Understanding wetting of immiscible liquids near a solid surface using molecular simulation

Coupled Monte Carlo and Molecular Dynamics Simulations on Interfacial Properties of Antifouling Polymer Membranes

We use molecular simulation to study the wetting behavior of antifouling polymer-tethered membranes. We obtain the interfacial properties (e.g., contact angle) of water at various temperatures for five polymer membranes, including a base polysulfone (PSF) membrane and four other PSF membranes grafted with antifouling polymers (two poly(ethylene glycol) (PEG) tethers and two zwitterionic tethers). We implement a coupled Monte Carlo (MC)/molecular dynamics (MD) approach to determine the interface potentials of water on the membrane surfaces in an efficient manner. Within this method, short MC and MD simulations are performed in cycles to collect the surface excess free energy of a thin water film on polymer membrane surfaces. Simulation results show that the grafting of zwitterionic tethers provides a more significant enhancement in the hydrophilicity of the PSF membrane than that of the PEG tethers. Water completely wets the surface of zwitterionic polymer membranes.

Molecular Dynamics Study of Wetting and Adsorption of Binary Mixtures of the Lennard-Jones Truncated and Shifted Fluid on a Planar Wall

The wetting of surfaces is strongly influenced by adsorbate layers. Therefore, in this work, sessile drops and their interaction with adsorbate layers on surfaces were investigated by molecular dynamics simulations. Binary fluid model mixtures were considered. The two components of the fluid mixture have the same pure component parameters, but one component has a stronger and the other a weaker affinity to the surface. Furthermore, the unlike interactions between both components were varied. All interactions were described by the Lennard-Jones truncated and shifted potential with a cutoff radius of 2.5σ. The simulations were carried out at constant temperature for mixtures of different compositions. The parameters were varied systematically and chosen such that cases with partial wetting as well as cases with total wetting were obtained and the relation between the varied molecular parameters and the phenomenological behavior was elucidated. Data on the contact angle as well as on the mole fraction and thickness of the adsorbate layer were obtained, accompanied by information on liquid and gaseous bulk phases and the corresponding phase equilibrium. Also, the influence of the adsorbate layer on the wetting was studied: for a sufficiently thick adsorbate layer, the wall's influence on the wetting vanishes, which is then only determined by the adsorbate layer.

Effect of Carboxylic Acid on the Wetting Properties of a Model Water-Octane-Silica System

Monte Carlo simulations are employed to determine the effects of acetic acid on the wetting properties of a model water-octane-silica system. We first compute the bulk liquid-vapor saturation properties of pure acetic acid and subsequently explore the bulk liquid-liquid saturation properties of the ternary water-octane-acid system. We introduce an expanded ensemble approach to compute the coexistence properties of the ternary system. An interface potential approach is then used to capture the evolution of the wetting properties of the water-octane-silica system upon the addition of acetic acid. We track the change in the octane-water interfacial tension and the contact angle of water droplet on a silica substrate in a mother octane fluid over a range of acetic acid activities. The structure of the fluid, including the partitioning of acetic acid within the interfacial system, is also considered at several state points. We observe that acetic acid has a strong tendency to adsorb at the octane-water interface, resulting in a reduction in the octane-water interfacial tension. The response of the contact angle is more sensitive to the temperature and the hydrophilicity of the silica substrate.

Highly non-additive symmetric mixtures at a wall

This paper discusses the results of the grand canonical ensemble Monte Carlo simulation of the wetting behavior of non-additive symmetric mixtures at non-selective walls. We have focused on the mixtures that exhibit closed immiscibility loops in the bulk, and the results obtained have demonstrated that such systems show a rather complex wetting behavior. In particular, such mixtures may exhibit a complete wetting at temperatures below the bulk demixing point, and an incomplete wetting at higher temperatures. Such a situation occurs when the adsorbed film remains mixed up to the bulk coexistence. However, close to the bulk tricritical point, being the onset of the continuous demixing transition (λ-line), a second wetting transition takes place. On the other hand, in the systems in which the adsorbed films undergo the demixing transition, a complete wetting may occur below as well as at and above the bulk demixing transition temperature. It has also been demonstrated that the wetting behavior depends strongly on the strength of the surface-fluid interaction.

Test-area surface tension calculation of the graphene-methane interface: Fluctuations and commensurability

The surface tension (γ) of methane on a graphene monolayer is calculated by using the test-area approach. By using a united atom model to describe methane molecules, strong fluctuations of surface tension as a function of the surface area of the graphene are evidenced. In contrast with the liquid-vapor interfaces, the use of a larger cutoff does not fully erase the fluctuations in the surface tension. Counterintuitively, the description of methane and graphene from the Optimized Potentials for Liquid Simulations all-atom model and a flexible model, respectively, led to a lessening in the surface tension fluctuations. This result suggests that the origin of fluctuations in γ is due to a model-effect rather than size-effects. We show that the molecular origin of these fluctuations is the result of a commensurable organization between both graphene and methane. This commensurable structure can be avoided by describing methane and graphene from a flexible force field. Although differences in γ with respect to the model have been often reported, it is the first time that the model drastically affects the physics of a system.