Paper BI-ThP6
Determination of the Surface pK_d of Carboxylic Acid and Amine-Terminated Alkanethiols Using Surface Plasmon Resonance

Thursday, October 18, 2007, 5:30 pm, Room 4C

Alkanethiol self-assembled monolayers (SAMs) are widely used as model surfaces because they form very well-characterized monolayers over a broad range of surface chemistries. An important property for SAMs with ionizable functional groups is the surface dissociation constant (pK_d), which defines the charge-density for a given pH solution. Using surface plasmon resonance (SPR) spectroscopy, we developed a method for the direct measurement of the pK_d of COOH and NH₂-SAMs by combining the ability of SPR to detect the changes in mass concentration close to a surface and the shift in ion concentration over the surface as a function of surface charged density. An analytical study was first performed to theoretically predict the general shape of the expected SPR plots by calculating the excess mass of salt ions over the SAMs as a function of the difference between the solution pH and surface pK_d. SPR studies were then conducted to measure the shift in salt concentration as a function of bulk pH, with the resulting data being used to determine the pK_d for HS-(CH₂)₁₁-COOH SAMs to be 7.4 ± 0.2 (N=4, mean ± 95% C.I.) and 6.5 ± 0.4 (N=4, mean ± 95% C.I.) for HS-(CH₂)₁₁-NH₂ SAMs . These methods present a way to calculate the pK_d for charged SAM surfaces that is non-disruptive and minimally interactive with the surface, thus providing an accurate and direct measure of the pK_d of the surface.