Molecular dynamics simulations of hydrophobins and cellulose nanomaterials near fluid and solid interfaces

Abstract

Classical molecular dynamics simulations using the Martini coarse-grained force field were performed to study oil nanodroplets surrounded by fungal hydrophobin (HP) proteins in seawater. The class I EAS and the class II HFBII HPs were studied along with two model oils, benzene and n-decane. Both HPs exhibit free energy minima at the oil-seawater interface, which are much deeper in benzene systems than in interfaces with n-decane. Larger constraint forces are required to keep both HPs within the n-decane phase compared to inside benzene, with HFBII being more affine to benzene compared to EAS. Smaller surface tensions are observed at benzene-seawater interfaces coated with HPs compared to their n-decane counterparts, in which the surface tension remains unchanged upon increases in the surface concentration of HPs, which in contrast lead to surface tension reductions at the benzene-seawater interface. EAS has a larger tendency to cluster together in the interface compared to HFBII, with both HPs having larger coordination numbers when surrounding benzene droplets compared to when they are around n-decane nanoblobs. The HP/oil nanostructures in seawater examined have radii of gyration ranging between 2-12 nm, where the n-decane structures are larger and have more irregular shapes (as visualized from simulation snapshots and quantified through different shape measures) compared to the benzene blobs. The n-decane molecules within the nanostructures form a compact spherical core, with the HPs partially covering its surface and clustering together, conferring irregular shapes to the nanostructures. The EAS/n-decane structures are larger and have more irregular shapes compared to their HFBII/n-decane counterparts. In contrast, in the HP/benzene structures both HPs tend to penetrate into the benzene part of the droplet. The HFBII/benzene structures having the larger benzene/HP ratios examined tend to be more compact and spherical compared to their EAS/benzene counterparts; however, some of the HFBII/benzene systems that have smaller benzene/HP ratios have a more elongated structure compared to their EAS counterparts. This simulation study provides insights into HP/oil nanostructures that are smaller than the oil droplets and gas bubbles recently studied in experiments, and thus might be challenging to examine with experimental techniques. In another project, classical molecular dynamics simulations using atomistic force fields were used to fundamentally understand the behavior of cellulose nanomaterials (CNMs) in the proximity of montmorillonite (MMT) clays in the presence of seawater. Two types of CNMs were considered, cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs, which have no crystalline structure). Results from MD simulations and potential of mean force calculations show that the 100 facet of the CNC has a strong affinity with the MMT basal surface, whereas the 010 facet of the CNC and the CNF have weaker interactions with the MMT. Analysis of hydrogen bonds between the CNMs and the MMT surface reveals that the 100 facet of the CNC has almost 2.5 times more hydrogen bonds than the CNF. Seventy-five percent of the hydrogen bonds formed between the CNMs and the MMT involve a hydrogen atom attached to a carbon atom from the CNM linking with an MMT oxygen atom. These simulation studies give insights into the behavior between CNMs and the MMT, reinforcing experimental studies that suggest that CNMs can be effective additives to water-based drilling fluids used in oil drilling operations. This computational study and the experimental studies propose that the CNMs can create an impermeable barrier aiming to prevent excessive water uptake by the MMT clays in the oil well, which is responsible for significant economic losses (>$500M/year) during drilling operations.--Author's abstract