Correlation between reversed-phase retention and solute molecular surface type and area : IV. Influence of eluent water content on the retention produced by different surface types

In reversed-phase systems, the retention of non-ionized solutes increases with increasing eluent water content. For all investigated solutes, the ln k′ increase is linear from 0 to about 25% (v/v) of water in methanol. It parallels an eluent surface tension increase which, in turn, is related to a linear increase of cavity formation free energy. If normalized to equal solute molecules surface area, changes in cavity free energy are the same for all solutes larger than the eluent molecules. However, quite remarkable differences in solute behaviour towards changes of eluent water content are observed. The solute retention index may drop, stay constant, or rise with increasing eluent water content. The strongest index decrease is observed with rigid symmetric solutes the surface of which does not carry protons. Both changes of the sorbed solute entropy and the existence of ▪ interaction between reference alkanes and eluent methanol seem to be responsible for this effect. Solutes having a strong interaction with methanol, like alkynes, exhibit a marked index increase when water is added to the eluent since these interactions are suppressed when the methanol is claimed by the water. Aromatic surface reacts less strongly on eluent composition changes than aliphatic surface, possibly due to an increasing polarization of π systems by water dipoles.