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中国物理学会期刊

甲醇主介电弛豫速率的疏水端与亲水端协同影响

CSTR: 32037.14.aps.73.20241261

Synergistic influence of hydrophobic and hydrophilic ends on the primary dielectric relaxation rate of methanol

CSTR: 32037.14.aps.73.20241261
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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. 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 molecule plays a major role in the primary dielectric relaxation process through a hydrogen bonding network, while the hydrophobic end mainly exerts an indirect effect by influencing the formation of intermolecular hydrogen bonds. In this work, the factors influencing the primary dielectric relaxation process of methanol are systematically investigated by using molecular dynamics simulations. Studying methanol, a simplest alcohol molecule, 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 influence of hydrogen bond partner concentration on the primary dielectric relaxation rate of the system. In this work, we systematically investigate the factors influencing the primary dielectric relaxation rate of methanol by adjusting the O—H bond length (dOH), the C—O bond length (dCO), and the methyl diameter (σmethyl) of methanol molecules, respectively, and significantly extend the “wait-and-switch” model. 1) By adjusting dOH, we find that stronger total hydrogen bond energy (UHB) in the system can enhance the correlation of molecular motion, slow down the reorientation rate of molecules and, consequently, the primary dielectric relaxation process of the system. 2) By adjusting dCO, we discover 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 find that an excessively small σmethyl is detrimental to the stability of the hydrogen bond network, while an excessively large σmethyl hinders thehydrogen bonds from forming. Both of these situations will have a negative influence on the correlation of molecular motion. When σmethyl is at a moderate level, the main dielectric relaxation process of the system is the slowest. Ultimately, it is found that factors such as UHB and related motion volume (VCM), as well as the concentration of hydrogen bond partners in the system, collectively constitute the key factors affecting the primary dielectric relaxation rate of the system. Our results can reasonably explain experimental phenomena that the original “wait-and-switch” model cannot explain. This study contributes to a more in-depth understanding of the relaxation processes of alcohol molecules and their physical origins.

     

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