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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Molecular simulation study of water mobility in aerosol-OT reverse micelles
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Molecular simulation study of water mobility in aerosol-OT reverse micelles

机译：气溶胶-OT反胶团中水迁移率的分子模拟研究

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摘要

In this work, we present results from molecular dynamics simulations on the single-molecule relaxation of water within reverse micelles (RMs) of different sizes formed by the surfactant aerosol-OT (AOT, sodium bis(2-ethylhexyl)- sulfosuccinate) in isooctane. Results are presented forRMwater content w 0 = [H2O]/[AOT] in the range from 2.0 to 7.5. We show that translational diffusion of water within the RM can, to a good approximation, be decoupled from the translation of the RM through the isooctane solvent. Water translational mobility within the RM is restricted by the water pool dimensions, and thus, the water mean-squared displacements (MSDs) level off in time. Comparison with models of diffusion in confined geometries shows that a version of the Gaussian confinement model with a biexponential decay of correlations provides a good fit to the MSDs, while a model of free diffusion within a sphere agrees less well with simulation results. We find that the local diffusivity is considerably reduced in the interfacial region, especially as w_0 decreases. Molecular orientational relaxation is monitored by examining the behavior of OH and dipole vectors. For both vectors, orientational relaxation slows down close to the interface and as w_0 decreases. For the OH vector, reorientation is strongly affected by the presence of charged species at the RM interface and these effects are especially pronounced for water molecules hydrogenbonded to surfactant sites that serve as hydrogen-bond acceptors. For the dipole vector, orientational relaxation near the interface slows down more than that for the OH vector due mainly to the influence of ion-dipole interactions with the sodium counterions. We investigate water OH and dipole reorientation mechanisms by studying the w_0 and interfacial shell dependence of orientational time correlations for different Legendre polynomial orders.
机译：在这项工作中，我们提供了分子动力学模拟的结果，这些分子是由表面活性剂气溶胶-OT（AOT，双（2-乙基己基）-磺基琥珀酸钠）在异辛烷中形成的不同大小的反胶束（RM）中水的单分子弛豫的结果。给出了RM水含量w 0 = [H2O] / [AOT]的范围为2.0至7.5的结果。我们表明，RM内水的平移扩散可以很好地近似于通过异辛烷溶剂的RM的平移。 RM中水的平移流动性受水池尺寸的限制，因此水均方位移（MSD）会随着时间的推移而趋于平稳。与受限几何体中扩散模型的比较表明，具有双指数相关性衰减的高斯约束模型版本与MSD非常吻合，而球体内的自由扩散模型与仿真结果不太吻合。我们发现界面区域的局部扩散率大大降低，尤其是当w_0减小时。通过检查OH和偶极载体的行为来监测分子取向弛豫。对于这两个向量，取向松弛在靠近界面处减慢，并且随着w_0的减小。对于OH载体，RM界面上带电物质的存在会严重影响重新定向，并且对于氢键合至充当氢键受体的表面活性剂位点的水分子，这些影响尤为明显。对于偶极子向量，界面附近的取向弛豫比OH向量的慢得多，这主要是由于离子-偶极子与钠抗衡离子相互作用的影响。我们通过研究不同勒让德多项式阶数的取向时间相关性的w_0和界面壳相关性，研究水的OH和偶极子重取向机制。

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