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单羟基醇的主介电弛豫过程通常表现出典型的Debye特征，近年来，影响其速率的因素成为研究的热点。一般认为，醇分子的亲水端(即羟基)在主介电弛豫过程中通过氢键网络发挥主要作用，而疏水端则主要通过“稀释”体系中的羟基浓度，间接影响该过程。本研究通过经典分子动力学模拟系统地探讨了影响甲醇主介电弛豫速率的因素。研究结果表明，甲醇的疏水端不仅在稳固体系氢键网络方面对主介电弛豫过程产生间接影响，甚至还能直接作用于弛豫过程。甲醇的主介电弛豫过程是其亲水端和疏水端协同作用的结果。此外，研究还发现，甲醇的主介电弛豫速率可能并不像著名的“等待-切换”模型所描述的那样，主要受“氢键伙伴”浓度的影响，而是多种因素共同作用的结果。在某些情况下，“氢键伙伴”浓度的影响甚至被其他因素产生的影响所掩盖，成为次要因素。本研究有助于加深对醇类分子弛豫过程及其物理起源的理解。The primary dielectric relaxation process of monoalcohols typically exhibits characteristic Debye behavior, and the factors influencing its rate have become a research focus in recent years. It is generally believed that the hydrophilic end (i.e., the hydroxyl group) of alcohol molecules plays a major role in the primary dielectric relaxation process through hydrogen bonding networks, while the hydrophobic end mainly exerts an indirect effect by influencing the formation of intermolecular hydrogen bonds. This study systematically investigates the factors influencing the primary dielectric relaxation process of methanol using molecular dynamics simulations. As the simplest alcohol molecule, studying methanol can provide insights into the common characteristics of monohydroxy alcohols and even alcohols in general. The well-known "wait-and-switch" model currently emphasizes the impact of hydrogen bond partner concentration on the primary dielectric relaxation rate of the system. In this study, we systematically investigated the factors influencing the primary dielectric relaxation rate of methanol by independently adjusting the O-H bond length (doh), the C-O bond length (doc), and the methyl diameter (σmethyl) of methanol molecules, and provided significant extensions to the "wait-and-switch" model:1) By adjusting doh, we found that a stronger total hydrogen bond energy (UHB) in the system enhances the correlation of molecular motion, slowing down the reorientation rate of molecules and, consequently, the primary dielectric relaxation process of the system. 2) By adjusting dco, we discovered that a longer hydrophobic end not only slows down the primary dielectric relaxation process by stabilizing the intermolecular hydrogen bond network but also directly reduces the rate of this process. 3) By adjusting σmethyl, we found that an excessively small σmethyl is detrimental to the stability of the hydrogen bond network, while an excessively large σmethyl hinders the formation of hydrogen bonds. Both cases negatively affect the correlation of molecular motion. The primary dielectric relaxation process of the system is slowest when σmethyl is at a moderate level. It was ultimately found that factors such as UHB and the volume of the correlated motion (VCM), along with the concentration of hydrogen bond partners in the system, collectively form the key elements influencing the primary dielectric relaxation rate of the system. Our results can reasonably explain experimental phenomena that the original "wait-and-switch" model could not account for. This study contributes to a deeper understanding of the relaxation processes of alcohol molecules and their physical origins.
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Keywords:
- Methanol /
- Dielectric Relaxation /
- Molecular Dynamics Simulations
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