Adsorption at the liquid-vapor surface of a binary liquid mixture

In a binary liquid mixture, the component possessing the lowest surface tension preferentially adsorbs at the liquid-vapor surface. In the past this adsorption behavior has been extensively investigated for critical binary liquid mixtures near the mixture's critical temperature T(c). In this fluctuation-dominated regime the adsorption is described by a universal function of the dimensionless depth zxi where xi is the bulk correlation length. Fewer studies have quantitatively examined adsorption for off-critical mixtures because, in this case, one must carefully account for both the bulk and surface crossover from the fluctuation-dominated regime (close to T(c)) to the mean-field dominated regime (far from T(c)). In this paper we compare extensive liquid-vapor ellipsometric adsorption measurements for the mixture aniline+cyclohexane at a variety of critical and noncritical compositions with the crossover theory of Kiselev and co-workers [J. Chem. Phys. 112, 3370 (2000)].     

How critical fluctuations influence adsorption properties of a van der Waals fluid onto a spherical colloidal particle

A recently proposed 3rd-order thermodynamic perturbation theory (TPT) is extended to its 5th-order version and non-uniform counterpart by supplementing with density functional theory (DFT) and a number of ansatzs for a bulk 2nd-order direct correlation function (DCF). Employment of the ansatzs DCF enables the resultant non-uniform formalism devoid of any adjustable parameter and free from numerically solving an Ornstein–Zernike integral equation theory. Density profiles calculated by the present non-uniform formalism for a hard core attractive Yukawa (HCAY) fluid near a spherical geometry are favorably compared with corresponding simulation data available in literature, and are more accurate than those based on a previous 3rd + 2nd-order perturbation DFT. The non-uniform 5th-order TPT is employed to investigate adsorption of the HCAY fluid onto a colloidal particle; it is disclosed that a depletion adsorption can be induced when the coexistence bulk fluid is situated in neighborhood of a critical point or near a bulk vapor–liquid coexistence gaseous phase or liquid phase density. A physical interpretation is given for such depletion adsorption and for its connection with parameters of the potential under consideration, which is ascribed to critical density fluctuations existing within a wide region of the bulk diagram. For a large spherical external potential inducing wetting transition, it is found that only round wetting transition is found instead of 1st-order pre-wetting transition in the case of a planar wall external potential, and the wetting transition temperature increases relative to that for the planar wall external potential. The present theoretical results for wetting transitions are supported by previous investigation based on thermodynamic considerations and a phenomenological Landau mean field theory, and are also in conformity with the present qualitative physical interpretation.     

Density functional theory for the recognition of polymer at nanopatterned surface

The recognition of homopolymer at nanopatterned surface has been investigated by density functional theory (DFT). Chain conformation and pattern transfer parameter predicted from the DFT are in good agreement with Monte Carlo simulation results. The theory describes satisfactorily the transition from depletion at low packing fractions to adsorption and double-layer adsorption at high packing fractions and also accounts for the crucial effect of the segment-wall interaction. It is found that homopolymer is better recognized at a low bulk density and a stronger interaction with the surface. The polymer can not only recognize the surface but also invert the surface at high bulk densities. The chain in the solution-wall interface exhibits a typical "brush" conformation with a length approximated by half the length of polymer chain.     

Structure, phase behavior, and inhomogeneous fluid properties of binary dendrimer mixtures

The effective pair potentials between different kinds of dendrimers in solution can be well approximated by appropriate Gaussian functions. We find that in binary dendrimer mixtures the range and strength of the effective interactions depend strongly upon the specific dendrimer architecture. We consider two different types of dendrimer mixtures, employing the Gaussian effective pair potentials, to determine the bulk fluid structure and phase behavior. Using a simple mean field density functional theory (DFT) we find good agreement between theory and simulation results for the bulk fluid structure. Depending on the mixture, we find bulk fluid-fluid phase separation (macrophase separation) or microphase separation, i.e., a transition to a state characterized by undamped periodic concentration fluctuations. We also determine the inhomogeneous fluid structure for confinement in spherical cavities. Again, we find good agreement between the DFT and simulation results. For the dendrimer mixture exhibiting microphase separation, we observe a rather striking pattern formation under confinement.     

Confinement effect on the adsorption from a binary liquid system near liquid/liquid phase separation

The preferential adsorption of one component of a binary system at the inner surfaces of mesoporous silica glasses was studied in a wide composition range at temperatures close to liquid/liquid phase separation. Confinement effects on the adsorption were investigated by using three controlled-pore glass (CPG-10) materials of different mean pore size (10 to 50 nm). For the experimental system (2-butoxyethanol+water), which exhibits an upper miscibility gap, strong preferential adsorption of water occurs, as the coexistence curve is approached at bulk compositions, at which water is the minority component. In this strong adsorption regime the area-related surface excess amount of adsorbed water decreases with decreasing pore width, while the shift in the volume-related mean composition of the pore liquid shows an opposite trend, i.e., greatest deviation from bulk composition occurring in the most narrow pores. A simple mean-field lattice model of a liquid mixture confined by parallel walls is adopted to rationalize these experimental findings. This model reproduces the main findings of the confinement effect on the adsorption near liquid/liquid phase separation.     

Theory for an order-driven disruption of the liquid state in water

Water is known to exhibit a number of peculiar physical properties because of the strong orientational dependence of the intermolecular hydrogen bonding interactions that dominate its liquid state. Recent full-atom simulations of water in a nanolayer between graphite plates submersed in an aqueous medium have raised the possibility of a new addition to this list of peculiarities: they show that application of a strong, uniform electric field normal to and between the plates can cause a pronounced decrease in particle density, rather than the increase expected from electrostriction theory for polarizable fluids [Vaitheeswaran et al., J. Phys. Chem. B 70, 6629 (2005)]. However, in seeming contradiction to this result, another study that simulated a range of similar systems has reported a less surprising electrostrictive increase in particle density upon application of the field [Bratko et al., J. Am. Chem. Soc. 129, 2504 (2007)]. In this work, we attempt to reconcile these conflicting simulation phenomena using a statistical mechanical lattice liquid model of water in an applied field. By solving the model using mean-field theory, we show that a field-induced transition to a markedly lower-density phase such as that observed in recent simulations is possible within a certain parameter regime, but that outside of this regime, the more conventional electrostrictive result should be obtained. Upon modifying the model to treat the case of bulk water under constant pressure in an applied field, we predict a density drop with rising field, and subsequently observe the predicted behavior in our own molecular dynamics simulations of liquid water. Our findings lead us to propose that the model considered here may be useful in a variety of contexts for describing the trade-off between orientational ordering of water molecules and their participation in the liquid phase.     

云母表面吸附烷基伯胺对其疏水性的影响

矿物表面的疏水性受吸附在其表面的表面活性剂的影响,进而影响矿物的浮选行为.本文通过表面接触角测量、原子力显微镜(AFM)观测以及密度泛函理论(DFT)和分子动力学(MD)模拟计算,研究了吸附在云母表面的烷基伯胺的链长对其疏水性的影响.通过比较氧密度和氢键数量分布,发现每个水分子在碳氢链尾端和水相接触的界面上相对于在体相中形成氢键的能力有所降低,而吸附烷基伯胺的云母由亲水性转化为疏水性.研究结果还表明,在单分子层吸附状态下,吸附十八胺的云母的疏水性比吸附十二胺的云母的疏水性要强,且由于十八胺的临界半胶束浓度(HMC)要远低于十二胺,十八胺更易在云母表面形成多层吸附,证明烷基伯胺的碳链越长,其对云母表面疏水性改善的能力越强.实验结果与理论计算结果吻合良好.     

Wetting of Crystalline Solids by Associating Fluids

The wetting behavior of Lennard-Jones associating fluid on a crystalline surface is studied using Monte Carlo simulation in the isobaric-isothermal ensemble. We explore the structure of the adsorbed film, characterized by the distribution of all particles and the distributions of centers of mass of dimers, as well as by orientational distributions of the dimers. The simulations performed indicate the existence of a prewetting-like behavior of the adsorption isotherm. The results for a crystalline surface are compared with simular results obtained for a structureless surface. Finally, we present some density functional theory evaluations and compare them with the simulation results at a high temperature. Copyright 2000 Academic Press.     

Orientational dynamics for an amphiphilic-solvent solution

In this work, we performed Monte Carlo simulations on a lattice model for spontaneous amphiphilic aggregation, in order to study the orientational and hydrogen-bonding dynamics of water on different regions inside the micellar solution. We employed an associating lattice gas model that mimics the aqueous solvent, which presents a rich phase diagram with first- and second-order transition lines. Even though this is a simplified model, it makes possible to investigate the orientational dynamics of water in an equilibrium solution of amphiphiles, as well as the influence of the different phases of the solvent in the interfacial and bulk water dynamics. By means of extensive simulations, we showed that, at high temperatures, the behavior of the orientational relaxation and hydrogen bonding of water molecules in the bulk, first, and second hydration shells are considerable different. We observe the appearance of a very slow component for water molecules in the first hydration shell of micelles when the system reaches a high-density phase, consistent with previous theoretical and experimental studies concerning biological water. Also, at high temperatures, we find that water molecules in the second hydration shell of micelles have an orientational decay similar to that of bulk water, but with a generally slower dynamics. Otherwise, at low temperatures, we have two components for the orientational relaxation of bulk water in the low density liquid phase, and only a single component in the high density liquid (HDL) phase, which reflect the symmetry properties of the different phases of the solvent model. In the very dense region of water molecules in the first hydration shell of micelles at low temperatures, we find two components for the orientational relaxation on both liquid phases, one of them much slower than that in the single component of bulk water in the HDL phase. This happens even though our model does not present any hindrance to the water rotational freedom caused by the presence of the amphiphiles.     

Entropic wetting and the free isotropic-nematic interface of hard colloidal platelets

We study bulk and interfacial properties of a model suspension of hard colloidal platelets with continuous orientations and vanishing thickness using both density functional theory, based on either a second virial approach or fundamental measure theory (FMT), and Monte Carlo (MC) simulations. We calculate the bulk equation of state, bulk isotropic-nematic (IN) coexistence, and properties of the (planar) free IN interface and of adsorption at a planar hard wall, where we find complete wetting of the nematic phase at the isotropic-wall interface upon approaching bulk IN coexistence. We investigate in detail the asymptotic decay of correlations at large distances. In all cases, the results from FMT and MC agree quantitatively. Our findings are of direct relevance to understanding interfacial properties of dispersions of colloidal platelets.     

Homogeneous nucleation in vapor-liquid phase transition of Lennard-Jones fluids: a density functional theory approach

Density functional theory (DFT) with square gradient approximation for the free energy functional and a model density profile are used to obtain an analytical expression for the size-dependent free energy of formation of a liquid drop from the vapor through the process of homogeneous nucleation, without invoking the approximations used in classical nucleation theory (CNT). The density of the liquid drop in this work is not the same as the bulk liquid density but it corresponds to minimum free energy of formation of the liquid drop. The theory is applied to study the nucleation phenomena from supersaturated vapor of Lennard-Jones fluid. The barrier height predicted by this theory is significantly lower than the same in CNT which is rather high. The density at the center of the small liquid drop as obtained through optimization is less than the bulk density which is in agreement with other earlier works. Also proposed is a sharp interface limit of the proposed DFT of nucleation, which is as simple as CNT but with a modified barrier height and this modified classical nucleation theory, as we call it, is shown to lead to improved results.     

Phase separations in liquid crystal-colloid mixtures

We present a mean-field theory to describe phase separations in mixtures of a nematic liquid crystal and a colloidal particle. The theory takes into account an orientational ordering of liquid crystals and a crystalline ordering of colloidal particles. We calculate phase diagrams on the temperature-concentration plane, depending on interactions between a liquid crystal and a colloidal surface and a coupling between nematic and crystalline ordering. We find various phase separation processes, such as a nematic-crystal phase separation and nematic-isotropic-crystal triple point. Inside binodal curves, we find new unstable and metastable regions which are important in phase ordering dynamics. We also find a stable nematic-crystalline (NC) phase, where colloidal particles dispersed in a nematic phase can form a crystalline structure. The coexistence between two NC phases with different concentrations can be appear though the coupling between nematic and crystalline ordering.     

Analysis of the subcritical carbon dioxide-water interface

We follow the evolution of the H(2)O/CO(2) interface at 300 K from the low pressure limit to near-critical pressures in molecular dynamics simulations using the SPC water and EPM2 carbon dioxide models. The intrinsic structure of the interface is elucidated by accumulating density profiles relative to the fluctuating capillary wave surface. Our main finding is that a carbon dioxide film of increasing density and thickness grows in two stages at the interface while the structure of the water surface barely changes. At low density, the entire film density profile grows linearly with the bulk CO(2) density. This regime continues up to a bulk CO(2) density of roughly 0.00095 ?(-3). At pressures above this point, we observe a distinct second peak in the CO(2) density, along with a tail of excess density that decays exponentially with distance from the interface. The decay length of the exponential tail diverges with increasing CO(2) pressure according to an inverse power law decay. Over the entire range of pressures, the CO(2) film had no detectable effect on the orientational order of the water surface. As expected, when the film of excess CO(2) at the interface grows, we find that the surface tension drops with increasing pressure. This is in qualitative accord with existing measurements, although the rate at which the surface tension falls with increasing pressure according to the SPC and EPM2 models is too small, indicating that the surface excess of CO(2) is underestimated by these models.     

The extent of molecular orientation at liquid/vapor interface of pyridine and its alkyl derivatives by molecular dynamics simulation

In this paper, molecular dynamics simulation was performed to investigate the liquid∕vapor interfacial structure of neat polar liquids. Large-scale ensembles of liquid pyridine and its alkyl derivatives, 4-methylpyridine and 4-ethylpyridine, were simulated by classical molecular dynamics at 298 K. For the liquid system of low polarity, the surface density profile of the atoms meet exactly at the middle of interfacial region, and atoms of hydrophobic nature can be hardly discriminated from hydrophilic ones in either vapor or liquid sides. For a liquid system of high polarity, the density profile of atoms with different nature is highly discriminated all over the interfacial region, and as the polarity increases, a dense region of atomic density is clearly developed in the subsurface region. The recognized bivariate method was also used to study the molecular orientational distribution quantitatively. Orientational analysis of the three liquid systems indicates that the pyridine ring plane in the outmost surface tends to be vertical. Its tendency in the innermost interfacial region is parallel. The orientational states available to 4-ethylpyridine and pyridine are discriminated by predicting the possibility of a bisector-wise tumbling for the ring plane in pyridine and a side-wise tumbling in 4-ethylpyridine. The orientational distribution maps explain the trend of experimental surface tension and surface entropy. As the dipole moment of these liquids increases with the alkyl chain length, the surface structural profile changes from a regular definite one to a surface of complex atomic structure involving a dense phase near the interface. The development of dense region in alkyl derivatives is the result of segregation of molecules due to the alkyl group, which is captured and discriminated by molecular dynamics simulation even when the length of a short alkyl chain is increased by one carbon atom.     

Simplified crossover droplet model for adsorption of pure fluids in slit pores

We present a generalized crossover (GC) model for the excess adsorption of pure fluids at a flat solid-liquid interface, which reproduces scaling behavior of the excess adsorption in the critical region and is reduced to the classical, van der Waals-type analytical model far away from the bulk critical point. In developing this model, we used the density-functional theory (DFT) approach for the order parameter profile calculations with a generalized corresponding states model for the local free-energy density. The GC DFT model well represents the available experimental adsorption data for Kr/graphite, C2H4/graphite, C3H8/graphite, CO2/silica, and SF6/graphite systems in the entire density range 0 < rho < or = 3rhoc and temperatures up to 1.7Tc. In the critical region 0.5 rhoc < r < or = 1.5rhoc and T < or = 1.15Tc, the GC DFT model is consistent with the predictions of the asymptotic renormalization-group crossover model for the critical adsorption in a semi-infinite system developed earlier. For the excess adsorption on the critical isochore, both theories predict a scaling-law behavior Gamma proportional tau(-nu+beta), but fail to reproduce a "critical depletion" of the excess adsorption along the critical isochore of the SF6/graphite system near Tc. We show that an anomalous decrease of adsorption observed in this system at tau = T/Tc - 1 < 10(-2) can be explained by finite-size effect and develop a simplified crossover droplet (SCD) model for the excess adsorption in a slit pore. With the effective size of the pore of L = 50 nm, the SCD model reproduces all available experimental data for SF6/graphite, including the critical isochore data where tau-->0, within experimental accuracy. At L > xib (where xib is a bulk correlation length) the SCD model is transformed into the GC DFT model for semi-infinite systems. Application of the SCD model to the excess adsorption of carbon dioxide on the silica gel is also discussed.     

Analysis of the validity of perturbation density functional theory: based on extensive simulation for simple fluid at supercritical and subcritical temperature under various external potentials

Because of the scarcity of available simulation data for confined hard-core attractive Yukawa model fluid, extensive Monte Carlo (MC) simulation research for this fluid under the influence of various external potentials were carried out. The present MC simulation results were employed to test a performance of the third-order perturbation density functional theory (DFT) based on a high order direct correlation function (DCF) [S. Zhou and E. Ruckenstein, Phys. Rev. E. 61, 2704 (2000)]. It was found that the present perturbation DFT formalism is soundly structured only if the imported second-order DCF is reliable. In this case, the accuracy of the results can be satisfactory or even very high for various types of external potentials. Further, the associated adjustable parameter can be universal, i.e., independent of the particular external field responsible for the generation of a nonuniform density profile. Dependence of both the maintenance of the reliability of the formalism and holding of the universality of the adjustable parameter on the accuracy of the imported bulk second-order DCF can be strengthened by the large difference between the external field investigated and that caused by a single hard wall used for specification of the adjustable parameter. In case the gaseous density in the subcritical region is below the coexistence density, an excellent performance of the present formalism is observed even for the mean spherical approximation's second-order DCF as an input. This advantageous property, combined with the fact that the present formalism needs only the second-order DCF of fluid at the coexistence state as an input, enables the present formalism to be a very good theoretical tool for the investigations of wetting and prewetting transitions.     

高阶FMSA展开研究缔合流体界面张力

结合一阶平均球近似(First-order mean-spherical approximation, FMSA)与重整化群(Renormalization group, RG)变换计算了流体全局性相行为. 应用FMSA理论解析得到的径向分布函数(Radial distribution function, RDF)和直接相关函数(Direct correction function, DCF)建立密度泛函方法, 并在其展开项中考虑了高阶微扰项作用, 即考虑了主体流体密度不一致性, 避免原有方法在计算密度分布时存在难以收敛、误差大等问题. 将高阶展开扩展应用到缔合流体, 计算表明, 和分子模拟数据相比, 界面密度分布和界面张力较之原有的密度泛函方法均有了明显改善.     

Structure, dynamics, and the free energy of solute adsorption at liquid-vapor interfaces of simple dipolar systems: molecular dynamics results for pure and mixed Stockmayer fluids

We have performed molecular dynamics simulation studies of the structural, thermodynamic, and dynamical properties of liquid-vapor interfaces of pure and binary Stockmayer fluids of different polarity. The density profiles, the width of the liquid-vapor interface, and the orientational structure of the interfaces are calculated to characterize the structural aspects of the interfaces. Among the thermodynamic properties, we have computed the surface tension and also the free energy of transfer of a charged solute across the liquid-vapor interface for both pure and mixed fluids. Among the dynamical properties of the interfaces, we have calculated the time dependence of the velocity and angular velocity autocorrelation functions, continuous and intermittent survival probabilities, mean square displacements, diffusion coefficients, and also the dipole correlation functions and orientational relaxation times of interfacial solvent molecules. It is found that the width of the interfaces decreases with increase of concentration of the more polar component. The dipole vectors of the interfacial molecules tend to align parallel to the surfaces and this alignment is enhanced with increasing dipole moment of the fluid molecules. Also, the surface tension shows an increasing trend with increase of dipole moment of the molecules. The dynamical properties of the interfaces are found to be different from those of the corresponding bulk liquid phases. In general, the molecules at the interfaces are found to rotate and translate in the parallel direction at a somewhat faster rate than the bulk molecules. Also, on increase of concentration of the more polar component, the diffusion and orientational relaxation of interfacial molecules are found to show a weaker slowing down than those of the bulk molecules, which can be attributed to the preferential presence of the more polar component in the bulk liquid regions. The temporal behavior of the interfacial survival probabilities reveals a decrease of the survival times with increasing polarity, which can be attributed to a corresponding decrease in the interfacial thickness. Results are presented for both continuous and intermittent survival times and the origins of their differences are discussed. The free energy calculations reveal no minimum at the interfaces for adsorption of a charged solute, which shows that the ions would prefer to stay in the interior of the liquid phases, rather than at interfaces, for these model dipolar systems.