Authors: Murilo Sodré Marques
Source: LA Referencia (Red Federada de Repositorios Institucionales de Publicaciones Científicas) (article, 2020-01-01)
Field: Chemistry — Physical and Theoretical Chemistry

Molecular modeling of fluids : colloids, water, alcohols and their mixtures Core-softened fluids comprise a large number of systems whose molecular constituents have competitive interactions. The most prominent example is water whose molecules tend to form hydrogen bonds. The competition between these two conformations leads to a series of anomalous behaviors which can manifest themselves both in colloids and in mixtures of water and short-chain alcohols. The successful description of colloidal systems by two-length-scale potentials is related to the composition of these systems, usually made up of molecular subunits forming a compact central agglomeration and a less dense peripheral area with a prevalence of entropic effects. This core-corona structure can be described by a hard core and a soft corona that well represents the competition between enthalpy and entropic effects. Alcohol molecules bring even more competition to this complex scenario, since the majority do not have spherical symmetry, therefore incorporating anisotropic effects, and because they carry, in addition to the nonpolar carbonic part, hydrophilic hydroxyl groups, capable of forming hydrogen bonds, giving them a high degree of miscibility with water. This thesis focuses on the intricate molecular mechanisms that underlie the behavior of these complex fluids. To this end, we perfom large-scale Molecular Dynamics simulations of distinct systems. First, we analyzed the behavior of a two-dimensional system of polymer-grafted nanoparticles. Effective core-softened potentials were obtained from coarse-grained simulations for two cases: one in which the polymers are free to rotate around the nanoparticle core and a second in which the polymers are fixed, with a 90∘ angle between them. Due to the competition in the system we observe waterlike anomalies, such as the temperature of maximum density (TMD) and the diffusion anomaly. We observe that for the fixed polymers case the waterlike anomalies originate in the competition between the characteristic length scales in the potential, while for the freely rotating case, the anomalies arise due to a smaller region of stability in the phase diagram, with no competition between scales. Subsequently, we propose a two-site model of tert-butanol in which the interactions involving hy- drogen bonding are represented by a Stillinger-Weber potential. The potential model is optimized to yield a reasonable description of experimental excess enthalpies and volumes over the whole composition range of the mixture. This model is able to reproduce the maximum in the change of the temperature of maximum density for very low alcohol mole fractions, followed by a considerable decrease until the density anomaly itself disappears. We have correlated this behavior with changes in the local structure of water and compared it with the results of all-atom simulations of water/tert-butanol mixtures. The third studied system was a core-softened representation of water-alcohol mixtures. This approach allowed us to explore the connection between the spontaneous crystallization observed in the supercooled regime in the vicinity of the liquid-liquid phase transition (LLPT), and the density anomaly by performing extensive Molecular Dynamics simulations of a model mixture of coresoftened water and methanol. Our results illustrate the relation between the vanishing of the density anomaly and an increase in the temperature of spontaneous crystallization. This peculiar feature illustrates how fine-tuning the competitive interactions determines the anomalous behavior of water/alcohol mixtures. Finally, we explored the supercooled regime of pure water and mixtures of water and short-chain alcohols (methanol, ethanol and propanol) using core-softened potential models. Our aim is to understand the influence of chain size on the density anomaly, the liquid-liquid phase Our aim is to understand the influence of chain size on the density anomaly, the liquid-liquid phase transition and on the polymorphism observed in these models. Colloid ; Chemistry ; Environmental science ; Organic chemistry Various Chemistry Research Topics