Wojciech Rżysko

Concentration-Dependent Supramolecular Engineering of Hydrogen-Bonded Nanostructures at Surfaces: Predicting Self-Assembly in 2D

We report a joint computational and experimental study on the concentration-dependent self-assembly of a flat C3-symmetric molecule at surfaces. As a model system we have chosen a rigid molecular module, 1,3,5-tris(pyridine-4-ylethynyl)benzene, which can undergo self-association via hydrogen bonding (H-bonding) to form ordered 2D nanostructures. In particular, the lattice Monte Carlo method, combined with density functional calculations, was employed to explore the spontaneous supramolecular organization of this tripod-shaped molecule under surface confinement. We analyzed the stability of different weak H-bonded patterns and the influence of the concentration of the starting molecule on the 2D supramolecular packing. We found that ordered, densely packed monolayers and 2D porous networks are obtained at high and low concentrations, respectively. A concentration-dependent scanning tunneling microscopy investigation of the molecular self-assembly at a graphite-solution interface revealed supramolecular motifs, which are in perfect agreement with those obtained by simulations. Therefore, our computational approach represents a step forward toward the deterministic prediction of molecular self-assembly at surfaces and interfaces.

One building block, two different nanoporous self-assembled monolayers: a combined STM and Monte Carlo study.

With the use of a single building block, two nanoporous patterns with nearly equal packing density can be formed upon self-assembly at a liquid-solid interface. Moreover, the formation of both of these porous networks can be selectively and homogenously induced by changing external parameters like solvent, concentration, and temperature. Finally, their porous properties are exploited to host up to three different guest molecules in a spatially resolved way.

Capillary condensation in non-uniform pores: Density functional approach

This paper reports on studies of adsorption in energetically and geometrically non-uniform slit-like pores. The calculations have been carried out by the density functional approach. We have studied the influence of both types of non-uniformity on the capillary condensation. We have found that the pore non-uniformity may qualitatively change the phase diagram of confined fluids. In particular, geometrical non-uniformity may lead to a splitting of the hysteresis loop into two separate parts, associated with the filling of different parts of the pore.

Density functional approach to the adsorption of spherical molecules on a surface modified with attached short chains

A density functional and Monte Carlo simulation study of end-grafted polymers immersed by simple fluids is presented. The polymer molecules are modeled as freely jointed tangent hard spheres with the end segments linked to the surface. The authors analyze an influence of the chain length, the grafting density, and a nature of solvent on the brush structure. Adsorption of hard-sphere mixtures on the modified surface is also discussed. The theory precisely approximates simulation data.

Phase behavior of a two-dimensional and confined in slitlike pores square-shoulder, square-well fluid.

We report the phase diagrams for a square-shoulder, square-well fluids in two dimensions (2D), as well as confined in slitlike pores. The diagrams are obtained by histogram reweighting Monte Carlo simulations. The critical points are located by using finite size scaling analysis. Our calculations indicate that the phase behavior of the systems investigated is distinct from that observed in three dimensions. In particular, we have not found the liquid-liquid transition that occurs in the bulk.

Self-assembly of molecular tripods in two dimensions: structure and thermodynamics from computer simulations

Controlled self-organization of molecular building blocks into low-dimensional ordered superstructures is a promising method of fabrication of functional materials with tunable physico-chemical properties. In this contribution we use the Parallel Tempering Monte Carlo simulation method to study the self-assembly of tripod molecules adsorbed on a triangular lattice being an analog of a graphite surface. In the adopted approach the molecules were assumed to be flat rigid C3-symmetric structures composed of a central segment connected with three n-membered arms. Our primary objective was to examine the effect of molecular size, n on the topology of the corresponding phase diagrams and to identify stable ordered phases with different morphologies. It was demonstrated that for the tripod molecules with sufficiently long arms (n > 1), the self-assembly leads to the formation of scalable chiral porous networks with hexagonal void spaces. For these systems the phase coexistence was found to be described by phase diagrams with the same overall topology. On the other hand, the simulations performed for the small tripods with n = 1 revealed the formation of compact patterns, resulting in a substantial change in the shape of the phase diagram. The insights from our theoretical investigations can be helpful in designing 2D self-assembled molecular architectures comprising C3-symmetric functional units.

Liquid films on heterogeneous surfaces: a density functional approach

A weighted density functional theory has been applied to calculate the detailed, spatial structure of a Lennard-Jones fluid in contact with a heterogeneous surface. The surface has been assumed to be formed of parallel strips possessing different adsorbing energy. The calculations indicate that the presence of parts of adsorbing surfaces with different energy can seriously affect the wetting behaviour in comparison with a model homogeneous surface.

Phase behavior of a fluid confined in slitlike pores with walls modified by preadsorbed chain molecules

We have studied the microscopic structure, thermodynamics of adsorption, and phase behavior of Lennard-Jones fluid in slitlike pores with walls modified due to preadsorption of chain molecules. The chain species are grafted at the walls by terminating segments. Our theoretical considerations are based on a density functional approach in the semigrand canonical ensemble. The applied constraint refers to the constant number of grafted chain molecules in the pore without restriction of the number of chains at each of the walls. We have observed capillary condensation of Lennard-Jones fluid combined with the change of the distribution of chains from nonsymmetric to symmetric with respect to the pore walls. The phase diagrams of the model are analyzed in detail, dependent on the pore width, length of chains, and grafted density.

Phase behavior of decorated soft disks in two dimensions

Molecular dynamics simulations are used to investigate the phase behavior of disks decorated with small ligands in two-dimensional films. We consider disks with four ligands, which are fixed at vertices of a square or slide over the circle delimiting the core. For selected model systems, phase diagrams are evaluated and discussed. We show that ligand mobility can change the topology of phase diagrams. In particular, it can affect fluid-solid transitions, changing the solid phase symmetry. Moreover, the mobility of ligands can either hamper or facilitate crystallization.

Two-dimensional binary mixtures of patchy particles and spherical colloids

Using Monte Carlo simulation we study two dimensional mixtures of patchy and spherically symmetric particles. Such mixtures can be synthesized experimentally by covering colloids with appropriate types of DNA strands [L. Feng, et al., Adv. Mater., 2013, 25, 2779]. We focus on finding out the ordered structures that can be formed in such systems. The type of ordered phase strongly depends on the valency, size and binding energy of the patchy particles. If the patch size is small enough, i.e. it allows only one spherically symmetric particle to be bound, the ordered structure follows either a hexagonal or a tetragonal pattern depending on the valency of the patchy particles. Moreover, we find stable quasicrystals of dodecagonal symmetry. Additional structures can be obtained if the patches are larger and the binding energy is higher. Depending on the valency of the patchy particles we find either lanes or branched structures forming polygons of the spherically symmetric particles with few patchy particles inside. For pentavalent patchy particles we find stable quasicrystals of decagonal symmetry.