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..

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