A 3D-RISM-KH Molecular Theory of Solvation Study of the Effective Stacking Interactions of Kaolinite Nanoparticles in Aqueous Electrolyte Solution Containing Additives
S Hlushak and SR Stoyanov and A Kovalenko, JOURNAL OF PHYSICAL CHEMISTRY C, 120, 21344-21357 (2016).
We develop a predictive model to study how flocculant additives alter the interactions among clay particles in colloidal suspensions, such as industrial mining tailings. Fully atomistic models of kaolinite nanoplatelets constructed from X-ray crystal structure data feature a highly polarized charge distribution, which defines their solvation, adsorption, and association properties. Effective interactions, in the form of potential of mean force (PMF), between kaolinite platelets in aqueous electrolyte solution are calculated using the three-dimensional reference interaction site model with the Kovalenko Hirata closure relation (3D-RISM-KH) molecular theory of solvation based on the first- principles of statistical mechanics. This theory is also employed to study the adsorption of ions and flocculant additive building blocks onto kaolinite surfaces. Three main mutual orientations of platelets are studied in aqueous electrolyte solutions of several polymer building blocks represented by acrylamide, acrylic acid, acrylate, and styrene. The results indicate that Na+ are predominantly adsorbed onto the siloxane surface of kaolinite while the chloride and acrylate anions prefer the aluminum hydroxide surface of kaolinite. Weaker adsorption preference is observed for the neutral monomers. The PMF between platelets depends nontrivially on the concentration of solvent components and exhibits a complex oscillating behavior with several minima and maxima that correspond to important solvation and aggregation energy barriers. Among the three studied mutual orientations of the nanoplatelets, the most stable one corresponds to the direct contact of the aluminum hydroxide with siloxane surfaces. Other highly probable arrangements correspond to nanoplatelets separated by a single layer of solvent. The effect of additives on interparticle interactions is correlated with the strength of adsorption on kaolinite relative to water, as strongly and weakly adsorbing species cause increase and decrease, respectively, of the PMF at short distances. Moreover, hydrophobic additives cause a decrease in the local solvent density between nanoparticles and consequently a decrease in the PMF. These results provide valuable -insights into the mechanism of interactions of kaolinite nanoplatelets in thermodynamic conditions relevant to clay dispersions, as occurring in tailings produced by the process of hot water extraction of bitumen from oil sands and other milling tailings.
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