A NUCLEAR MAGNETIC RESONANCE METHOD FOR DETERMINATION OF CRITICAL MICELLE CONCENTRATION

An NMR technique was used to determine CMC values at 35°C for octanoic, decanoie, laurie, and myristic acids in 95.5% sulfuric acid and the laurylammonium salts of butyric, caprylic, and laurie acids in CC14. The results are consistent with reported data from other methods. Values for the CMC of some of the compounds could not be detected owing to very small values for the acids and to chemical shifts beyond the range of the Varian A-60 instrument produced by some of the salts. The slopes of the δ-concentration curves above the CMC of the acids show a chain-length dependence with decanoic acid having anomalous behavior. The results can be explained by the relative loss or gain of structure breaking and polarization factors involving NMR sensitive protons. A large upfield chemical shift for the butyrate salt on micelle formation indicates large loss of proton polarization and is interpreted as the result of inverted micelles. The small chemical shift of added water protons at the CMC of the butyrate salt indicates solubiliaation of the water in the outer region of the micelles. © 1997 Elsevier Science Ltd. All rights reserved.