Effect of PEG Molecular Structure on Interfacial Stabilization and Laboratory Stability of High-Internal-Phase Emulsified Asphalt

High-internal-phase emulsified asphalt (HIPEA), characterized by a high asphalt volume fraction, may reduce water demand relative to conventional emulsified asphalt. However, the relationship between polyethylene glycol (PEG) molecular structure, asphalt–water interfacial organization, and laboratory stability remains incompletely quantified. This study compared four model systems containing octadecyltrimethylammonium chloride (STAC) alone or STAC combined with PEG with ethylene oxide (EO) numbers of 22, 45, and 55 (P22, P45, and P55). Molecular dynamics (MD) simulations were combined with dispersion observations, fluorescence microscopy, storage monitoring, centrifugation, differential scanning calorimetry (DSC), and exploratory Pearson correlation analysis. The results showed that STAC alone did not produce a uniform emulsion under the tested preparation conditions, whereas the PEG-containing formulations formed oil-in-water structures. The initial mean droplet diameter decreased from 4.20 μm for P22 to 2.57 μm for P55. After 22 days, the mean diameters were 6.70, 6.20, and 5.42 μm for P22, P45, and P55, respectively, indicating less pronounced droplet growth for the longer PEG formulations under the tested storage conditions. Under the same centrifugation protocol, the reported separated continuous-phase mass decreased from 0.233 g for P22 to 0.059 g for P55, corresponding to a 74.7% reduction based on these mass values. The characteristic DSC peak temperature increased from 49.853 °C for P22 to 61.105 °C for P55; this shift indicates a change in thermal response but does not, by itself, establish improved overall thermal stability. The MD results suggested differences in interfacial organization among the constructed systems. Pearson analysis identified exploratory associations between molecular descriptors and macroscopic indicators, but its interpretation is limited by the four-system dataset and the lack of strict compositional normalization. Overall, the findings provide molecular-level insights into the tested STAC/PEG systems and their laboratory stability. Further composition-controlled simulations and pavement-related testing are required before drawing conclusions about field performance.