Istvan SZILAGYI1, Dóra TAKÁCS1, Bojana KATANA1
1MTA-SZTE Momentum Biocolloids Research Group, University of Szeged, Szeged, Hungary
Nanoparticles (NPs) dispersed in ionic liquids (ILs) are important class of systems due to the growing demand in IL-assisted synthesis of novel materials in catalysis, sensing and energy storage. Low (1D and 2D) dimensional NPs are widely used as building blocks of composite materials, while fine particle dispersions are required during the preparation, since aggregation processes give rise to loss in surface area, phase separation and unprocessable samples. Therefore, optimizing the colloidal stability of NPs in ILs is necessary and requires thorough investigation of the interfacial assembly of IL constituents and the interparticle forces acting under relevant experimental conditions.
In our group, colloidal stability of 1D halloysite nanotubes (HNTs) [1] and 2D layered double hydroxide nanosheets (LDHs) [2] has been comprehensively studied in IL solutions through determination of surface charge and aggregation rates at different pH, particle and IL concentrations with scattering techniques. The structure of ILs were varied, ethylammonium, methylimidazolium, methylpyridinium, methylpiperidinium and methylpyrrolidinium derivatives were used as cations, while chloride, bromide, nitrate, thiocyanide and dicyanamide as anions. Moreover, latex particles (LPs) [3] were also applied to investigate the influence of water and inert salts on the structure of the solid-IL interface. Based on the new insights on the stability of particle-IL dispersions, the following conclusions can be made. First, increasing water and salt content induce particle aggregation due to the disruption ofthe self-assembled IL layer on the particle surfaces, which otherwise responsible for the colloidal stabilization of the particles. However, this effect can be masked by surface functionalization by imidazolium-based polymers. Second, 1-alkyl-3-methylimidazolium cations of longer alkyl chains tend to adsorb stronger on oppositely charged HNTs leading to dispersion destabilization at low concentrations. This effect was the most pronounced in the HNT-1-hexyl-3-methylimidazolium samples, in which charge reversal of the particles occurred owing to the strong adsorption of the IL cations. Third, IL constituent ions possess different affinities to the surfaces of LDHs and HNTs and these ions can be ordered into the Hofmeister series based on their destabilization power. Such a series for the above NPs was extended with IL coions and counterions making the prediction of the colloidal stability of the particles in ILs possible.
These findings provide novel insights into the interfacial behavior of ILs in dispersions of low dimensional NPs leading to better understanding of the colloidal stability of NP-IL dispersions, which can be optimized by choosing the ILs of appropriate structure or physicochemical properties. Such a knowledge helps in designing processable NP-IL samples for preparation of composite materials, for instance.
References
[1] B. Katana, D. Takács, A. Szerlauth, S. Sáringer, G. Varga, A. Jamnik, F.D. Bobbink, P.J. Dyson, I. Szilagyi, Langmuir 37 (2021) 11869.
[2] D. Takács, B. Katana, A. Szerlauth, D. Seb?k, M. Tomši?, I. Szilagyi, Soft Matter 17 (2021) 9116.
[3] D. Takács, M. Tomsic, I. Szilagyi, Colloid Interfac. 6 (2022) 2.