液态水的分子动力学模拟

用分子动力学(MD)模拟方法在150～376K的温度范围内对液态水的微正则系综进行了研究.考察了液态水的结构及其性质.模拟采用了由从头算得出的柔性水-水相互作用势MCYL.对时间和空间的平均得出了液态中水分子几何构型及温度改变所引起的液态水结构变化.对径向分布函数g_(OH),goo,g_(HH)及配位数的分析表明,在所考察的温度范围内,每个水分子与相邻分子形成的氢键数为2～3,水分子在参与的2个氢键中同时作为接受体.结合对振动谱的研究表明在低温时液态水形成的网络结构可能随温度的升高而形成小的簇结构.     

温度对液态水结构的影响：中子及X射线散射研究

采用中子散射和X射线散射研究了液态水在298～373 K温度范围内的结构,通过偏径向分布函数(PDF)、配位数分布(CN)、角分布(ADF)及空间密度分布(SDF)等讨论了温度对液态水结构的影响。整体来看,液态水具有"不规则四面体"氢键网络的短程有序结构,该有序度可延续到第三水合层。液态水分子的第一水合层中,围绕中心水分子约有4.8个水分子,然而其中仅有约3.3个水分子与中心水分子通过氢键相键合,约1/3进入到第一水合层的水分子并未与中心水分子直接键合,也正是这些间隙水分子的存在加剧了液态水结构的复杂性。温度对液态水的有序度存在一定的影响,在298～373 K的有限温度变化范围内,温度对液态水中氢键的键长、键角分布及第一水合层SDF的影响不大。从298K升温到373K,O(W)-O(W)距离仅增加0.03?,氢键数目也仅有微小减少,温度对第二和第三水合层的影响则要显著很多。     

聚N,N-二乙基丙烯酰胺低聚物水溶液的分子动力学研究

对50个单元构成的聚N,N-二乙基丙烯酰胺(PDEA)低聚物的水溶液体系进行了分子动力学的研究,分别模拟了300 K时的伸展链、310 K时的伸展链以及紧缩链与水构成的体系,对溶液中PDEA周围溶剂水分子的分布情况以及水分子形成氢键的情况进行了统计,结果表明在PDEA周围的水产生了比本体水更有序的结构,形成了更多的氢键,这种有序结构维持到第二水合层甚至更远.发生相分离后,PDEA与水分子形成的氢键大部分未被破坏,水合层中每个水分子形成的氢键数也没有明显变化,但水合层(形成有序结构的水分子)内水分子数目的减少使得总的氢键数目减少,从而造成体系能量增加及熵增加.同时还研究了聚合物及水分子的自扩散系数,表明PDEA影响周围水分子结构的同时,对水的动力学性质也产生了很大影响.     

液态水几何构型的分子动力学模拟

利用ＭＣＹＬ势对液态水进行分子动力学模拟，以探求近程和远程相互作用及液体的有序结构对水分子几何构型的影响，并确定对液态中水分子几何构型起作用的主要范围。模拟给出的液态水中原子对径向分布函数和水分子键长键角的变化与实验值能较好地符合。研究结果表明，液态水分子几何构型除与氢键的形成有关外，还与液体的近程有序结构有很大关系。键长的变化对这种近程有序结构虽不敏感，但键角的变化则对此非常灵敏。液态中第二水合     

离子对水结构的影响

液态水中广泛存在的氢键网络使其作为一种结构性很强的液体在自然界中发挥着独一无二的作用。电解质溶于水中后,以水合离子形式存在,离子附近强大的电场使水合层中的偶极水分子发生重排,这些水分子的结构与本体水的结构存在差异。一个多世纪以来,科学家做了大量关于离子对水结构影响的研究。本文从静态谱学和溶剂动力学两方面综述了离子对水结构影响的研究进展,概括了水中氢键的研究情况,并就离子对水结构影响在生物化学方向的重大意义-Hofmeister效应作了解释。     

水化镁基蒙脱石的分子动力学模拟

利用分子动力学(MD)模拟了300 K时镁基蒙脱石(粘土)层间水和镁离子的结构和动力学性质.模拟结果显示水在粘土层间分为二层,只有一小部分水被粘土表面吸附,与粘土结构中的羟基形成氢键,不同分布位置的水处于动态平衡.层间水分子氢键配位数比普通水少24%左右,水在粘土中自扩散系数D＝5.355×10^-10 m²·s^-1,约为主体相水的1/4.镁离子在粘土层间形成一层,其与水分子配位数约为6.进一步讨论了温度对粘土层中水的结构和动力学性质的影响.随着温度升高,水层的局部密度ρ(z)降低,水在XY方向的扩散系数不断增大.当温度达到600 K后,层间水分子间的氢键断裂,与超临界状态下水的结构相似,层间水的扩散系数达最大值,温度进一步升至700 K时,其值基本无变化.     

分子动力学方法研究聚甲基乙烯基醚／水体系中氢键和准氢键的结构与分布

用分子动力学方法模拟室温下不同浓度的聚甲基乙烯基醚／水体系的微观溶剂化结构．得到的径向分布函数和氢键给体和受体距离分布表明,聚合物与水形成的氢键比水之间形成的氢键短约0．005nm．准氢键C—H…O的数目是范德华作用对的7．2％．我们发现,在各浓度下,水分子并不能均匀地分布在聚合物结构单元上,即使在很稀的溶液（3．3％,质量分数）中,仍然有10％左右的醚氧没有和水分子形成氢键．这说明在溶液中,不但高分子链间有紧密的接触,而且高分子链内的链段间也有紧密的接触,导致链上的一些醚氧不能和水分子有效地接触而形成氢键．准氢键随浓度的变化和氢键的变化趋势类似,但形成准氢键的结构单元数目与形成氢键的结构单元数目比值在0．2附近．文献上用动态DSC测量低分子量聚甲基乙烯基醚（PVME）水溶液的相转变焓发现,在浓度为30％左右有一转折,与本模拟所得出的在浓度为27％左右氢键和准氢键比例的转折相关,这给相转变焓的转折点提供了分子尺度的微观解释．另外,浓度小于54％的溶液中存在“自由水”,在86％的浓溶液中每个结构单元大约与1．56个水分子缔合．     

液态水氢键的分析方法及研究进展

氢键已经发现了100多年,迄今为止它仍然是科学研究的重要课题。氢键的研究涉及从无机到有机众多学科,例如材料科学、生物化学、分子医学等。液态水是一种重要的化学溶剂,它很多特殊的性质都源于水分子间形成的氢键体系。特别是最近几年,水体系间氢键的研究取得了巨大的进步,关于氢键本质的新认识不断被提出。本文以水为例,重点介绍水体系中氢键的分析方法及研究进展,以期对氢键的研究有一个整体的认识。     

液态水热容的量子校正

在不同温度下对液态水进行分子动力学模拟，研究各温度下液态水中各个原子的速度自相关函数密度谱，以考察液态水热容的量子校正随温度的变化规律．研究结果表明，水分子的三个内部振动模式对热容的量子校正不随温度变化，而转动和分子距平衡位置的摆动运动模式的量子校正随温度升高而逐步减小．对于分子动力学模拟结果经温度涨落计算所得的热容进行了量子校正，校正结果与实验值能符合．     

油纸复合介质中水分子扩散行为的分子动力学模拟

对不同温度下水分子在油纸复合介质中的扩散行为进行了分子动力学模拟研究. 通过分析水分子与纤维素形成的氢键发现, 油中的水分子在模拟过程中会逐渐扩散到纤维素内并与之形成氢键, 而纤维素内的水分子则与纤维素形成氢键后被束缚于纤维素中. 通过分析水分子的扩散系数发现, 由于油和纤维素的极性不同, 使得水分子在油和纤维素两种单介质中的扩散行为有较大差别, 而在复合介质中的扩散系数受水分子在油和纤维素中的比例影响较大, 两者表现出很强的相关性. 水分子和两介质的相互作用与两介质的极性也存在很大的关系, 且不同温度下水分子与两介质的相互作用能和水分子在油和纤维素中的比例也表现出了较强的相关性. 不同温度下水分子的不同分布弱化了温度对扩散系数的影响.     

Molecular dynamics investigation into the structural features and transport properties of C60 in liquid argon

Molecular dynamics (MD) simulations were performed to investigate the structural features and transport properties of C60 in liquid argon. The results reveal that an organized structure shell of liquid argon is formed close to the surface of a C60 fullerene molecule, thereby changing the solid/liquid interfacial structure. Furthermore, the simulation indicates that the C60-liquid argon fluid becomes structurally more stable as the C60 molecule volume fraction and the temperature increase. The viscosity of the fluid increases significantly as the C60 molecule loading is increased, particularly at a lower temperature. The thermal conductivity enhancement of the fluid in the present simulations is anomalously an order of magnitude higher than the theoretical predictions from either the Maxwell or the Lu and Liu models, and is found to vary approximately linearly with the C60 molecule volume fraction. The increased thermal conductivity is attributed to the nature of heat conduction in C60 molecule suspensions and an organized structure at the solid/liquid interface.     

Contrasting nonaqueous against aqueous solvation on the basis of scaled-particle theory

Normal hexane is adopted as a typical organic solvent for comparison with liquid water in modern theories of hydrophobic hydration, and detailed results are worked-out here for the C-atom density in contact with a hard-sphere solute, rhoCG(R), for the full range of solute radii. The intramolecular structure of an n-hexane molecule introduces qualitative changes in G(R) compared to scaled-particle models for liquid water. Also worked-out is a revised scaled-particle model implemented with molecular simulation results for liquid n-hexane. The classic scaled-particle model, acknowledging the intramolecular structure of an n-hexane molecule, is in qualitative agreement with the revised scaled-particle model results, and is consistent in sizing the methyl/methylene sites which compose n-hexane in the simulation model. The classic and revised scaled-particle models disagree for length scales greater than the radius of a methyl group, however. The liquid-vapor surface tension of n-hexane predicted by the classic scaled-particle model is too large, though the temperature variation is reasonable; this contrasts with the classic scaled-particle theory for water which predicts a reasonable magnitude of the water liquid-vapor surface tension, but an incorrect sign for the temperature derivative at moderate temperatures. Judging on the basis of the arbitrary condition that drying is indicated when G(R)<1, hard spheres dry at smaller sizes in n-hexane than in liquid water.     

Influence of temperature on the structure of liquid water

Molecular dynamics simulations have been carried out for liquid water at 7 different temperatures to understand the nature of hydrogen bonding at molecular level through the investigation of the effects of temperature on the geometry of water molecules. The changes in bond length and bond angle of water molecules from gaseous state to liquid state have been observed, and the change in the bond angle of water molecules in liquid against temperature has been revealed, which has not been seen in literature so far. The analysis of the radial distribution functions and the coordinate numbers shows that, on an average, each water molecule in liquid acts as both receptor and donor, and forms at least two hydrogen bonds with its neigbors. The analysis of the results also indicates that the water molecules form clusters in liquid.     

Liquid structures of water, methanol, and hydrogen fluoride at ambient conditions from first principles molecular dynamics simulations with a dispersion corrected density functional

Using first principles molecular dynamics simulations in the isobaric-isothermal ensemble (T = 300 K, p = 1 atm) with the Becke-Lee-Yang-Parr exchange/correlation functional and a dispersion correction due to Grimme, the hydrogen bonding networks of pure liquid water, methanol, and hydrogen fluoride are probed. Although an accurate density is found for water with this level of electronic structure theory, the average liquid densities for both hydrogen fluoride and methanol are overpredicted by 50 and 25%, respectively. The radial distribution functions indicate somewhat overstructured liquid phases for all three compounds. The number of hydrogen bonds per molecule in water is about twice as high as for methanol and hydrogen fluoride, though the ratio of cohesive energy over number of hydrogen bonds is lower for water. An analysis of the hydrogen-bonded aggregates revealed the presence of mostly linear chains in both hydrogen fluoride and methanol, with a few stable rings and chains spanning the simulation box in the case of hydrogen fluoride. Only an extremely small fraction of smaller clusters was found for water, indicating that its hydrogen bond network is significantly more extensive. A special form of water with on average about two hydrogen bonds per molecule yields a hydrogen-bonding environment significantly different from the other two compounds.     

Water simulation model with explicit three-molecule interactions

Much effort has been directed at developing models for the computer simulation of liquid water. The simplest models involve effective two-molecule interactions, parametrized from experiment, for use in classical molecular dynamics simulations. These models have been very successful in describing the structure and dynamics of liquid water at room temperature and one atmosphere pressure. A completely successful model, however, should be robust enough to describe the properties of liquid water at other thermodynamic points, water's complicated phase diagram, heterogeneous situations like the liquid/vapor interface, ionic, and other aqueous solutions, and confined and biological water. In this paper, we develop a new classical simulation model with explicit three-molecule interactions. These interactions presumably make the model more robust in the senses described above, and since they are short-ranged, the model is efficient to simulate. The model is formulated as a perturbation from a classical two-molecule interaction model, where the forms of the correction to the two-molecule term and the three-molecule terms result from electronic structure calculations on dimers and trimers. The magnitudes of these perturbations, however, are determined empirically. The resulting model improves upon the well-known two-molecule interaction models for both static and dynamic properties.     

卵磷脂类液晶乳液的性能

以卵磷脂为主乳化剂制备了一款液晶乳液。 该乳液在皮肤上具有良好的稳定性能,在皮肤表层停留6 h仍能保持完整的液晶结构,克服了一般液晶乳液中液晶织构无法在皮肤上长时间停留的问题。 液晶结构在皮肤温度范围内具有较高的稳定性,可为液晶乳液其他性能的发挥提供条件。 基于此,我们采用体外称重法研究了液晶乳液的锁水性,发现液晶乳液锁水性能明显优于传统乳液,实验7 h后仍能保持原有水分总质量的50%;体外透析实验证实,液晶乳液表现出显著的缓释性能且能释放完全,7 h后目标分子(水杨酸钠)释放率可达96%;耐温性能研究表明,液晶乳液耐高温性能及低温冻融性能优良,且温度对乳液中层状液晶的影响是可恢复的;采用离心法、高低温循环和高低温长时间(3个月)放置的方法探究了乳液储运稳定性,结果表明,液晶乳液产品储运稳定性极佳,便于实际的应用和储运。 该研究不仅可为开发多效的高端化妆品提供必要信息和参考数据,而且有望拓展卵磷脂类液晶乳化剂体系的应用范畴。     

Molecular dynamics simulations of structural features and diffusion properties of fullerene-in-water suspensions

This study performs molecular dynamics (MD) simulations to investigate the structural features and diffusion properties of fullerene-in-water suspensions. The numerical results reveal that an organized structure of liquid water is formed close to the surface of the fullerene molecule, thereby changing the solid/liquid interfacial structure. The organized structure formation becomes more pronounced as the fullerene size is reduced. This observation implies that a transition zone exists between the organized liquid water layers and the random distribution region. Furthermore, the results indicate that the structural stability of fullerene-in-water suspensions improves as the fullerene volume fraction increases, but is insensitive to changes in the fullerene size. Finally, the simulation results reveal that the diffusion coefficient of the water molecules varies as a linear function of the fullerene loading, but is independent of the fullerene size.