An introduction to classical molecular dynamics simulation for experimental scattering users

Molecular dynamics simulation of synergistic behavior at the air-water interface: mixed cationic-anionic fluorocarbon-hydrocarbon surfactants

Ecological concerns surrounding conventional aqueous film-forming foam extinguishing agents, predominantly composed of long-chain fluorocarbon surfactants, have spurred the need for innovation in eco-compatible substitutes, such as short-chain fluorocarbon surfactants. Molecular dynamics simulations are a valuable tool for studying the behavior of mixed surfactant systems at the air-water interface. We have conducted molecular dynamics simulations to investigate the interfacial behavior of a mixed cationic-anionic surfactant system, including N-[3-(dimethylamino)propyl]perfluorobutanesulfonamide hydrochloride (PFB-MC) and 1-octanesulfonic acid (1-OA). The simulations explored the effects of varying PFB-MC and 1-OA ratios on aggregation and adsorption. The results indicate that the equimolar 1 : 1 ratio produced more compact aggregates at the interface and achieved the most effective reduction in surface tension and the formation of a dense interfacial film. The study highlights the competitive adsorption phenomena between surfactants and counterions at the interface, providing insights through 1D and 2D density analyses into the impact of counterbalancing ions on aggregation. An increased PFB-MC concentration resulted in decreased hydrogen bonding with water, while 1-OA showed a higher tendency for hydrogen bonding, underscoring its hydrophilicity. These findings provide valuable insights into surfactant interfacial behavior and are instrumental in the development of advanced foam extinguishing agents suitable for environmental and industrial use.

Illuminating the nanostructure of diffuse interfaces: Recent advances and future directions in reflectometry techniques

Diffuse soft matter interfaces take many forms, from end-tethered polymer brushes or adsorbed surfactants to self-assembled layers of lipids. These interfaces play crucial roles across a multitude of fields, including materials science, biophysics, and nanotechnology. Understanding the nanostructure and properties of these interfaces is fundamental for optimising their performance and designing novel functional materials. In recent years, reflectometry techniques, in particular neutron reflectometry, have emerged as powerful tools for elucidating the intricate nanostructure of soft matter interfaces with remarkable precision and depth. This review provides an overview of selected recent developments in reflectometry and their applications for illuminating the nanostructure of diffuse interfaces. We explore various principles and methods of neutron and X-ray reflectometry, as well as ellipsometry, and discuss advances in their experimental setups and data analysis approaches. Improvements to experimental neutron reflectometry methods have enabled greater time resolution in kinetic measurements and elucidation of diffuse structure under shear or confinement, while innovation in analysis protocols has significantly reduced data processing times, facilitated co-refinement of reflectometry data from multiple instruments and provided greater-than-ever confidence in proposed structural models. Furthermore, we highlight some significant research findings enabled by these techniques, revealing the organisation, dynamics, and interfacial phenomena at the nanoscale. We also discuss future directions and potential advancements in reflectometry techniques. By shedding light on the nanostructure of diffuse interfaces, reflectometry techniques enable the rational design and tailoring of interfaces with enhanced properties and functionalities.

Investigation of the Dynamic Behaviour of H2 and D2 in a Kinetic Quantum Sieving System

Porous organic cages (POCs) are nanoporous materials composed of discrete molecular units that have uniformly distributed functional pores. The intrinsic porosity of these structures can be tuned accurately at the nanoscale by altering the size of the porous molecules, particularly to an optimal size of 3.6 Å, to harness the kinetic quantum sieving effect. Previous research on POCs for isotope separation has predominantly centered on differences in the quantities of adsorbed isotopes. However, nuclear quantum effects also contribute significantly to the dynamics of the sorption process, offering additional opportunities for separating H2 and D2 at practical operational temperatures. In this study, our investigations into H2 and D2 sorption on POC samples revealed a higher uptake of D2 compared to that of H2 under identical conditions. We employed quasi-elastic neutron scattering to study the diffusion processes of D2 and H2 in the POCs across various temperature and pressure ranges. Additionally, neutron Compton scattering was utilized to measure the values of the nuclear zero-point energy of individual isotopic species in D2 and H2. The results indicate that the diffusion coefficient of D2 is approximately one-sixth that of H2 in the POC due to the nuclear quantum effect. Furthermore, the results reveal that at 77 K, D2 has longer residence times compared to H2 when moving from pore to pore. Consequently, using the kinetic difference of H2 and D2 in a porous POC system enables hydrogen isotope separation using a temperature or pressure swing system at around liquid nitrogen temperatures.