利用超额拉曼光谱研究硝酸镁水溶液中的离子对

利用超额拉曼光谱研究了室温下硝酸镁(Mg(NO₃)₂)溶液的离子缔合情况. 测量了该溶液羟基(－OH)伸 缩振动谱段和NO₃^-全对称伸缩振动谱段的拉曼光谱, 利用超额拉曼光谱及光谱拟合分析了这些光谱数据. － OH伸缩振动谱段的超额拉曼光谱显示, 低浓度(<2.3 mol·kg^-1)下阴离子第一水合层的水分子含量随溶液浓度 的升高呈线性关系增加, 在较高浓度时(>2.3 mol·kg^-1), 该含量变化偏离了线性关系, 这是因为Mg(NO₃)₂溶液 在高浓度时存在直接接触离子对导致的. 同样的转折点浓度也在对NO₃^- 全对称伸缩振动谱段的分析中被观测 到. 除了直接接触离子对, 还观测到三种溶剂分隔型离子对. 对该谱段下所有浓度的拉曼光谱和超额光谱进行 同时拟合, 给出了不同浓度下各种离子对的相对含量, 结果显示在0.23-4.86 mol·kg^-1浓度范围内都有溶剂分 隔型离子对和直接接触型离子对. 当Mg(NO₃)₂浓度低于2.3 mol·kg^-1时, 所有离子对的相对含量随浓度增加呈 现直线上升, 在高于这个浓度后直接接触离子对的相对含量急剧增加, 一种溶剂分隔型离子对的相对含量增加 变缓, 另一种溶剂分隔型离子对的相对含量逐渐减少, 还有一种溶剂分隔型离子对相对含量的增加趋势保持不 变, 在Mg(NO₃)₂浓度大于3.5 mol·kg^-1后, 其相对含量不再发生明显变化.

Ion Pairs in Aqueous Magnesium Nitrate Solution by Excess Raman Spectroscopy

We study the ion associations in magnesium nitrate solution by Raman spectroscopy. Utilizing the excess spectra and peak decomposition, we analyze the －OH stretching and NO₃^- symmetric stretching regions. Analysis of the －OH stretching region demonstrates that the amount of water molecules in the first hydration shell of the anion follows a linear relationship in the low concentration range (<2.3 mol·kg^-1), but deviates from this linear relationship at high concentrations (>2.3 mol·kg^-1). The contact ion-pairs at high concentration result in the nonlinear variation. In the NO₃^- symmetric stretching region, the contact ion-pairs and solvent separated ion pairs exist in the Mg(NO₃)₂ concentration region of 0.23-4.86 mol·kg^-1. From a global fitting of the Raman spectra over the whole concentration region, we deduce that the concentration of ion pairs varies. When the Mg(NO₃)₂ concentration is below 2.3 mol·kg^-1, the relative amounts of all ion-pairs increase with the concentration. However, above this concentration, the relative amount of contact ion pairs increases sharply, and solvent separated ion pairs either decrease or increase in their slower rate, until the Mg(NO₃)₂ concentration is 3.5 mol·kg^-1, and above this concentration the relative amount of one kind of solvent separated ion-pair does not change.