用激光拉曼光谱研究液态乙醇的水合作用过程

Hydration of Liquid Ethanol Probed by Raman Spectra

为研究室温下乙醇-水二元混合物内部的分子间缔合情形，测得了不同体积配比溶液的拉曼光谱，发现位于2800-3050cm^-1波数区间的C—H伸缩振动频率随乙醇中加入水量的增加整体呈现蓝移趋势，而位于1048cm^-1附近的CO伸缩振动频率的变化规律却与此相反。分析认为，这种现象主要由溶液内部分子间发生的不同水合作用所致，并据此阐明了液态乙醇的水合作用过程：水分子首先与纯乙醇中的自缔合短链发生氢键缔合作用，形成了含有较多乙醇分子数的乙醇水合团簇，直到溶液中水的体积含量达到50％时，乙醇的水合作用达到暂时饱和；而当水的体积含量继续增加到70％以后，水分子致使原有乙醇水合团簇解离形成较小尺寸的团簇，并与解离点位上的乙醇分子羟基发生进一步水合作用；而后，当水体积含量增至一定程度后，还会导致乙醇分子疏水基CH基团与水分子间形成弱氢键C—H…O。

In order to study the concentration dependence of the molecular interactions in ethanol-water hydrogen bonded system, Raman spectra of ethanol-water mixtures with different water contents were obtained at room temperature. It was found that the positions of the 3 C-H stretching vibration bands of ethanol molecule located in the range of 2 800-3 050 cm^-1 would generally present blue shifts when more water was added into the mixture; at the same time, however, the C-O stretching vibration band located at around 1 048 cm^-1 showed an opposite behavior. The different hydration, which is induced by hydrogen bonding, which happened in different concentration mixtures, was thought to be responsible for this situation, the hydration process of liquid ethanol was thus deduced： when adding a small amount of water into pure ethanol, and clusters containing one water molecule and several ethanol molecules were formed instead of ethanol self-association short chain clusters existing in pure ethanol; the clusters would combine more water molecules to form ringlike clusters through hydrogen bond association when adding more water into the mixture, then a temporary saturation would be seen when the volume percent of water reached 50%, and this saturation state would last until the water content reached 70 vol% ; after that, the large number of water molecules would dissoci- ate the ringlike clusters to smaller clusters and then associate to the ends of these dissociated clusters through hydrogen bonding; in addition, the improper hydrogen bonding between oxygen atom of water molecule and C-H bond of ethanol molecule is considered to be formed after the content of water reached a high value.