The Effect of Electrolyte Concentration on the Chemical Force Titration Behavior of ω-Functionalized SAMs: Evidence for the Formation of Strong Ionic Hydrogen Bonds

Abstract

The chemical force titration behavior of carboxylic and phosphonic acid-functionalized tips and substrates has been found to be very strongly dependent on electrolyte concentration. Under low electrolyte concentration conditions, these force titrations take the form of peaks, which change to monotonic sigmoidal curves with a concomitant shift to lower pH as the electrolyte concentration of the buffer is increased. The appearance of a peak in the low electrolyte concentration force titrations is attributed to the formation of strong hydrogen bonds between neutral and ionized species on the tip and substrate, which is prevented in the case of high electrolyte concentration by the formation of an electric double layer. There is strong evidence that the measured pK1/2(which corresponds to the pH of the bulk solution at which half of the surface groups are ionized) of these acid groups lies at the position of the peak of the low electrolyte concentration titration curves (carboxylic acid, pKa = 8; phosphonic acid, pKa1 = 4.6, pKa2 = 8.4) and cannot be measured under high electrolyte concentration conditions by this adhesion method. JKR theory of contact mechanics cannot be used to describe the low electrolyte concentration force titration data of these acid SAMs. The shapes of the force titration curves are described very well at all electrolyte concentrations by a simple model in which strong ionic and weak neutral hydrogen bonds contribute to the total adhesion force, which strongly supports our hypothesis. The results of fitting the force titration data to this model indicate that the strong ionic hydrogen bonds are on the order of 16 times stronger than a neutral hydrogen bond, which agrees well with theoretical predictions.