José Antonio Moreno-Razo

Liquid-crystal-mediated self-assembly at nanodroplet interfaces

Technological applications of liquid crystals have generally relied on control of molecular orientation at a surface or an interface. Such control has been achieved through topography, chemistry and the adsorption of monolayers or surfactants. The role of the substrate or interface has been to impart order over visible length scales and to confine the liquid crystal in a device. Here, we report results from a computational study of a liquid-crystal-based system in which the opposite is true: the liquid crystal is used to impart order on the interfacial arrangement of a surfactant. Recent experiments on macroscopic interfaces have hinted that an interfacial coupling between bulk liquid crystal and surfactant can lead to a two-dimensional phase separation of the surfactant at the interface, but have not had the resolution to measure the structure of the resulting phases. To enhance that coupling, we consider the limit of nanodroplets, the interfaces of which are decorated with surfactant molecules that promote local perpendicular orientation of mesogens within the droplet. In the absence of surfactant, mesogens at the interface are all parallel to that interface. As the droplet is cooled, the mesogens undergo a transition from a disordered (isotropic) to an ordered (nematic or smectic) liquid-crystal phase. As this happens, mesogens within the droplet cause a transition of the surfactant at the interface, which forms new ordered nanophases with morphologies dependent on surfactant concentration. Such nanophases are reminiscent of those encountered in block copolymers, and include circular, striped and worm-like patterns.

Liquid crystal nanodroplets, and the balance between bulk and interfacial interactions

Molecular dynamics simulations of a coarse grain model are used to explore the morphology of thermotropic liquid crystal nanodroplets. The characteristic length of the droplets is such that different contributions to the energy, including interfacial and bulk-like terms, have comparable magnitudes. Depending on the relative strength of such contributions, a wide variety of mesophases can be identified. These range from a completely disordered isotropic phase at elevated temperatures, to ordered radial and smectic phases at low temperatures. Bipolar, uniaxial and axial phases are also observed. Our results suggest that according to the ratio between perpendicular and planar anchoring strengths, an isotropic–radial transition may occur through several intermediate phases. In contrast, a direct bipolar–radial transition is never observed. Our results are summarized in the form of a generic phase diagram for spherical nanodroplets as a function of anchoring strength. The diagram exhibits a number of common features with phase transitions that have been observed in experiments with larger, micron-sized droplets. Perhaps more importantly, it serves to emphasize the balance that exists in nanodroplets between surface and bulk interactions, droplet size and temperature, and how that balance influences the behavior of the system.

Phase and interfacial behavior of partially miscible symmetric Lennard-Jones binary mixtures

We have carried out extensive equilibrium molecular-dynamics simulations to study quantitatively the topology of the temperature versus density phase diagrams and related interfacial phenomena in a partially miscible symmetric Lennard-Jones binary mixture. The topological features are studied as a function of miscibility parameter, alpha = epsilonAB/epsilonAA. Here epsilonAA = epsilonBB and epsilonAB stand for the parameters related to the attractive part of the intermolecular interactions for similar and dissimilar particles, respectively. When the miscibility varies in the range 0 < alpha < 1, a continuous critical line of consolute points Tcons(rho)--critical demixing transition line--appears. This line intersects the liquid-vapor coexistence curve at different positions depending on the values of alpha, yielding mainly three different topologies for the phase diagrams. These results are in qualitative agreement to those found previously for square-well and hard-core Yukawa binary mixtures. The main contributions of the present paper are (i) a quantitative analysis of the phase behavior and (ii) a detailed study of the liquid-liquid interfacial and liquid-vapor surface tensions, as function of temperature and miscibility as well as its relationship to the topological features of the phase diagrams.

Analytical equation of state with three-body forces: Application to noble gases

We developed an explicit equation of state (EOS) for small non polar molecules by means of an effective two-body potential. The average effect of three-body forces was incorporated as a perturbation, which results in rescaled values for the parameters of the two-body potential. These values replace the original ones in the EOS corresponding to the two-body interaction. We applied this procedure to the heavier noble gases and used a modified Kihara function with an effective Axilrod-Teller-Muto (ATM) term to represent the two- and three-body forces. We also performed molecular dynamics simulations with two- and three-body forces. There was good agreement between predicted, simulated, and experimental thermodynamic properties of neon, argon, krypton, and xenon, up to twice the critical density and up to five times the critical temperature. In order to achieve 1% accuracy of the pressure at liquid densities, the EOS must incorporate the effect of ATM forces. The ATM factor in the rescaled two-body energy is most important at temperatures around and lower than the critical one. Nonetheless, the rescaling of two-body diameter cannot be neglected at liquid-like densities even at high temperature. This methodology can be extended straightforwardly to deal with other two- and three-body potentials. It could also be used for other nonpolar substances where a spherical two-body potential is still a reasonable coarse-grain approximation.

Metastable liquid lamellar structures in binary and ternary mixtures of Lennard-Jones fluids.

We have carried out extensive equilibrium molecular dynamics simulations to investigate the liquid-vapor coexistence in partially miscible binary and ternary mixtures of Lennard-Jones fluids. We have studied in detail the time evolution of the density profiles and the interfacial properties in a temperature region of the phase diagram where the condensed phase is demixed. The composition of the mixtures is fixed, 50% for the binary mixture and 33.33% for the ternary mixture. The results of the simulations clearly indicate that in the range of temperatures 78<T<102 K-in the scale of argon-the system evolves towards a metastable alternated liquid-liquid lamellar state in coexistence with its vapor phase. These states can be achieved if the initial configuration is fully disordered-that is, when the particles of the fluids are randomly placed on the sites of an fcc crystal or the system is completely mixed. As temperature decreases these states become very well defined and more stables in time. We find that below 90 K, the alternated liquid-liquid lamellar state remains alive for 80 ns, in the scale of argon, the longest simulation we have carried out. Nonetheless, we believe that in this temperature region these states will be alive for even much longer times.

Computer simulations of strongly interacting dipolar systems : performance of a truncated Ewald sum

Using strongly interacting, three-dimensional dipolar fluids as model systems we compare computer simulation results obtained with the full, conventional Ewald summation with results based on a truncated Ewald sum. The truncation consists of entirely neglecting the Fourier part of the full Ewald expression, which saves a large amount of the otherwise required computational time. In order to test the truncated version we consider two types of dipole-driven phase transitions: the isotropic-to-ferroelectric transition of one-component dipolar soft sphere fluids, on one hand, and the demixing transition in mixtures of dipolar and neutral soft spheres, on the other. Comparing various thermodynamic and structural data we find that the truncated Ewald sum yields surprisingly accurate results even at large dipolar coupling strength and even when subtle quantities such as the dielectric constant and the pressure tensor are considered. Our work thus suggests that the truncated version can safely be used to obtain a first estimate of the properties of interest even under strongly coupled conditions.

Isotropic-nematic phase transition in the Lebwohl-Lasher model from density of states simulations.

Density of states Monte Carlo simulations have been performed to study the isotropic-nematic (IN) transition of the Lebwohl-Lasher model for liquid crystals. The IN transition temperature was calculated as a function of system size using expanded ensemble density of states simulations with histogram reweighting. The IN temperature for infinite system size was obtained by extrapolation of three independent measures. A subsequent analysis of the kinetics in the model showed that the transition occurs via spinodal decomposition through aggregation of clusters of liquid crystal molecules.

Molecular aspect ratio and anchoring strength effects in a confined Gay–Berne liquid crystal

Phase diagrams for Gay–Berne (GB) fluids were obtained from molecular dynamics simulations for GB(2, 5, 1, 2) (i.e. short mesogens) and GB(3, 5, 1, 2) (i.e. long mesogens), which yield isotropic, nematic, and smectic-B phases. The long-mesogen fluid also yields the smectic-A phase. Ordered phases of the long-mesogen fluid form at higher temperatures and lower densities when compared to those of the short-mesogen fluid. The effect of confinement under weak and strong substrate couplings in slab geometry was investigated. Compared to the bulk, the isotropic–nematic transition does not shift in temprature significantly for the weakly coupled substrate in either mesogen fluid. However, the strongly coupled substrate shifts the transition to lower temperature. Confinement induces marked stratification in the short-mesogen fluid. This effect diminishes with distance from the substrate, yielding bulk-like behaviour in the slab central region. Fluid stratification is very weak for the long-mesogen fluid, but the strongly coupled substrate induces ‘smectisation’, an ordering effect that decays with distance. Orientation of the fluid on the substrate depends on the mesogen. There is no preferred orientation in a plane parallel to the substrate for the weakly coupled case. In the strongly coupled case, the mesogen orientation mimics that of adjacent fluid layers. Planar anchoring is observed with a broad distribution of orientations in the weakly coupled case. In the strongly coupled case, the distribution leans toward planar orientations for the short-mesogen fluid, while a marginal preference for tilting persists in the long-mesogen fluid.

Separating the effects of repulsive and attractive forces on the phase diagram, interfacial, and critical properties of simple fluids

Molecular simulations in the canonical and isothermal-isobaric ensembles were performed to study the effect of varying the shape of the intermolecular potential on the phase diagram, critical, and interfacial properties of model fluids. The molecular interactions were modeled by the Approximate Non-Conformal (ANC) theory potentials. Unlike the Lennard-Jones or Morse potentials, the ANC interactions incorporate parameters (called softnesses) that modulate the steepness of the potential in their repulsive and attractive parts independently. This feature allowed us to separate unambiguously the role of each region of the potential on setting the thermophysical properties. In particular, we found positive linear correlation between all critical coordinates and the attractive and repulsive softness, except for the critical density and the attractive softness which are negatively correlated. Moreover, we found that the physical properties related to phase coexistence (such as span of the liquid phase between the critical and triple points, variations in the P-T vaporization curve, interface width, and surface tension) are more sensitive to changes in the attractive softness than to the repulsive one. Understanding the different roles of attractive and repulsive forces on phase coexistence may contribute to developing more accurate models of liquids and their mixtures.

Structure and Translational Diffusion in Liquid Crystalline Phases of a Gay-Berne Mesogen: A Molecular Dynamics Study

Structures and self-diffusion coefficients of Gay-Berne (GB) mesogens with parameterizations GB(3.0, 5.0, 2.0, 1.0) and GB(4.4, 20.0, 1.0, 1.0) were extracted from NVT Molecular Dynamics simulations. These parameterizations are commonly used in the study of mesogenic systems. Structural features of accessible phases were characterized through translational [\(g_{\parallel}(r_{\parallel})\)] and positional [\(g_{\perp}(r_{\perp})\)] radial distribution functions. Translational self-diffusion coefficients parallel (\(D_{\parallel}\)) and perpendicular (\(D_{\perp}\)) to the global director were determined. Upon cooling a mesogenic system with parameterization GB(3.0, 5.0, 2.0, 1.0), a solid-like phase forms (as deduced from diffusivity) without attaining a smectic phase. Instead, the GB(4.4, 20.0, 1.0, 1.0) parameterization yields a range of liquid crystalline phases that follows the sequence isotropic \(\to\) nematic \(\to\) smectic A \(\to\) smectic B, for which the smectic B phase exhibits small, but measurable diffusivity. Collectively, results point to the GB(4.4, 20.0, 1.0, 1.0) parameterization as being a better candidate in capturing the typical gamut of liquid crystalline phases.