Séminaire PHENIX | Ryo Kanzaki "Ionization thermodynamics of poly(acrylic acid) in ionic liquids"

Ionic liquids, regarded as an extremely condensed electrolyte solvents, are attracting colloidal dispersion media. According to DLVO theory, the addition of electrolytes is generally the primary cause for reduced colloidal stability for charged nanoparticles in water. Therefore, ionic liquids could not be “good” dispersing media of colloids. Even those predictions, techniques to yield stable dispersions of nanoparticles have already been obtained in various ionic liquids. We so far investigated the pH response of the colloidal stability of maghemite magnetic nanoparticles, whose surface is functionalized with polyacrylic acid (pAA), namely coated nanoparticles (CNps). [1,2] In aqueous solutions, CNps aggregate to precipitate in an acidic condition, showing a single threshold pH, while the same CNps aggregate in both acidic and basic conditions and flocculate in the medium pH region in protic ionic liquids (PILs), ethylammonium nitrate (EAN) and N,N-diethylethanolammonium trifluoromethanesulfonate (Et2HyNH+×TfO−), showing two switching pHs. In the current study, the ionization thermodynamics of pAA in ionic liquids are studied in order to reveal the role of pAA on the dispersion of CNps. Figure 1(a) shows the enthalpogram of pAA ionization titration, and Figure 1(b) shows that of CNp in EAN. CNps disperse under the acidic condition (titration ratio < 0), flocculate when the titration ratio exceeds zero, and then redisperse when the titration proceeds (titration ratio > 0.8) to achieve an adequate degree of ionization. These enthalpograms are very similar, indicating that the ionization behavior of the surface pAA on CNp resembles that in the bulk. Interestingly, this works out even in the flocculation pH region, implying that the chemical potential of bulk pH penetrates inside the CNp aggregation to govern the ionization equilibria of the inside pAA. Figure 3(c) shows the pAA titration curve in Et2HyNH+×TfO−. Prior to the ionization of the pAA, a heat generation not to be attributed to the ionization was observed. This has also been observed for CNp in the same PIL, [2] which may arise from the condensation of the solvent cation at the vicinity of the surface pAA. Since the entropy of the subsequent ionization was large and negative, further condensation of the solvent cation around CNps may occur with increasing degree of ionization. Thanks to such strong solvations of the surface pAA of the CNps, the pH region of the stable dispersion of the CNp is considered to be wider than that in EAN. In both PILs, the ionization thermodynamics of CNp resemble those of pAA in each solvent, and thus, the ionization and solvation of pAA are confirmed to contribute to the dispersion stability of CNps. Notably, the dispersion mechanism of CNps in EAN and Et2HyNH+×TfO− is different, even though both are PILs.

[1] R. Kanzaki, C. Guibert, J. Fresnais, V. Peyre, J. Colloid Interface Sci., 2018, 516, 248.
[2] R. Kanzaki, M. Sako, H. Kodamatani, T. Tomiyasu, C. Guibert, J. Fresnais, V. Peyre, J. Mol. Liq., 2022, 349, 118146.