Chemistry Department, Bar-Ilan University, Ramat-Gan 52900, Israel.
J Phys Chem B. 2010 May 6;114(17):5755-63. doi: 10.1021/jp909657m.
There is much renewed interest in the arrangement and kinetic of hydrogen bonds in water and heavy water. D(2)O forms a higher average number of hydrogen bonds per molecule (10% more) compared to the case for H(2)O, which cause a larger entropic cost for solvating molecules in D(2)O. Here we used isothermal titration calorimetry (ITC) to investigate the enthalpy of titration of D(2)O-H(2)O solutions with different D/H isotope ratios. We found significant enthalpy deviations (exothermic contributions) relative to the computed enthalpy for the limit of ideal mixing both for dilution titration and for concentration titration (injection of solutions with lower D/H ratios into solutions with higher ratios and vice versa). We propose that the observed exothermic deviations might be connected to entropic effects associated with differences in the H and D arrangements that depend on the D/H ratio of the solutions. This ratio varies during the titration processes, leading to the entropy production beyond that of ideal mixing. We also used the ITC in the nonstirring mode to measure the titration kinetics and found long relaxation times of up to tens of minutes for the concentration titrations (but not for the dilution titrations). These observations are consistent with slow propagation of the reaction H(2)O + D(2)O <--> 2HDO that involves hopping of deuterium and rearrangements of the H and D bonding.
人们对水和重水中氢键的排列和动力学重新产生了浓厚的兴趣。与 H(2)O 相比，D(2)O 每个分子形成的氢键数量更多（多 10%），这导致在 D(2)O 中溶剂化分子的熵成本更高。在这里，我们使用等温滴定量热法（ITC）研究了不同 D/H 同位素比的 D(2)O-H(2)O 溶液的滴定焓。我们发现相对于理想混合极限的计算焓，无论是稀释滴定还是浓度滴定（将较低 D/H 比的溶液注入较高比的溶液中，反之亦然），都存在显著的焓偏差（放热贡献）。我们提出，观察到的放热偏差可能与与 H 和 D 排列差异相关的熵效应有关，这些差异取决于溶液的 D/H 比。在滴定过程中，该比值发生变化，导致产生的熵超过理想混合的熵。我们还使用非搅拌模式的 ITC 测量了滴定动力学，发现浓度滴定的弛豫时间长达数十分钟（但稀释滴定则没有）。这些观察结果与反应 H(2)O + D(2)O <--> 2HDO 的缓慢传播一致，该反应涉及氘的跳跃和 H 和 D 键的重排。
