Stability of Titania Nanomaterials Dispersed in Aqueous Solutions of Ionic Liquids of Different Alkyl Chain Lengths

Rouster, Paul and Pavlovic, Marko and Cao, Tianchi and Katana, Bojana and Szilágyi, István (2019) Stability of Titania Nanomaterials Dispersed in Aqueous Solutions of Ionic Liquids of Different Alkyl Chain Lengths. JOURNAL OF PHYSICAL CHEMISTRY C, 123 (20). pp. 12966-12974. ISSN 1932-7447 (print); 1932-7455 (online)

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Abstract

Charging and aggregation of titania nanosheets (TNS) and spherical titania nanoparticles (TNP) were studied in aqueous solutions of ionic liquids. The pH and the length of the alkyl chain of the IL cations [1-methylimidazolium (MIM+), 1-ethyl-3-methylimidazolium (EMIM+), and 1-butyl 3-methylimidazolium (BMIM+)] were systematically varied in the experiments. No detectable interaction was observed between the IL cations and the positively charged TNS or TNP surfaces at low pH, where the imidazolium derivatives are the co-ions. For the negatively charged titania nano-objects, significant adsorption of MIM+ and EMIM+ took place, leading to charge neutralization and overcharging at appropriate concentrations. The BMIM+ behaved like a simple salt constituent causing charge screening. For both TNS and TNP, the MIM+ < EMIM+ < BMIM+ counterion order was obtained in the critical coagulation concentrations, indicating that MIM+ was the most effective in destabilization of the dispersions. The major interparticle forces were of electrostatic origin; however, viscous stabilization was also observed at high IL concentrations. The same aggregation mechanism and charging behavior were found for the titania nano-objects irrespective of their shape. The results shed light on the hydrophilic nature of the surface of the TNS and TNP of negative charge, in contrast to earlier findings with hydrophobic colloidal particles, where the increasing alkyl chain length gave rise to higher destabilization power. The charging properties were governed by specific adsorption of the IL constituents, while the major interparticle forces were qualitatively well-predicted by the Derjaguin, Landau, Verwey, and Overbeek theory.

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