Charging of the Ice/FCS Interface Revealed by Ice Zeta Potential Measurements

Abstract

This chapter proposes a simple and versatile method for the determination of the zeta potentials of frozen solvents (Inagawa et al. in J Colloid Interface Sci 532:231–235, [1]; J Phys Chem C 123:6062–6069, [2]). A microchannel was fabricated in a solidified solvent. The zeta potential of the frozen solvent was determined by measuring the apparent mobility of the probe microspheres. Thus, the ice zeta potentials under various conditions, including the dependences of temperature, salt concentration, and pH, were determined. Based on the above results, the charging mechanism at the ice/solution interface as well as the ionic behavior was revealed using the Stern double-layer model. The zeta potential of ice is generated by the deprotonation of dangling OH bonds, ion adsorption on the ice surface, and ion uptake in the ice crystal lattice. The deprotonation of the dangling OH bonds on the ice surface (pK_a, ∼3) is enhanced compared to that in bulk liquid water. Interestingly, only 1.41% of the total dangling OH bonds on the ice surface are deprotonated, even at pH > 6, suggesting that the deprotonation of a dangling bond suppresses further dissociation of the neighboring OH sites. This is caused by the facilitated reorientation of the water molecules in ice in the presence of L-defects. The ion adsorption constants indicate that the interaction of ions, other than H⁺ ions, with the ice surface is mainly driven by the coordination of the dangling bonds with the ions. Therefore, smaller ions are adsorbed more readily on the ice surface than their larger counterparts. Additionally, the uptake of F⁻ ions in the ice crystal lattice is suggested.