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hydrocarbons, aromatic hydrocarbons and weakly polar solvents such as the alkyl ethers all appeared to give isotherms of the type shown in figure 18.   The same process of solvent adsorption occurs with reverse phases only dispersive substance are held strongly on the surface as opposed to polar materials on silica gel. The adsorption isotherms are Langmuir in form and a monolayer coating of the solvent is deposited on the hydrocarbon chains bonded to the surface. The adsorption isotherms of acetic and propionic acid are shown in figure 19. It is seen that the acids exhibit the same type of curve as chloroform on silica gel. It is also apparent that when the solvent mixture contains about 10 %w/v of acid the reverse phase is covered with carboxyl groups from the adsorbed acid. Figure 19. Adsorption Isotherms of Acetic and Propionic Acid on a Reveres Phase

, the magnitude of solute retention. The curves shown in figure 44 only cover a range of 0 to 0.05 g.ml-1. In order to show the shapes of the adsorption isotherms for the higher alcohols in proportion to those of the lower alcohols with reasonable clarity, the same curves are shown in figure 45 for an alcohol concentration range of 0-100% (which is approximately 0-0.8g/ml). It should be noted that the mass adsorbed is expressed as g.cm-2   Figure 44. The Adsorption Isotherms of a Homologous Series of Aliphatic Alcohols over the Concentration Range of 0 to 0.0.5 g.ml-1

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

the surface of the reverse phase is being modified over one third of the methanol concentration range. The reverse phase surface can be modified in a controlled manner, over the range of 0 to about 40 % methanol, but between methanol concentrations of 40% and 100 % the nature of the reverse phase surface remains sensibly constant and it is the solute interactions in the mobile phase that are progressively modified. Acetonitrile and tetrahydrofuran behave in a similar manner but there adsorption isotherms are closer in magnitude to those of ethanol than of methanol.   The types of interactions that can take place between the solute and the reverse phase are similar to those that can take place between the solute and the silica gel surface. Solutes can interact by the sorption process, the displacement process or a combination of both. The same rules apply; if the solvent interacts more strongly with the surface than the solute then the solute interacts with the adsorbed layer of

stationary phase may well be presented with two, quite different types of surface with which to interact. The probability that a solute molecule will interact with one particular type of surface will be statistically controlled by the proportion of the total surface area that is covered by that particular solvent. Dispersive solvents appear to be adsorbed from a solvent mixture on the surface of silica gel according to the Langmuir adsorption isotherm (33). Examples of mono-layer adsorption isotherms obtained for benzene, chloroform and butyl chloride are shown in figure 40.   Figure 40. Langmuir Adsorption Isotherms for Benzene, Butyl Chloride and Chloroform

Figure 21. Individual and Combined Adsorption Isotherms for Ethyl Acetate on Silica Gel The individual isotherms for the two adsorbed layers of ethyl acetate are included in figure (21). The two curves are of the same form but quite different in magnitude. The first layer is complete when the concentration of  ethyl acetate in the mobile phase is only about 1%w/w. As the concentration of ethyl acetate rises above 1%w/w the second layer is only just started forming. The second layer of ethyl acetate forms much slower and obviously the