H-1-NMR STUDIES ON AGGREGATION-INDUCED CONFORMATIONAL-CHANGES IN LINEAR ALKYLBENZENESULFONATES

The proton NMR chemical shifts of several linear alkylbenzenesulfonate s (NaLAS) and commercial NaLAS have been measured at concentrations ra nging from well below to well above critical micelle concentration (cm c). Upon micellization of various NaLAS isomers and also commercial Na LAS, significant changes occur in their H-1 NMR spectra. These changes are the largest for terminal methyl groups, meta protons, and the met hylene groups attached to the methine group. The changes in the chemic al shifts of terminal methyl groups upon micellization indicate that t he two chains, experience dissimilar environments due to micellization . This dissimilarity of chemical environment can be seen to increase w ith increasing difference in the number of methylene groups between th e 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 mic elle. The two protons on each methylene group attached to the methine group are magnetically nonequivalent below cmc and become equivalent a bove cmc. The aromatic protons meta to the sulfonate group show large upfield shift on micellization whereas the aromatic protons in the ort ho positions remain unshifted. Thus, the water boundary in NaLAS micel les lies between the ortho and meta protons of the phenyl ring. Aggreg ation-induced conformational changes in NaLAS isomers are shown to dep end on the position of the phenyl ring along the alkyl chain.