H-1 NMR studies on aggregation-induced conformational changes in linear alkylbenzenesulfonates

The proton NMR chemical shifts of several linear alkylbenzenesulfonates (NaLAS) and commercial NaLAS have been measured at concentrations ranging from well below to well above critical micelle concentration (cmc). Upon micellization of various NaLAS isomers and also commercial NaLAS, significant changes occur in their H-1 NMR spectra. These changes are the largest for terminal methyl groups, meta protons, and the methylene groups attached to the methine group. The changes in the chemical shifts of terminal methyl groups upon micellization indicate that the two chains, experience dissimilar environments due to micellization. This dissimilarity of chemical environment can be seen to increase with increasing difference in the number of methylene groups between the two branches of the alkyl chain. On the basis of these results it is concluded that the shorter chain loops back toward the palisade layer of the micelle while the other occupies the central region of the micelle. The two protons on each methylene group attached to the methine group are magnetically nonequivalent below cmc and become equivalent above cmc. The aromatic protons meta to the sulfonate group show large upfield shift on micellization whereas the aromatic protons in the ortho positions remain unshifted. Thus, the water boundary in NaLAS micelles lies between the ortho and meta protons of the phenyl ring. Aggregation-induced conformational changes in NaLAS isomers are shown to depend on the position of the phenyl ring along the alkyl chain.