Which statement best describes polar vs nonpolar behavior on silica gel?

Which statement best describes polar vs nonpolar behavior on silica gel?

• A. Polar compounds move fastest
• B. Nonpolar compounds are retained more strongly
• C. Polar compounds interact with silica and move slowly
• D. Nonpolar compounds move slowly

Answer: (C)

On a polar silica gel stationary phase, retention in normal-phase chromatography is governed by how strongly an analyte interacts with the surface. Polar compounds can form hydrogen bonds and dipole interactions with the silanol groups on silica, so they are pulled into the stationary phase and travel more slowly with the mobile phase. Nonpolar compounds interact weakly with the polar surface, staying mostly in the mobile phase and eluting faster. Therefore, polar compounds interact with silica and move slowly is the best description. The other statements contradict this pattern: polar compounds wouldn’t move fastest when retention is driven by surface interactions; nonpolar compounds wouldn’t be retained more strongly on a polar surface; and nonpolar compounds wouldn’t typically move slowly on silica gel.

On a polar silica gel stationary phase, retention in normal-phase chromatography is governed by how strongly an analyte interacts with the surface. Polar compounds can form hydrogen bonds and dipole interactions with the silanol groups on silica, so they are pulled into the stationary phase and travel more slowly with the mobile phase. Nonpolar compounds interact weakly with the polar surface, staying mostly in the mobile phase and eluting faster. Therefore, polar compounds interact with silica and move slowly is the best description. The other statements contradict this pattern: polar compounds wouldn’t move fastest when retention is driven by surface interactions; nonpolar compounds wouldn’t be retained more strongly on a polar surface; and nonpolar compounds wouldn’t typically move slowly on silica gel.