Competition between surfactants and apolar fragrances in micelle cores

Abstract

The impact of surfactant structure and composition on the micellar partitioning of hydrophobic fragrance molecules was studied. Based on molecular diffusion coefficients derived from self-diffusion NMR spectroscopy the partition coefficient KM between water and micellar phases turned out to be directly proportional to the octanol-water partition coefficient PO/W of the fragrance molecules. By partially replacing the main surfactant sodium laureth sulfate with low- and high-molecular weight co-surfactants a shift of the fraction of fragrance molecules towards the water phase could be observed. The shift is related to a change in the hydrophobic environment inside the micelles as evidenced by fluorescence spectroscopy. A micellar affinity factor was defined and free energy differences were calculated that take into account the compatibility between solute and micellar core. A variation of micellar affinity of up to a factor of 10 was observed between various surfactant systems and mixtures, affecting significantly the proportions of the fragrance molecules in micellar and water phases. According to the Henry’s law constant, the availability of volatile fragrance molecules in the aqueous phase has a direct impact on the headspace concentration in a liquid-gas equilibrium. The shift of fragrance molecules towards the water phase and a subsequent concentration increase in the gas phase was experimentally confirmed by dynamic headspace measurements. The selection of type and composition of surfactant has thus a direct consequence on the concentration of volatiles in the gas phase and on the final perceived fragrance intensity.