Materials with nanopores of regular and tunable morphology are finding emerging applications in different fields, besides their traditional uses in separations and catalysis. As an example, templated mesoporous silicas MCM-41 and SBA-15 are being used in microelectronics as low dielectric constant materials to insulate integrated circuits and microchips, and as templates for fabrication of opto-electronic nanodevices, ordered carbon materials and nanowires. In those processes the host molecules often experience phase changes inside the nanopores; moreover, gas adsorption is a key component of the characterization of the porous materials. The interpretation of experimental results is complicated by issues such as metastability and an incomplete characterization of the pore morphology. As a result, molecular simulations have been invaluable in the study of confined systems. Here we present Monte Carlo simulation results for capillary condensation and freezing of fluids inside realistic models of carbon nanotubes, MCM-41 and SBA-15. A rich phase behavior with multiple transition temperatures and different phases was observed for these confined systems. Our results show that intermolecular interactions and pore morphology have a profound influence on the gas-liquid and freezing transitions in confinement.
Our second research topic is based on the driven assembly of nanoparticles immersed in liquid crystals. These systems have attracted great attention for their potential uses in optical sensors for chemicals and biomolecules, as well as for microemulsions and colloidal dispersions with tailored physical properties. The binding of analytes at the surface of the particles can perturb the local ordering of the liquid crystal, triggering the formation of inhomogeneous textures that can be detected using a microscope and polarized light. The elastic distortions of the liquid crystal also give rise to long-range interparticle interactions and spatial reorganization of the colloids. Increases in selectivity and sensitivity of these sensors, as well as controlled colloidal assemblies, can be obtained by tuning the size and shape of the particles, by engineering their surfaces, or by manipulating the physical properties of the liquid crystal. Optimization of these applications requires a theoretical formalism linking macroscopic measurements (collective optical properties) with events occurring at smaller length scales (particle aggregation, liquid crystal reordering and binding events at the surface of the particles). Here we present numerical simulations for systems of spherical colloids dispersed in a liquid crystal. We have adopted a hybrid strategy that includes particle-based modeling of the colloids and a continuous field treatment of the liquid crystal. The effect of several physical variables is analyzed and discussed. We also show results for anisotropic, spherocylindrical particles immersed in liquid crystals, for which it was found that the interparticle energies were up to three times stronger than those observed for spherical colloids of comparable diameters.