Nature of the specific anion response of a hydrophobic weak polyelectrolyte brush revealed by AFM force measurements

Complementary interaction force measurements between an atomic force microscope (AFM) tip or colloid probe and a weak polybasic brush have been shown to yield a number of fundamental characteristics of the brush and its response to the presence of specific anions in aqueous solution. Stretching of the poly(2-diisopropylamino)ethyl methacrylate (PDPA) chains physisorbed to the AFM tip and modeling the resultant force curves allowed the persistence and contour lengths, molecular weight, and thus grafting density of the brush to be determined. In kosmotropic acetate, high osmotic forces associated with the swollen PDPA brush repelled the colloid probe during both approach and retraction. For mildly chaotropic nitrate the behavior was similar, but at high ionic strength and during retraction, the interaction was strongly adhesive partly because of decreased brush solvation. For strongly chaotropic thiocyanate, the interaction was adhesive over the entire concentration range studied. Here, physical contact between the poorly solvated brush and the colloid resulted in an attractive force.