The effect of electrolyte concentration on the chemical force titration behavior of omega-functionalized SAMs: Evidence for the formation of strong ionic hydrogen bonds

The chemical force titration behavior of carboxylic and phosphonic acid-fun ctionalized tips and substrates has been found to be very strongly dependen t on electrolyte concentration. Under low electrolyte concentration conditi ons, these force titrations take the form of peaks, which change to monoton ic 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 formati on of strong hydrogen bonds between neutral and ionized species on the tip and substrate, which is prevented in the case of high electrolyte concentra tion by the formation of an electric double layer. There is strong evidence that the measured pK(1/2) (which corresponds to the pH of the bulk solutio n at which half of the surface groups are ionized) of these acid groups lie s at the position of the peak of the low electrolyte concentration titratio n curves (carboxylic acid, pK(a) = 8; phosphonic acid, pK(a1) = 4.6, pK(a2) = 8.4) and cannot be measured under high electrolyte concentration conditi ons by this adhesion method. JKR theory of contact mechanics cannot be used to describe the low electrolyte concentration force titration data of thes e acid SAMs. The shapes of the force titration curves are described very we ll at all electrolyte concentrations by a simple model in which strong ioni c and weak neutral hydrogen bonds contribute to the total adhesion force, w hich strongly supports our hypothesis. The results of fitting the force tit ration 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 agre es well with theoretical predictions.