Nevertheless, these fields are critical in electrochemistry and energy conversion schemes, as well as to the function of biological membranes and enzyme active sites. Despite their importance in electrochemistry, a major complication is disentangling the apparent (applied) electric field from the one experienced by molecules at the double layer. Now, work by Delley, Nichols and Mayer explores this discrepancy by studying mixed monolayers of 4-mercaptobenzoic acid (4-MBA) and 4-mercaptobenzonitrile (4-MBN) on a gold substrate using surface-enhanced IR spectroscopy. The change in the pK1/2 of the carboxylate group in 4-MBA is monitored as the electric field strength is simultaneously quantified through Stark effects on the nitrile stretching frequency of 4-MBN (Delley, M. F., Nichols, E. M. & Mayer, J. M. J. Am. Chem. Soc. 143, 10778–10792 (2021)).
Spectroelectrochemical experiments enabled the population of (de)protonated COO–/COOD groups to be determined by tracking the changes in the intensity of the carbonyl stretching frequencies (pictured, a and b). Interestingly, the pK1/2 of the carboxylic acid (where [COO–] = [COOD]) was found to be pD and potential dependent, but not in the expected Nernstian way where a 59 mV change would be enough to shift the pK1/2 by 1. Instead, the behaviour was drastically non-Nernstian and a ~600 mV change was necessary for the pK1/2 to be shifted to the same degree (pictured, c, blue line). This observation raised the question of if 4-MBA was experiencing a reduced electric field because of the intervening monolayer and distance from the electrode surface or, more fundamentally, if the pK1/2 was resistant to the influence of the field.
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