Micellar solubilization: structural and conformational changes investigated by 1H and 13C liquid-state NMR.

A combination of (1)H self-diffusion measurements and (13)C chemical shift analysis has been used for the study of the solubilization of amphiphilic additives C(n)H(2n+1)X (n=4, 6; X=OH, NH(2)) in cetyltrimethylammonium bromide (CTAB) micelles. This approach, which could be extended to other mixed systems, allows complementary data on structures of micelles and conformations of alkyl chains to be obtained. Previous results on these systems are confirmed and new features emerge. All the additives studied behave as cosurfactants. Their degree of solubilization is determined solely by their alkyl chain length. In the case of cosurfactants with n=6, polar head group differences have been shown to modify micellar structures. This effect has been linked to differences in cosurfactant penetration into micelles. In parallel, the area of CTAB head groups and the length of cetyl chains gradually decrease on average when solubilizing more cosurfactants. Cetyl chain compression is strong in the case of cosurfactants with n=4, whereas it is slightly compensated by an extension below the micellar surface in the case of longer cosurfactants. These conformational changes are related respectively to the formation of smaller spherical micelles and of anisotropic or swollen micelles.