# Anti-Aging Performance and Action Mechanism of Asphalt Modified by Composite Modification
## 2 Materials and Methodology
### Methodology
#### 2.2.6 Molecular Dynamics Simulation  
(1) Asphalt molecular model  
Molecular dynamics (MD) simulations are employed to explore the effects of composite modifiers on the aging resistance of asphalt. Using the Materials Studio 2023.1 software, both base asphalt and modified asphalt models are constructed. These models are optimized to achieve equilibrium states. The analysis of the aging resistance of composite-modified asphalt is based on parameters such as solubility parameters, free volume, and diffusion coefficients. The AAA-1 four-component asphalt molecular model (Figure 2) is selected for molecular dynamics simulations [20]. The molecular formulas and quantities of the components in the AAA-1 model are detailed in Table 3.  
Figure 2: Molecular structure of each component [20]. (2)  
Modifier molecular models  
The UV-928 ultraviolet absorber has the molecular formula C\({}_{29}\)H\({}_{35}\)N\({}_{3}\)O. The model of the UV-928 ultraviolet absorber, constructed using the Visualizer module, is shown in Figure 3a. Hydrotalcite features a unique layered structure with the general formula \([M_{1}^{2+}{}_{x}M_{x}^{3+}(OH)_{2}](A^{n}\ \ )_{x/n}mH_{2}O\). The model of hydrotalcite is depicted in Figure 3b [10]. (3) Aging Resistance Evaluation Criteria Based on Molecular Dynamics  
In this study, the mean square displacement (MSD) curves for both base and modified asphalt were calculated. Based on these curves, the diffusion coefficients for each type of asphalt were also computed. By comparing the diffusion displacements and diffusion rates of the asphalt molecules, the study analyzes the impact of the composite modification effects on the aging resistance of asphalt. The specific evaluation criteria are as follows:  
The solubility parameter is a measure of intermolecular forces and is commonly used to assess the compatibility between materials. A smaller difference in solubility parameters between materials indicates better compatibility and higher thermodynamic stability [21]. The cohesive energy density represents the energy required to overcome all intermolecular forces within 1 mole of a substance and is a physical quantity characterizing the strength of intermolecular interactions. The square root of the cohesive energy density is the solubility parameter (8), calculated as follows [22]. In this study, the solubility parameters of base asphalt, ultraviolet absorber, hydrotalcite, and composite modifiers are calculated and compared. This comparison helps to investigate the differences in solubility parameters between various modifiers and asphalt, clarifying the impact of composite modification on the thermal stability of modified asphalt. \[\delta=\sqrt{\frac{E_{coh}}{V}} \tag{3}\]  
Figure 3: Molecular modeling of modifiers [10]. \begin{table}
\begin{tabular}{c c c c} \hline
**Components** & **Molecule Name** & **Molecular Formula** & **Molecular Number** \\ \hline \multirow{4}{*}{Saturates} & Hopanoid & C\({}_{30}\)H\({}_{62}\) & 4 \\  & Squalane & C\({}_{35}\)H\({}_{62}\) & 4 \\  & Dioctylcyclohexynnphthalene & C\({}_{35}\)H\({}_{44}\) & 13 \\  & Phenanthrenes & C\({}_{30}\)H\({}_{46}\) & 11 \\  & Trimethylbenzothiophenes & C\({}_{40}\)H\({}_{59}\)N & 15 \\  & Pyridine patchouli & C\({}_{40}\)H\({}_{60}\)S & 4 \\  & Trimethylphenylene & C\({}_{18}\)H\({}_{102}\) & 5 \\  & Thicoartenoids & C\({}_{36}\)H\({}_{57}\)N & 4 \\  & Quinolino patchouli & C\({}_{29}\)H\({}_{50}\)O & 4 \\  & Phenols & C\({}_{42}\)H\({}_{56}\)O & 3 \\  & Pyrroles & C\({}_{66}\)H\({}_{81}\)N & 2 \\  & Thiophenes & C\({}_{51}\)H\({}_{62}\)S & 3 \\ \hline \end{tabular}
\end{table} 
Table 3: Matrix asphalt model allocation ratios by group. where \(\delta\) is the solubility parameter, \(\left(\frac{1}{\text{cm}^{3}}\right)^{1/2}\); E\({}_{\text{coh}}\) is the cohesive energy density, J; V is the true density volume, cm\({}^{3}\). This study employs the free volume theory to assess the UV aging resistance of different asphalts.