How calcium and magnesium ions affect the stability of asphalt with anionic emulsifiers: A molecular dynamics study

Based on molecular dynamics, modelling and analysis of the state of salt-free ionic emulsified asphalt system with the addition of calcium ions and magnesium ions (Ca2+and Mg2+, respectively). The effect of calcium and magnesium ions on the stability of sodium dodecyl sulfate (SDS)-emulsified asphalt was investigated. To determine the effect of the metal cations on the stability of anionic emulsified asphalt, the interfacial morphological distribution, interaction of the emulsifier with water molecules, and mobility of water molecules in the emulsifier head group/metal cations was investigated. The results of the simulation analysis show that, compared to a salt-free environment, the addition of Ca2+ or Mg2+ to a salt-containing system induces aggregation between the head groups of the SDS emulsifier molecules and reduces the thickness of the SDS emulsifier headgroup distribution along the Z-axis. This, in turn, weakens the stability of the anionic emulsified asphalt. Compared to the Ca2+-emulsified asphalt system, the Mg2+-emulsifier headgroup molecules are more easily bound to water molecules and more difficult to dissociate from them. Thus, the addition of Mg2+ is more likely to reduce the stability of the emulsion. Furthermore, the results of the simulated data analysis indicate that the addition of cations to the emulsified asphalt system, particularly Mg2+, reduces the mobility of the water molecules surrounding the head group, thereby reducing the stability of the emulsion. The results of the molecular simulations are consistent with the experimental results in the literature. Thus, the validity of using molecular dynamics to analyze the effect of metal cations on anionic emulsified asphalt was verified.