and the 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

Mono-layer Adsorption A solvent can be adsorbed from a solvent mixture on the surface of silica gel according to the Langmuir adsorption isotherm. Consider the situation depicted in figure 17.     Figure 17. The Distribution of Solvents A and B as a Mono-layer on a Silica Gel Surface Let the surface be in contact with a solution of solvent (B) at a concentration (c) g/ml in solvent (A) and let the fraction of the area covered by solute (B) molecules be (a) and consequently, the area fraction occupied by solute (A) will be (1-a). Under equilibrium conditions, if the number of molecules of

isotherm become constant, see The Mechanism of Chromatographic Retention). It should be noted that there is no significant difference between the retention volume of water and that of methanol over the complete range of solvent compositions examined, which confirms the validity of this method for measuring the thermodynamic dead volume. Again, however, the lower concentrations of methanol, where the surface area of the stationary phase was not completely covered with methanol and the Langmuir adsorption isotherm would apply, can not be used. It must also be stressed, that this method of measuring thermodynamic dead volume will only be valid for small molecules. Larger molecules will be partially excluded and, thus, their dead volumes will be commensurably smaller. Alhedai et al. also examined the exclusion properties of the reversed phase material The stationary phase was a C8 hydrocarbon bonded to the silica, and the mobile phase chosen was n-octane. As the solutes,

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

conditions. (K) is a dimensionless constant and, in gas/liquid and liquid/liquid systems, (Xs) and (Xm) can be measured as mass of solute per unit volume of phase. In gas/solid and liquid/solid systems, (Xs) and (Xm) can be measured as mass of solute per unit mass of phase. Equation (1) reiterates the general distribution law and presumes the adsorption isotherm as linear. In both gas/solid chromatography (GSC) and liquid/solid chromatography (LSC), virtually all the solutes exhibit Langmuir type isotherms between the two phases which, over a wide concentration range, is certainly not linear. However, at the extremely low solute concentrations employed in chromatography, (i.e., that portion of the isotherm that is pertinent) the isotherm can be considered as linear

in n-heptane (13) is shown in figure 18. The variables given in the graph are not in the form of equation (29), but relate concentration of chloroform on the silica gel to the concentration of chloroform in the mobile phase. It is seen that the silica gel surface does not become completely covered with chloroform (and thus, have consistent interactive properties with respect to any solute) until the concentration of chloroform in the mobile phase is in excess of 40%w/w. Figure 18. Langmuir Isotherm for Chloroform on Silica Gel