Dissociation constants of aromatic carboxylic acids spread at the air/water interface

The surface potential technique is employed to estimate the dissociation co nstant (K) of a series of aromatic carboxylic acids in the form of Langmuir monolayers. Using electric surface potential (Delta V) measurements of mon olayers spread on aqueous subphases, containing different concentrations of Nai, and applying the Gouy-Chapman theory as well as the Henderson-Hasselb alch equation, the surface pK for the investigated compounds falls in the r ange of 4.5-5.6 with the mean value of 5.1. The other method, based on surf ace potential measurements of monolayers of the parent, unsubstituted compo und (5'-phenyl-m-terphenyl-4-carboxylic acid) spread on aqueous solutions w ith distinct pHs, leads to a pK of ca. 6.5 which corresponds to the bulk co nditions. This value is shifted towards neutral pH as compared to the one c alculated with the Henderson-Hasselbalch equation (5.3), owing to the inhom ogeneous distribution of hydrogen ions caused by partial ionization of carb oxylic groups of the monolayer molecules. (C) 2000 Elsevier Science S.A, Al l rights reserved.