Development and Applications of ReaxFF Methodologies for Electrolytes and Polymers
Open Access
- Author:
- Dasgupta, Nabankur
- Graduate Program:
- Engineering Science and Mechanics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- October 05, 2021
- Committee Members:
- Kristen Fichthorn, Outside Unit, Field & Minor Member
Charles Bakis, Major Field Member
Francesco Costanzo, Co-Chair of Committee
Adri van Duin, Co-Chair & Dissertation Advisor
Albert Segall, Program Head/Chair - Keywords:
- ReaxFF force field
molecular dynamics
electrolytes
polymers
mesoporous silica materials
bond boost - Abstract:
- The study of chemical reaction dynamics and solvation in liquids goes back to at least fifty years. Water is one such universal liquid which can dissolve a wide range of substances due to its polarity and ability to form hydrogen bonds with species like electrolytes and hydrophilic polymers. The presence of charge on electrolytes allow them to establish a hydrogen bonding network with water. Electrolytes can act either as “structure maker” or “structure breaker” in water. Polymers on the other hand are often neutral species that do not dissolve immediately in water. The dissolution of polymers is controlled by either the disentanglement of the chains or by diffusion of the chain through a boundary layer adjacent to the polymer-solvent interface. The solvation and reaction dynamics of these species in solvents can be greatly modulated by thermodynamic conditions. A thorough understanding of the interplay between thermodynamic conditions, solvation and reaction of these species in water at microscopic level is vital. A computational method is required that can simulate the complex physics and chemistry of complex solvation process and capture the possible reaction pathways without any prior user input. The ReaxFF reactive force field method is one such method. ReaxFF can simulate these processes in large (>> 1000 atoms) dynamic systems and can provide atomistic-scale insights of the critical reaction steps. In order to elucidate these phenomena, we studied the solvation and reactivity of ions and polymers in ambient and supercritical water using ReaxFF molecular dynamics simulations in the following five areas: (a) ReaxFF molecular dynamics simulations have been performed to study the effect of cations Li+, Na+ and K+ and anion Cl- on the structural and dynamical properties of water, using the force field recently developed by Fedkin and co-workers. The structural relationship of ion and water has been analyzed from the radial distribution function and angular distribution. Comparisons of ReaxFF angle variation of ions and water within the first solvation shell were made and found to be in good agreement with experimental and computational literature. The disruption of hydrogen bond network of water by ions is elucidated by ion-water residence times, water-water hydrogen bond dynamics and reorientational dynamics. ReaxFF diffusion coefficient and residence times of electrolyte water system were compared with ab initio and non-reactive potentials to analyze the difference in dynamics. We gained insight into the ion interaction with water and how it can accelerate or decelerate water dynamics. ReaxFF outlines the formation and dissolution of metal hydroxides and metal chlorides over the course of simulation to explain the diffusion dynamics of water in salt solutions, allowing us to elucidate the impact of concentration on the self-diffusivity of water and ions in solutions, and to reveal that this effect always decreases the mobility and is not at all ion-specific. (b) ReaxFF molecular dynamics simulations of alkali metal-chlorine pairs in different water densities at supercritical temperature (700 K) have been performed to elucidate the structural and dynamical properties of the system. Radial distribution function and the angular distribution function explains the inter-ionic structural and orientational arrangements of atoms during the simulation. Coordination number of water molecules in the solvation shell of ions increases with increase in radius of ions. We find the self-diffusion coefficient of metal ions increases with decrease in density at supercritical conditions due to formation of voids within the system. The hydrogen bond dynamics has been analyzed by tracking the residence time distribution of various ions, which shows Li+ having the highest water retaining capability. The void distribution within the system has been analyzed by using Voronoi polyhedra algorithm providing an estimation of void formation within the system at high temperatures. We see the formation of salt clusters of Na+ and K+ at low densities due to the loss of dielectric constants of ions. The diffusion of ions gets altered dramatically due to the formation of voids and nucleation of ions in the system. (c) The reactivity and mechanical properties of poly(1,6-hexanediol-co-Citric acid) via accelerated ReaxFF molecular dynamics simulations. We implement an accelerated scheme within the ReaxFF framework to study the hydrolysis reaction of the polymer, which is provided with a sufficient amount of energy, known as the restrain energy, after a suitable pre-transition state configuration is obtained to overcome the activation energy barrier and the desired product is obtained. The validity of the ReaxFF force field is established by comparing the ReaxFF energy barriers of ester and ether hydrolysis with benchmark DFT values in literature. We perform chemical and mechanical degradation of polymer chain bundles at 300 K. We find that ester hydrolyzes faster than ether due to lower activation energy barrier of the reaction. The selectivity of the bond-boost scheme has been demonstrated by lowering the boost parameters of the accelerated simulation which almost stops the ether hydrolysis. We find that the tensile modulus of the polymers increase with increase in strain rates which shows that polymers show a strain dependent behavior. The tensile modulus of the Polyester-ether is higher than Polyester but reaches yield-stress faster than Polyester. This makes Polyester more ductile than Polyester-ether. (d) A new ReaxFF reactive force field has been developed for metal carbonate systems including Na+, Ca2+, and Mg2+ cations and the CO32- anion. This force field is fully transferable with previous ReaxFF water and water/electrolyte descriptions. The Me-O-C three-body valence angle parameters and Me-C non-reactive parameters of the force field have been optimized against quantum mechanical calculations including equations of states, heats of formation, heats of reaction, angle distortions and vibrational frequencies. The new metal carbonate force field has been validated using molecular dynamics simulations to study solvation and reactivity of metal cation and carbonate anions in water at 300 K and 700 K. The coordination radius and self-diffusion coefficient show good consistency with existing experiments and simulations results. The angular distribution analysis explains the structural preference of carbonate ions to form carbonates and bicarbonates, where Na+ predominantly forms carbonates due to lesser angular strain, while Ca2+ and Mg2+ prefer to form bicarbonates monodentate in nature. The formation and dissolution of bicarbonates and carbonates in the solution were explored on the basis of protonation capability in different systems. The nucleation phenomenon of metal carbonates at ambient and supercritical conditions is explained from the perspective of clusters formation over time: Ca2+ ions can form prenucleation clusters at ambient temperature but shows a saturation with temperature, whereas Na+ and Mg2+ ions show rapid increase in cluster size and amount upon increasing time and temperature. (e) A combination of non-reactive (OPLS-AA) and reactive (ReaxFF) molecular dynamics simulations is used to synthesize and post-heal mesoporous silica materials. The self-assembly of pluronic P123 polymer in water was studied using OPLS-AA simulations. We find a micelle formation in water showing a proper segregation of the hydrophilic and hydrophobic part in the water. Followed by it, the condensation of silica precursor around the micelle is performed by accelerated ReaxFF molecular dynamics. We find agglomeration of silica precursor in the system. The carbonaceous micelle along with water is then calcinated from the system which induces pores within the system. Post-healing synthesis of these mesoporous silica materials are performed by carbonizing polymers inside silica pores which can act as coatings on the silica inner pore. The formation of tar and gaseous compounds on carbonization is deduced from the carbon ring analysis and reaction analysis. Finally, the formation of carbon fibers inside silica pore on prolonged pyrolysis is observed from the hybridization analysis. Polyethylene and Lignite serve as the two best candidates for coating as the carbon fiber network obtained from it serves as the best blocking agent for unreacted silica precursors inside the pore.