Monte Carlo simulations of racemic liquid mixtures: thermodynamic properties and local structure

Self-Assembly of Mesophases from Nanoparticles

A growing number of crystalline and quasi-crystalline structures have been formed by coating nanoparticles with ligands, polymers, and DNA. The design of nanoparticles that assemble into mesophases, such as those formed by block copolymers, would combine the order, mobility, and stimuli responsive properties of mesophases with the electronic, magnetic, and optical properties of nanoparticles. Here we use molecular simulations to demonstrate that binary mixtures of unbound particles with simple short-ranged pair interactions produce the same mesophases as block copolymers and surfactants, including lamellar, hexagonal, gyroid, body-centered cubic, face-centered cubic, perforated lamellar, and semicrystalline phases. The key to forming the mesophases is the frustrated attraction between particles of different types, achieved through control over interparticle size and over strength and softness of the interaction. Experimental design of nanoparticles with effective interactions described by the potentials of this work would provide a distinct, robust route to produce ordered tunable liquid crystalline mesophases from nanoparticles.

Effects of geometrical and energetic nonadditivity on the phase behavior of two-component symmetric mixtures

Using Monte Carlo simulation methods in the grand-canonical ensemble, we have studied the phase behavior of three-dimensional symmetric binary mixtures of Lennard-Jones particles. We have also elucidated the effects of geometric and energetic nonadditivity on the phase behavior. Phase diagrams for several systems have been evaluated. We have demonstrated that in completely miscible mixtures the geometrical nonadditivity (negative as well as positive) stabilizes a liquid phase leading to a gradual increase of the critical temperature. The mechanism leading to such behavior is different when the system shows negative and positive geometrical nonadditivity. In the case of systems with negative energetic nonadditivity, which may exhibit demixing transition in the liquid phase, their phase behavior is also strongly affected by the geometric non-additivity. The systems with negative geometric nonadditivity have been demonstrated to show reentrant miscibility, while those with positive geometric nonadditivity show enhanced tendency toward mixing at sufficiently high temperatures. We have evaluated phase diagrams for several systems.

Reentrant miscibility in two-dimensional symmetrical mixtures

The Monte Carlo simulation method in the grand canonical ensemble is used to study the phase behavior of two-dimensional symmetrical binary mixtures of Lennard-Jones particles with negative nonadditivity and the weaker interaction between the pairs of unlike than between the pairs of like particles. We have determined the evolution of the phase diagram topology when the parameters describing the interaction between unlike particles vary. It has been found that such systems may exhibit reentrant miscibility in the liquid and the solid phases.

The phase behavior of two-dimensional symmetrical mixtures

Using Monte Carlo simulation methods in the grand canonical and semigrand canonical ensembles, we study the phase behavior of two-dimensional symmetrical binary mixtures of Lennard-Jones particles. We discuss the interplay between the demixing transition in a liquid and the freezing in detail. Phase diagrams for several systems characterized by different parameters describing interactions in the system are presented. It is explicitly demonstrated that different scenarios involving demixing and freezing transitions, described in our earlier paper [A. Patrykiejew and S. Sokołowski, Phys. Rev. E, 81, 012501 (2010)], are possible. In one class of systems, the λ-line representing a continuous demixing transition in a liquid phase starts at the liquid side of either the vapor-liquid or liquid-solid coexistence. The second class involves the systems in which the λ-line begins at the liquid side of the vapor-liquid coexistence, in the lower critical end point, and then terminates at the liquid side of the liquid-solid coexistence, in the upper critical end point. It is also shown that in such systems the solid phase may undergo a demixing transition at the temperature above the upper critical end point.

Chiral separation on a model adsorbent with periodic surface heterogeneity

Optimization of enantioselectivity in heterogeneous catalysis and chiral chromatography is a challenging task for the production of enantiopure chemicals. Enantioselective adsorbents usually consist of a surface with chiral receptors being either chiral molecules linked to the surface or chiral pockets formed by molecular templating of the surface. In both cases, the enantioselectivity is controlled mainly by the strength of the receptor-enantiomer interaction, such that one-to-one correspondence is usually preserved. The authors use Monte Carlo calculations to show that this steric requirement is not a necessary condition for the effective separation of chiral molecules. In particular, they propose a way in which a chiral surface can be constructed by a suitable spatial distribution of active sites for which the classical concept of a chiral receptor is no longer useful. Their calculations indicate that the effectiveness of the separation is affected mainly by the difference in shape of the adsorption energy distribution functions corresponding to the enantiomers.

On the separation of nonadditive symmetric mixtures in nanoscopic slitlike pores: A simple model for racemic fluids

A grand canonical ensemble Monte Carlo simulation method is used to study the adsorption of nonadditive symmetric mixtures of Lennard-Jones spherical particles in nanoscopic slitlike pores. The walls of the pore are assumed to be formed by the parallel (100) planes of the model face centered cubic crystal of adjustable corrugation potential. It is demonstrated that depending on the nonadditivity effects in the mixture and the pore width the condensed phases formed inside the pore may have different structures. In particular, it is shown that the mixture may separate into layers containing only one component each and the stacking may depend on the pore width and properties of the mixture.

Kinetics and equilibrium of multicomponent adsorption on chiraly templated surfaces

In this contribution we propose a simple model of adsorption of a binary (racemic) mixture on a chiraly templated surface. As an example, the adsorption of a liquid mixture of enantiomers on a chiral stationary phase (CSP) is considered. In particular, we study the effect of the lateral interactions in the adsorbed phase on the kinetic and equilibrium isotherms of the enantiomers. Additionally, we investigate the influence of the composition of the surface on the performance of the CSP in the presence of the lateral interactions. To that end, the adsorption of the mixture is modeled by using Monte Carlo simulations as well as by applying an analytical approach involving rate equations coupled with the Mean Field Approximation (MFA). The predictions of the theory are found to be in good agreement with the results of the simulations.