Theory of the liquid-vapour interface of a binary mixture of Lennard-Jones fluids

We present results of calculations of the equilibrium surface tension and density profiles for the liquid-vapour interface of a binary mixture of Lennard-Jones 12-6 fluids. The calculations are based on a density-functional theory for the Helmholtz free energy of the inhomogeneous mixture. This is a ‘microscopic’ generalization of the van der Waals-Cahn-Hilliard theory for the interface of a binary mixture.

Our calculations cover the full range of liquid-vapour coexistence and the whole range of concentration. We find a correlation between the excess surface tension of the mixture and the surface segregation (adsorption) of the species with the lower surface tension. The ways in which segregation and excess surface tension depend on the Lennard-Jones parameters of the pure components are briefly discussed. Our results for the excess surface tension of mixtures of Ar and N₂ and Ar and CH₄ are compared with experiment; the agreement is reasonable.     

HYDRODYNAMIC STABILITY OF TWO-COMPONENT FLUID HEATED FROM BELOW AND WITH SURFACE ADSORPTION

The stability of a two-component Benard fluid with surface adsorption in reduced- gratuity condition is examined by using linear stability analysis. Results show that the surface tension effect of the active solute plays a dual role on the stability of the fluid, A few dimensionless combinations are introduced to delimit the regions where the stabilizing or destabilizing role, or both, of the solute tension come into plan.     

Surface tension of an electrolyte-air interface: a Monte Carlo study

We present a new method for calculating the surface tension of an electrolyte-air interface using Monte Carlo (MC) simulations with an implicit solvent in a spherical drop geometry. The boundary conditions for the electric field at the interface are accounted for using image and counter-image charges. The density profiles obtained from the simulations are used to calculate the excess surface tension of the electrolyte-air interface using the Gibbs adsorption isotherm equation. The results are found to be in good agreement with experiments and the earlier theoretical calculations.     

Segregation of silicone acrylate from acrylate mixture at resin-mold interface and its effect on UV embossing

When silicone diacrylate was added in small amount (<5 wt.%) to ultraviolet (UV) curable formulations containing other oligomeric diacrylates, there was segregation of the silicone additive at the solid substrate-formulation interface. The amount was quantified by X-ray photoelectron spectroscopy measurement of the UV cured film surface composition. The effect of silicone diacrylate concentration, resin formulation and substrate polarity on silicone surface excess was systematically studied. Young's-Gibbs adsorption theory was applied to the prediction of the silicone surface excess at the solid substrate interface for these oligomeric mixtures. Further, we proposed a simplified Young's-Gibbs adsorption theory equation to predict the variation of surface excess from only formulation surface tension and substrate critical surface tension. The selective segregation is beneficial to demolding in UV embossing since only small amount of release added can result in large decrease of the mold-resin interfacial energy difference leading to easy demolding and high replication fidelity.     

Shift of whispering-gallery modes in microspheres by protein adsorption

Biosensors based on the shift of whispering-gallery modes in microspheres accompanying protein adsorption are described by use of a perturbation theory. For random spatial adsorption, theory predicts that the shift should be inversely proportional to microsphere radius R and proportional to protein surface density and excess polarizability. Measurements are found to be consistent with the theory, and the correspondence enables the average surface area occupied by a single protein to be estimated. These results are consistent with crystallographic data for bovine serum albumin. The theoretical shift for adsorption of a single protein is found to be extremely sensitive to the target region, with adsorption in the most sensitive region varying as 1/R(5/2). Specific parameters for single protein or virus particle detection are predicted.     

Line thermodynamics: adsorption at a membrane edge

We report a novel experimental study of line thermodynamics. Our system consists of detergent molecules adsorbing at the edges of freestanding lipid bilayers. Adsorption reduces the line tension T of the membrane edges. Measuring T as a function of the bulk detergent concentration C, we obtain a line adsorption isotherm. Using an extension of Gibbs's surface thermodynamics to lines, we estimate the "line excess density" of adsorbants and the energy of adsorption per site.     

Novel surface tension isotherm for surfactants based on local density functional theory

In this Letter we report a new general method for calculating of surface tension isotherms in the presence of surfactants, based on a local density functional. We illustrate this method by deriving the interfacial tension isotherm for nonionic surfactants at an air-water or oil-water interface by using the self-consistent field theory of polymer brushes. We consider a particular case of local density functional to calculate explicitly how the interfacial tension and the surfactant adsorption depend on the surfactant bulk concentration. Experimental data for the surface tension and the surfactant adsorption isotherm for nonionic surfactants were interpreted with the help of the new isotherm. Very good agreement between the adsorption of n-dodecyl pentaoxyethylene glycol ether (C12E5) at an air-water interface, calculated from the surface tension isotherm and small-angle neutron-scattering is obtained.     

Effect of contact line curvature on solid-fluid surface tensions without line tension

For sessile droplets partially wetting a solid surface, it has been observed experimentally that the value of the contact angle depends on the contact line curvature and this dependence has been attributed to tension in the contact line. But previous analyses of these observations have neglected adsorption at the solid-liquid interface and its effect on the surface tension of this interface. We show that if this adsorption is taken into account the relation between the contact angle and contact line curvature is completely accounted for without introducing line tension. Further, from the observed relation between the contact angle and contact line curvature, the adsorption at the solid-liquid interface can be determined, as can the surface tensions of the solid-liquid and solid-vapor interfaces.     

Statistical thermodynamics of isolated rigid rod-like molecules near a surface

The configurational behaviour and thermodynamic properties of a dilute gas of rigid rod-like molecules in the vicinity of a macroscopic planar adsorption surface are investigated using statistical mechanics. The interaction energy between the surface and a rod-like molecule is determined as a function of both its molecular centre of mass separation R and its orientation relative to the surface. In calculating this interaction energy, each rod segment and molecule comprising the surface is assumed to interact through a Lennard-Jones pair potential. The average molecular order parameter is then determined as a function of R. We find that an isolated rod-like molecule tends to align nearly parallel to the surface for small separations. However, as R increases the order parameter first passes through a maximum then decays to zero as R ^-5 for large R. The configurational behaviour of an isolated rod-like molecule located between two parallel adsorption surfaces is also considered briefly. The surface spreading pressure, excess surface energy and entropy are also obtained for a dilute gas of rod-like molecules near a surface. We find that the extent of surface binding increases nearly exponentially with molecular length at constant temperature and surface density, and that the excess surface energy and entropy are essentially proportional to the molecular length.     

Instabilities in droplets spreading on gels

We report a novel surface-tension driven instability observed for droplets spreading on a compliant substrate. When a droplet is released on the surface of an agar gel, it forms arms or cracks when the ratio of surface-tension gradient to gel strength is sufficiently large. We explore a range of gel strengths and droplet surface tensions and find that the onset of the instability and the number of arms depend on the ratio of surface tension to gel strength. However, the arm length grows with an apparently universal law L proportional t(3/4).     

The diffusion-control limit revisited

We consider nonequilibrium adsorption to a freshly formed surface. Owing to the initial lack of equilibrium, the common diffusion-control assumption is inconsistent at small times. A uniform small-time asymptotic approximation is constructed for a Langmuir-type system in terms of the small parameter epsilon representing the ratio of the respective kinetic and diffusive time scales of the problem. The diffusion-control approximation becomes valid only when t>epsilon. The adsorption results are applied to the calculation of the dynamic surface tension.     

Vapour-to-Liquid Nucleation in Associating Lennard-Jones Fluids with Multiple Association Sites

The excess Helmholtz free energy functional for associating Lennard-Jones （L J） fluid is formulated in terms of a weighted density approximation for short-ranged interactions and a Weeks-Chandler-Andersen approximation for long-range attraction. Within the framework of density functional theory, phase equilibria, vapour-liquid surface tension and vapour-liquid nucleation properties including the density profile, work of formation, excess number of particles and critical supersaturation are investigated for associating LJ fluids with different numbers of association sites （M =1,2, 3, 4） per particle. The influences of association energy and association sites on phase equilibria, surface tension and vapour-liquid nucleation properties are discussed.     

Adsorption and interfacial phenomena of a Lennard-Jones fluid adsorbed in slit pores: DFT and GCMC simulations

ABSTRACT

Confinement of fluids in porous media leads to the presence of solid–fluid (SF) interfaces that play a key role in many different fields. The experimental characterisation of SF interfacial properties, in particular the surface tension, is challenging or not accessible. In this work, we apply mean-field density functional theory (DFT) to determine the surface tension and also density profile of a Lennard-Jones fluid in slit-shaped pores for realistic amounts of adsorbed molecules. We consider the pore walls to interact with fluid molecules through the well-known 10-4-3 Steele potential. The results are compared with those obtained from Monte Carlo simulations in the Grand Canonical Ensemble (GCMC) using the test-area method. We analyse the effect on the adsorption and interfacial phenomena of volume and energy factors, in particular, the pore diameter and the ratio between SF and fluid–fluid dispersive energy parameters, respectively. Results from DFT and GCMC simulations were found to be comparable, which points to their reliability.     

Density, ultrasonic velocity, surface tension, excess volume and viscosity of quaternary fluid solutions

Density, ultrasonic velocity, surface tension, excess volume and viscosity of quaternary non-electrolyte solution n-pentane+tolune+n-heptane+cyclohexane has been experimentally determined at 25 °C over a wide range of composition. Assuming a quaternary fluid solution to be made of six binaries, a statistical approach of Prigogine–Flory–Patterson (PFP) theory has been extended to develop the expressions for dynamic viscosity, surface tension excess volume and ultrasonic velocity of quaternary fluid system. A reasonable agreement has been achieved between theory and experiment. An attempt has also bee made to explain the nature of molecular interactions involved in the light of excess thermodynamic functions whose sign and magnitude depend upon the chain length and size of the component fluids and also studied the role of order breakers (toluene and cyclohexane) on these properties.     

Quantum Monte Carlo calculations of H2 dissociation on Si(001)

The dissociative adsorption of H2 on the Si(001) surface is theoretically investigated for several reaction pathways using quantum Monte Carlo methods. Our reaction energies and barriers are at large variance with those obtained with commonly used approximate exchange-correlation density functionals. Our results for adsorption support recent experimental findings, while, for desorption, the calculations give barriers in excess of the presently accepted experimental value, pinpointing the role of coverage effects and desorption from steps.     

Investigation of vapour--liquid nucleation properties for spherical and chain-like fluids by density functional theory

The excess Helmholtz free energy functional for nonpolar chain-like molecules is formulated in terms of a weighted density approximation （WDA） for short-range interactions and a Weaks Chandler Andersen （WCA） approximation and a Barker Henderson （BH） theory for long-range attraction. Within the framework of density functional theory （DFT）, vapour liquid interracial properties including density profile and surface tension, and vapour-liquid nucleation properties including density profile, work of formation and number of particles are investigated for spherical and chain- like molecules. The obtained vapour liquid surface tension and the number of particles in critical nucleus for Lennard- Jones （L J） fluids are consistent with the simulation results. The influences of supersaturation, temperature and chain length on vapour liquid nucleation properties are discussed.[第一段]     

Chain length dependencies of the bending modulus of surfactant monolayers

The effect of the surfactant chain length n on the bending modulus kappa of surfactant monolayers is simulated with a mesoscopic oil-water-surfactant model. We confirm a power law, kappa is proportional to np, as predicted by mean-field theory and found experimentally, and find p approximately 1.5 at a constant surface density and p approximately 1.0 at a constant interfacial tension. This agrees quite well with both mean-field theory (p=2-3, assuming constant surface density) and experiments (at constant surface tension). Our results suggest that the previously reported agreement between theory and experiment may be fortuitous and caused by the difference in surfactant types.