Electrical Aspects of Adsorbing Colloid Flotation. XX. NMR Studies of Laurie Acid Solubilization

Abstract

Mixed micelles of dodecanoic acid with sodium dodecylsulfate, Triton X-100 (TX-100, an alkylphenylpolyethoxy alcohol), and dodecyltrimethylammonium chloride were studied. T₁ and nuclear Overhauser enhancement (NOE) measurements were made using dodecanoic acid C-13 labeled at the carboxyl group. Viscosity measurements were made and titration curves run of dodecanoic acid in these mixed micelles. The pK_a of dodecanoic acid increases, as one goes, from cationic to neutral to anionic micelles. Generally the mixed micelles are apparently small and roughly spherical; in acidic solution, however, dodecanoic acid-TX-100 micelles are large with an extended structure. NOE measurements indicate that the carboxyl group is relatively close to the protons in the micelle core, and that the carboxyl group moves out slightly into the aqueous phase on becoming ionized. The NOE of dodecanoic acid-TX-100 micelles indicates interaction between the carboxyl carbon and the protons in the ethoxy chain of TX-100.