Probe Molecules for Pulsed-Field-Gradient Diffusion Nuclear Magnetic Resonance Experiments on Micelles

摘要

Nuclear magnetic resonance (NMR) diffusion measurements of surfactants suffer from fast exchange of the surfactant between the micellar aggregate and bulk solution. Therefore, hydrophobic probe molecules are commonly used to directly measure aggregate diffusion under the assumption that the probe molecules remain solubilized inside the micelles. Aggregate size is then determined or estimated from the aggregate diffusion value. Unfortunately, the probe molecule also experiences rapid exchange between the aggregate and bulk, leading to inaccurate or unreasonable micelle diffusion values, a fact that has often been ignored in the literature. In this article, we present a systematic evaluation of probe molecules in cationic and anionic surfactants, obtained by measuring the diffusion of probe molecules with varying hydrophobicity. We find that an octanol-water partition coefficient of at least similar to 5 is required for correct measurements of aggregate diffusion in the systems studied. Notably, some commonly used probes have a partition coefficient much lower than five and are therefore not suitable for aggregate diffusion measurements. Consideration of these results will help researchers obtain accurate results for micelle sizing or partitioning studies with pulsed-field-gradient NMR.