Force-field dependence on the interfacial structure of oil–water interfaces

We investigate the performance of different force-fields for alkanes, united (TraPPE) and all atom (OPLS-AA) models, and water (SPC/E and TIP4P-2005), in the prediction of the interfacial structure of alkane (n-octane, and n-dodecane)–water interfaces. We report an extensive comparison of the interfacial thermodynamic properties as well as the interfacial structure (translational and orientational). We use the recently introduced intrinsic sampling method, which removes the averaging effect of the interfacial capillary waves and provides a clear view of the interface structure. The alkane interfacial structure is sensitive to the environment, i.e. alkane–vapour or alkane–water interfaces, showing a stronger structure when it is in contact with the water phase. We find that this structure is fairly independent of the level of detail, full or united atom, employed to describe the alkane phase. The water surface properties show a small dependence on the water model. The dipole moment of the SPC/E model shows asymmetric fluctuations, with a tendency to point both towards the alkane and water phases. On the other hand the dipole moment of the TIP4P-2005 model shows a tendency to point towards the water phase only. Analysis of the intrinsic electrostatic field indicates that the surface water potential is confined to an interfacial region of about 8 Å. Overall we find that the intrinsic structure of alkane–water interfaces is a robust interfacial property, which is independent of the details of the force-field employed. Hence, it should provide a good reference to interpret experimental data.     

Intrinsic structure of hydrophobic surfaces: the oil-water interface

We investigate the water-oil interface using molecular dynamics simulations of realistic models of alkanes and water. The intrinsic density profiles are computed using a methodology that removes the smoothing effect of the capillary waves. We show that at 300 K the intrinsic width of the gap separating the oil and water phases spans little more than one water molecule diameter, and undergoes very weak short-ranged fluctuations, indicating that the water-oil interface is a rigid molecular structure at ambient temperature. Only near the drying transition (above 500 K for dodecane), the gap features uncoupled fluctuations of the oil and water surfaces, as expected in a typical drying structure. We find that the intrinsic structure of water next to the oil phase is remarkably similar to the bare water-vapor interface.     

Interfacial roughness in a near-critical binary fluid mixture: X-ray reflectivity and near-specular diffuse scattering

Measurements are presented of the X-ray specular reflectivity and near-specular diffuse scattering of the interface in a near-critical mixture of hexane and perfluorohexane. A lineshape analysis of the scattered intensity at each temperature yields values for the interfacial tension and interfacial width. The temperature variation of the tension and width so-obtained are consistent with current understanding of this interface, which holds that there is, firstly, an intrinsic width over which the fluid density varies smoothly from one coexistence composition to the other, and, secondly, that the interface acquires an additional and larger statistical interfacial width as a result of capillary fluctuations. Received 1 April 1998     

The capillary-wave Hamiltonian for an interface in the presence of a weakly corrugated substrate

We derive within the mean-field version of the Landau-Ginzburg -Wilson theory the expression for the capillary-wave Hamiltonian of the interface between twophases of an uniaxial ferromagnet fluctuating in the presence of a weakly corrugated, asymptotically flat substrate. The coefficients of interfacial stiffness as well as the coefficient multiplying the coupling between the ondulations of the interface and the corrugation of the substrate are shown to depend on the local distance l between the interface and the substrate. This distance dependence goes via the exponentially decaying terms which are multiplied by polynomials in l. Using this capillary-wave Hamiltonian we show that the weak corrugation of the substrate does not lead to a shift of the critical wetting temperature as compared to the flat substrate case.     

Interfacial width and shape fluctuations and extensions of the Gaussian model of capillary waves

Interfacial density fluctuations are studied at the level of the Gaussian model of capillary waves by means of density functional theory. We consider nonrigid fluctuations and arrive at exact Triezenberg-Zwanzig-type expressions for new interfacial coefficients. These include a width tension, a width rigidity, and other coefficients linked to both shape and width distortions. We find for these coefficients magnitudes of the same orders as those of their tangential counterparts. The corresponding capillary-wave model describes the effect of fluctuations when the density is slowly varying, and the recognition of the additional quantities and their roles may help in the understanding of ellipsometric studies near critical points.     

Effects of finite thickness on interfacial widths in confined thin films of coexisting phases

The capillary broadening of a 2-phase interface is investigated both experimentally and theoretically. When a binary mixture in a thin film with thickness D segregates into two coexisting phases the interface between the two phases may form parallel to the substrate due to preferential surface attraction of one of the components. We show that the interfacial profile (of intrinsic width w₀) is broadened due to capillary waves, which lead to fluctuations, of correlation length of the local interface positions in the directions parallel to the confining walls. We postulate that acts as an upper cutoff for the spectrum of capillary waves on the interface, so that the effective mean square interfacial width w varies as . In the limit of large D this yields or respectively for the case of short- or long-range forces between walls and the interface. We used the Nuclear Reaction Analysis depth profiling technique, to investigate this broadening effect directly in two binary polymer mixtures. Our results reveal that the interfacial width indeed increases with film thickness D, though the observed interfacial width is lower than the predicted w. This is probably due to surface tension effects imposed by the confining surfaces which are not taken into account in our model. Received: 19 February 1998 / Received in final form: 2 September 1998 / Accepted: 8 September 1998     

Molecular Dynamics Simulation of 4-N-Hexyl-4'-Cyanobiphenyl Adsorbed at the Air-Water Interface

The interfacial behavior of 4-n-hexyl-4'-cyanobiphenyl(6CB) molecules at the air-water interface is investigated by full atomistic molecular dynamics simulations. To understand the morphology and the structure of adsorbed 6CB molecules in detail, the snapshots and mass density profiles of the simulation system are generated. The average tilt angles between the interface normal and various vectors defined in the rigid and alkyl parts of 6CB are in good agreement with the experimental data available. The interfacial thickness and monolayer width are obtained from the mass density profiles of water and 6CB phase, respectively. The second and fourth rank orientational order parameters of cyanobiphenyl core are found to be larger than those of an elastic alkyl chain.Bond order parameters for 6CB are also calculated. The calculated oxygen-oxygen radial distribution function and hydrogen bonding statistics for bulk water are compared with those for the interfacial region. The surface tensions of the systems are calculated. All simulation results are compared with the available literature data.     

Theory of the interface between a classical plasma and a hard wall

The interfacial density profile of a classical one-component plasma confined by a hard wall is studied in planar and spherical geometries. The approach adapts to interfacial problems a modified hypernetted-chain approximation developed by Lado and by Rosenfeld and Ashcroft for the bulk structure of simple liquids. The specific new aim is to embody self-consistently into the theory a “contact theorem”, fixing the plasma density at the wall through an equilibrium condition which involves the electrical potential drop across the interface and the bulk pressure. The theory is brought into fully quantitative contact with computer simulation data for a plasma confined in a spherical cavity of large but finite radius. It is also shown that the interfacial potential at the point of zero charge is accurately reproduced by suitably combining the contact theorem with relevant bulk properties in a simple, approximate representation of the interfacial charge density profile.     

Interface states in two-dimensional electron systems with spin-orbital interaction

Interface states at a boundary between regions with different spin-orbit interactions (SOIs) in two-dimensional (2D) electron systems are investigated within the one-band effective mass method with generalized boundary conditions for envelope functions. We have found that the interface states unexpectedly exist even if the effective interface potential equals zero. Depending on the system parameters, the energy of these states can lie in either or both forbidden and conduction bands of bulk states. The interface states have chiral spin texture similar to that of the edge states in 2D topological insulators. However, their energy spectrum is more sensitive to the interfacial potential, the largest effect being produced by the spin-dependent component of the interfacial potential. We have also studied the size quantization of the interface states in a strip of 2D electron gas with SOI and found an unusual (non-monotonic) dependence of the quantization energy on the strip width.     

Interfacial properties of a driven diffusive system

A study is made of the low-temperature interfacial properties of a driven system with a single conserved density whose bulk properties were first analyzed, using computer simulations, by Katz, Lebowitz, and Spohn in 1983. The system corresponds to a nearest neighbor interacting lattice gas of charged particles (henceconserved order parameter), which are acted upon by a uniform, constant external electric fieldE. Starting from a bulk kinetic equation, an integral equation for the interface is derived. Nonlocal coupling between different parts of the interface arises from local particle conservation. The interface at any angle is shown to be stable against small deformations ofall wavelengths that are large compared to the interfacial width. However, the relaxation rate(k) for the interface exhibits a strongorientational dependence, which can be understood in terms of the modification of nonlocality byE. The wandering of the interface is considered. Also, the possible stabilizing effect of periodic boundary conditions on the orientation toward the direction ofE is discussed.     

Interfacial resonance state probed by spin-polarized tunneling in epitaxial Fe/MgO/Fe tunnel junctions

The direct impact of the electronic structure on spin-polarized transport has been experimentally proven in high-quality Fe/MgO/Fe epitaxial magnetic tunnel junctions, with an extremely flat bottom Fe/MgO interface. The voltage variation of the conductance points out the signature of an interfacial resonance state located in the minority band of Fe(001). When coupled to a metallic bulk state, this spin-polarized interfacial state enhances the band matching at the interface and therefore increases strongly the conductivity in the antiparallel magnetization configuration. Consequently, the tunnel magnetoresistance is found to be positive below 0.2 V and negative above. On the other hand, when the interfacial state is either destroyed by roughness-related disorder or not coupled to the bulk, the magnetoresistance is almost independent on the bias voltage.     

Development of novel materials through interface engineering

The impact of interfaces in the processing and properties of polycrystalline material is briefly discussed. The local properties of the interfacial layer are considered in terms of composition, structure and related properties that differ substantially from those of the bulk phase. It has been postulated that novel materials with desired properties for specific industrial applications may be processed through interface engineering rather than through bulk chemistry. This paper considers the impact of interfaces on the properties of materials for technological applications, such as electrochemical devices for reduction of greenhouse gases, through energy conversion and environmental monitoring. The procedures that may be applied for the modification of interfacial chemistry are considered.     

Force theory for multiphase bodies

We present a framework for the study of bodies wherein the deformation gradient may suffer a jump across an evolving nonmaterial interface. To formulate the kinematics relevant to such a situation, we use a global approach in which the configuration space has the structure of an infinite dimensional bundle. We show that a force, defined as an element of the cotangent bundle of the configuration manifold, may be represented by bulk and interfacial stress measures. The invariant decomposition of that force into bulk and interfacial components is discussed and we show that, in the case where the stress measures representing the force are given in terms of smooth densities, such a decomposition is determined by the average stress on the interface.     

Electronic structure of Si (111) interface between diamond and hexagonal phases

The electronic structure of the Si (111) interface between the diamond and hexagonal wurtzite structure is studied by first-principles LCAO method. No gap interfacial states or resonances are found; instead, the calculated density of states curve shows an oscillatory redistribution of electron states and a reduction of band gap of about 10%. It is concluded that the electronic structure of this interface can be approximated as the average of that of the bulk diamond and the hexagonal Si.     

Study of intrinsic anchoring in nematic liquid crystals based on modified Gruhn-Hess pair potential

A nematic liquid crystal slab composed of N molecular layers is investigated using a simple cubic lattice model, based upon the molecular pair potential which is spatially anisotropic and dependent on elastic constants of liquid crystals. A perfect nematic order is assumed in the theoretical treatment, which means the orientation of the molecular long axis coincides with the director of liquid crystal and the total free energy equals to the total interaction energy. We present a modified Gruhn-Hess model, which is relative to the splay-bend elastic constant K₁₃. Furthermore, we have studied the free nematic interfacial behavior (intrinsic anchoring) by this model in the assumption of the perfect nematic order. We find that the preferred orientation at the free interface and the intrinsic anchoring strength change with the value of modification, and that the director profile can be determined by the competition of the intrinsic anchoring with external forces present in the system. Also we simulate the intrinsic anchoring at different temperatures using Monte Carlo method and the simulation results show that the intrinsic anchoring favors planar alignment and the free interface is more disordered than the bulk.     

Suppression of thermally excited capillary waves by shear flow

We investigate the thermal fluctuations of the colloidal gas-liquid interface subjected to a shear flow parallel to the interface. Strikingly, we find that the shear strongly suppresses capillary waves, making the interface smoother. This phenomenon can be described by introducing an effective interfacial tension that increases with the shear rate. The increase of sigma(eff) is a direct consequence of the loss of interfacial entropy caused by the flow, which affects especially the slow fluctuations. This demonstrates that the interfacial tension of fluids results from an intrinsic as well as a fluctuation contribution.     

Theory of surface excess Miesowicz viscosities of planar nematic liquid crystal-isotropic fluid interfaces

An expression for the surface excess stress tensor for planar compressible interfaces between rod-like nematic liquid crystals and isotropic viscous fluids is derived using the classical surface excess theory formalism, adapted to capture the intrinsic anisotropy of the nematic orientational ordering. A required step in the theory is to find the actual stress tensor in the three-dimensional interfacial region, which is obtained by a decomposition of the kinematic fields (rate of deformation tensor and director Jaumann derivative) into tangential, normal, and mixed components with respect to the interface. The viscosity coefficients appearing in the surface excess stress tensor are expressed in terms of interfacial and bulk viscosities for planar, constant orientation, flows. The expressions are used to define the three fundamental surface excess Miesowicz shear viscosities, in analogy with the three bulk Miesowicz shear viscosities. The ordering in the magnitudes of the surface excess Miesowicz shear viscosities is shown to depend on the magnitude of the surface scalar nematic order parameter relative to that of the adjoining bulk nematic phase. When the surface scalar order parameter is greater than in the bulk, the classical ordering in terms of magnitudes of the three bulk Miesowicz shear viscosities is recovered. On the other hand, when the surface scalar order parameter is smaller than in the bulk, the classical ordering in terms of magnitudes of the three viscosities does not hold, and inequality transitions are predicted as the surface scalar order parameter increases towards the bulk value. Received 5 July 1999 and Received in final form 16 November 1999     

金刚石/硅(001)异质界面的分子动力学模拟研究

采用分子动力学方法模拟研究了未重构的金刚石/硅(001)面相接触时界面层原子的弛豫过程及所形成的异质界面的结构特征.硅碳二元系统中原子间的相互作用采用Tersoff多体经验势描述.弛豫前沿[110]与[110]方向界面碳硅原子数之比均为3∶2.界面碳硅原子总数之比为9∶4.弛豫后金刚石与硅界面处晶格匹配方式改变为[110]方向基本上以3∶2关系对准,而[110]方向大致以1∶1关系对准.相应地,界面碳硅原子总数之比接近3∶2.界面下方部分第二层硅原子在弛豫过程中向上迁移至界面是引起这种变化的原因,同时该层其他原子及其底下一到两个原子层厚度的区域在[001]方向上出现一定程度的无序化转变倾向.金刚石/硅异质界面处的硅碳原子发生强烈键合,形成平均键长为0.189nm的硅碳键.研究证实,晶格匹配主要呈现界面及其附近硅原子迎合界面碳原子排列的特点. 关键词： 金刚石 硅 异质界面 分子动力学     

光散射法同时测量气液二元体系的黏度、界面张力和热、质扩散系数的研究

当气液二元体系处于宏观热力学平衡状态时,表面与体相的波动弛豫时间具有不同量级,通过调节散射体积和改变采样时间,利用表面光散射实验系统可以同时探测到来自界面处表面波及体相中温度和浓度的波动信息。本文在已有的表面光散射实验系统上,以正庚烷与二氧化碳二元系统为例,同时实现了该体系黏度、界面张力和热、质扩散系数的测量,其扩展不确定度分布为3.0%,3.7%,20%,4.3%(k=2)。实验所得到的数据与文献进行了对比,其偏差在不确定度范围内,证明了本文提出的测量方法可以实现气液二元体系四种性质的同时测量,且测量精度可以满足一般工程应用。