Titration of fatty acids in sugar-derived (APG) surfactants:: A 13C NMR study of the effect of headgroup size, chain length, and concentration on fatty acid pKa at a nonionic micellar interface

The pK(a) of a C-13-enriched fatty acid probe, tetradecanoic acid, was investigated in a variety of sugar-derived (APG) surfactants by using C-13 NMR to monitor the chemical environment of the probe during titration with base. There was an increase of approximately 2 pK(a) units for the probe within the micellar systems compared to the value of acetic acid in water. Analysis shows that the interfacial environment of alkylglucoside surfactants is similar to, although slightly more polar than, that of ethylene glycol-derived (EO) nonionic surfactants, and there is no indication that the alkylglucoside surfactants are deprotonated at pH levels between 2 and 10. The average pK(a) of the fatty acid probe in the APGs is measured to be 6.3, and it is argued that this value is an appropriate choice for pK(a)(0), the intrinsic dissociation constant for an acid at a hydrophobic/hydrophilic interface, when calculating the surface potential of charged systems.