Nuclear magnetic resonance paramagnetic relaxation method for the determination of micellar solubilization equilibria applied to anionic micelles

Abstract

The NMR paramagnetic relaxation method has been developed (Z. Gao, R. E. Wasylishen and J. C. T. Kwak, J. Phys. Chem., 1989, 93, 2190) as an alternative procedure for the determination of micellar distribution equilibria. The 3-carboxyproxyl anion has been suggested as a suitable relaxation agent for the case of anionic micelles. In a recent paper (P. J. Bratt, H. Choudhury, P. B. Chowdhury, D. G. Gillies, A. M. L. Krebber and L. H. Sutcliffe, J. Chem. Soc. Faraday Trans., 1990, 86, 3313) it was noted that the acid form 3-carboxyproxyl is partially located in the micellar phase, and therefore cannot be used as a relaxation agent in this method. In this paper, the difference between 3-carboxyproxyl (acid form) and sodium-3-carboxyproxyl (salt form) is demonstrated by measuring ¹H T₁ values of sodiumdodecylsulphate alkyl protons and of completely solubilized tert-butylcyclohexane in the presence of these two relaxation agents. The paramagnetic 3-carboxyproxyl anion does not influence the T₁ values of surfactant or solubilizate protons in the micellar phase. It is concluded that the anionic form of 3-carboxyproxyl can be used to determine distribution coefficients of neutral solubilizates in anionic micelles.