U-shaped log k vs organic fraction plots for RP columns

[DJ]

For even C18 columns, the plot of log K as a function of volume fraction organic becomes U shaped at higher organic concentrations. This effect is more pronounced for some RP columns (pentafluoro) than others. However, the underlying these seems to be that that the silica support necessarily permits this unexpected "HILIC-like" (perhaps also ion exchange) mode of retention.

What about non-silica based supports, such as PS-DVB? Surely you would not expect retention factor to INCREASE at the upper extremes of organic volume fractions, presumably because there is no hydrophilic nature to the underivatized ps-dvb support.

What about PS-DVB functionalized with a high exchange capacity ion exchanger, such as PS-DVB +quarternary ammonium group? Would hilic retention at high MeCN% be possible then?

If not, could, perhaps, co-polymers of hydroxy-methyl, PEG, or cyano- substituted styrenes permit HILIC column made from ps-DVB co-polymers?

[tom jupille]

To oversimplify quite a bit, the definition of "reversed phase" is that the column is less polar than the eluant. This allows for a large gray area at high % organic where the polarities are comparable, and a reversal of the curve at very high % organic where the column may be unequivocally more polar than the eluant, hence the U-shaped plot. And yes, I would expect silanols to contribute to that effect. And, yes, I would agree with you that the effect is less likely with polystyrene packings, although the aromatic character is something of a wild card in that respect.

Would an anion exchanger function in "HILIC" mode? The way to find out is to try. If I were doing it, I'd stick with "macroporous" (high crosslink) materials to minimize shrink/swell problems with changes in organic solvent %.