Solute–solvent interactions on the surface of reverse phases. Interactive characteristics of some short-chain aliphatic moderators having different functional groups

The desorption–adsorption coefficients and distribution coefficients of a series of aliphatic alcohols, carboxylic acids and aldehydes between water and ODS2 reverse phase have been determined using a chromatographic procedure. The moderator concentrations over which the measurements were made were kept at sufficiently low levels so that moderator/moderator interactions in the mobile phase and moderator interactions with adsorbed moderator were kept minimal.It is shown that the desorption–adsorption coefficients decrease and the distribution coefficients between water and the reverse phase increase exponentially with the carbon number of the moderator confirming the system can be described by the Martin equation. As a result, substances having chains of five carbon atoms or more could produce complete coverage of the surface when present at a level of only a few percent in the mobile phase. Thus, the surface of the reverse phase can be strongly modified without significantly affecting the solvent characteristics of the mobile phase. It was shown that dispersive interactions between the reverse phase and the hydrocarbon chain of the moderator were the same for each moderator and from the slope of the log K against carbon-number curves, it was also shown that the logarithmic increment for each methylene group in the alkyl chain was the same for all moderators and independent of the functional group. The difference in the distribution coefficients between different moderators having alkyl chains of the same length depends solely on the dispersive interactions between the functional groups with the reverse phase and the functional group with any residual silanol groups that may be present. The method used can also determine the effective chromatographic surface area of a given reverse-phase column. It is suggested that the technique could also be used to determine the effective surface area of different types of bonded phases, give a measure of the residual polarity of the stationary phase and the magnitude of the interactive capacity of the hydrocarbon portion.