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.