Electrokinetic Potentials of Binary Self-Assembled Monolayers on Gold: Acid−Base Reactions and Double Layer Structure

Abstract

The electrokinetic charge density and the dissociation behavior of the surface carboxylic acid groups in binary self-assembled monolayers (SAMs) of 11-mercaptoundecanoic acid (MUA)/11-mercapto-undecanol (MUOH), and 16-mercaptohexadecanoic acid (MHA)/hexadecanethiol (HDT) on planar gold surfaces were studied by streaming potential and streaming current measurements in aqueous electrolyte solutions. For both two-component SAMs, the isoelectric point decreased with lower surface fractions (<0.25) of the carboxy-terminated alkanethiols, indicating that electrostatic interactions of the acid anions cause the significantly attenuated acidity of high coverage COOH-terminated SAMs. This agrees well with theoretical considerations on the influence of electrostatic energy on the interfacial acid-base equilibrium. Diffuse layer charge densities calculated from zeta potentials in monovalent electrolyte solutions show that the countercharge in the diffuse layer increases with decreasing surface charge but is due to inner layer counterion adsorption still significantly lower than predicted by the standard Gouy-Chapman theory. In the case of bivalent counterions, ion binding models successfully describe the observed electrokinetic potentials.