Interaction forces between hydrophobic and hydrophilic self-assembled monolayers

An investigation is presented of the interaction of charged self-assembled monolayers (SAMs) with a monoprotic ionizable acid functional group (-COOH) and uncharged SAMs with a methyl terminated functional group (-CH3). The s trength of the interactions are determined using an atomic force microscope . For all electrolyte conditions investigated the interactions are not well described by a summation of van der Waals attractions and electrostatic re pulsions in a manner suggesting that van der Waals attractions are screened . The repulsions are accurately described as corresponding to two surfaces of different charge interacting with surface charges that are independent o f separation (i.e., the constant charge model). A small adhesion force was observed under all conditions and its magnitude increased with NaCl concent ration. (C) 2000 Academic Press.