Quantum-Chemistry Based Atomistic Modeling of Asphalt Aging Mechanism and Rehabilitation
Objectives
Asphalt aging is an extremely complicated physical-chemical process. This research is aimed at understanding and predicting chemical structure, reactivity, and spectroscopy through accurate density functional, fast semi-empirical, and reactive force field computer programs.
Methodology
Quantum-Chemistry based atomistic modeling and simulation method.
Specifically, this atomistic simulation employs distance-dependent bond-order functions to represent the contributions of chemical bonding to the potential energy. ReaxFF is a novel bond order potential that bridges quantum-scale and classical MD approaches by explicitly modeling bond activity (reactions) and charge equilibration.
ReaxFF uses a general relationship between bond distance and bond order on one hand and between bond order and bond energy on the other hand that leads to proper dissociation of bonds to separated atoms. Other valence terms present in the force field (angle and torsion) are defined in terms of the same bond orders so that all these terms go to zero smoothly as bonds break. In addition, ReaxFF has Coulomb and Morse (van der Waals) potentials to describe nonbond interactions between all atoms (no exclusions). These nonbond interactions are shielded at short range so that the Coulomb and van der Waals interactions become constant as Rij -> 0.
Initial configuration of the oxidation system
Average asphalt molecules model --- A simulation box including oxygen molecules at size (40X33X87)
Mix1 asphalt molecules model --- a simulation box including oxygen molecules at size (25X28X89)
Mix2 asphalt molecules model --- a simulation box including oxygen molecules at size (25X28X94)
Results and Discussion
Products: various kinds of final and middle products, e.g. HO, H2O2, H2O, CO2, CH4. From the plot, we can see that the number of aveasph molecules reduces, while with the decomposing of aveasph part of the molecules separated from the big one and C100, C80, C60, etc. appeared in the domain. Then, the small carbonhydrate molecules react with the oxygen and form new kinds of oxidatives. Thus accelerate the oxidation procedure.
A big picture of the oxidation system
A closer look at one end of the oxidation system Lignin
Lignin (recycled polymer material) Modified Asphalt for Anti-Oxidation
Chemical Formula of 2 types of Lignin ---- Lignin-1: C72H88O26;
Lignin-2: C62H76O23
2 types of lignin are added to asphalt_mix1 model
Reaxff atomistic simulation is performed to investigate the anti-oxidation effects
Lignin is a waste of food industry. Put it into asphalt is for the purpose of recycling/reuse waste materials
200 oxygen molecules were add to each side of the simulation box, then chemical oxidation is performed.
