Detergency and foam studies on linear alkylbenzene sulfonate and secondary alkyl sulfonate

Abstract

Among anionic surfactants used in detergent products, the sodium salt of linear alkylbenzene sulfonate (NaLAS) is the only surfactant belonging to the aromatic class; the others are aliphatic, e.g., the sodium salt of secondary alkyl sulfonate (NaSAS). We observed earlier that certain conformational changes taking place in aromatic anionic surfactants (NaLAS) upon micellization can be brought about in aliphatic anionic surfactants (NaSAS) by addition of phenol. In this paper we examined how conformational changes at the molecular level translated into macroscopic properties such as foam and detergency. We performed foam and detergency measurements on NaLAS, NaSAS, and NaSAS/phenol systems. Foam behavior of these systems is shown to be dependent only upon calcium ion sensitivities of the surfactants whereas the detergency results have a dependence on conformational changes at the molecular level.Among anionic surfactants used in detergent products, the sodium salt of linear alkylbenzene sulfonate (NaLAS) is the only aromatic one; the others are aliphatic. In recent years, aggregation behavior of NaLAS has been extensively studied (1-6). Spectroscopic studies done in our laboratory on these classes of surfactants indicated some interesting aggregation-induced conformational changes. Specifically, in the case of NaLAS, the proton nuclear magnetic resonance (NMR) peaks corresponding to terminal methyl groups of the hydrocarbon chains appeared as a composite triplet below the critical micelle concentration (CMC). However, upon micellization, two well-defined triplets corresponding to each of the terminal methyl groups were observed. Based on these results, it was concluded that in NaLAS micelles, the shorter alkyl chain loops toward the palisade layer and experiences the phenyl ring effect (7). These conformational changes were not observed in the case of nonaromatic surfactants, specifically the sodium salt of secondary alkyl sulfonates/sulfates (NaSAS). However, the conformational changes encountered in NaLAS were brought about in NaSAS by the external incorporation of a phenyl ring by the addition of phenol (8). These phenomena indicate that the aggregation-induced conformation changes caused by the aromatic ring in NaLAS can be mimicked in the aliphatic anionic surfactant NaSAS by introducing a phenyl group externally.The objective of this paper was to examine how conformational changes observed at the molecular level translated into macroscopic properties. In view of this, the foam and detergency behavior of NaLAS, NaSAS, and NaSAS/phenol systems was studied. MATERIALS AND METHODSA sample of the sodium salt of LAS was prepared in the laboratory by neutralizing commercial linear alkylbenzene sulfonic acid (Reliance Industries, Mumbai, India) with sodium hydroxide solution in a Z-blade (Sigma) mixer. The resulting paste of NaLAS was dried in an oven at about 60°C, followed by vacuum drying. This dried NaLAS was subjected to Soxhlet solvent extraction using hexane to remove nondetergent organic matter. The paste was...

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