限域单分子层冰-水两相平衡体系的分子动力学模拟

限域水因有极其丰富的结构物相变化而成为近年来水科学研究的一个热点.然而,不同相限域水之间的相平衡结构与性质却鲜有报道.论文提出一套分子动力学模拟技术,可实现纳米尺度限域条件下冰和水的不同结构相间形成的低维固-液界面（线）的平衡态模拟.应用此模拟技术,我们探索了0.65 nm限域尺寸、5000 bar限域压强条件下,单分子层厚度的冰-水（固-液）两相平衡,计算了该平衡体系一系列热力学量在界线附近的分布.平衡态的分子模拟结果直观地展示了粗糙型固-液界线的热毛细涨落、界线固-液结构转变的微观机制、以及缺陷在固-液相变区附近的形成与输运.各种热力学量分布函数呈现了二维限域冰-水共存界面（线）的特殊性质,如：相平衡区域的尺寸异于块体材料固-液界面,固-液界线处于切向压缩状态等.     

应用极化水分子模型研究单分子层受限水的介电性质

受限条件下水的介电性质因测量极具挑战,其在诸多电化学过程与反应输运过程中如何扮演关键角色从未被定量地澄清.本工作利用平衡态分子动力学模拟和受限体系介电性质计算方法,系统性地探索了0.65 nm限域尺寸、5×10⁸ Pa限域压强、不同温度条件下单分子受限冰和受限水的介电性质.详细比较了恒定偶极矩SPC/E水分子模型和可极化的SWM4-NDP水分子模型在描述受限冰、水结构与介电性质上的优劣势,包括统计分析SWM4-NDP模型模拟的单分子层受限水和受限冰的瞬时分子偶极矩概率密度分布,计算每个模拟体系的静态结构因子、静态偶极空间关联函数、静态介电常数、体系偶极时间关联函数和德拜弛豫时间.首次发现了极化水分子模型描述的低维度受限水和受限冰的奇异分子极性变化,并观察到两种模型描述静态结构性质的效果相当,SWM4-NDP模型对于静态介电常数描述的优势会因受限条件的增强而被大幅削减.但在受限水介电极化弛豫动力学性质描述上SWM4-NDP模型明显优于SPC/E模型.我们推断SWM4-NDP模型在探索受限水结构相变动力学以及受限体系离子输运和溶剂化动力学等过程的模拟研究中是比SPC/E模型更好的选择.本工作将在进一步开展基于受限水系统储能、传感、输运的设计工作中提供一定的理论指导意义.     

界面扩张流变法研究C_(12)mimBr对Gemini12-2-12在空气/水界面聚集行为的影响

采用界面扩张流变技术研究了季铵盐偶联表面活性剂C12-(CH2)2-C12·2Br(Gemini12-2-12)及其与离子液体表面活性剂溴化1-十二烷基-3-甲基咪唑(C12mim Br)复配体系的动态界面张力、扩张流变性质和界面弛豫过程等,探讨了C12mim Br对C12mim Br/Gemini12-2-12混合体系界面性质的影响及C12mim Br对Gemini12-2-12界面聚集行为影响的机制.结果表明,随着离子液体表面活性剂的不断引入,体系界面吸附达到平衡所需的时间逐渐缩短,扩张模量和相角明显降低,界面吸附膜由粘弹性膜转变为近似纯弹性膜;同时,界面及其附近的弛豫过程也发生显著变化,慢弛豫过程消失,快弛豫过程占主导地位,且离子液体浓度越高,快弛豫的贡献越大.这些界面性质的变化主要归因于离子液体表面活性剂C12mim Br参与界面形成及两表面活性剂在界面竞争吸附的结果.少量离子液体表面活性剂C12mim Br的加入可以填补疏松的Gemini12-2-12界面上的空位,形成混合界面吸附膜.随着C12mim Br含量的增加,嵌入界面的C12mim Br分子数不断增多,导致界面上相互缠绕的Gemini12-2-12烷基链"解缠",在体相和界面分子扩散交换的过程中"解缠"的Gemini12-2-12分子从界面上解吸回到体相,与此同时,C12mim Br分子相对较小的空间位阻及较强的疏水作用促使其优先扩散至界面进而取代Gemini12-2-12分子,最终界面几乎完全被C12mim Br分子所占据.     

羟基取代烷基苯磺酸盐界面扩张粘弹性质

研究了2-羟基-3,5-二癸基苯磺酸钠(C10C10OHphSO3Na)表面和正癸烷-水界面上的扩张粘弹性质, 考察了平衡时间对界面性质的影响. 研究结果表明, 羟基取代烷基苯磺酸钠具有十分特异的界面性质, 其扩张模量比一般表面活性剂大一个数量级, 达到平衡的时间较长, 形成的界面膜弹性较大. 界面张力弛豫测定结果表明, 平衡时界面上存在特征时间长达103 s的慢过程. 上述实验结果可能是由于羟基间形成氢键造成的.     

不同结构烷基苯磺酸盐油水界面扩张粘弹性质

研究了2-甲基-5-(1-庚基辛基)苯磺酸钠、辛基苯磺酸钠和十六烷基苯磺酸钠在正辛烷-水界面上的扩张粘弹性质, 考察了链长变化和疏水基支链化对分子界面行为的影响. 研究结果表明, 链长增加导致分子间相互作用增强, 弹性增大; 疏水支链在界面上可能由于缠绕和变形产生界面慢弛豫过程, 导致较高的扩张模量.     

流动注射-分光光度法用于固-液吸附体系表观吸附速率常数测定

利用流动注射分析技术具有定时、定量取样、耗样量少、系统稳定性高等特点,将流动注射-光度检测法应用于固-液吸附体系的动力学研究中,结合固-液界面吸附动力学方程,测定固-液吸附体系表观吸附速率常数,作为一种新的动力学研究手段和方法,应用于物理化学的实验教学中。     

多元气-液-固复杂体系相平衡

石油流体中含有气相、液相及可能遇到的固相包括水合物、石蜡和沥青质等,涉及多元气-液-固复杂体系的相平衡问题.为防止这些沉积物堵塞造成安全隐患,需要确定水合物、石蜡、沥青质沉积起始条件以及沉积量.本文针对化学热力学理论在含水合物、石蜡和沥青质的多元-多相平衡研究中的应用进行了综述.水合物相平衡模型较为成熟,主要有两类,其一为基于等温吸附理论的van der Waals-Platteeuw型热力学模型;其二为基于双过程水合物生成机理的Chen-Guo水合物热力学模型.石蜡沉积一般采用活度系数法、状态方程法及多固相模型描述.沥青质絮凝、沉积则可采用溶解度参数模型、状态方程法、胶体模型和标度理论模型进行计算.同时对多元气-液-固复杂体系的相平衡研究发展方向进行了展望.     

界面扩张流变法研究C12mimBr对Gemini12-2-12在空气/水界面聚集行为的影响

采用界面扩张流变技术研究了季铵盐偶联表面活性剂C₁₂-(CH₂)₂-C₁₂·2Br (Gemini12-2-12)及其与离子液体表面活性剂溴化1-十二烷基-3-甲基咪唑(C₁₂mimBr)复配体系的动态界面张力、扩张流变性质和界面弛豫过程等, 探讨了C₁₂mimBr 对C₁₂mimBr/Gemini12-2-12 混合体系界面性质的影响及C₁₂mimBr 对Gemini12-2-12界面聚集行为影响的机制. 结果表明, 随着离子液体表面活性剂的不断引入, 体系界面吸附达到平衡所需的时间逐渐缩短, 扩张模量和相角明显降低, 界面吸附膜由粘弹性膜转变为近似纯弹性膜; 同时, 界面及其附近的弛豫过程也发生显著变化, 慢弛豫过程消失, 快弛豫过程占主导地位, 且离子液体浓度越高, 快弛豫的贡献越大. 这些界面性质的变化主要归因于离子液体表面活性剂C₁₂mimBr参与界面形成及两表面活性剂在界面竞争吸附的结果. 少量离子液体表面活性剂C₁₂mimBr 的加入可以填补疏松的Gemini12-2-12 界面上的空位, 形成混合界面吸附膜. 随着C₁₂mimBr 含量的增加, 嵌入界面的C₁₂mimBr 分子数不断增多, 导致界面上相互缠绕的Gemini12-2-12烷基链“解缠”, 在体相和界面分子扩散交换的过程中“解缠”的Gemini12-2-12分子从界面上解吸回到体相, 与此同时, C₁₂mimBr 分子相对较小的空间位阻及较强的疏水作用促使其优先扩散至界面进而取代Gemini12-2-12分子, 最终界面几乎完全被C₁₂mimBr分子所占据.     

Pb-Al二元体系液-固界面自由能的热力学理论计算

以复杂的Warren二元及赝二元常规系统下的液-固界面自由能理论为基础, 借助Pb-Al二元体系为例对其进行简化, 获得了二元非混溶体系液-固界面自由能物理模型, 然后对其热力学公式进行推导, 得出只含两个参变量的理论公式, 并对几种温度下液-固界面自由能(γSL)计算值及用多相平衡(MPE)法得到的实验值作了对比. 结果表明, 改进的物理模型及理论公式易于理解、计算简便, γSL的计算值取决于温度及Al原子分数的两个参变量, 与实验值较好地吻合, 证明了该模型具有结构简单、精度较高的优点, 并可作为其它非混溶体系γSL的计算模型, 为其推广应用奠定基础.     

界面张力弛豫法研究不同分子量原油活性组分界面扩张粘弹性

采用界面张力弛豫法研究了不同分子量原油活性组分在正癸烷-水界面上的扩张粘弹性质,阐述了界面扩张模量的弹性和粘性随扩张频率的变化规律.研究发现,随着原油活性组分分子量的增大,极限扩张粘度明显增大,而极限扩张弹性逐渐增大;当分子量大于某一数值后,极限扩张弹性变化不明显.对界面张力弛豫实验结果进行拟合得到的参数表明,界面上和界面附近的微观弛豫过程的数目随原油活性组分分子量的增加而增加,弛豫过程的特征频率也呈规律性变化.不同原油活性组分的界面扩张粘弹性质可从其不同特征的微观弛豫过程得到解释.     

Time autocorrelation function analysis of master equation and its application to atomic clusters

We derive the energy fluctuation Delta(2)E, and the time autocorrelation kappa(tau) and its Fourier transformation--the fluctuation spectra S(omega)--of the master-equation transition matrix. The contribution from each eigenmode of the transition matrix to these fluctuation quantities reveals the relevant importance of the individual mode in the relaxation processes. The time scales associated with these relaxation processes are determined by the corresponding eigenvalues. Unlike traditional time evolution analysis, the autocorrelation function and fluctuation spectra analysis does not involve an arbitrary initial population. It is also more suitable for analyzing the underlying dynamic, kinetic behavior near the equilibrium and the behavior of the long-time-scale rare events. We utilize our technique to analyze the solid-liquid phase coexistence of the 13-atom Morse cluster and the fcc-to-icosahedral structure transition of the 38-atom Lennard-Jones cluster. For the processes studied, the fluctuation spectra from the master equation simplify the analysis of the transition matrix, and the important relaxation modes are easily extracted.     

Growth inhibition mechanism of an ice-water interface by a mutant of winter flounder antifreeze protein: a molecular dynamics study

Molecular dynamics (MD) simulations of a growing ice-water interface of a pyramidal {2021} plane in the presence of a mutant of winter flounder antifreeze protein (AFP) were conducted. Simulation results indicated that the AFP was partially surrounded by ice grown at the pyramidal interface. The AFP stably bound to the interface only when AFP hydrophobic residues bound to ice. Simulation results also indicated a drastic decrease in the growth velocity of the ice surrounding the stably bound AFP, in agreement with ice growth inhibition processes that have been observed in real systems. We confirmed that the decrease in the growth velocity of ice was attributable to the melting point depression caused by the Gibbs-Thomson effect. Simulation results suggested that the growth of ice surrounding the AFP is needed to promote stable AFP binding to the interface and subsequent ice growth inhibition. MD simulations of a growing ice-water interface of a prismatic {10_10} plane were also conducted. Neither the stable binding of the AFP to the interface nor the decrease in the growth velocity occurred for the prismatic plane. These results agree with the fact that AFPs inhibit the growth of ice only on the pyramidal planes in real systems.     

Free energy of solvation of simple ions: molecular-dynamics study of solvation of Cl- and Na+ in the ice/water interface

Molecular-dynamics simulations of Cl(-) and Na(+) ions are performed to calculate ionic solvation free energies in both bulk simple point-charge/extended water and ice 1 h at several different temperatures, and at the basal ice 1 h/water interface. For the interface we calculate the free energy of "transfer" of the ions across the ice/water interface. For the ions in bulk water in the NPT ensemble at 298 K and 1 atm, results are found to be in good agreement with experiments, and with other simulation results. Simulations performed in the NVT ensemble are shown to give equivalent solvation free energies, and this ensemble is used for the interfacial simulations. Solvation free energies of Cl(-) and Na(+) ions in ice at 150 K are found to be approximately 30 and approximately 20 kcal mol(-1), respectively, less favorable than for water at room temperature. Near the melting point of the model the solvation of the ions in water is the same (within statistical error) as that measured at room temperature, and in the ice is equivalent and approximately 10 kcal mol(-1) less favorable than the liquid. The free energy of transfer for each ion across ice/water interface is calculated and is in good agreement with the bulk observations for the Cl(-) ion. However, for the model of Na(+) the long-range electrostatic contribution to the free energy was more negative in the ice than the liquid, in contrast with the results observed in the bulk calculations.     

Excess electron relaxation dynamics at water/air interfaces

We have performed mixed quantum-classical molecular dynamics simulations of the relaxation of a ground state excess electron at interfaces of different phases of water with air. The investigated systems included ambient water/air, supercooled water/air, Ih ice/air, and amorphous solid water/air interfaces. The present work explores the possible connections of the examined interfacial systems to finite size cluster anions and the three-dimensional infinite, fully hydrated electron. Localization site analyses indicate that in the absence of nuclear relaxation the electron localizes in a shallow potential trap on the interface in all examined systems in a diffuse, surface-bound (SB) state. With relaxation, the weakly bound electron undergoes an ultrafast localization and stabilization on the surface with the concomitant collapse of its radius. In the case of the ambient liquid interface the electron slowly (on the 10 ps time scale) diffuses into the bulk to form an interior-bound state. In each other case, the excess electron persists on the interface in SB states. The relaxation dynamics occur through distinct SB structures which are easily distinguishable by their energetics, geometries, and interactions with the surrounding water bath. The systems exhibiting the most stable SB excess electron states (supercooled water/air and Ih ice/air interfaces) are identified by their characteristic hydrogen-bonding motifs which are found to contain double acceptor-type water molecules in the close vicinity of the electron. These surface states correlate reasonably with those extrapolated to infinite size from simulated water cluster anions.     

Fast Water Relaxation through One‐Dimensional Channels by Rapid Energy Transfer

Water in carbon nanotubes is surrounded by hydrophobic carbon surfaces and shows anomalous structural and fast transport properties. However, the dynamics of water in hydrophobic nanospaces is only phenomenologically understood. In this study, water dynamics in hydrophobic carbon nanotubes is evaluated based on water relaxation using nuclear magnetic resonance spectroscopy and molecular dynamics simulations. Extremely fast relaxation (0.001 s) of water confined in carbon nanotubes of 1 nm in diameter on average is observed; the relaxation times of water confined in carbon nanotubes with an average diameter of 2 nm (0.40 s) is similar to that of bulk water (0.44 s). The extremely fast relaxation time of water confined in carbon nanotubes with an average diameter of 1 nm is a result of frequent energy transfer between water and carbon surfaces. Water relaxation in carbon nanotubes of average diameter 2 nm is slow because of the limited number of collisions between water molecules. The dynamics of interfacial water can therefore be controlled by varying the size of the hydrophobic nanospace.     

Reversible charging of the ice-water interface. II. Estimation of equilibrium parameters

Equilibrium parameters characterizing the ice-water interface were obtained from electrokinetic and potentiometric data. The dependency of surface potential on pH was measured by the ice electrode. Electrokinetic zeta potentials were measured by electrophoresis. In the interpretation, the 1-pK and 2-pK concepts of the surface complexation model, developed for metal oxides, were used. The equilibrium parameters (constants of surface reactions) were determined. The results were confirmed by numerical simulations. Both 1-pK and 2-pK models fit the experimental data well.     

Simulation of fluid-solid coexistence in finite volumes: a method to study the properties of wall-attached crystalline nuclei

The Asakura-Oosawa model for colloid-polymer mixtures is studied by Monte Carlo simulations at densities inside the two-phase coexistence region of fluid and solid. Choosing a geometry where the system is confined between two flat walls, and a wall-colloid potential that leads to incomplete wetting of the crystal at the wall, conditions can be created where a single nanoscopic wall-attached crystalline cluster coexists with fluid in the remainder of the simulation box. Following related ideas that have been useful to study heterogeneous nucleation of liquid droplets at the vapor-liquid coexistence, we estimate the contact angles from observations of the crystalline clusters in thermal equilibrium. We find fair agreement with a prediction based on Young's equation, using estimates of interface and wall tension from the study of flat surfaces. It is shown that the pressure versus density curve of the finite system exhibits a loop, but the pressure maximum signifies the "droplet evaporation-condensation" transition and thus has nothing in common with a van der Waals-like loop. Preparing systems where the packing fraction is deep inside the two-phase coexistence region, the system spontaneously forms a "slab state," with two wall-attached crystalline domains separated by (flat) interfaces from liquid in full equilibrium with the crystal in between; analysis of such states allows a precise estimation of the bulk equilibrium properties at phase coexistence.     

Thermodynamics of the formaldehyde-water and formaldehyde-ice systems for atmospheric applications

Formaldehyde (HCHO) is a species involved in numerous key atmospheric chemistry processes that can significantly impact the oxidative capacity of the atmosphere. Since gaseous HCHO is soluble in water, the water droplets of clouds and the ice crystals of snow exchange HCHO with the gas phase and the partitioning of HCHO between the air, water, and ice phases must be known to understand its chemistry. This study proposes thermodynamic formulations for the partitioning of HCHO between the gas phase and the ice and liquid water phases. A reanalysis of existing data on the vapor-liquid equilibrium has shown the inadequacy of the Henry's law formulation, and we instead propose the following equation to predict the mole fraction of HCHO in liquid water at equilibrium, X(HCHO,liq), as a function of the partial pressure P(HCHO) (Pa) and temperature T (K): X(HCHO,liq) = 1.700 × 10(-15)?e((8014/T))(P(HCHO))(1.105). Given the paucity of data on the gas-ice equilibrium, the solubility of HCHO and the diffusion coefficient (D(HCHO)) in ice were measured by exposing large single ice crystals to low P(HCHO). Our recommended value for D(HCHO) over the temperature range 243-266 K is D(HCHO) = 6 × 10(-12) cm(2) s(-1). The solubility of HCHO in ice follows the relationship X(HCHO,ice) = 9.898 × 10(-13)?e((4072/T))(P(HCHO))(0.803). Extrapolation of these data yields the P(HCHO) versus 1/T phase diagram for the H(2)O-HCHO system. The comparison of our results to existing data on the partitioning of HCHO between the snow and the atmosphere in the high arctic highlights the interplay between thermodynamic equilibrium and kinetics processes in natural systems.     

Desorption kinetics of surfactants at fluid interfaces by novel coaxial capillary pendant drop experiments

Surfactants appear in multiphase fluid systems in which the interface and the adjacent bulk phase have been removed from equilibrium. Here, a new method is described for the measurement of rate constants of desorption of surface-active materials from fluid/fluid interfaces and the extent to which adsorption is reversible: the coaxial capillary pendant drop experimental technique.

Kinetic constants are determined by desorption experiments in pendant drops in which the interface adjacent to a surfactant solution is removed from equilibrium by replacing the subphase of the drop with pure water. Further, we demonstrate that although the rate of subphase exchange is comparatively slow with respect to the desorption timescale, it is possible to resolve desorption processes which occur under local equilibrium with the adjacent bulk phase from those that are determined in part by sorption kinetics. Experiments which measure the desorption kinetic coefficient, , using a homologous series of n-alkyl (C₈, C₁₀, C₁₂, C₁₄) dimethyl phosphine oxides are presented.     

受限状态聚合物熔体的分子动力学模拟

用粗粒化的分子动力学(MD)模拟方法从分子层次研究了受限于粗糙壁内的聚合物熔体的动力学性质. 结果表明, 对于链长较短的受限聚合物熔体体系, 随着膜厚的增加, 体系内部高分子链的松弛时间逐渐减少; 然而对于链长较长的受限体系, 聚合物链的松弛时间随着膜厚的增加先减少后增加. 推测这种由于链长的变化所引起的动力学性质的差异源自受限熔体内聚合物链聚集状态的改变, 并且通过考察交叠参数对这种改变进行了分析. 结果表明, 在膜厚增加的过程中, 决定受限状态高分子长链松弛机理的因素逐渐从受限效应转变成为链间的缠结效应.