Liquid Chromatography - Solvent/Solute Interactions with the Reversed Phase Surface 1
Solvent/Solute Interactions with the Reversed Phase Surface
Solvents interact with the surface of a reverse phase in a similar manner to the surface of silica gel. In figure 44, the adsorption isotherms for a series of aliphatic alcohols are shown. The effect of the carbon chain-length of the alcohol on the strength of the adsorption is clearly seen from the shape of the curves. The most strongly adsorbed alcohol, butanol, (the alcohol with the longest chain and, thus, the most dispersive) has only a four carbon chain and yet the surface is completely covered when the butanol concentration is only about 2%w/v. Thus, any component of the mobile phase with a hydrocarbon chain length of four or more, will be rapidly adsorbed and modify the reverse phase surface extensively and, consequently, 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