Solid Monolayers Adsorbed at the Solid−Liquid Interface Studied by Incoherent Elastic Neutron Scattering

Structure and thermodynamics of supported lipid membranes on hydrophobic van der Waals surfaces

Understanding the adsorption and physical characteristics of supported lipid membranes is crucial for their effective use as model cell membranes. Their morphological and thermodynamic properties at the nanoscale have traditionally been studied on hydrophilic substrates, such as mica and silicon oxide, which have proved to facilitate the reconstruction of biomembranes. However, in more recent years, with the advent of the van der Waals crystals technology, two-dimensional crystals such as graphene have been proposed as potential substrates in biosensing devices. Membranes formed on these crystals are expected to behave differently owing to their intrinsic hydrophobicity, however thus far knowledge of their morphological and thermodynamic properties is lacking. Here we present a comprehensive nanoscale analysis of the adsorption of phosphatidylcholine lipid monolayers on two of the most commonly used van der Waals crystals, graphite and hexagonal boron nitride. Both morphological and thermodynamic properties of the lipid membranes were investigated using temperature-controlled atomic force microscopy. Our experiments show that the lipids adsorb onto the crystals, forming monolayers with their orientation dependent upon their concentration. Furthermore, we found that the hydrophobicity of van der Waals crystals determines a strong increase in the transition temperature of the lipid monolayer compared to that observed on hydrophilic substrates. These results are important for understanding the properties of lipid membranes at solid surfaces and extending their use to novel drug delivery and biosensing devices made of van der Waals crystals.

How Do Surfactants Control the Agglomeration of Clathrate Hydrates?

Clathrate hydrates can spontaneously form under typical conditions found in oil and gas pipelines. The agglomeration of clathrates into large solid masses plugs the pipelines, posing adverse safety, economic, and environmental threats. Surfactants are customarily used to prevent the aggregation of clathrate particles and their coalescence with water droplets. It is generally assumed that a large contact angle between the surfactant-covered clathrate and water is a key predictor of the antiagglomerant performance of the surfactant. Here we use molecular dynamic simulations to investigate the structure and dynamics of surfactant films at the clathrate-oil interface, and their impact on the contact angle and coalescence between water droplets and hydrate particles. In agreement with the experiments, the simulations predict that surfactant-covered clathrate-oil interfaces are oil wet but super-hydrophobic to water. Although the water contact angle determines the driving force for coalescence, we find that a large contact angle is not sufficient to predict good antiagglomerant performance of a surfactant. We conclude that the length of the surfactant molecules, the density of the interfacial film, and the strength of binding of its molecules to the clathrate surface are the main factors in preventing the coalescence and agglomeration of clathrate particles with water droplets in oil. Our analysis provides a molecular foundation to guide the molecular design of effective clathrate antiagglomerants.

Activity and Reactivity of Pyrogenic Carbonaceous Matter toward Organic Compounds

Pyrogenic carbonaceous matter (PCM) includes environmental black carbon (fossil fuel soot, biomass char), engineered carbons (biochar, activated carbon), and related materials like graphene and nanotubes. These materials contact organic pollutants due to their widespread presence in the environment or through their use in various engineering applications. This review covers recent advances in our understanding of adsorption and chemical reactions mediated by PCM and the links between these processes. It also covers adsorptive processes previously receiving little attention and ignored in models such as steric constraints, physicochemical effects of confinement in nanopores, π interactions of aromatic compounds with polyaromatic surfaces, and very strong hydrogen bonding of ionizable compounds with surface functional groups. Although previous research has regarded carbons merely as passive sorbents, recent studies show that PCM can promote chemical reactions of sorbed contaminants at ordinary temperature, including long-range electron conduction between molecules and between microbes and molecules, local redox reactions between molecules, and hydrolysis. PCM may itself contain redox-active functional groups that are capable of oxidizing or reducing organic compounds and of generating reactive oxygen species (ROS) from oxygen, peroxides, or ozone. Amorphous carbons contain persistent free radicals that may play a role in observed redox reactions and ROS generation. Reactions mediated by PCM can impact the biogeochemical fate of pollutants and lead to useful strategies for remediation.

Effect of surface texture on freezing in nanopores: surface-induced versus homogeneous crystallization

Freezing of argon in ordered and disordered carbon pores of a similar diameter D ∼ 2.4 nm is investigated using extensive molecular simulations with large system sizes up to 10(4) atoms. While crystallization in the atomistically smooth pore consists in a surface-induced phase transition occurring at a temperature larger than the bulk, crystallization in the disordered pores, which is only partial as it is spatially restricted to the pore center, occurs through homogeneous crystallization. These results shed light on solidification in pores by showing that there is a crossover between surface-induced and homogeneous crystallization upon increasing the surface disorder of the host material. In the latter case, the Gibbs-Thomson equation, in which crystallization is assumed to occur when the crystal size equals the pore size corrected for the thickness of the unfreezable layer at the pore surface, is in reasonable agreement with the observed freezing temperature.

Direct observation of prefreezing at the interface melt-solid in polymer crystallization

Crystallization is almost always initiated at an interface to a solid. This observation is classically explained by the assumption of a reduced barrier for crystal nucleation at the interface. However, an interface can also induce crystallization by prefreezing (i.e., the formation of a crystalline layer that is already stable above the bulk melting temperature). We present an atomic force microscopy (AFM)-based in situ observation of a prefreezing process at the interface of a polymeric model system and a crystalline solid. Explicitly, we show an interfacial ordered layer that forms well above the bulk melting temperature with thickness that increases on approaching melt-solid coexistence. Below the melting temperature, the ordered layer initiates crystal growth into the bulk, leading to an oriented, homogeneous semicrystalline structure.

Functional group effects on the enthalpy of adsorption for self-assembly at the solution/graphite interface

The thermodynamics of self-assembly have long been explored by either experimental or theoretical investigations which are often unable to account for all the factors influencing the assembly process. This work interrogates the thermodynamics of self-assembly at a liquid/solid interface by measuring the enthalpy of adsorption encompassing analyte-analyte, analyte-solvent, analyte-substrate, and solvent-substrate interactions. Comparison of the experimental data with computed lattice energies for the relevant monolayers across a series of aliphatic analytes reveals similar ordering within the series, with the exceptions of the fatty acid and bromoalkane adsorbates. Such a discrepancy could arise when the lattice energies do not account for important interactions, such as analyte-analyte interactions in solution. Flow microcalorimetry provides a uniquely inclusive view of the thermodynamic events relevant to self-assembly at the liquid/solid interface.

Freezing in the bulk controlled by prefreezing at a surface

We use Monte Carlo simulations of the Lennard-Jones model to study the nucleation of a crystal phase at a flat surface. Our motivation is the observation that crystal phases almost always nucleate at a surface. We find that a surface phase transition (prefreezing) can control nucleation of the bulk crystal. This finding should be general and so surface phase behavior should be considered whenever nucleation of bulk phases at surfaces is considered. Also, nucleation of the bulk crystal transforms smoothly into the nucleation of a surface crystal layer as the bulk transition is crossed.

Effect of solvation and confinement on the trans-gauche isomerization reaction in n-butane

The effect of solvation and confinement on the conformational equilibria and kinetics of n-butane is examined using molecular dynamics simulations of the bulk and confined fluids and compared to appropriately chosen reference states. Clear evidence for a solvent shift of the preferred conformation in bulk n-butane is found. At a temperature of 292 K and a density of 6.05 nm-3 a small solvent shift in favor of gauche is observed (similar to previously reported values), and the shift increases substantially with an increase in density to 8.28 nm-3. The rate of torsional interconversion from the trans to the gauche state, calculated using the relaxation function method, was found to increase with increasing temperature and density. The rate constants kTG and kGT have an Arrhenius temperature dependence yielding activation energies significantly lower than the trans-gauche and gauche-trans barrier heights in the torsional potential for a free molecule, depending on the density. In the confined phase, we considered the same densities as simulated in the bulk phase, and for four different values of the physical pore width (approximately 1.5-4.0 nm). At the high density, we find that the position of the trans-gauche equilibrium is displaced towards excess trans compared with the bulk phase, reflecting the confinement and interactions of the molecules with the pore wall. The isomerization rate is found to decrease with decreasing pore width. Again, we find that the kinetics obeys an Arrhenius rate law and the activation energy for the trans-gauche and gauche-trans interconversions is slightly smaller than that of the bulk fluid at the same density.

Effects of confinement on freezing and melting

We present a review of experimental, theoretical, and molecular simulation studies of confinement effects on freezing and melting. We consider both simple and more complex adsorbates that are confined in various environments (slit or cylindrical pores and also disordered porous materials). The most commonly used molecular simulation, theoretical and experimental methods are first presented. We also provide a brief description of the most widely used porous materials. The current state of knowledge on the effects of confinement on structure and freezing temperature, and the appearance of new surface-driven and confinement-driven phases are then discussed. We also address how confinement affects the glass transition.