[orgo814]

I'm having trouble conceptualizing this... Why would having a more polar phase in a reverse HPLC increase retention time of the compounds. And similarily, if your mobile phase is similiar to stationary retention time decreases?

[Arkcon]

I'm having trouble conceptualizing this... Why would having a more polar phase in a reverse HPLC increase retention time of the compounds.

It really depends on the sens in which you mean "more polar."  Are you comparing C8 to C18?  Or are we discussing embedded polar groups. And of course, even more significantly, the analyate.

And similarily, if your mobile phase is similiar to stationary retention time decreases?

Generally, yes.  If you're doing reverse phase, and so have a nonpolar column, the more non polar mobile phase causes quicker elution.

[Babcock_Hall]

I like to think that molecules of the mobile phase compete against the analyte for binding to the stationary phase.  The more polar the mobile phase, the less well it binds to the very non polar stationary phase.  This allows the analyte molecules to stay bound for long periods of time and to have a long retention time.  If the mobile phase is less polar then it competes more effectively versus the analyte for binding to the stationary phase, and the analyte molecules are more likely to be displaced from the stationary phase.