On the solubility of long n-alkanes in water at room temperature

Insights into short chain polyethylene penetration of phospholipid bilayers via atomistic molecular dynamics simulations

The escalation of global plastic production, reaching an annual output of 400 million tons, has significantly intensified concerns regarding plastic waste management. This has been exacerbated by improper recycling and disposal practices, contributing to the impending crisis of plastic pollution. Predictions indicate that by 2025, the environment will bear the burden of over ten billion metric tons of accumulated plastic waste. This situation has led to the concerning release of microplastics and nanoplastics (NPs) into the environment as plastic materials degrade, thereby posing risks to both ecosystems and human health. Nanoparticle interactions with living organisms have garnered significant attention due to their potential to disrupt vital biological processes. Of particular interest are lipid membranes, acting as crucial gatekeepers, underscoring the importance of comprehending the intricate process of NP penetration. Molecular dynamics (MD) simulations serve as a robust tool, offering molecular-level insights into these intricate interactions. In this study, we leverage all-atom MD simulations to delve into the interactions between lipid bilayers and polyethylene (PETH) chains of varying lengths. The investigation spans diverse lipid bilayer compositions-ranging from pure POPC to POPC:DPPC mixtures-revealing how PETH accommodates itself, adopts extended conformations, and influences membrane structure and ordering. Significantly, while longer PETH chains demonstrate limited passive diffusion, their potential to penetrate bilayers over extended timescales emerges as a significant revelation. Overall, this research significantly advances our comprehension of NP-membrane interactions, shedding light on the potential environmental and health implications that lie ahead.

On the size, shape and energetics of the hydration shell around alkanes

A large number of clathrate-like cages have been proposed as the very first hydration shell of alkanes. The cages include canonical structures commonly found in clathrate hydrates and many others, not previously reported, derived from the carbon fullerene cavities. These structures have a rich and variegated form, which can adapt to the shape and conformation of the solute. They avoid "wasting" hydrogen bonds, while minimizing the volume cage and maximizing the solute-solvent van der Waals interactions. DFT/M06-2X and MP2 ab initio calculations give comparable structural and energetic results although the latter predicts slightly larger cages for a given solute. It is shown that the van der Waals interactions are substantial and the large exoenergetic values found for isobutane and cyclopentane provide an explanation for the surprising high melting points of related hydrates at room pressure. The encaging enthalpy for various hydrocarbons is similar to the enthalpy of solution measured at a temperature just above the melting point of aqueous hydrocarbon solutions, thus indicating that water molecules should not deviate too much from the configuration with O-H bonds tangentially oriented with respect to the solute surface. The computed trend differs from the enthalpy of solution measured at room temperature, thus the very first hydration shell departs, up to a certain degree, from the clathrate-like structures.

Impact of water on the lubricating properties of hexadecane at the nanoscale

Fluid lubricants are routinely used to reduce friction in a wide range of applications, from car engines to machinery and hard-disk drives. However, their efficiency can be significantly influenced by the ambient conditions they are exposed to, in particular humidity. Our understanding of the molecular mechanisms responsible for the well-documented impact of water on lubrication remains limited, hindering the improvement of tribological formulations. Here, we use Atomic Force Microscopy (AFM) and shear force spectroscopy to investigate the structural and dynamical behaviour of a model lubricant, hexadecane, confined between an AFM probe and a hydrophilic mica surface at different temperatures and humidities. We show that both the nanoscale structure and the tribological behaviour of the system are dominated by the nucleation of water nanodroplets at the interface. The process is favoured at higher temperature and can be explained with classical nucleation theory whereby the droplets become stable when larger than 20 nm to 50 nm size, depending on the ambient conditions. Below this threshold, a molecularly thin film of water molecules coats the surface uniformly. Highly localised shear measurements demonstrate a detrimental impact of the nanodroplets on shear with a twofold increase in the lubricated friction force. However, this can be mitigated by the adjunction of an amphiphilic additive, here oleic acid.

On the effect of sodium salts on the coil-to-globule transition of poly(N-isopropylacrylamide)

Addition of sodium salts to water increases the density, thereby increasing the magnitude of the solvent-excluded volume effect and stabilizing the globule state of PNIPAM.