表面扩散的Monte Carlo初探

利用ＭｏｎｔｅＣａｒｌｏ方法模拟了理想表面和分形表面上的扩散过程；通过模拟可以发现，表面扩散系数不仅与表面浓度有关，而且还与扩散的时间、表面的几何形貌等有关。在表面覆盖度比较高时，表面扩散系数有一极大值。与理想表面相比，分形表面会使扩散系数减小。     

Effect of organic acid on lauroamide propyl betaine surface dilatational modulus and foam performance in a porous medium

The purpose of this work was to study the effect of surface tension and surface dilatational modulus on foam performance in high-salinity water in a porous medium. In order to clarify the role of the surface dilatational property in foam flow in a porous medium, three systems were established: a system with low surface dilatational modulus and high surface tension, a system with low surface dilatational modulus and low surface tension, and a system with high surface dilatational modulus and low surface tension. Measurement of dilatational modulus and surface tension showed that lauroamide propyl betaine (LAB) could not reduce surface tension and that surface dilatational modulus was low. The addition of lauric acid (LCOOH) to LAB could not achieve high surface dilatational modulus; however, it could reach lower surface tension. The addition of myristic acid (MCOOH) to LAB could achieve high surface dilatational modulus and lower surface tension. Unlike the other two systems, the results of a dilatational modulus comprised of a mixture of MCOOH and LAB were not a constant, as demonstrated by varied surface area deformation outcomes. With the increase of deformation, surface dilatational modulus decreased. Results of foam flow tests showed that among the two lower surface dilatational modulus systems, LAB foam had higher flow resistance regardless of flow rate. Among the two systems of similar lower surface tension, the mixture of LAB and MCOOH showed higher flow resistance than the mixture of LAB and LCOOH. However, with the increase of flow rate, pressure differences between the two systems became smaller, which corresponded to the decrease of surface dilatational modulus with an increase of deformation.     

二氧化钛(TiO2)表面能的理论研究

用密度泛函理论和虚拟原子轨道方法对二氧化钛-金红石(TiO2)(110)表面的表面能进行了理论计算. 结果表明, 二氧化钛的表面能与表面缺陷的百分率相关. 完整的表面具有最低的表面能, 表面能随着表面缺陷百分率的增大而升高, 这与自然环境下二氧化钛-金红石(TiO2)具有规整的(110)表面一致. 在光催化实验中利用二氧化钛表面的缺陷作催化剂需要考虑到表面的稳定性. 另一方面, 在完整的表面五配位Ti4+上填加氧原子与表面作用时, 表面能起初变化很小, 直到50%的五配位Ti4+被填充后表面能才开始升高.     

Oscillating bubble SHG on surface elastic and surface viscoelastic systems: new insights in the dynamics of adsorption layers

Surface rheology governs a great variety of interfacial phenomena such as foams or emulsions and plays a dominant role in several technological processes such as high-speed coating. Its major difference with bulk rheology resides in the high compressibility of the surface phase, which is the direct consequence of the molecular exchange between adsorbed and dissolved species. In analogy to bulk rheology, a complex surface dilational modulus, epsilon, which captures surface tension changes upon defined area changes of the surface layer, can be defined. The module epsilon is complex, and the molecular interpretation of the dissipative process that gives rise to the imaginary part of the module is subject to some controversy. In this contribution, we used the oscillating bubble technique to study the surface dilational modulus in the mid-frequency range. The dynamic state of the surface layer was monitored by a pressure sensor and by surface second-harmonic generation (SHG). The pressure sensor measures the real and imaginary part of the modulus while SHG monitors independently the surface composition under dynamic conditions. The experiment allows the assessment of the contribution of the compositional term to the surface dilational modulus epsilon. Two aqueous surfactant solutions have been characterized: a surface elastic and a surface viscoelastic solution. The elastic surface layer can be described within the framework of the extended Lucassen-van den Tempel Hansen model. The change in surface concentration is in phase with the relative area change of the surface layer, which is in strong contrast with the results obtained from the surface viscoelastic solution. Here, surface tension, area change, and surface composition are phase-shifted, providing evidence for a nonequilibrium state within the surface phase. The data are used to assess existing surface rheology models.     

New Thermodynamics for Evaluating the Surface‐phase Enrichment in the Lower Surface Tension Component

Regarding the surface phase of liquid mixtures as a thermodynamic phase, ideal surface phases are designed so that at fixed bulk‐phase composition, real and ideal surface phases have the same chemical composition and identical limiting slopes for the dependence of surface tension on mole fraction. Standard chemical potentials are introduced for surface phase components, and quasi‐exact expressions are worked out to compute ideal surface tensions and surface‐phase compositions of real liquid mixtures. Guidelines for choosing molecular models to estimate the molar surface area of pure constituents are given. Ideal and excess surface tensions are calculated by using literature data for aqueous ethanol solutions at 298 K. These results show treatment based on Butler’s equations grossly overestimate predicted surface tensions, thus leading to lower ethanol content in the surface phase. These inaccuracies are ascribed to the use of molar surface areas in model equations that are too small.     

关于表面张力和表面自由能的讨论

简要地回顾了表面物理化学中表面张力和表面吉布斯自由能(以下简称为表面自由能)两个基本概念的发展历史,讨论了表面张力和表面自由能形成的机制,以及键能与表面张力的关系,最后指出了尽管表面相可以在理论上抽象为几何面,但它是真实存在的独立相,而且由于表面自由能的原因它始终处于亚稳定状态.     

Smart Multiple Wetting Control on ZnO Coated Shape Memory Polymer Arrays

Recently, surfaces with intelligent wetting controllability have aroused increased attention. Endowing the surface with stimuli-responsive surface chemistry and tunable surface microstructure can achieve a surface with smart wetting performances. However, almost all existing surfaces only focused on single surface chemistry or micromorphology, thus to achieve smart multiple wetting regulation is still difficult. Herein, we report a ZnO coated shape memory polymer(SMP) surface, and the surface chemistry and micromorphology can be synergistically regulated. ZnO can provide adjustable surface chemistry under UV irradiation, and SMP can offer tunable micromorphology due to its shape memory effect(SME). Based on the combined effect between the above two features, surface wetting performance can be smartly regulated among multiple states. Moreover, due to the excellent controllability of the surface, the application in directional droplet transportation was also demonstrated. This paper offers a new surface with tunability in both surface chemistry and micromorphology, and given the excellent wetting controllability, the surface is believed to be applied in a lot of fields, such as droplet manipulation, fluidic devices and selective catalysis.     

Kinetics and layer organization during the polymerization of coniferyl alcohol at the air-water interface

The initial stage of the polymerization of coniferyl alcohol catalyzed by a peroxydase was studied at the air-water interface. The properties of the monolayers were investigated at constant area and at constant surface pressure by surface pressure, surface potential and ellipticity measurements. On the basis of the results obtained at constant surface area, it is proposed that the formation of a 2D layer occurs up to the inflection point of the surface pressure-area isotherm, and that for larger surface pressures a 3D structure is formed during the polymerization. If the barostat is set at the inflection point (the surface pressure is constant while the surface area increases), the 2D organization of the monolayer is retained. A kinetic model describing the adsorption of the reaction products in a 2D layer is proposed. The kinetic constants were determined from the surface potential, ellipsometry, surface pressure and surface area data.     

A topography/chemical composition gradient polystyrene surface: toward the investigation of the relationship between surface wettability and surface structure and chemical composition

In this paper, we have prepared of a topography/chemical composition gradient polystyrene (PS) surface, i.e., an orthogonal gradient surface, to investigate the relationship between surface wettability and surface structure and chemical composition. The prepared surface shows a one-dimensional gradient in wettability in the x, y, and diagonal directions, including hydrophobic to hydrophilic, superhydrophobic to hydrophobic, superhydrophobic to superhydrophilic gradients, and so forth. These one-dimensional gradients have different gradient values, gradient range, and contact angle hysteresis, which lie on both the surface roughness and the surface compositions. From the trend of variation of contact angle hysteresis, it can be concluded that the transition from the Cassie's model to the Wenzel's model occurs both by decreasing surface roughness and by increasing surface hydrophilic compositions. Moreover, the transition is more effective via changing surface chemical composition than changing surface roughness herein.     

渗透汽化膜表面化学结构设计研究

综述了近年来渗透汽化膜表面结构设计调控的研究进展。膜表面结构的设计与优化是提高其分离性能的重要方法。然而高分子表面具有环境响应性,这往往导致高分子材料在使用环境中失去在表面设计时所期待的性能。因此,高分子膜表面的环境响应性是在对膜表面进行设计和调控过程中必须考虑的因素。本文介绍了渗透汽化膜表面结构设计的方法,重点阐述了高分子膜表面环境响应特性对膜表面性质以及渗透汽化性能的影响。指出了利用高分子膜的表面重构行为可以对其表面结构进行优化,从而有效地提高膜的分离选择性。     

Surface dilational moduli of poly (ethylene oxide), poly (methyl methacrylate), and their blend films

Surface dilational moduli of poly (ethylene oxide) (PEO), poly (methyl methacrylate) (PMMA), and compatible PEO/PMMA blend films spread at the air-water interface were investigated as a function of surface concentration. The surface dilational modulus of an expanded PEO film increased as the surface concentration increased to 0.4mg/m(2), which corresponds to the limiting surface area of PEO. After peaking at this value, the surface dilational modulus decreased with an increase in the PEO concentration. Lissajous orbits of PEO films exhibited positive hysteresis loops for all surface concentration ranges. On the other hand, the surface dilational modulus of a condensed PMMA film steeply increased as the surface concentration increased. Lissajous orbits of PMMA films changed from positive hysteresis loops to negative loops at the surface concentration at which the surface pressure reached in the plateau region. The magnitude of the surface dilational modulus of PMMA was larger than that of PEO at a fixed surface concentration. The surface dilational moduli of the PEO/PMMA blend films increased with the total surface concentration and their magnitudes were less than those of the individual PMMA films and larger than those of the individual PEO films at fixed surface concentrations. Lissajous orbits of the PEO/PMMA blend films also changed from positive hysteresis loops to negative loops beyond the surface concentration at which the plateau surface pressure of PEO was attained.     

Three-Dimensional Aspect of the Surface Tension: An Approach Based on the Total Pressure Tensor

The conditions of mechanical equilibrium were considered, and the generalized notion of the surface tension at an arbitrarily curved surface layer was analyzed on the basis of the total nondiagonal pressure tensor including the external fields and anisotropic even in the bulk phases. It was shown that the transverse surface tension can be eliminated using the selection of a dividing surface; however, in the general case, this surface does not exhibit the properties of the tension surface. On the whole, three-dimensional and nondiagonal character of the tensors of excess surface stresses determined by the integration over the volume and the cross section of the surface layer is retained at any selection of the dividing surface.     

Analytical expressions for the electrical potential near planar, cylindrical, and spherical surfaces for symmetric electrolytes

The conjecture of Tuinier (J. Colloid Interface Sci. 258 (2003) 45) for the electrical potentials near a cylindrical surface and near a spherical surface under the conditions of symmetric electrolyte and large scaled radius are derived by solving the corresponding Poisson-Boltzmann equation. The surface charge density-surface potential relations for these surfaces are also derived under the conditions of constant surface potential. We show that the level of surface charge density for planar, cylindrical, and spherical surfaces follows the order spherical surface > cylindrical surface > planar surface.     

Monte Carlo Simulation of Polymers Confined Between Two Surfaces in the Presence of Diluent

Structure and surface properties of polymers confined between two surfaces are studied in the presence of diluent by using an off‐lattice Monte Carlo method. When the diluent–chain interaction is weak, the density of diluent beads near the surface increases sharply with increasing the diluent–surface interaction while the chain bead density near the surface decreases gradually. The total bead density near the surface increases with increasing the diluent–surface interaction until it exceeds the total bulk density. This is mainly due to a large increase in the diluent bead density near the surface. Because of the increase of diluent at the surface, chains near the surface change their conformation, i.e., long tails become abundant while short trains and loops are relatively depleted. On the other hand, when the diluent–chain interaction becomes strong, the total bead density near the surface increases slightly, but it exceeds the total bulk density only when the diluent–surface interaction is strong enough, because the diluents approaching the surface induce chain beads to move to the surface due to strong interaction between the chain and diluent.     

多晶铑表面上碳氧共存体系的研究

用紫外光电子能谱（ＵＰＳ）和俄歇电子能谱（ＡＥＳ）对碳氧共存的多晶铑表面及ＣＯ、Ｏ_２、Ｃ_２Ｈ_４在该表面上的吸附进行了研究，发现铑表面上共存的表面碳和表面氧存在相互作用，由于表面氧的存在，外来吸附物如ＣＯ、Ｏ_２、Ｃ_２Ｈ_４等可以覆盖在表面碳上面，使表面碳占据表面位抑制其它物种吸附的屏蔽效应消失。该表面具有氧化性，可以把吸附的乙烯氧化。     

Surface thermodynamic functions of dilute solutions of methylcyclohexanols in ethylene glycol

Surface thermodynamic functions (surface entropy, surface enthalpy, and surface composition) of dilute solutions of 2-, 3-, and 4-methylcyclohexanol in ethylene glycol were obtained using surface tension measurements at various temperatures. Surface excess values and surface mole fractions were obtained from Gibbs equation and extended Langmuir model respectively. The results show that all methylcyclohexanols are surface active in ethylene glycol. The lyophobicity of solutes decreases with increasing temperature. The presence of a maximum point in the surface entropy diagram in all systems is explained by the formation of clathrate-like solvates at the surface of these systems.     

Polyion adsorption onto catanionic surfaces. A Monte Carlo study

The adsorption of a single and negatively charged polyion with varying flexibility onto a surface carrying both negative and positive charges representing a charged membrane surface has been investigated by using a simple model employing Monte Carlo simulations. The polyion was represented by a sequence of negatively charged hard spheres connected with harmonic bonds. The charged surface groups were also represented by charged hard spheres, and they were positioned on a hard surface slightly protruding into the solution. The surface charges were either frozen in a liquidlike structure or laterally mobile. With a large excess of positive surface charges, the classical picture of a strongly adsorbed polyion with an extended and flat configuration emerged. However, adsorption also appeared at a net neutral surface or at a weakly negatively charged surface, and at these conditions the adsorption was stronger with a flexible polyion as compared to a semiflexible one, two features not appearing in simpler models containing homogeneously charged surfaces. The presence of charged surface patches (frozen surface charges) and the ability of polarization of the surface charges (mobile surface charges) are the main reasons for the enhanced adsorption. The stronger adsorption with the flexible chain is caused by its greater ability to spatially correlate with the surface charges.     

Piperidine adsorption on hydrated alpha-alumina (0001) surface studied by vibrational sum frequency generation spectroscopy

The adsorption of piperidine vapor on the hydrated alumina (alpha-Al2O3, corundum) (0001) surface was investigated using vibrational broad bandwidth and scanning sum frequency generation (SFG) spectroscopy. The interfacial vibrational signature in the C-H stretching region of piperidine at the alumina (0001) surface is shown to be a sensitive spectroscopic probe revealing the adsorption mechanism. The neat piperidine surface, aqueous piperidine surface, and aqueous piperidium chloride surface were also investigated in the C-H stretching region by SFG to establish vibrational reference frequencies. After piperidine adsorption, piperidine vapor was removed and piperidine was found to be chemisorbed onto the alumina (0001) surface through protonation by surface hydroxyl groups. The O-H stretching region of the alumina surface before and after piperidine adsorption was also investigated, and the results revealed the decrease of the surface number density of alumina surface hydroxyl groups.     

Surface layering in ionic liquids: an X-ray reflectivity study

The surface structure and thermodynamics of two ionic liquids, based on the 1-alkyl-3-methylimidazolium cations, were studied by X-ray reflectivity and surface tensiometry. A molecular layer of a density approximately 18% higher than that of the bulk is found to form at the free surface of these liquids. In common with surface layering in liquid metals and surface freezing in melts of organic chain molecules, this effect is induced by the lower dimensionality of the surface. The concentrations of the oppositely charged ions within the surface layer are determined by chemical substitution of the anion. The temperature-dependent surface tension measurements reveal a normal, negative-slope temperature dependence. The different possible molecular arrangements within the enhanced-density surface layer are discussed.     

水分子和二氧化铈(111)表面相互作用的DFT+U研究

采用引入Hubbard参数U修正的密度泛函理论(DFT+U)方法, 对水分子在二氧化铈(111)表面的吸附作用进行了研究. 计算结果表明: 在氧化的二氧化铈(111)表面, 水分子以单氢键构型吸附在二氧化铈表面, 但是不能自发解离; 在还原的二氧化铈(111)表面, 水分子或化学吸附在衬底上, 或自发解离成表面羟基结构. 与氢气在氧化的二氧化铈(111)表面上物理吸附体系的总能相比, 羟基化表面构型是能量更低的构型, 所以羟基解离形成氢气, 从而使表面被氧化的过程需要有外部条件, 反应不能自发进行. 因此, 水分子在还原的二氧化铈(111)表面有两个可能的状态, 即无氢键结构的化学吸附和表面羟基结构的解离吸附. 在一定的外部条件下, 表面羟基结构进一步解离形成氢气, 并使还原的二氧化铈(111)表面得以氧化.