Stability of dodecyl sulfate emulsified asphalt: The overlook effect of dissociated counterions

Abstract

As an environmentally friendly technology, emulsified asphalt can conserve resources and reduce hazardous fuming during paving, which is pivotal for promoting sustainable road engineering. However, asphalt emulsions with poor stability resulted in performance degradation especially after long-term storage or long-distance transport. The dissociation of anionic asphalt emulsions generates counterions that can potentially induce the emulsifier agglomeration at the asphalt-water interface, subsequently resulting in emulsion destabilization. In this study, two dodecyl sulfate emulsifiers (i.e., sodium lauryl sulphate (SDS) with Na+ counterion, and potassium lauryl sulfate (PDS) with K+ counterion) were employed in the production of asphalt emulsions. The effect of counterion type and concentration on the stability of dodecyl sulfate emulsified asphalt were investigated using storage stability, particle size distribution, and differential scanning calorimetry (DSC) tests. Molecular simulation was used to elucidate the mechanism of how the counterions affected the stability of dodecyl sulfate emulsified asphalt. Due to the fact that K+ counterion has a smaller electrostatic potential energy and a weaker electrostatic interaction with the O atoms in water molecules, PDS emulsified asphalt displayed superior homogeneity, storage stability, and heat stability. The emulsifier molecules adsorbed at the asphalt-water interface diffused into the water phase due to electrostatic attraction when the counterion doping exceeded the crucial value (SDS: 5 wt%, PDS: 4 wt%). Subsequently, the stable monolayer film at the interface dissolved, lowering the stability of the emulsified asphalt. It was challenging for SDS to compress the bilayer created by the headgroup since a limited number of Na+ counterions were accumulated near the headgroup. This hindered SDS from forming a monolayer at the asphalt-water interface, potentially explaining its inferior interfacial stability. Overall, our findings highlight the critical role of counterions in determining the stability of dodecyl sulfate emulsified asphalt, with K+ counterion in PDS emulsified asphalt showing superior stability.