Calculation of the Saturation Properties of a Model Octane–Water System Using Monte Carlo 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.

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.

Designing Molecular Building Blocks for the Self-assembly of Complex Porous Networks

The creation of molecular or colloidal building blocks which can self-assemble into complex, ordered porous structures has been long sought-after, and so are the guiding principles behind this creation. The pursuit of this goal has led to the creation of novel classes of materials like metal organic frameworks (MOFs) and covalent organic frameworks (COFs). In theory, a tremendous number of structures can be formed by these materials due to the variety of geometries available to their building blocks. However, most realized crystal structures tend to be simple or homoporous and typically assemble from building blocks with high degrees of symmetry. Building blocks with low degrees of symmetry suitable for assembly into the more complex structures tend to assemble into polymorphous or disordered structures instead. In this work, we use Monte Carlo simulations of patchy vertex-like building blocks to show how the addition of chemical specificity via orthogonally reacting functional sites can allow vertex-like building blocks with even asymmetric geometries to self-assemble into ordered crystallites of various complex structures. In addition to demonstrating the utility of such a strategy in creating ordered, heteroporous structures, we also demonstrate that it can be used as a means for tuning specific features of the crystal structure, accomplishing such aims as the control of relative pore sizes. We also discuss heuristics for properly designing molecules so that they can assemble into target structures.