The Mechanism of Chromatographic Retention - Solvent/Solute Interactions with Adsorbent Surfaces > Bi-layer Adsorption > Page 45
The adsorbed acids can provide ionic interactions with a solute, in fact, the so-called ion pair reagents function largely as adsorbed ion exchangers. A typical ion pair reagent is tertiary butyl ammonium bromide whic is strongly adsorbed on a reversed phase as a result of the strong dispersive interacions with the butyl chains and acts as an adsorbed cation exchanger..
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