Underlying Molecular Interactions between Sodium Montmorillonite Clay and Acidic Bitumen

Moisture damage is one of the common distresses in pavements; it is mainly associated with failure of interfacial bonds between bitumen and the stone aggregates that form the skeleton of pavement. The presence of acid compounds at the interface of bitumen and stone aggregates, especially siliceous aggregates, is implicated as a potential cause of interfacial failure, especially in the presence of moisture. This study investigates the merits of doping bitumen with sodium montmorillonite clay (MMT) as a sink for acid molecules, preventing them from reaching the interface of bitumen and stone aggregates. In a case study performed on bitumen doped with hexadecanoic acid, atomic force microscopy images clearly showed excessive growth of acid microstructures around the MMT particles, accompanied by a notable reduction of acid crystallization at the interface of bitumen and stone aggregates, indicating that MMT filler could be used to adsorb alkyl acid compounds. The high adsorption capacity of sodium montmorillonite clay was studied through quantum-based density functional theory (DFT) calculations. Based on the DFT results, three adsorption sites were identified for the mineral clay: hydroxyl groups protruding from the broken edges of clay, siloxane sites of the basal surface, and exchangeable cations in interlayer space. The adsorption on to edge surfaces is well stabilized by an extensive H-bonding network in this domain via the silanol groups (Si-OH). The dipole-dipole interactions between an acid and a siloxane surface carrying a negative charge make siloxane an active site for trapping acid molecules from bitumen binder. The interaction of acid molecules with the surface through Na⁺ cations was associated with a considerable adsorption energy, mostly due to the ion-dipole interactions between the oxygen of a carbonyl group (C=O) and an interlayer cation (Na⁺). Self-assembly of the acid molecules induced by H-bonding interactions between the head groups and van der Waals interactions between the tail alkyl groups reinforces the adsorption to form mono- or polylayers of the organic compound on the mineral surface. In agreement with recent findings, our DFT calculations corroborate the capability of saturated fatty acid to intercalate Na-montmorillonite clay sheets. This shows that acid molecules are adsorbed between adjacent clay layers, highlighting the role of clay as a highly efficient sorbent for saturated organic acids. The study results provide insights about the mechanisms by which clay minerals adsorb acids and prevent their migration to the bitumen-aggregate interface, consequently enhancing interfacial strength against moisture damage.