Torsion-induced phase transitions in fluids confined between chemically decorated substrates

Effects of the Chemical and Structural Properties of Silane Monolayers on the Organization of Water Molecules and Ions at Interfaces, from Molecular Dynamics Simulations

Understanding the organization of the hydration layer at functionalized silica surfaces is relevant for a large range of biosensing applications or surface phenomena such as biomolecule adsorption. Silane monolayers are widely used to functionalize silica surfaces. Using molecular dynamics simulations, we have investigated the role of silane molecule head-group charge, alkyl chain length, and surface coverage in the structural organization and dynamic properties of Na⁺ ions, Cl^- ions, and water molecules at the interface. The silane molecules studied are 3-aminopropyldimethylethoxysilane, n-propyldimethylmethoxysilane, octadecyldimethylmethoxysilane, and (dimethylamino)dimethylsilylundecanoate. Our results suggest that the distribution of interfacial ions is sensitive to the 2D dispersion of the silane-charged head groups. Also, while charged silane monolayers show a strong orientation of interfacial water molecules, which leads to a rupture in the hydrogen bond network and disturbs their tetrahedral organization, the arrangement of water molecules at the interface with uncharged silane monolayers seems to be related to the surface roughness and to alkyl chain length. In line with these results, the diffusion of ions and water molecules is higher at the CH₃ long monolayer interface than at the CH₃ short monolayer interface and at the charged monolayer interfaces. Also, whatever the silane molecules studied, bulk properties are recovered around 0.7 nm above the interface. The interfacial water organization is known to impact biomolecule adsorption. Therefore, these results could further help in optimizing the functionalization layers to capture analytes.

Fluid bridges confined between chemically nanopatterned solid substrates

We discuss equilibrium properties of classical fluids confined to nanoscopic volumes by solid substrates. The substrates themselves are endowed with wettable chemical patterns of variable symmetry. We develop a thermodynamic description suitable for these highly anisotropic systems. Based upon a combination of Monte Carlo simulations in the grand canonical ensemble and lattice density functional theory at mean-field level we analyze the structure and phase behaviour of the confined fluid. Under suitable thermodynamic conditions the fluid may condense partially in regions controlled by the wettable nanopatterns. The resulting fluid bridges are established as thermodynamic phases and exhibit unique rheological features.

Nanoscopic Liquid Bridges between Chemically Patterned Atomistic Walls

A binary liquid mixture, containing the Lennard-Jones molecules A and B, in equilibrium with a bulk liquid reservoir near the point of phase separation, confined between atomistic chemically patterned walls, is studied by grand canonical Monte Carlo simulations. In the bulk, the B-rich phase is stable and the A-rich phase is metastable. The walls bear patches attractive to A; when the walls are close, A-rich liquid bridges condense between the patches. The normal and lateral forces on the walls are measured as a function of the wall separation and of the lateral displacement between the patches on opposite walls. When there are one or two molecular layers in the bridge and the wall lattice constant is close to that of crystalline A, the normal and lateral forces depend strongly on the registry of the wall lattices, varying in an oscillatory manner.