Special Issue "Third Edition: Advances in Molecular Simulation".

Molecular simulation is one of the fastest growing fields in science [...].

Molecular Dynamics Simulations of Different Nanoparticles at Substrates.

We report the results of large-scale molecular dynamics simulations of adsorption nanoparticles on solid surfaces. The particles were modeled as stiff aggregates of spherical segments. Three types of particles were studied: rods, rectangles, and triangles built of the same number of segments. We show how the particle shape affects the adsorption, the structure of the surface layer, and the degree of the removal of particles from the solvent. The systems with different segment-segment and segment-surface interactions and different concentrations of particles were investigated. The ordered structures formed in adsorption monolayers were also analyzed. The results are consistent with experimental observations.

Special Issue "2nd Edition: Advances in Molecular Simulation".

Molecular simulation is becoming a standard tool for researchers working in different fields, such as physics, chemistry, material science, biology, medicine, engineering, and many others [...].

Hybrid Nanoparticles at Fluid-Fluid Interfaces: Insight from Theory and Simulation.

Hybrid nanoparticles that combine special properties of their different parts have numerous applications in electronics, optics, catalysis, medicine, and many others. Of the currently produced particles, Janus particles and ligand-tethered (hairy) particles are of particular interest both from a practical and purely cognitive point of view. Understanding their behavior at fluid interfaces is important to many fields because particle-laden interfaces are ubiquitous in nature and industry. We provide a review of the literature, focusing on theoretical studies of hybrid particles at fluid-fluid interfaces. Our goal is to give a link between simple phenomenological models and advanced molecular simulations. We analyze the adsorption of individual Janus particles and hairy particles at the interfaces. Then, their interfacial assembly is also discussed. The simple equations for the attachment energy of various Janus particles are presented. We discuss how such parameters as the particle size, the particle shape, the relative sizes of different patches, and the amphiphilicity affect particle adsorption. This is essential for taking advantage of the particle capacity to stabilize interfaces. Representative examples of molecular simulations were presented. We show that the simple models surprisingly well reproduce experimental and simulation data. In the case of hairy particles, we concentrate on the effects of reconfiguration of the polymer brushes at the interface. This review is expected to provide a general perspective on the subject and may be helpful to many researchers and technologists working with particle-laden layers.

Shape Transformations and Self-Assembly of Hairy Particles under Confinement.

Molecular dynamics simulations are used to investigate the behavior of polymer-tethered nanoparticles between two inert or attractive walls. The confinement in pores creates new possibilities for controlling the shape transformation of individual hairy particles and their self-organization. We introduce a minimalistic model of the system; only chain-wall interactions are assumed to be attractive, while the others are softly repulsive. We show how the shape of isolated particles can be controlled by changing the wall separation and the strength of the interaction with the surfaces. For attractive walls, we found two types of structures, "bridges" and "mounds". The first structures are similar to flanged spools in which the chains are connected with both walls and form bridges between them. We observed various bridges, symmetrical and asymmetrical spools, hourglasses, and pillars. The bridge-like structures can be "nano-oscillators" in which the cores jump from one wall to the other. We also study the self-assembly of a dense fluid of hairy particles in slit-like pores and analyze how the system morphology depends on interactions with the surfaces and the wall separation. The hairy particles form layers parallel to the walls. Different ordered structures, resembling two-dimensional crystalline lattices, are reported. We demonstrate that hairy particles are a versatile soft component forming a variety of structures in the slits.

Adsorption on Ligand-Tethered Nanoparticles.

We use coarse-grained molecular dynamics simulations to study adsorption on ligand-tethered particles. Nanoparticles with attached flexible and stiff ligands are considered. We discuss how the excess adsorption isotherm, the thickness of the polymer corona, and its morphology depend on the number of ligands, their length, the size of the core, and the interaction parameters. We investigate the adsorption-induced structural transitions of polymer coatings. The behavior of systems involving curved and flat "brushes" is compared.

Adsorption-induced co-assembly of hairy and isotropic particles.

We use coarse-grained molecular dynamics simulations to study the behavior of polymer-tethered particles immersed in fluids of isotropic particles. Particles modified with weakly anchored, mobile ligands are considered. We discuss how the concentration of fluid particles affects the morphology of an isolated hairy particle. It is shown that hairy particles present different morphologies including typical core-shell, octopus-like and corn-like, depending on fluid-segment interactions and the fluid density. The mechanism of changes in the shape of hairy particles is explained. The reconfiguration of the polymer corona arises from adsorption of fluid particles "on chains". The adsorbed fluid particles form bridges between the chains. This causes the mobile ligands to merge into clusters on the core surface. A part of the core remains empty so the hairy particle becomes a Janus-like object. We also study co-assembly in mixtures of hairy and isotropic particles. Depending on the strength of fluid-segment interactions, hairy particles with fluid particles trapped inside their coronas remain isolated or form mixed clusters of different structures. The aggregation of hairy particles results from the formation of bridges between chains belonging to different cores by fluid particles.

Self-assembly of hairy disks in two dimensions - insights from molecular simulations.

We report the results of large scale molecular dynamics simulations conducted for sparsely grafted disks in two-dimensional systems. The main goal of this work is to show how the ligand mobility influences the self-assembly of particles decorated with short chains. We also analyze the impact of the chain length on the structure of dense phases. A crossover between the systems controlled by the core-core or by the segment-segment interactions is discussed. We prove that the ligand mobility determines the structure of the system. The particles with fixed tethers are found to order into different structures, an amorphous phase, hexagonal or honeycomb lattices, and a "spaghetti"-like phase containing single strings of cores, depending on the length of attached chains. The disks with mobile monomers assemble into a hexagonal structure, while the particles with longer mobile chains attached to them form a lamellar phase consisting of double strings of cores.

Molecular dynamics simulations of mono-tethered particles at solid surfaces.

We use molecular dynamics simulations to study the behavior of mono-tethered nanoparticles on solid surfaces. In our model particle-particle and particle-chain interactions are repulsive, while chain-chain interactions are attractive. Two surfaces are considered: the first one attracts particles and the other attracts chains. Excess adsorption isotherms are presented for both the surfaces and different lengths of tethers. The mechanism of adsorption is discussed. We find that depending on the assumed parameters the mono-tethered particles can be adsorbed as single particles or as different aggregates. Our main goal is to explore the structure of surface films. We show that the morphology of the adsorbed layer depends mainly on the type of the surface but the influence of the particle diameter, the chain length and the density is also important. We prove that the shape of aggregates changes near the substrate. For certain parameters the aggregates can break under the influence of the surface.

Janus Ligand-Tethered Nanoparticles at Liquid-Liquid Interfaces.

We investigate the structural properties of Janus ligand-tethered nanoparticles at liquid-liquid interfaces using coarse-grained molecular dynamics simulations. The effect of interactions between different chains and liquids is discussed. We consider the Janus particles with symmetrical interactions with the liquids which correspond to supplementary wettability and particles with uncorrelated interactions. Simulation results indicate that the Janus hairy particles trapped in the interface region have different configurations characterized by the vertical displacement distance, the orientation of the Janus line relative to the interface, and the particle shape. The Janus hairy particles present abundant morphologies, including dumbbell-like and typical core-shell, at the interface. The shape of adsorbed particles is analyzed in detail. The simulation data are compared with those predicted by a simple phenomenological approach. This work can promote the applications of Janus hairy particles in nanotechnology.

Amphiphilic Janus Particles Confined in Symmetrical and Janus-Like Slits.

We use Monte Carlo simulations to investigate the behavior of Janus spheres composed of attractive and repulsive parts confined between two parallel solid surfaces. The slits with identical and competing walls are studied. The adsorption isotherms of Janus particles are determined, and the impact of the density in the pore on the morphology is discussed in detail. So far, this issue has not been systematically investigated. New, unique structures are observed along the isotherms, including the bilayer and three-layer structures located at different distances from the walls. We analyze how selected parameters affect the positional and orientational ordering in these layers. In some cases, the particles form highly ordered hexagonal lattices.

Adsorption of Polymer-Tethered Particles on Solid Surfaces.

We explore the behavior of polymer-tethered particles on solid surfaces using coarse-grained molecular dynamics simulations. Segment-segment, segment-core, and core-core interactions are assumed to be purely repulsive, while the segment-substrate interactions are attractive. We analyze changes in the internal structure of single hairy particles on the surfaces with the increasing strength of the segment-substrate interactions. For this purpose, we calculate the density profiles along the x, y, z axes and the mass dipole moments. The adsorbed hairy particles are found to be symmetrical in a plane parallel to the substrate but strongly asymmetric in the vertical direction. On stronger adsorbents, the particle canopies become flattened and the cores lie closer to the wall. We consider the adsorption of hairy nanoparticles dispersed in systems of different initial particle densities. We show how the strength of segment-substrate interactions affects the structure of the adsorbed phase, the particle-wall potential of the average force, the excess adsorption isotherms, and the real adsorption isotherms.

Theory of retention in reversed phase liquid chromatography with ternary mobile phases.

The partition model of retention is developed for reversed phase liquid chromatography with multicomponent mobile phases. Simple equations for the retention and selectivity in ternary mobile phases are derived. For the systems in which the ratio of volume fractions of organic modifiers remains fixed, new linear dependences for retention factor and selectivity are proposed. These equations are successfully used to describe experimental data found in the literature. An influence of the nature of organic solvents and proportion in which they are mixed on retention and selectivity is discussed.

Adsorption model for retention in normal-phase liquid chromatography with ternary mobile phases.

The review gives a link between the theory of adsorption from multicomponent solutions and liquid chromatography. The article surveys the methods developed to describe the retention in normal-phase chromatography with ternary mobile phases with emphasis on the results of the authors. In the model used the driving force for the separation is the difference in adsorption of a solute and all solvents onto the solid surface. The general equation generates a series of simple linear relationships to predict the retention factor in ternary mobile phase for which certain parameters remain fixed. Theoretical concepts are tested by comparison with experimental data. The correlations between parameters characterizing retention in ternary, binary and pure solvents are discussed.