Alejandro Cuetos

Phase behavior of hard colloidal platelets using free energy calculations

We investigate the phase behavior of a model for colloidal hard platelets and rigid discotic molecules: oblate hard spherocylinders (OHSC). We perform free energy calculations using Monte Carlo simulations to map out the phase diagram as a function of the aspect ratio L∕D of the particles. The phase diagram displays a stable isotropic phase, a nematic liquid crystal phase for L∕D≤0.12, a columnar phase for L∕D≲0.3, a tilted crystal phase for L≲0.45, and an aligned crystal phase for L∕D≳0.45. We compare the results to the known phase diagram of hard cut spheres. Thin cut spheres are almost cylinder-shaped, while the interactions between real discotic mesogens and colloidal platelets are more consistent with the toroidal rims of the OHSC. Since the shapes of the OHSC and the cut spheres are otherwise similar, the phase diagrams of the two types of particles are quite akin. However, the tilted crystal phase for OHSC, which is of a crystal type that is frequently found in experiments on disklike molecules, has not been found for hard cut spheres. Furthermore, although we have found a cubatic phase, it was shown to be definitely unstable, whereas the stability of the cubatic phase of cut spheres is still disputed. Finally, we also show that the phase boundaries differ significantly from those for cut spheres. These are remarkable consequences of a subtle change in particle shape, which show that for a detailed comparison with the phase behavior of experimental particles, the OHSC should be used as a model particle.

Monte Carlo study of liquid crystal phases of hard and soft spherocylinders

We report on a Monte Carlo study of the liquid crystal phases of two model fluids of linear elongated molecules: (a) hard spherocylinders with an attractive square-well (SWSC) and (b) purely repulsive soft spherocylinders (SRS), in both cases for a length-to-breadth ratio L*=5. Monte Carlo simulations in the isothermal–isobaric ensemble have been performed at a reduced temperature T*=5 probing thermodynamic states within the isotropic (I), nematic (N), and smectic A (Sm A) regions exhibited by each of the models. In addition, the performance of an entropy criterion to allocate liquid crystalline phase boundaries, recently proposed for the isotropic–nematic transition of the hard spherocylinder (HSC) fluid, is successfully tested for the SWSC and the SRS fluids and furthermore extended to the study of the nematic–smectic transition. With respect to the more extensively studied HSC fluid, the introduction of the attractive square well in the SWSC model brings the I–N and N–Sm A transitions to higher pressur...

Columnar phases of discotic spherocylinders

The liquid crystal phase diagram of the discotic hard spherocylinder fluid is investigated by Monte Carlo simulations. Thickness-to-diameter aspect ratios within L/D=0.2-0.5 are considered. Three distinct columnar phases are found, namely, a hexatic interdigitated phase (D(hi)), a hexatic ordered phase (D(ho)), both with long-range spatial correlations, and a hexatic disordered phase (D(hd)), in which the columns become fluidlike. Local domains of stacked particles are also observed in the isotropic phase. The stability of the D(ho) and D(hd) phases is favored with increasing anisotropy of the particle shape. As a consequence, the packing fraction versus the aspect ratio representation of the phase diagram features D(ho)-D(hd)-I and D(hi)-D(ho)-I triple points. The study involved the development of an efficient algorithm to compute the shortest distance between two oblate spherocylinder particles. The study provides a general coarse-grain methodology to explore discotic behavior, with fundamental advantages against alternative molecular models.

A novel orientation-dependent potential model for prolate mesogens

An intermolecular potential is introduced for the study of molecular mesogenic fluids. The model combines distinct features of the well-known Gay-Berne and Kihara potentials by incorporating dispersive interactions dependent on the relative pair orientation to a spherocylinder molecular core. Results of a Monte Carlo simulation study focused on the liquid crystal phases exhibited by the model fluid are presented. For the chosen potential parameters, molecular aspect ratio L*=5 and temperatures T*=2, 3, and 5, isotropic, nematic, smectic-A, and hexatic phases are found. The location of the phase boundaries as well as the equation of state of the fluid and further thermodynamical and structural parameters are discussed and contrasted to the Kihara fluid. In comparison to this latter fluid, the model induces the formation of ordered liquid crystalline phases at lower packing fractions and it favors, in particular, the appearance of layered hexatic ordering as a consequence of the greater attractive interaction assigned to the parallel side-to-side molecular pair configurations. The results contribute to the evaluation of the role of specific interaction energies in the mesogenic behavior of prolate molecular liquids in dense environments.

Columnar phases of discotics with orientation-dependent interactions

The liquid crystal phase diagram of fluids of rigid discotics with soft interactions has been investigated by means of Monte Carlo simulations. The particles are modeled by spherocylinders or Gay-Berne ellipsoids with thickness/diameter aspect ratios of L/D=0.2. The study includes a variety of pair interaction potentials, featuring different energetic dependencies on the orientation of the particles. Three distinct types of models are considered: (i) models with a homogeneous interaction around the molecular core, (ii) models favoring stacked pair configurations, and (iii) models favoring edge-to-edge configurations. The stability and internal structure of the isotropic, nematic, and the different hexatic columnar phases exhibited by these fluids are discussed. The results indicate that the spherocylinder and ellipsoidal models differ in fundamental trends of their phase behavior. The spherocylinder fluids display more extended ranges of stability and longer pair correlation lengths in the columnar phases than the ellipsoidal models. As a consequence, as opposed to ellipsoids, the nematic phase for spherocylinders with the title aspect ratio tends to be entropically suppressed, even under favorable energetics.

Rotational viscosities of Gay-Berne mesogens

Rotational viscosities γ1 are calculated for three Gay-Berne models for a wide range of state points in the nematic phase. There was a strong density dependence in the results, with γ1 increasing with increasing density. Away from the clearing point, the temperature dependence of γ1 was described by simple Arrhenius-like behaviour. A comparison of the values of γ1 and the Arrhenius activation energies with real mesogens pointed to a number of problems with the Gay-Beme potential, when used as a model for real mesogenic systems.

Sedimentation of charged colloids: the primitive model and the effective one-component approach.

Sedimentation-diffusion equilibrium density profiles of suspensions of charge-stabilized colloids are calculated theoretically and by Monte Carlo (MC) simulations, both for a one-component model of colloidal particles interacting through pairwise screened-Coulomb repulsions and for a three-component model of colloids, cations, and anions with unscreened-Coulomb interactions. We focus on a state point for which experimental measurements are available [C. P. Royall, J. Phys.: Condens Matter 17, 2315 (2005)]. Despite the apparently different picture that emerges from the one- and three-component models (repelling colloids pushing each other to high altitude in the former, versus a self-generated electric field that pushes the colloids up in the latter), we find similar colloidal density profiles for both models from theory as well as simulation, thereby suggesting that these pictures represent different viewpoints of the same phenomenon. The sedimentation profiles obtained from an effective one-component model by MC simulations and theory, together with MC simulations of the multicomponent primitive model are consistent among themselves, but differ quantitatively from the results of a theoretical multicomponent description at the Poisson-Boltzmann level. We find that for small and moderate colloid charge the Poisson-Boltzmann theory gives profiles in excellent agreement with the effective one-component theory if a smaller effective charge is used. We attribute this discrepancy to the poor treatment of correlations in the Poisson-Boltzmann theory.

Simulation study of discotic molecules in the vicinity of the isotropic–liquid crystal transition

The equilibrium and microscopic properties of systems of discotic molecules have been investigated with Monte Carlo (MC) simulations. The study focuses on the behaviour of the fluid in the isotropic phase in the vicinity of the first liquid crystal transition, which involves either a nematic or a columnar phase. The molecules are modelled by rigid oblate spherocylinders with various types of interaction potentials. Molecular thickness/diameter ratios within L/D = 0.1–0.5 are considered. The MC equations of state are compared with theoretical predictions for hard convex bodies, based on molecular shape and virial expansions. A good agreement is found for the hard spherocylinder system, although discrepancies arise for L/D < 0.4 at sufficiently large packing fraction. Particular efforts are also devoted to characterising the formation of domains of stacked molecules in the isotropic phase for the different repulsive and attractive interaction models.

Gibbs ensemble simulation of the vapour-liquid equilibrium of square well spherocylinders

Gibbs ensemble Monte Carlo simulations have been performed for systems of square-well spherocylinders of different length-to-breadth ratio. The results are used to test a recent perturbation theory proposed for this kind of system. In addition, the results are compared to similar simulations performed for a Kihara fluid of elongated molecules. An unexpected good agreement is found for the coexistence thermodynamic and structural properties of both model fluids, hence suggesting that the hard spherocylinder plus square-well interaction should be considered as a reference potential for a perturbative treatment of more complex fluid models.

Experimental Evidence of the Relevance of Orientational Correlations in Photoinduced Bimolecular Reactions in Solution

A major problem in the extraction of the reaction probability in bimolecular processes is the disentanglement from the influence of molecular diffusion. One of the strategies to overcome it makes use of reactive solvents in which the reactants do not need to diffuse to encounter each other. However, most of our quantitative understanding of chemical reactions in solution between free partners is based on the assumption that they can be approximated by spheres because rotation averages their mutual orientations. This condition may not be fulfilled when the reaction takes place on time scales faster than that of molecular reorientation. In this work, the fluorescence quenching of two very similar polyaromatic hydrocarbons with different electric dipole moments is measured. The concentration of a liquid electron-donating quencher is varied from very dilute solutions to pure quencher solutions. In both cases, the thermodynamics of the reactions are very similar and, according to the Marcus expression, the kinetics are expected to proceed at similar rates. However, one of them is 10 times faster in the pure quencher solution. This difference starts at relatively low quencher concentrations. An explanation based on the fluorophore-solvent dipole-dipole interaction and the consequent orientational solvent structure is provided. The orientational correlation between fluorophore and quencher is calculated by means of computer simulations. Important differences depending on the fluorophore dipole moment are found. The kinetics can be explained quantitatively with a reaction-diffusion model that incorporates the effects of the presence of the dipole moment and the rotational diffusion, only in the highest quencher concentration case, but not in dilute solutions, most likely due to fundamental limitations of the kinetic theory.