Article pubs.acs.org/JPCC
Cite This: J. Phys. Chem. C 2019, 123, 15513−15522
Underlying Molecular Interactions between Sodium Montmorillonite Clay and Acidic Bitumen Masoumeh Mousavi,† Elham H. Fini,*,‡ and Albert M. Hung† †
North Carolina A&T State University, 1601 E. Market St., Greensboro, North Carolina 27411, United States Arizona State University, 660 S. College Avenue, Tempe, Arizona 85287-3005, United States
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ABSTRACT: 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.
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INTRODUCTION An increasing body of evidence correlates the physicochemical characteristics of the bitumen in pavement to moisture susceptibility of the bitumen−stone interface. Moisture damage in the asphalt matrix is associated with the degradation of the mechanical properties of the material due to the presence of moisture, which results in adhesive failure at the bitumen−stone aggregate interface and/or cohesive failure within the binder.1−3 Moisture susceptibility of paving materials is strongly affected by the chemical composition of bitumen as well as the surface chemistry of stone aggregates: in aggregates, the hydrophilic nature of acidic material increases the susceptibility to moisture damage; and in bitumen, individual components such as carboxylic acids and sulfoxides reduce moisture resistance.4,5 Saturated alkane acids or fatty acids are a class of carboxylic acids readily found in bitumen. Most natural fatty acids are primarily built from a two-carbon unit, so they have an even number of carbons in their n-alkane backbone, with one © 2019 American Chemical Society
carboxyl group at one end. Fini’s group recently studied the effect of acid-terminated modifiers on the surface morphology of bitumen and its interaction with stone aggregate; they observed a brushlike layer of crystallized acids at the bindersilica interface.6,7 These interfacial microstructures appeared to be potential agents for undermining the moisture resistance of the bitumen. Whether the acid compounds are added to bitumen intentionally (many biobased additives contain significant amounts of acid) or absorbed passively from the environment, tracking them in bitumen and developing techniques to reduce their accumulation at the interface of bitumen-stone could be a practical way to improve moisture resistance.6,7 The merits of using mineral dopants (or so-called active fillers) to adsorb acids and consequently alleviate the Received: March 1, 2019 Revised: May 26, 2019 Published: June 3, 2019 15513
DOI: 10.1021/acs.jpcc.9b01960 J. Phys. Chem. C 2019, 123, 15513−15522
Article
The Journal of Physical Chemistry C
mineral interface, 2 wt % of selected mineral powders were introduced to acid-doped bitumen followed by hand-mixing for 5 min at roughly 120 °C. Immediately after mixing, the molten bitumen was sucked up in the capillary stem of a pipet. A heat gun was used to keep the pipet warm while the bitumen was being drawn up, and excess bitumen on the outside of the tube was cleaned off with laboratory tissues and acetone. The bitumen-filled glass capillary tube was broken off the pipet and annealed at 120 °C for 30 min in a convection oven. Atomic Force Microscopy. To prepare the samples for AFM, the filled capillary tube was first chilled for at least 20 min at −4 °C. Short segments of the tube were then snapped off under a stream of dry N2 gas to prevent moisture condensation on the sample. Using a glass scribing tool to scribe the glass capillary before chilling also helped promote a clean, brittle fracture. The fractured segments were immediately mounted vertically and imaged under a large-stage 5600LS atomic force microscope (Keysight Technologies) in tapping mode with TAP-300 silicon cantilever tips (Budget Sensors,
