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Schweiß, R. ; Pleul, D. ; Simon, F. ; Janke, A. ; Welzel, P. ; Voit, B. ; Knoll, W. ; Werner, C.
Electrokinetic Potentials of Binary Self-Assembled Monolayers on Gold: Acid-Base Reactions and Double Layer Structure

The electrokinetic charge density and the dissociation behavior of the surface carboxylic acid groups in binaryself-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 bystreaming potential and streaming current measurements in aqueous electrolyte solutions. For bothtwo-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 significantlyattenuated acidity of high coverage COOH-terminated SAMs. This agrees well with theoretical considerationson the influence of electrostatic energy on the interfacial acid-base equilibrium. Diffuse layer charge densitiescalculated from zeta potentials in monovalent electrolyte solutions show that the countercharge in the diffuselayer increases with decreasing surface charge but is due to inner layer counterion adsorption still significantlylower than predicted by the standard Gouy-Chapman theory. In the case of bivalent counterions, ion bindingmodels successfully describe the observed electrokinetic potentials.

Journal of Physical Chemistry / B 108



March 2004


Polymer Interfaces