Atomistic Approach towards Sodium Citrate Role on Surface Properties of Silica and Clay Minerals
Creators: Nazemi Ashani,Mahsa
Publisher: name: University of Alberta Library
Year: 2022

<p>The use of sodium citrate as a secondary processing aid in combination with caustic significantly improved bitumen recovery, specifically in poor oil sands ores. The role of sodium citrate on wettability alteration of solid surfaces, benefiting bitumen liberation as well as improving bitumen coalescence were mainly attributed to the strong chelating ability of citrate molecules. It was observed that the addition of citrate to the system enhanced bitumen liberation through significantly increased negative zeta potential of bitumen, silica, and clay surfaces and consequently stronger electrostatic repulsion between bitumen and solid surfaces. Atomic calculations are applied to further understand the underlying mechanism of the role of sodium citrate on interfacial properties and possible interactions with silica and alumina surfaces. To study the effect of sodium citrate on the wettability of clay minerals and considering the possible effect of surface structure on the adsorption behavior, both basal and edge sites of gibbsite-like substrates were studied. While edge sites are highly reactive and possibly contribute to inner-sphere adsorption of ligands present in the system, basal planes of gibbsite-like octahedral surface can contribute to outer-sphere adsorption of these molecules. The former interaction was investigated as ligand exchange reaction mechanism, where citrate was concluded to be a better complexing agent in comparison with naphthenic acids (natural surfactants present in bitumen). Furthermore, the exchange of naphthenic acids with citrate in outer-sphere adsorption mode was calculated as a favorable reaction specifically on the edge plane. The better complexing ability of citrate compared to common naphthenates present in oil sands was attributed to the unique structure of this molecule. While it is not necessary for all functional groups of citrate to coordinate with alumina surfaces, the remaining can contribute to modifying surface charges.<br>
To study the effect of sodium citrate on wettability alteration of silica surfaces as a common host present in oil sands, a three-phase system containing silica substrate, model oil droplet, and solution was used. Applying hydration film theory into the simulations, a thin film containing different compositions of monovalent and divalent ions was placed between the silica and droplet interface. While cations, and specifically divalent ions, contribute to the bridging of polar organic compounds (naphthenic acids) present in the oil droplet to the substrate, introduction of sodium citrate to the solution phase considerably changed this behavior. It was observed that the adsorption of citrate on cations present on the surface reduced their accessibility for naphthenic acid molecules and recovered the surface wettability as observed quantitively with contact angle calculation. Moreover, the effect of sodium citrate on the dynamical properties of the oil droplet was investigated through the definition of Debye-Waller factor  for the oil molecules present in the droplet. It was observed that the remaining citrate molecules in the solution contributed to the higher local mobility of decane as well as naphthenic acids molecule, resulting in a softer droplet, especially at the oil-water interface. <br>
From these studies, it can be observed that sodium citrate contributes to the solid wettability alteration of both silica and alumina surfaces through competitive adsorption on the substrate, where surfaces get dominated by the presence of negatively charged citrate anions, and further adsorption of organic compounds present in the oil phase are prohibited.</p>