Proton nuclear magnetic resonance studies on the aggregation-induced conformational changes in secondary alkyl sulfate/sulfonate plus phenol system

Many interesting aggregation-induced conformational changes in sodium salts of linear alkylbenzene sulfonates (NaLAS) previously have been reported. Some of these findings include the looping back of one of the hydrocarbon chains toward the palisade layer and the upfield shift of the meta protons of the phenyl ring upon micellization. The objective of this work is to compare the aggregation behavior of sodium salts of secondary alkyl sulfates (NaSAS) and secondary alkane sulfonates (NaSAlkS) in the presence of added phenol with that of NaLAS. The aggregation behavior of commercial NaSAlkS and several pure NaSAS has been studied in the absence and presence of phenol by H-1 nuclear magnetic resonance (NMRI spectroscopy. In the absence of phenol, the protons belonging to the two terminal methyl groups on the two hydrocarbon chains give rise to one triplet above and below the critical micelle concentration (CMC), indicating a similar chemical environment. However, separation of the H-1 NMR signals of the terminal methyl groups corresponding to the two hydrocarbon chains is noticed when phenol is added to the NaSAS or NaSAlkS micellar solution. These results indicate that phenol inserts itself into the micelle palisade layer and exerts a phenyl ring effect on the hydrocarbon chains. Because the terminal methyl group of the shorter chain is closer to the phenyl ring, it experiences more ring current effect and hence a separation of the two signals. Corresponding chemical-shift changes in the proton resonances of phenol are also observed when it is incorporated into the micelle. The ortho protons of phe nol remain essentially unshifted while meta and para protons shift upfield. These results indicate that ortho protons of phenol experience an aqueous environment while meta and para protons experience a nonpolar environment confirming the insertion of phenol into the surfactant micelles. Based on these studies, it is concluded that NaSAS/phenol and NaSAlkS/phenol systems mimic the NaLAS system in terms of conformational changes. These results also highlight the fact that the presence of a phenyl ring not only within the molecule las in the case of NaLAS) but also outside the molecule (added phenol) can be effective in exerting phenyl-ring effects.