Hoffmann Markus M, Too Matthew D, Vogel Michael, Gutmann Torsten, Buntkowsky Gerd
Department of Chemistry and Biochemistry, State University of New York College at Brockport, Brockport, New York 14420, United States.
Institute of Condensed Matter Physics, Technical University Darmstadt, Hochschulstraße 6, Darmstadt 64289, Germany.
J Phys Chem B. 2020 Oct 15;124(41):9115-9125. doi: 10.1021/acs.jpcb.0c06124. Epub 2020 Oct 2.
An experimental study is presented for the reverse micellar system of 15% by mass polydisperse hexaethylene glycol monodecylether (CE) in cyclohexane with varying amounts of added water up to 4% by mass. Measurements of viscosity and self-diffusion coefficients were taken as a function of temperature between 10 and 45 °C at varying sample water loads but fixed CE/cyclohexane composition. The results were used to inspect the validity of the Stokes-Einstein equation for this system. Unreasonably small reverse average micelle radii and aggregation numbers were obtained with the Stokes-Einstein equation, but reasonable values for these quantities were obtained using the ratio of surfactant-to-cyclohexane self-diffusion coefficients. While bulk viscosity increased with increasing water load, a concurrent expected decrease of self-diffusion coefficient was only observed for the surfactant and water but not for cyclohexane, which showed independence of water load. Moreover, a spread of self-diffusion coefficients was observed for the protons associated with the ethylene oxide repeat unit in samples with polydisperse CE but not in a sample with monodisperse CE. These findings were interpreted by the presence of reverse micelle to reverse micelle hopping motions that with higher water load become increasingly selective toward CE molecules with short ethylene oxide repeat units, while those with long ethylene oxide repeat units remain trapped within the reverse micelle because of the increased hydrogen bonding interactions with the water inside the growing core of the reverse micelle. Despite the observed breakdown of the Stokes-Einstein equation, the temperature dependence of the viscosities and self-diffusion coefficients was found to follow Arrhenius behavior over the investigated range of temperatures.
本文介绍了一项实验研究，该研究针对质量分数为15%的多分散六甘醇单癸醚（CE）在环己烷中的反胶束体系，添加了质量分数高达4%的不同量的水。在10至45°C的温度范围内，在不同的样品水负载量但固定的CE/环己烷组成下，测量了粘度和自扩散系数。结果用于检验该体系中斯托克斯-爱因斯坦方程的有效性。使用斯托克斯-爱因斯坦方程得到的反平均胶束半径和聚集数不合理地小，但使用表面活性剂与环己烷自扩散系数的比值得到了这些量的合理值。虽然本体粘度随水负载量的增加而增加，但仅在表面活性剂和水上观察到自扩散系数同时预期的降低，而环己烷的自扩散系数与水负载量无关。此外，在具有多分散CE的样品中，观察到与环氧乙烷重复单元相关的质子的自扩散系数存在差异，但在具有单分散CE的样品中未观察到这种差异。这些发现可以通过反胶束到反胶束的跳跃运动来解释，随着水负载量的增加，这种运动对具有短环氧乙烷重复单元的CE分子的选择性越来越高，而具有长环氧乙烷重复单元的分子由于与反胶束不断增长的核心内的水的氢键相互作用增加而被困在反胶束内。尽管观察到斯托克斯-爱因斯坦方程失效，但在研究的温度范围内，粘度和自扩散系数的温度依赖性遵循阿伦尼乌斯行为。
