George Yiapanis

Comparative study of commonly used molecular dynamics force fields for modeling organic monolayers on water

This study compares the performance of the all-atom molecular dynamics force fields OPLS-AA and COMPASS, and the united-atom GROMOS96 ff53a6 force field, for organic monolayers at aqueous interfaces, as a function of surface density, temperature, and system size. Where possible, comparison with experimental data was undertaken and used to scrutinize the performance of each force field. We find close agreement between the all-atom force fields (OPLS and COMPASS) and experiment for the description of organic monolayers on water. However, the united-atom force field 53a6 tends to exhibit poorer agreement than the all-atom force fields.

Molecular mechanism of stabilization of thin films for improved water evaporation protection.

All-atom molecular dynamics simulations and experimental characterization have been used to examine the structure and dynamics of novel evaporation-suppressing films where the addition of a water-soluble polymer to an ethylene glycol monooctadecyl ether monolayer leads to improved water evaporation resistance. Simulations and Langmuir trough experiments demonstrate the surface activity of poly(vinyl pyrrolidone) (PVP). Subsequent MD simulations performed on the thin films supported by the PVP sublayer show that, at low surface pressures, the polymer tends to concentrate at the film/water interface. The simulated atomic concentration profiles, hydrogen bonding patterns, and mobility analyses of the water-polymer-monolayer interfaces reveal that the presence of PVP increases the atomic density near the monolayer film, improves the film stability, and reduces the mobility of interfacial waters. These observations explain the molecular basis of the improved efficacy of these monolayer/polymer systems for evaporation protection of water and can be used to guide future development of organic thin films for other applications.

Nanoscale wetting and fouling resistance of functionalized surfaces: a computational approach.

A computational modeling methodology has been developed and employed to characterize the nanoscale wettability and antifouling properties of functionalized hard and deformable surfaces, with a specific focus on poly(ethylene glycol) grafted substrates and their resistance to graphitic carbons. Empirical evidence suggests that the antifouling behavior of polyethylene PEG is associated with two main mechanisms: steric repulsions and hydration via formation of a structured water layer. However, there is also little attention paid to the contribution of steric repulsion vs surface hydration. We examine these two mechanisms through a combination of in silico contact angle and force measurements at the nanoscale level. We investigate the properties of the grafted functional chains and the underlying substrate, responsible for resisting surface deposition of graphitic contaminants in aqueous solution. Our results reveal that the fouling-release efficiency is enhanced when PEG chains are grafted onto hard hydrophilic substrates such as silica in contrast to deformable polymer substrates where surface modifications are effectively mitigated during interfacial contact with a hard contaminant. We conclude that the contribution of steric repulsion vs surface hydration to the antifouling ability of surfaces is strongly dependent on the nanoscale structure and deformability of the substrate. This generic method can be applied to examine individual contribution of steric repulsions and surface hydration to antifouling performance of grafted chains.

Molecular interactions behind the synergistic effect in mixed monolayers of 1-octadecanol and ethylene glycol monooctadecyl ether.

Mixed monolayers of 1-octadecanol (C18OH) and ethylene glycol monooctadecyl ether (C18E1) were studied to assess their evaporation suppressing performance. An unexpected increase in performance and stability was found around the 0.5:0.5 bicomponent mixture and has been ascribed to a synergistic effect of the monolayers. Molecular dynamics simulations have attributed this to an additional hydrogen bonding interaction between the monolayer and water, due to the exposed ether oxygen of C18E1 in the mixed system compared to the same ether oxygen in the pure C18E1 system. This interaction is maximized around the 0.5:0.5 ratio due to the particular interfacial geometry associated with this mixture.

Surface crosslinking effects on contamination resistance of functionalised polymers

We have applied a theoretical modelling approach to aid in the rational design of contamination resistant coatings. Using in silico nano-indentation we have characterised the adhesion between a contaminant particle and engineered polymer surfaces that include functionalised-surface-crosslinked and functionalised non-surface-crosslinked polyesters. The roughness, density and morphology of the surfaces were dynamically monitored as the coatings responded to the approaching particle. Our results suggest that surface crosslinking provides a stable platform for incorporation of functional groups that would otherwise migrate into the bulk substrate upon aging. However surface crosslinking with rigid cyclic curing agents renders the coatings far too stiff, which can result in slip whereby surface crosslinkers shift to the side of the incoming contaminant particle, leading to both the exposure of the flexible substrate and the envelopment of the contaminant. This type of deformation negates steric repulsions of the functional groups, crucial for preventing undesirable adhesion. In contrast, functionalisation combined with flexible acyclic surface crosslinks results in superior contamination resistance.

Dynamic Performance of Duolayers at the Air/Water Interface. 1. Experimental Analysis

Understanding, and improving, the behavior of thin surface films under exposure to externally applied forces is important for applications such as mimicking biological membranes, water evaporation mitigation, and recovery of oil spills. This paper demonstrates that the incorporation of a water-soluble polymer into the surface film composition, i.e., formation of a three-duolayer system, shows improved performance under an applied dynamic stress, with an evaporation saving of 84% observed after 16 h, compared to 74% for the insoluble three-monolayer alone. Canal viscometry and spreading rate experiments, performed using the same conditions, demonstrated an increased surface viscosity and faster spreading rate for the three-duolayer system, likely contributing to the observed improvement in dynamic performance. Brewster angle microscopy and dye-tagged polymers were used to visualize the system and demonstrated that the duolayer and monolayer system both form a homogeneous film of uniform, single-molecule thickness, with the excess material compacting into small floating reservoirs on the surface. It was also observed that both components have to be applied to the water surface together in order to achieve improved performance under dynamic conditions. These findings have important implications for the use of surface films in various applications where resistance to external disturbance is required.

Size-Dependent Fullerene–Fullerene Interactions in Water: A Molecular Dynamics Study

Nested fullerenes display a range of unique properties influenced by their size and shape. In this paper, the size- and shape-dependent aggregation of nested fullerenes in water is studied using explicit solvent molecular dynamic simulations. It is shown that water forms a layered structure near the surface of the particle, with the density of interfacial water increasing with increasing particle size. Meanwhile, water molecules near the extended facets of large nested fullerenes are unable to maintain their hydrogen bonding network, leading to a shape and size mediated structuring of surrounding waters. These distortions affect the overall association kinetics of particles in water with spherically shaped particles transitioning quickly into contact, while larger fullerenes, characterized by a lower sphericity, cluster at a much slower rate.

Dynamic Performance of Duolayers at the Air/Water Interface. 2. Mechanistic Insights from All-Atom Simulations

The novel duolayer system, comprising a monolayer of ethylene glycol monooctadecyl ether (C18E1) and the water-soluble polymer poly(vinylpyrrolidone) (PVP), has been shown to resist forces such as wind stress to a greater degree than the C18E1 monolayer alone. This paper reports all-atom molecular dynamics simulations comparing the monolayer (C18E1 alone) and duolayer systems under an applied force parallel to the air/water interface. The simulations show that, due to the presence of PVP at the interface, the duolayer film exhibits an increase in chain tilt, ordering, and density, as well as a lower lateral velocity compared to the monolayer. These results provide a molecular rationale for the improved performance of the duolayer system under wind conditions, as well as an atomic-level explanation for the observed efficacy of the duolayer system as an evaporation suppressant, which may serve as a useful guide for future development for thin films where resistance to external perturbation is desirable.

Effect of substrate on the responsive behaviour of functionalised surfaces: insights from molecular simulation

Abstract Responsive surfaces have been suggested to enhance longevity and antifouling performance of materials in many applications from industrial coatings to tissue engineering and drug delivery. We present a molecular dynamics study investigating de-swelling and swelling of some of the most commonly used responsive materials – PEG-functionalised silica and polymer surfaces – as a function of hydration and temperature. We show that PEG chains grafted onto the hard silica substrates exhibit a dehydration-induced collapse that is far more pronounced compared to chains grafted onto the soft polyester surface. The difference between the hard and soft substrates is particularly notable at low coverage densities where the chains are sufficiently separated from one another. We also show that inter-molecular hydrogen bonding responsible for the conformational state of the tethered chains in water can be temperature controlled. It can be suggested that the hard substrates with the intermediate-to-high coverage densities of low molecular weight hydrophilic grafts may be more appropriate for anti-fouling applications due to their ability to trap greater amount of water molecules. Soft substrates may be detrimental for the efficient response of the functionalised surfaces to changes in hydration and enhancement of the surface hardness must be considered when designing responsive surfaces for solution-based applications, such as antimicrobial coatings for industry and biomedicine.

Molecular rationale for the structure of cyclic poly(methyl methacrylate) stereocomplexes

Using all-atom classical molecular dynamics simulations, we have determined the structure of stereocomplexes composed of double-stranded helices of linear isotactic and cyclic syndiotactic poly(methyl methacrylate) in agreement with experimental X-ray diffraction data.