Flocculates of Ag nanoparticles (AgNPs) were formed using p-mercaptobenzoic acid (PMBA) to utilize coupled localized surface plasmon, providing enormous enhancement in surface enhanced Raman scattering. Electrostatic interaction between dissociated PMBA and counter ions, as well as van der Waals force between protonated PMBAs on AgNPs plays a crucial role in the flocculation. Dissociation of PMBA on AgNPs is determined not only by pH in solutions but also by surface coverage of PMBA. Mono- and divalent cations modified the nu(COO-) band at 1420 cm(-1) by 20 cm(-1) indicating distinct interaction of the cations with dissociated carboxylate anions of PMBA on AgNPs. (c) 2013 Elsevier B.V. All rights reserved.