In Situ Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy of Carboxylate-Bearing, Siloxane-Anchored, Self-Assembled Monolayers: A Study of Carboxylate Reactivity and Acid-Base Properties

Fourier transform infrared spectroscopy in the internal reflectance (attenuated total reflectance) mode (ATR-FTIR) has been applied to the study of carboxylate functionalized thin films. Both the monitoring of the self-assembly of an alkyl carboxylate-bearing siloxane-anchored monolayer and the use of pH difference spectra to directly assess (in situ) the acid-base properties of such a functionalized film have been achieved and are reported herein. This work has shown that the effective pK_a of carboxylic acid groups in such an assembly varies linearly with the coverage of the ionized carboxylate, with a limiting value of 5.4 as this coverage approaches zero. This acid-base behavior is in agreement with that reported for other carboxylic acid bearing surfaces, as determined by a variety of other techniques.