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and acts as an adsorbed cation exchanger.. Bi-layer Adsorption The adsorption isotherms of the more polar solvents, ethyl acetate, isopropanol and� tetrahydrofuran from n-heptane solutions on silica gel were also determined experimentally by Scott and Kucera (12). They found that experimental results for the more polar solvents, did not fit the simple mono-layer adsorption equation. As a consequence, the possibility of bi-layer adsorption on the silica gel surface was examined. Bi-layer adsorption is not uncommon and the development of the bi-layer adsorption isotherm equation is a simple extension of the procedure used for the mono-layer equation. Consider the bi-layer adsorption of solvent (B), from a solution in solvent (A), on a silica gel surface, as depicted in figure 20. Figure 20. The Distribution of Solvents A and B as a Bi-layer on a Silica Gel Surface

dispersive characteristics that do not produce peak tailing. Although the major contributors to adsorption by the support are the silanol groups, a residual adsorption results from the presence of trace quantities of heavy metals such as iron. which can be largely removed by acid washing prior to silanization. All three types of support are commercially available. None of these supports, however, are completely devoid of adsorptive properties and in may cases the effect of the residual adsorption must be further reduced by suitable stationary phase additives.   To try to completely eliminate adsorption effects from the support, Teflon was explored as a possible alternative to a diatomaceous earth. Teflon powder proved to have little adsorption, but also proved to be extremely difficult to pack into a column. So difficult, that it is very rarely used in general GLC analyses. Its inert character makes it useful for the separation of certain highly

If (kd) is the desorption rate constant then the mean desorption time (td) for the adsorbed molecule will be . Correspondingly, if the adsorption rate constant is (ka), then the mean adsorption time for a free molecule in the mobile phase will be . Consider a peak moving down a column. During this migration process, adsorption and desorption steps will constantly and frequently occur and each occurrence will be a random event. Now a desorption step will be a random movement forward as it releases a molecule into the mobile phase. Conversely, an adsorption step will be a random movementbackward, as it is a period of immobility for the molecule while it resides in the

The adsorption isotherms of the more polar solvents, ethyl acetate, isopropanol and� tetrahydrofuran from n-heptane solutions on to the silica gel surface did not fit the simple mono-layer adsorption equation but did fit the bi-layer adsorption isotherm which is a simple extension of the monolayer formation process. The bi-layer adsorption isotherm for ethyl acetate on silica gel is shown in figure 41. The curve is theoretical� and the points experimental.   The individual isotherms for the two adsorbed layers of ethyl acetate are included in figure 41. The two curves, although of the same form, are quite different in magnitude. The first layer is very strongly held to

amount of ethyl acetate remaining in the n-heptane. The data obtained was fitted to a bi-layer Langmuir adsorption isotherm employing a simple iterative computer program and the constants for adsorption isotherm for the two layers identified. The theoretical curve was then constructed and the experimental points superimposed on the theoretical curve. The results obtained are shown in figure (21). It is seen that an excellent fit is obtained between the equation for the bi-layer Langmuir adsorption isotherm and the experimental data. It is also clear by comparison of the curve shape given in figure (21) with that given in figure (18) that a simple single layer adsorption function could not fit the experimental data. It is seen that the initial adsorption of the ethyl acetate on the silica surface to form the first layer increases very rapidly with the ethyl acetate concentration in the solvent

Silica gel adsorbs relatively large quantities of water� which was explained on the basis of multi-layer adsorption. This concept was supported by Vleeskens (18,19) and experimentally validated by gravimetric measurements (20). An example of one type of multi-layer adsorption is shown in figure 32.   Figure 32. Multi-Layers of Physically Adsorbed Water The multi-layer adsorption depicted in figure 32 is much over simplified, as adsorption could also take place the surface of siloxane bonds as well