The Mechanism of Chromatographic Retention - Solvent/Solute Interactions with Adsorbent Surfaces > Bi-layer Adsorption > Page 49


where (j) is the mass of a single layer of the polar solvent covering the surface of one gram of silica gel.

Scott and Traiman (14) (measured the weight of ethyl acetate adsorbed on a sample of silica gel when in contact with a solutions containing a range of concentrations of the ethyl acetate in n-heptane. Known masses of ethyl acetate were added to the silica and n-heptane which were contained in a stoppered flask situated in a thermostat. After equilibrium was established, the concentration of ethyl acetate in the n-heptane were determined by GC. The concentration of ethyl acetate on the silica gel was determined from the difference between the amount of ethyl acetate added to the system 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..