Properties of Concentrated Aqueous Electrolyte Solution in a Vicinal Region of Coexisting Solid Surface
Authors: Kunikata Nobuaki , Matsui Masaki , Maki Hideshi , Mizuhata Minoru 
Published: February 2020

Numerous electrochemical devices are expected to operate in higher output using large amount of active materials and concentrated electrolyte solution in limited volume of each device. Therefore, little amount of concentrated electrolyte solution is penetrated in active materials for electrode. Although a lot of researchers have been investigating for electrolyte solution, the properties of electrolyte solution coexisting with solid materials are not studied very much. It is well-known that the solid surface affects the properties of water nearby the solid/liquid interface1. We have been studying the properties of the coexisting system consisting of an aqueous electrolyte solution and an inorganic powder, and confirmed that various kinds of solutions change its properties depending on a distance from solid surface2. In the present study, we focused on zinc aqueous solution because strong interaction of Zn2+ makes large hydration sphere, therefore interaction from solid surface might be much observable. While almost all 2:2 electrolytes are slightly soluble due to their strong interaction, ZnSO4 can dissolve up to 3 mol L-1 at ambient temperature. As Rudolph et. al reported using Raman spectroscopy, Zn2+ and SO4
2- forms a 1:1 inner-sphere contact ion pair even in a dilute solution3. Dehydration occurred by forming contact ion pair is supposed to affect water activity and this behavior might change in a solid/liquid coexisting system by the effect from solid surface. We measured liquid properties of ZnSO4aqueous solution coexisting with silica nanoparticles in order to discuss the change in liquid property caused by silica surface in the vicinal region of solid phase.
Aqueous solutions containing prescribed concentration were prepared by stirring commercial salts in deionized water, and mixed with fumed silica as 85 – 100 vol% of liquid phase using agate mortar. We carried out DSC, Raman spectroscopy, 1H NMR, and proton relaxation measurement. The employed scanning temperature range in the DSC measurement was between -70 and 50°C and a scan rate was 5°C per minute. Cooling and heating processes were repeated three times in dry N2 atmosphere. Raman spectroscopy and the proton relaxation measurement were carried out at 25oC.
From results of DSC measurement, H2O-ZnSO4 system exhibited binary phase diagram which have eutectic composition at 0.050 mole fraction of ZnSO4, therefore this suggests 19 water molecules and one ZnSO4 salt exhibit phase transition at once. This ratio of salt to water molecule at eutectic composition is very low value compared with other eutectic system. This indicates a combination of zinc cation and sulfonate anion have strong influence on water molecules. Because it is known that Zn2+ have 6 water molecules as first hydration sphere and 12 water molecules as second hydration sphere4, phase transition at the eutectic composition was supposed to take along almost all water molecules of first and second hydration sphere. In the coexisting system with silica nanoparticles, single endothermic peak of eutectic mixture exhibited splitting into some endothermic peaks, suggesting heterogeneous phase transitions were occurred. From Raman spectroscopy in solid/liquid coexisting system shown as Fig. 1, the ν1-SO4
2- band of 1 mol L-1 (ZnSO4 · 55.2H2O) shifted to higher wavenumber with liquid phase volume fraction decrease, and closed to the value of 3 mol L-1 (ZnSO4 · 7.7H2O) which have much less water molecules per 1 salt than eutectic composition or sum of the first and second hydration sphere. In the vicinity of silica nanoparticle, Zn2+ was not supposed to make hydration sphere sufficiently due to sharing water molecules with solid surface. This result agrees with the previous study for the α-Al2O3 powder/ZnCl2 aqueous solution coexisting system2, and it suggests that the structural change of the ionic species affect ionic conduction behavior.
Consequently, the tendency of zinc cation and sulfate anion to form contact ion pair was enhanced in the vicinity region of silica solid surface, while that in higher concentration was negligible. This behavior was similar to the liquid phase volume fraction dependences of 1H spin-spin relaxation time which asymptotically closed to a constant value with decrease of liquid phase volume fraction, regardless of the bulk concentration.
【References】
[1] W. Drost-Hansen, Ind. Eng. Chem.,
61(11), 10 (1969).
[2] M. Mizuhata, et al., Phys. Chem. Chem. Phys., 6, 1944 (2004).
[3] W. W. Rudolph, et al., Z. Phys. Chem., 209, 181 (1999).
[4] W. W. Rudolph, et al., Journal of Solution Chemistry, 28, 9 (1999).

Figure 1