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Article

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 pK_a at a Nonionic Micellar Interface

Christy R. Whiddon,*^† Clifford A. Bunton,^‡ and Olle Söderman^†

Department of Physical Chemistry 1, Lund University, P.O. Box 124, S-221 00 Lund, Sweden, and Department of Chemistry and Biochemistry, University of California at Santa Barbara, Santa Barbara, California 93106

J. Phys. Chem. B, 2003, 107 (4), pp 1001–1005

DOI: 10.1021/jp0263875

Publication Date (Web): December 21, 2002

To whom correspondence should be addressed. E-mail: Christy. whiddon@fkem1.lu.se. Phone: +46-(46)-222-8142. Fax: +46-(46)-222-4413.

†

Lund University.

‡

University of California at Santa Barbara.

Abstract

The pK_a of a ¹³C-enriched fatty acid probe, tetradecanoic acid, was investigated in a variety of sugar-derived (APG) surfactants by using ¹³C 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 p , the intrinsic dissociation constant for an acid at a hydrophobic/hydrophilic interface, when calculating the surface potential of charged systems.