Kokubo Hironori, Pettitt B Montgomery
Department of Chemistry, University of Houston, Houston, Texas 77204-5003, USA.
J Phys Chem B. 2007 May 17;111(19):5233-42. doi: 10.1021/jp067659x. Epub 2007 Apr 21.
We performed molecular dynamics simulations of urea solutions at different concentrations with two urea models (OPLS and KBFF) to examine the structures responsible for the thermodynamic solution properties. Our simulation results showed that hydrogen-bonding properties such as the average number of hydrogen bonds and their lifetime distributions were nearly constant at all concentrations between infinite dilution and the solubility limit. This implies that the characterization of urea-water solutions in the molarity concentration scale as nearly ideal is a result of facile local hydrogen bonding rather than a global property. Thus, urea concentration does not influence the local propensity for hydrogen bonds, only how they are satisfied. By comparison, the KBFF model of urea donated fewer hydrogen bonds than OPLS. We found that the KBFF urea model in TIP3P water better reproduced the experimental density and diffusion constant data. Preferential solvation analysis showed that there were weak urea-urea and water-water associations in OPLS solution at short distances, but there were no strong associations. We divided urea molecules into large, medium, and small clusters to examine fluctuation properties and found that any particular urea molecule did not stay in the same cluster for a long time. We found neither persistent nor large clusters.
我们使用两种尿素模型（OPLS和KBFF）对不同浓度的尿素溶液进行了分子动力学模拟，以研究决定热力学溶液性质的结构。我们的模拟结果表明，诸如氢键平均数量及其寿命分布等氢键性质在从无限稀释到溶解度极限的所有浓度下几乎保持不变。这意味着，在摩尔浓度尺度上，尿素水溶液近乎理想的特性是由易于形成的局部氢键而非整体性质导致的。因此，尿素浓度并不影响形成氢键的局部倾向，仅影响氢键的满足方式。相比之下，KBFF尿素模型形成的氢键比OPLS模型少。我们发现，在TIP3P水中的KBFF尿素模型能更好地重现实验密度和扩散常数数据。优先溶剂化分析表明，在OPLS溶液中，短距离内存在较弱的尿素-尿素和水-水缔合，但不存在强缔合。我们将尿素分子分为大、中、小簇来研究其波动性质，发现任何一个特定的尿素分子都不会长时间处于同一簇中。我们既未发现持久簇，也未发现大簇。
