further approximations will prove necessary as we proceed. 

The experimental e values of binary solvents in thin-layer systems often appear low because of solvent demixing. Preferential adsorption of B by the adsorbent depletes the advancing solvent front of B, leaving a solvent which is weaker than the original mixture. The seriousness of this effect can be estimated from the likelihood of extensive solvent demixing (see below). 

In the bed-development separation of sample mixtures which are not subject to the general elution problem (e.g., a two-component sample), solvent demixing is usually an unwelcome complication. The preceding relationships for describing the effect of the solvent on separation [e.g., Eqs. (8-4), (8-5), (8-10)] become inapplicable, and it is difficult to predict satisfactory separation conditions or to define the relative strength of a binary solvent. As in the case of polyzonal chromatography, there may be a tendency for two or more sample components to travel with one of the solvent fronts and hence remain unseparated. In extreme cases a satisfactory separation of a particular sample may be impossible with a given binary A-B of any composition, even where sample K° values are... 

The composition of the solvent may change during separation (solvent demixing). 

Aside from the complication of solvent demixing (see below), solvent mixtures are chromatographically equivalent to pure solvents (e.g.. Fig. 

A variety of problems can arise during the course of a separation, apart from poor sample resolution. Some of these problems (solvent demixing, adsorbent activation by solvent, etc.) have already received attention. A few additional problems will now be examined. 

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