How do secondary interactions affect an extraction?

Secondary interactions can be beneficial or harmful.  Unendcapped surface silanol groups are acidic with a pK_a between 4-5, above which the [Si-O]^- form exist dominantly.  Charged silanol groups can interact with polar basic compounds to increase their retention in a reversed phase material.  Depending on the goal of the extraction, these interactions can be good or bad.  Secondary interactions can be eliminated by acidifying the material to keep these silanol groups protonated, at a pH under their pK_a.