Adsorption of Anionic Thiols on Silver Nanoparticles

Abstract

The adsorption of negatively charged 3-mercaptopropanesulfonate (MPS) on the surface of citrate-stabilized Ag nanoparticles in water is investigated using colloidal particle surface sensitive techniques. The adsorption of this negatively charged thiol appears to be qualitatively different from that of neutral thiols and highlights the importance of repulsive interactions of electrostatic and steric origins pertaining to charged thiols. For the charged MPS thiol, the adsorption process occurs in two phases. At low surface coverage, where the intermolecular repulsion is negligible and the adsorption is dominated by the formation of the S–Ag bond, MPS molecules need to overcome an activation energy barrier <i>E</i>_<b>a</b> = (7.5 ± 0.9) kcal/mol with an associated free energy change Δ<i>G</i>_ads = −(14.3 ± 0.3) kcal/mol and behave similar to neutral thiols. On the other hand, at high surface coverage where the repulsive interactions among MPS molecules cannot be neglected, the adsorption is characterized by a higher <i>E</i>_a = (12.4 ± 0.5) kcal/mol and lower Δ<i>G</i>_ads = −(7.4 ± 0.1) kcal/mol