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 ~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.

中文翻译：

---

胶束上脉冲场梯度扩散核磁共振实验的探针分子

表面活性剂的核磁共振（NMR）扩散测量结果表明，表面活性剂在胶束聚集体和本体溶液之间快速交换。因此，在探针分子保持可溶在胶束内部的假设下，疏水探针分子通常用于直接测量聚集体的扩散。然后根据聚集扩散值确定或估计聚集尺寸。不幸的是，探针分子还经历了聚集体和本体之间的快速交换，从而导致胶束扩散值不准确或不合理，这一事实在文献中经常被忽略。在本文中，我们对阳离子和阴离子表面活性剂中的探针分子进行了系统的评估，方法是通过测量具有不同疏水性的探针分子的扩散来获得。我们发现，在所研究的系统中，为了正确测量聚集体扩散，至少需要〜5的辛醇-水分配系数。值得注意的是，一些常用探针的分配系数远低于5，因此不适合进行聚集体扩散测量。考虑这些结果将有助于研究人员使用脉冲场梯度NMR获得胶束大小或分配研究的准确结果。