P. Dynarowicz-latka et al., Dissociation constants of aromatic carboxylic acids spread at the air/water interface, THIN SOL FI, 360(1-2), 2000, pp. 261-267
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.