Chiroptical Spectroscopy of Surfactants

Abstract

Three different chiroptical spectroscopic methods, namely, optical rotation, electronic circular dichroism (ECD), and vibrational circular dichroism (VCD) have been evaluated for studying the aggregation of sodium dodecylsulfate (SDS), an achiral surfactant, using garcinia acid disodium salt (GADNa) as a chiral probe. The specific rotation and ECD of GADNa are found to be altered by the aggregation of SDS, suggesting for the first time that achiral surfactants can be characterized with chiroptical spectroscopy using appropriate chiral probes. In addition, a chiral compound, fluorenyl methyloxy carbonyl l-leucine sodium salt (FLNa) is found for the first time to behave as a surfactant in water, with 205 Å(2) surface area per molecule at the air-water interface, critical micelle concentration (CMC) of 0.18 M, and Gibbs energy of micellization of -14 kJ/mol. The specific rotation of FLNa in water is found to increase with concentration beyond CMC, suggesting the formation of chiral aggregates. Different conformations of FLNa amenable to micellization have been identified using quantum chemical conformational analysis and their specific rotations calculated. The formation of lamellar aggregates of FLNa in water is suggested to be the cause for increase in specific rotation with concentration beyond CMC.