Molecular Interactions on Polystyrene Stationary Phases

The strength of the interactions between individual molecules and solvent-swollen polystyrene gels can be correlated using Lewis acid-Lewis base arguments. Polystyrene may be viewed as a Lewis base because it may serve as an electron-pair donor to a suitable electron-pair acceptor. Mobile phases such as toluene and tetrahydrofuran (THF) are also Lewis bases. The stationary phase gel and mobile phases such as toluene and THF will compete for a Lewis acid solute, with the stronger base always interacting preferentially. For a THF mobile phase, acidic solute-mobile phase interactions predominate and size exclusion chromatography (SEC) will be the only separation mechanism occurring. Polystyrene appears to be a stronger base than toluene, and acidic solutes always interact with the gel. Retention in this system includes contributions from both SEC and gel-solute interactions. Chloroform is a Lewis acid and will interact strongly with polystyrene. Acidic solute-gel interaction must then be stronger than the chloroform-gel interaction for non-SEC retention to occur. The relative strength of the Hydrogen-bonding interactions can be predicted using Drago's E and C constants. Dipole-dipole interactions can be estimated using simple solubility parameter theory. This approach unifies experimental data reported earlier and allows a qualitative prediction of the performance of other SEC mobile phases.