Interaction between solute molecules in medium density solvents

Solvation properties of solutes in supercritical, medium density solvents have been analysed using hypernetted-chain theory with the emphasis on the solvent-mediated interaction between solute molecules. The solvent and solute molecules are Lennard-Jones particles, and the solute is present at infinite dilution. Also a pair of solute molecules separated by different distances has been considered using reference interaction site model theory. Mainly, solvents at two typical densities (1.09_pc and 2.91_pc; pc is the critical density) that are in medium and high density regions, respectively, are treated. The temperature is set at 1.04T_C (T_c is the critical temperature). When the solute size is larger than the solvent size and the strength of the solute-solvent attractive interaction is greater than that of the solvent-solvent in the medium density region, the solvent structure confined between a pair of solute molecules is largely different from that near a single solute molecule. The confined solvent becomes denser and more stabilized as the distance between the solute molecules decreases, and an attractive interaction is induced between them. The interaction becomes even more attractive as the strength of the solute-solvent attractive interaction increases. The observations are qualitatively different from those in the high density region. Another high density region, which is well below the critical temperature, has been considered, but the behaviour observed is similar to that in the high density region above the critical temperature.     

Rotational diffusion of coumarin laser dye studied by fluorescence depolarization technique

Solute-solvent interactions play a major role in determining the physiochemical properties of solutions. Yet our understanding of this subject is far from complete. Rotational diffusion studies of medium-sized molecules provide a useful means to probe these interactions. The rate of diffusion is sensitive to the shape of the molecule and interaction between the solute and solvent molecules. Because of continuous interactions with their neighbours, molecules rotating in liquid, experience friction. By modeling this friction using various continuum-based theories, we can get better insight into the nature of the solute-solvent interactions. In the present work steady-state and time-resolved fluorescence polarization studies have been carried out with coumarin 30. The rotational reorientation of this probe has been measured in butanol at higher values of viscosity over temperature. However, it was found that coumarin 30 rotates faster in butanol compared to n-octanenitrile.     

Influence of Solvent-Solvent and Solute-Solvent Interaction Properties on Solvent-Mediated Potential

A recently proposed universal calculational recipe for solvent-mediated potential is applied to calculate excess potential of mean force between two large Lennard-Jones (LJ) or hard core attractive Yukawa particles immersed in small LJ solvent bath at supercritical state. Comparison between the present prediction with a hypernetted chain approximation adopted for solute-solute correlation at infinitely dilute limit and existing simulation data shows high accuracy for the region with large separation, and qualitative reliability for the solute particle contact region. The calculational simplicity of the present recipe allows for a detailed investigation on the effect of the solute-solvent and solvent-solvent interaction details on the excess potential of mean force. The resultant conclusion is that gathering of solvent particles near a solute particle leads to repulsive excess PMF, while depletion of solvent particles away from the solute particle leads to attractive excess PMF, and minor change of the solvent-solvent interaction range has large influence on the excess PMF.     

Influence of Solvent-Solvent and Solute-Solvent Interaction Properties on Solvent-Mediated Potential

A recently proposed universal calculational recipe for solvent-mediated potential is applied to calculate excess potential of mean force between two large Lennard-Jones （LJ） or hard core attractive Yukawa particles immersed in small LJ solvent bath at supercritical state. Comparison between the present prediction with a hypernetted chain approximation adopted for solute-solute correlation at infinitely dilute limit and existing simulation data shows high accuracy for the region with large separation, and qualitative reliability for the solute particle contact region. The calculational simplicity of the present recipe allows for a detailed investigation on the effect of the solute-solvent and solvent-solvent interaction details on the excess potential of mean force. The resultant conclusion is that gathering of solvent particles near a solute particle leads to repulsive excess PMF, while depletion of solvent particles away from the solute particle leads to attractive excess PMF, and minor change of the solvent-solvent interaction range has large influence on the excess PMF.     

Phase diagram of highly asymmetric binary mixtures: A study of the role of attractive forces from the effective one-component approach

The phase diagram of an asymmetric solute-solvent mixture is investigated at the level of the effective one-component fluid. The solvent is taken into account by computing the potential of mean force between solute particles at infinite dilution for different models of solvent-solvent and solute-solvent short range interactions. Fluid-fluid and fluid-solid coexistence lines are determined from the free energy in the reference hypernetted chain theory for the fluid branch and from a variational perturbation theory for the solid one. The phase boundaries so determined compare well with recently published Monte Carlo data for mixtures of pure hard spheres. The influence of solute-solvent and solvent-solvent short range attractive forces is then investigated. When compared with pure hard core interactions, these forces are found to produce dramatic changes in the phase diagram, especially on the solvent packing fractions at which a dense fluid of solutes can be stable and on the separation of the fluid-fluid and fluid-solid coexistence lines. Finally, the connection of these results with the behavior of some colloidal suspensions is emphasized.     

Influence of isotopic substitution on the diffusion and thermal diffusion coefficient of binary liquids

The mutual mass diffusion coefficient (D) and the thermal diffusion coefficient ( D _T) of the liquids acetone, benzene, benzene-d ₁, benzene-d ₃, benzene-d ₅, benzene-d ₆, benzene- ¹³C₆, n-hexane, toluene, 1, 2, 3, 4-tetrahydronaphtalene, isobutylbenzene, and 1, 6-dibromohexane in protonated and perdeuterated cyclohexane have been measured with a transient holographic grating technique at a temperature of 25 ^°C. The mass diffusion coefficient shows a pronounced concentration dependence. Perdeuteration of cyclohexane only leads to marginal changes of the mass diffusion coefficient. The Stokes-Einstein equation describes the limiting tracer diffusion coefficients well if the solute molecule is smaller than the solvent. It is not capable to describe the small isotope effect of a few percent. On the other hand, the isotope effect, which is independent of concentration, is in agreement with the Enskog theory, that does not provide the absolute value of the mass diffusion coefficient of the liquid mixtures. The thermal diffusion coefficient of all the binary mixtures shows a moderate and almost linear concentration dependence. Its isotope effect, which is the change of D _T upon deuteration of cyclohexane, varies with mole fraction. The thermophoretic force acting on any tracer molecule in cyclohexane changes by the same amount when cyclohexane is perdeuterated, irrespective of the magnitude of the thermophoretic force before deuteration. This change of the thermophoretic force is equal but of opposite sign to the difference between the thermophoretic forces acting on cyclohexane and perdeuterated cyclohexane as tracers in any of the above liquids.     

模耦合理论计算纳米粒子在聚合物熔体中的含时扩散系数与常规扩散常数 (cited: 2)

分析和计算了纳米粒子在聚合物熔体中的含时扩散系数与常规扩散常数. 采用广义朗之万方程描述扩散动力学,并通过模耦合理论计算摩擦记忆内核.为简单起见,只考虑了来自两体碰撞和溶剂密度涨落耦合作用两类微观因素对摩擦记忆内核的贡献. 采用聚合物参考作用点模型以及Percus-Yevick闭合条件计算了聚合物-纳米粒子复合溶液的平衡态结构信息函数;详尽分析了纳米粒子的尺寸与聚合物链的尺寸对扩散动力学的影响. 揭示了结构函数、摩擦记忆内核以及扩散系数等随着纳米粒子半径和聚合物链长的变化关系. 结果表明,对于小尺寸的纳米粒子或者短链的聚合物,短时间的非马尔可夫扩散 动力学特征比较显著,含时扩散系数需要更长的时间弛豫到常规扩散常数. 微观因素对扩散常数的贡献随着纳米粒子尺寸的增加而减小,却随着聚合物链长的增加而增大. 此外,模耦合理论得到的扩散常数与Stokes-Einstein关系的预测值进行比较,发现对于小尺寸的纳米粒子或者长链的聚合物,微观因素对扩散常数的的贡献占主导地位. 相反,当纳米粒子较大或者聚合物链长较短时,流体力学的贡献会发挥重要作用.     

对利用动态光散射法测量颗粒粒径和液体黏度的改进

光散射技术通过测量悬浮液中布朗运动颗粒的平移扩散系数,得到颗粒流体力学直径或液体黏度.本文由单参数模型入手,建立了低颗粒浓度下,单颗粒平移扩散系数与颗粒集体平移扩散系数和颗粒浓度之间的线性依存关系并将其引入光散射法中,从而对现有的测量方法进行了改进.改进后的测量方法可实现纳米尺度球型颗粒标称直径的测量和液体黏度的绝对法测量.以聚苯乙烯颗粒+水和二氧化硅颗粒+乙醇两个分散系为参考样本,通过实验,验证了改进后方法的可行性.此外,还针对上述两个分散系,实验探讨了温度和颗粒浓度对颗粒集体平移扩散系数的影响规律,发现聚苯乙烯颗粒+水分散系中,颗粒间相互作用表现为引力;二氧化硅颗粒+乙醇分散系中,颗粒间相互作用表现为斥力.讨论了颗粒集体平移扩散系数随颗粒浓度变化规律与第二渗透维里系数的关系.     

计算平均力势的理论方法

提出了一个用于计算平均力势的普适性的理论框架,方法克服了以前的方法的缺陷,仅仅需要溶剂粒子在单个溶质粒子附近的密度分布作为输入.计算了两个大尺寸溶质粒子浸在小尺寸硬球溶剂浴中的平均力势,理论预言与可能的模拟数据符合.调查了溶剂-溶质相互作用势、溶剂密度、溶质粒子尺寸对过量平均力势的影响.结论是:溶剂粒子在单个溶质粒子附近的减少导致吸引的过量平均力势,而溶剂粒子在单个溶质粒子附近的聚集导致排斥的过量平均力势,高溶剂密度与大溶质粒子尺寸能强化这种趋势.讨论了这种空耗吸引-聚集排斥与生物学中的疏水吸引-水化排斥的联系.     

Reaction dynamics for heavy particle charge transfer reactions in polar solvents and ion dielectric friction

The authors' theory of a solvent's influence on heavy particle charge transfer reactions in polar solvents with complex frequency-dependent dielectric constant is applied to treat the dielectric friction on a moving ion. The aim is to demonstrate a relation between the approach used in the theory and other approaches used for treating solute-solvent Coulomb interactions. As a result Zwanzig's formula is recovered. In addition, because this formula is derived by the other method, the relationship between the dielectric friction and the dynamics of nonequilibrium solvent polarization is demonstrated in a very transparent manner.     

Tracer diffusion in glassforming liquids

In the last decades, a wide collection of experimental evidence has been found in the study of supercooled glassformers on the existence of a crossover between two dynamical regimes at a temperature T_c. We discuss the validity of the Vogel-Fulcher-Tammann in both regions. The breakdown of the Stokes-Einstein relation below T_c is presented, indicating that the diffusion coefficient of a tracer becomes decoupled from the viscosity through an exponent ξ, and the diffusion process is intensified. We verify that a temperature shift on the diffusion coefficient introduces the same effect as the Stokes-Einstein breakdown equation. We present the dependence of this exponent on the ratio between the radii of the tracer and the host liquid molecule.     

Microscopic theory of intrinsic shear and bulk viscosities

A microscopic theory of intrinsic shear and bulk viscosities of solutions is given for a model of particles that interact with hard-sphere cores and weak longrange attraction. The approximation considered (the velocity chaos assumption of the Enskog theory) can be expected to yield quantitatively useful values for viscosities of the model solute-solvent system when the solute particles are not much larger than the solvent particles. Under solute-solvent mixing conditions of constant pressure and temperature we find that the intrinsic viscosities of a hard-sphere solute in a hard-sphere solvent can be positive or negative, depending upon size and mass ratios; for solute and solvent particles whose mass ratio equals their volume ratio, the intrinsic shear and bulk viscosities are always positive for solute particles larger than solvent particles: in the opposite case, the intrinsic shear viscosity is always negative while the intrinsic bulk viscosity is for the most part negative, becoming positive again when the solute particle is sufficiently small. For solute particles smaller than solvent particles, this result is sensitive to change in mass ratio. The addition of solvent-solvent attraction is found to lower the intrinsic viscosities substantially; the addition of solute-solvent attraction raises it. Detailed quantitative analysis of these effects is given.     

Can we understand (and model) aqueous solutions without any long range electrostatic interactions?

A computer simulation experiment has been conducted to study the extent to which long range Coulombic interactions are indispensable when modelling aqueous solutions of electrolytes. A simple molecular model, which accounts explicitly for the molecular structure of water but which does not incorporate any long range Coulombic interactions is employed. The solvent is primitive water (EPM5-4 model) and the solute molecules are hard spheres interacting with the interaction sites of the water molecule by means of either repulsive (like-charge interaction) or attractive (unlike-charge interaction) short range triangular-well tails. The structural changes (hydrophobic ordering, structure breaking, and structure enhancement) which take place in an infinitely dilute solution upon ‘charging’ the solute were studied, in terms of the correlation functions and of the orientational distribution functions and of the average binding energy of the water molecules around the solute in terms of their dependence on the solute-water oxygen distance. The main thermodynamic property reflecting these changes is the residual entropy. This quantity is found to exhibit an asymmetric double maximum, in agreement with the findings for a realistic counterpart of this simple model that employs long range Coulombic interactions.     

Stokes-Einstein relation in supercooled aqueous solutions of glycerol

The diffusion of glycerol molecules decreases with decreasing temperature as its viscosity increases in a manner simply described by the Stokes-Einstein relation. Approaching the glass transition, this relation breaks down as it does with a number of other pure liquid glass formers. We have measured the diffusion coefficient for binary mixtures of glycerol and water and find that the Stokes-Einstein relation is restored with increasing water concentration. Our comparison with theory suggests that adding water postpones the formation of frustration domains.     

低密度溶液中溶剂的重组织性质

分析了溶液的微观结构 ,结果表明 ,单个溶质粒子影响其周围的溶剂的结构 ,溶质粒子间的相互作用也将影响溶剂的结构 ,溶质对溶剂结构的影响称作溶剂的重组织 .提出了二阶重组织能及二阶重组织熵等概念 ,可以描述在两个溶质粒子发生碰撞时对其周围溶剂结构的影响 .利用二元系的集团展开理论 ,给出了溶剂的一阶、二阶重组织能和重组织熵的表达式 .统计热力学分析给出了溶剂 溶剂径向分布函数与溶质和溶剂化学势之间的关系 ,给出了无限稀溶液模型是否成立的宏观判据 .提出的理论可用于低密度的二元溶液 .     

Interaction induced Raman light scattering as a probe of the local density structure of binary supercritical solutions

Interaction induced Raman light scattering is presented as a unique tool for the understanding of solvation processes from the solute's point of view in weakly interacting solute-solvent systems. A review of pertinent literature shows that this technique should be useful at least in single-phase binary mixtures such as supercritical solutions. Methane is used here as a probe molecule at 10mol% concentration (as the solute) and 90mol% CO and CO₂ are the solvents. The light scattering results, i.e., the dependence of the anisotropic intensities divided by density (I/d) on the density, are interpreted by use of the Duh-Haymet-Henderson closure (bridge) function of the Ornstein-Zernike integral equation. These data, together, are examined in the context of known supercritical solution thermodynamics and statistical mechanical results. It is shown that the light scattering I/d data versus density yield maxima in both attractive and repulsive solute-solvent systems. The local number density maxima were found near these same densities by the integral equation calculations for both methane + carbon monoxide or carbon dioxide using only Lennard-Jones single-centre parameters as input. The methane + carbon monoxide system is identified as weakly attractive (augmenting), whereas the methane + carbon dioxide system is identified as repulsive (avoidance).     

γ-丁内酯溶剂效应的红外光谱

在19种不同有机溶剂中测定γ-丁内酯的红外光谱,研究溶质-溶剂间相互作用,并进行了溶质的羰基伸缩振动频率位移与溶剂参数相关性分析。结果表明,在所考察的溶剂(除正己烷和环己烷外)中,γ-丁内酯与溶剂间形成非键缔合物。缔合物的羰基伸缩振动频率(带)和游离γ-丁内酯的羰基伸缩振动频率(带)位移与溶剂参数AN、S值、ET(30)值、G值、Swain参数Aj和Bj以及LSER间具有较好的线性相关性。溶剂与γ-丁内酯相互作用中,溶剂的接受电子对的能力和极性起着重要作用。这意味着溶剂与γ-丁内酯形成非键缔合物时的主要作用力是氢键作用和偶极-偶极作用。     

Three-phase osmotic equilibria using the Gibbs ensemble simulation method

The Gibbs Ensemble Monte Carlo method has been used to simulate osmotic equilibria for Lennard-Jones mixtures. When the simulations are performed with two independent boxes, one containing solvent and the other a mixture of solute and solvent significantly negative osmotic pressures (Π) develop. Following a sugestion of Powles et al. (1997, Molec. Phys., 90, 665), we have extended these simulations to include a third box and the possibility of modelling three coexisting phases. The simulations show that the two phase equilibria with negative values of 77 are metastable and that the system spontaneously separates into three phases: pure solvent, dilute solute-solvent and dense solute-solvent with a resulting osmotic pressure that is normally small and positive.