Size, Hydration, and Shape of SDS/Heptane Micelles Investigated by Time-Resolved Fluorescence Quenching and Electron Spin Resonance

Abstract

Time-resolved fluorescence quenching (TRFQ) and electron spin resonance (ESR) are used to investigate sodium dodecyl sulfate/heptane micelles in water in the presence of varying amounts of sodium chloride (NaCl). Aggregation numbers are obtained from TRFQ and the micelle hydration from ESR. At any zero or any fixed sodium chloride concentration ≤375 mM, micelles grow with heptane while maintaining the volume fraction of water in the polar shell of the micelle, referred to as hydration index, constant. Core−shell model calculations, using simple geometry, show that some hydrocarbon density extends into the polar shell. The shell comprises about 15−17% of the core hydrocarbon in addition to the headgroups and hydration water. The rate of micelle growth with sodium chloride is smaller in the presence of heptane, and at higher salt concentrations (>375 mM), the micelles decrease in size with heptane. The rate of variation of hydration index with [NaCl] or aggregation number is indicative of micelle shape and shows that the sphere-to-rod transformation occurs at higher aggregation numbers and higher salt concentrations when heptane is present.