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-SO42- 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.
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 M. Mizuhata, et al., Phys. Chem. Chem. Phys., 6, 1944 (2004).
 W. W. Rudolph, et al., Z. Phys. Chem., 209, 181 (1999).