RESUMEN
This research addresses interaction mechanisms of water-soluble polymers used as soil mineral stabilizers via atomistic classical molecular dynamics (MD). Specifically, this study addresses polyelectrolyte interactions with kaolinite, a ubiquitous clay mineral, in soils. The two water-soluble polymeric species evaluated are PSS: poly(4-sodium styrenesulfonate) and PDADMAC: poly(diallyldimethylammonium chloride). The primary focus is the evaluation of water migration through a polymer-kaolinite composite system, the resulting molecular arrangement and interactions, and the extents of water migration through the polymeric phase-binding kaolinite interfacial planes. Mean square displacement (MSD) analysis was used to quantify the motion of the system species from the MD trajectories by calculation of self-diffusion coefficients and comparison of the curves obtained. The MD results indicate that water infiltrates the polyelectrolyte phase adhering to the mineral interfaces. Nevertheless, the MSD analysis results indicate a 55.8% reduction in water self-diffusion with respect to pure mineral-confined water. This is a compelling indication that polyelectrolytes can hinder water movement. Most importantly, MSD analysis of both polyelectrolyte species shows that the movement of the chains is negligible relative to that of water. These results strongly suggest that the movement of polymer phases is restricted only to local chain mobility and a rather bound state to the mineral surfaces prevails.
RESUMEN
Hydrated lime is widely used as a mineral filler to improve several properties of bituminous materials such as reducing the susceptibility of the composite to moisture-induced damage. Although experimental evidence supports the efficacy of using hydrated lime as a mineral filler, the molecular scale mechanism of reactivity of hydrated lime within the bitumen to reduce moisture damage is not understood. This is important when considering the durability of structural applications of bituminous materials such as asphalt concrete pavements subjected to both environmental and loading extremes. In this study, the interaction between hydrated lime and the key molecular building blocks of bitumen is modeled using density functional theory and compared against analogues of other common fillers such as calcite and quartz. Free energies of dissociation (ΔG dissoc) are calculated, and the nature of the bonds is characterized with contour maps of the Laplacian of the electron density. Hydrated lime is capable of reacting with specific functional groups in bitumen moieties and developing strong, water-resistant complexes. Among the functional groups investigated, carboxylic acids are the preferential reaction sites between hydrated lime and the bitumen moieties. Values as high as ΔG dissoc = +49.42 kcal/mol are reported for hydrated lime with water as the surrounding solvent. In contrast, analogues of calcite (ΔG dissoc = +15.84 kcal/mol) and quartz (ΔG dissoc = +4.76 kcal/mol) are unable to chemically react as strongly as hydrated lime in the presence of water. Contour maps of the Laplacian of the electron density indicate that the bonds between hydrated lime and model asphalt moieties are of an ionic nature. The atomistic modeling results correlate with thermodynamic calculations derived from experimental constants and are consistent with infrared spectrometric data.