微孔中简单流体扩散行为的分子动力学模拟研究

用分子动力学模拟方法研究了受限在微孔中的简单流体氩的扩散行为,考察了微孔类型、孔径、温度和密度对微孔中流体扩散系数的影响.研究发现,微孔中流体的扩散系数均小于体相流体,并且随孔径的减小而减小,同时沿孔道或狭缝方向的扩散系数分量远大于沿孔径方向的分量,并且流体在通道型微孔中的扩散系数小于在狭缝型微孔中的扩散系数,温度和密度也是影响微孔中扩散的重要因素.     

甘油-水-氯化钠三元溶液中甘油浓度对甘油自扩散系数的影响

利用分子动力学模拟方法研究了浓度对甘油-水-氯化钠三元溶液中甘油自扩散系数的影响. 随着甘油浓度的增大, 甘油的自扩散系数逐渐减小. 氢键分析表明, 甘油自扩散系数的减小来源于其参与的甘油-水氢键数目的减少和甘油-甘油氢键数目的增加.     

CTAB反相微乳液中水辛烷分子的自扩散研究

利用电导测量确定了CTAB/正丁醇/正辛烷/水四组分体系的相态,在反相微乳液区有明显的渗滤现象,通过脉冲梯度场核磁共振方法研究了反相微乳液中水和辛烷分子的自扩散行为,采用双指数拟合得出辛烷的自扩散系数。对于体系中出现的渗滤现象以及水和辛烷分子自扩散系数反常的原因,从分子水平上进行了解释。水和自扩散系数(2～7×10^-10m·s^-1)比一般文献值高,其原因：一是水在油相中有较大的溶解度;二是渗滤现象的存在,由于液滴间相互作用的增强和：粘性“碰撞而导致液滴的融合,以及由此而形成的瞬时水相连续通道,使水的自扩散系数增大。而辛烷自扩散系数比一般文献值小,则是由于在微乳液液滴的栅栏中表面活性剂分子烃链与辛烷分子间范德华引力所造成。     

CTAB反相微乳液中水辛烷分子的自扩散研究

利用电导测量确定了CTAB/正丁醇/正辛烷/水四组分体系的相态,在反相微乳液区有明显的渗滤现象,通过脉冲梯度场核磁共振方法研究了反相微乳液中水和辛烷分子的自扩散行为,采用双指数拟合得出辛烷的自扩散系数。对于体系中出现的渗滤现象以及水和辛烷分子自扩散系数反常的原因,从分子水平上进行了解释。水和自扩散系数(2～7×10^-10m·s^-1)比一般文献值高,其原因：一是水在油相中有较大的溶解度;二是渗滤现象的存在,由于液滴间相互作用的增强和：粘性“碰撞而导致液滴的融合,以及由此而形成的瞬时水相连续通道,使水的自扩散系数增大。而辛烷自扩散系数比一般文献值小,则是由于在微乳液液滴的栅栏中表面活性剂分子烃链与辛烷分子间范德华引力所造成。     

金属-有机骨架材料中吸附气体的扩散速率

采用分子动力学方法,以甲烷为探针分子研究了不同压力条件下气体在具有不同孔道结构的金属-有机骨架材料(MOFs)中的扩散速率.通过计算气体在八种材料中的自扩散系数,并结合气体分子在材料中的质心分布图等,讨论了气体扩散速率与孔道结构之间的关系.研究结果表明:对于同时含有孔笼(pocket)和三维正交孔道(channel)结构的MOF材料(P-C材料),低压时甲烷气体吸附在孔笼结构中,随着压力的升高,气体分子开始进入正交孔道,同时其自扩散系数增加;而对于只含有三维立方孔道结构的IRMOF(isoreticular MOF)系列材料,在中低压范围内,气体分子在其中的自扩散系数随压力变化较小.当压力进一步升高时,气体分子在材料孔道中的吸附逐渐接近饱和,其自扩散系数均降低.因此,在不同MOF材料中气体分子扩散速率的差异主要取决于孔道结构的不同.对P-C材料,中低压下通过控制压力可以控制气体在其中的扩散速率,从而为MOF材料在气体存储、分离等方面的实际应用提供参考信息.     

石英玻璃高温分子动力学模拟中的势函数

根据石英玻璃高温下的分子动力学研究, 分析了势函数中多体势在高温应用下的局限性, 认为离子型对势在模拟石英玻璃高温结构方面优于多体势. 在原子电荷转移方面, 计算并分析了Si和O原子电荷大小对计算原子自扩散系数的影响, 发现用原子电荷转移较少的Morse势函数计算的原子自扩散激活温度比BKS势函数计算的低, 而且在同一温度下, 自扩散系数的计算值也随着原子电荷的减小而增大, 因此, 较小的原子电荷转移应该有利于石英玻璃在高温下的动力学性能的研究.     

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.     

多孔离子交换树脂内Eu3+离子的分步扩散

用同位素交换法研究了Eu³⁺离子在D72和D751树脂内的扩散过程.应用分步孔道扩散方程将粒内有效扩散系数D_e分解为孔道扩散系数D_p和固相扩散系数D_g,表明该方程可用于描述多孔树脂内的动力学过程.实验表明,D_e、D_p、D_g均随反应温度的升高而增大.计算了实验条件下的Eu³⁺的自扩散活化能;D72树脂的D_p和D_g对温度的响应比D751树脂大,其D_e、D_p、D_g值亦均大于D751树脂;Eu³⁺在溶液中的自扩散系数D_s＞D_p,说明离子在树脂孔道内的自扩散不能完全等同于其在溶液中的自扩散.     

动力学因素对热诱导相分离法制备亲水性乙烯-丙烯酸共聚物微孔膜结构的影响

选择 3种不同丙烯酸含量的乙烯 丙烯酸共聚物 (EAA)为原材料 ,二苯醚 (DPE)为稀释剂 ,研究了淬冷温度、粗化时间等影响液滴生长的动力学因素对热诱导相分离法 (TIPS)制备EAA DPE亲水性高分子微孔膜结构的影响 .淬冷温度的高低决定了EAA DPE体系是发生液 液相分离还是固 液相分离 ,而产生相分离的机理不同将影响稀释剂液滴的生长 ,最终影响微孔膜的孔径 .实验结果表明 ,在相同粗化时间的条件下 ,随着EAA1 41 0 DPE、EAA3 0 0 2 DPE、EAA3 0 0 3 DPE三体系冷却温度的逐渐升高 ,孔径逐渐变大 .在结晶温度以下 ( 0℃、3 0℃、60℃ )粗化时间相同时 ,温度对微孔膜的孔径影响较小 ,例如 0℃和 3 0℃的恒温条件粗化 1 0min,微孔膜的孔径在 1～ 3 μm之间 ;在 60℃的恒温条件粗化 1 0min ,微孔膜的孔径在 3～ 5 μm之间 .而在 90℃的恒温条件粗化相同的时间 ,由于体系始终处于结晶温度线以上 ,体系始终处在液 液相分离区域 ,最终得到微孔膜的孔径达到了 6～8μm .在结晶温度以下 ( 3 0℃ )进行恒温粗化 ,由于体系的过冷程度很大 ,液滴相的粗化过程被抑制住 ,所以粗化时间对微孔膜的孔径影响不大 ;而在结晶温度以上 ( 90℃ )进行恒温粗化时 ,则是随着粗化时间的延长 ,微孔膜的孔径逐渐变大     

孔道型微孔和介孔分子筛的改性及超级绝热性能

在水热体系中合成了具有规则孔道结构的微孔分子筛ZSM-5和介孔分子筛MCM-41,SBA-15,MAS-5,通过改变材料表面的电性对介孔材料进行了化学修饰.采用X射线衍射(XRD)和扫描电子显微镜(SEM)对样品的结构、形貌进行了表征;通过氮气吸附-脱附测试了产物的比表面积,采用BJH法计算孔分布和孔容;将制得的样品压制成绝热材料后,进行导热性质测定.常温(25℃)常压下,有序介孔分子筛MCM-41的导热系数为0.038 W·m-1·K-1,具有少量微孔结构的MAS-5的导热系数为0.035W·m-1·K-1,二者均为超级绝热材料.材料经改性后,绝热性能有所提高:MCM-41的导热系数降至0.028 W·m-1·K-1,MAS-5的导热系数降至0.017 W·m-1·K-1.结合纳米介孔材料导热理论模型进行分析,发现纳米孔绝热材料的孔径越小,孔隙率越大,绝热性能好;介孔分子筛的导热系数与其孔壁厚度、孔径大小以及孔隙率有关.     

Transport Properties of Fluids in Micropores by Molecular Dynamics Simulation

The transport properties of fluid argon in micropores, i.e. diffusivity and viscosity, were studied by molecular dynamics simulations. The effects of pore width, temperature and density on diffusivity and viscosity were analyzed in micropores with pore widths from 0.8 to 4.0 nm. The results show that the diffusivity in micropores is much lower than the bulk diffusivity, and it decreases as the pore width decreases; but the viscosity in micropores is significantly larger than the bulk one, and it increases sharply in narrow micropores. The diffusivity in channel parallel direction is obviously larger than that in channel perpendicular direction. The temperature and density are important factors that obviously affect diffusivity and viscosity in micropores.     

Molecular Simulation on Transport Properties of Confined Water

The diffusivity and viscosity of water confined in micropores were studied by molecular dynamics simulations. The effects of pore width and density were analyzed at pore widths from 0.9 to 2.6nm. The diffusivity in micropores is lower than that of the bulk, and it decreases as pore width decreases and as density increases. But the viscosity in micropores is much larger than that of the bulk, and it increases as pore width decreases and as density increases. The diffusivity in channel parallel direction is obviously larger than that in channel perpendicular directions.     

Transport properties and distribution of water molecules confined in hydrophobic nanopores and nanoslits

The transport properties, including the diffusivity and viscosity, of water confined in hydrophobic nanopores and nanoslits were studied by molecular dynamics simulations. The results show that the diffusion coefficient in nanopores and nanoslits is markedly lower than that in the bulk. But the viscosity is much larger than that in bulk. The parallel diffusion coefficient is obviously larger than the perpendicular ones. The diffusion coefficient in the channel pore is ever less than that in the slit pore at the same pore width, but the viscosity is larger. The temperature and density affect significantly the diffusivity and viscosity in nanopores and nanoslits. Lower density water exhibits some special characteristics on density profiles in nanopores and nanoslits at lower temperatures, and the density profiles show a change from homogeneous to inhomogeneous as the pore width is reduced. Even clusters occurred in micropores.     

Density inhomogeneity and diffusion behavior of fluids in micropores by molecular-dynamics simulation

The density profiles and the diffusion behavior of fluid argon confined in micropores were studied by molecular-dynamics simulations. The effects of pore size (width), temperature and number density on the density profiles and the self-diffusion coefficients in micropores were simulated with pore widths from 0.6 to 4.0 nm. The density profiles are greatly affected by the pore size. Strong inhomogeneities in the channel direction and vapor-liquid phase separation in the micropores were observed when initial conditions were chosen in the coexistence region of the fluid. The self-diffusion coefficient in the channel direction in the pores was found to be much lower than in the bulk, and decreasing with decreasing pore size, decreasing temperature, and increasing density.     

Effects of water nanochannel diameter on proton transport in proton‐exchange membranes

The effects of water nanochannel diameter on proton transport pathways and properties have been studied using reactive molecular dynamics simulations. The proton distributions and diffusivities have been evaluated using the cylinder model of water domains at various diameters that is the most typical proposed morphological model in proton‐exchange membranes. The proton distributions are analyzed to clarify proton pathways by classifying the water channel into two regions in parallel: an inner channel (free water) and an outer channel (bound water). For all the water contents, the nonmonotonic trends that show a peak at a certain diameter are found to be observed in the proton diffusivity, which is dominated by the proton diffusivity in the free water region and has a strong correlation with the proton distribution that is controlled by the balance between the volume fraction of free water and the surface density of sulfonate groups. The electroosmotic drag coefficients are found to increase monotonically with increasing channel diameter as a result of the increase in the volume fraction of free water. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 867–878     

Negative thermal expansion of water in hydrophobic nanospaces

The density and intermolecular structure of water in carbon micropores (w = 1.36 nm) are investigated by small-angle X-ray scattering (SAXS) and X-ray diffraction (XRD) measurements between 20 K and 298 K. The SAXS results suggest that the density of the water in the micropores increased with increasing temperature over a wide temperature range (20-277 K). The density changed by 10%, which is comparable to the density change of 7% between bulk ice (I(c)) at 20 K and water at 277 K. The results of XRD at low temperatures (less than 200 K) show that the water forms the cubic ice (I(c)) structure, although its peak shape and radial distribution functions changed continuously to those of a liquid-like structure with increasing temperature. The SAXS and XRD results both showed that the water in the hydrophobic nanospaces had no phase transition point. The continuous structural change from ice I(c) to liquid with increasing temperature suggests that water shows negative thermal expansion over a wide temperature range in hydrophobic nanospaces. The combination of XRD and SAXS measurements makes it possible to describe confined systems in nanospaces with intermolecular structure and density of adsorbed molecular assemblies.     

Fluid structure and transport properties of water inside carbon nanotubes

The fluid structure and transport properties of water confined in single-walled carbon nanotubes (CNTs) with different diameters have been investigated by molecular-dynamics simulation. The effects of CNT diameter, density of water, and temperature on the molecular distributions and transport behaviors of water were analyzed. It is interesting that the water molecules ordered in helix inside the (10, 10) CNT, and the layered distribution was clearly observed. It was found that the axial and radial diffusivities in CNTs were much lower than that of the bulk, and it ever decreased as the diameter of CNT decreases. The axial thermal conductivity and shear viscosity in CNTs are obviously larger than that of the bulk and those in the radial direction, they increase sharply as the diameter of CNT decreases, which is clearly in contrast to the diffusivity. The inner space of CNT and the interactions between water molecules and the confining walls play a key role in the structure and transport properties of water confined in the CNTs.     

Estimation of micropore size distribution in active carbons from adsorption isotherms of water vapor

A possibility of estimation of the micropore size distribution in the carbon adsorbents with the developed micro-and mesoporous structure by analysis of the adsorption isotherms of water vapors was considered. At saturation water condenses in micropores in a form of a weakly compressed liquid. However, water molecules in micropores are packed not so closely as in the liquid because of steric hindrance. Therefore, the real density of water adsorbed in the micropores is lower than that of water adsorbed on an open surface and lower than the density of the normal liquid. An analysis of the adsorption isotherms of water vapors with account for the both opposite effects on the water density gives reliable data on the micropore sizes of the carbon adsorbents. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 40–43, January, 2007.     

A diffusion model for the fluids confined in micropores

The self-diffusion coefficients were calculated by molecular dynamics simulations and the effects of pore width, temperature, and fluid density on diffusion behavior of simple fluid argon and polar fluid water confined in micropores were analyzed and studied. A mathematical model describing diffusion behavior of fluids confined in micropores was proposed from the theories of molecular dynamics and molecular kinematics, and validated on the basis of the simulation results at various conditions. The model indicates that the diffusion coefficient is proportional to the square root of the pore width and to the temperature divided by the density squared. It is applicable to either liquid or gas states and only two parameters are required.