For I think when separate polar compounds, choose a column with higher carbon content can give longer retention time, and it is more convenient than using HILIC or NPLC.
I suspect that approach may be misguided for at least a couple of reasons:
First of all, carbon content by itself is only a crude indicator of reversed-phase retention. Surface area and bonding chemistry will also get into the act. A thick polymeric phase on a low-surface area silica for example, may be less retentive than a monolayer coverage on a high-surface-area silica despite having a higher carbon content. If you want a better measure of reversed-phase retentivity, go to the PQRI database on the USP web site (
http://www.usp.org/USPNF/columns.html)and look at the "H" parameters. The documentation on that site gives a good explanation of the underlying principles.
Second, if you are looking for better retention of polor compounds (since you mentioned HILIC as an alternative), a more hydrophobic stationary phase would make things worse. Arguably, a better approach would be to try one of the "embedded polar group" columns (sometimes called "amide" columns which provide at least some opportunity for your analytes to hydrogen-bond to the stationary phase.
