Isentropic compressibility, shear relaxation time, and raman spectra of aqueous calcium nitrate and cadmium nitrate solutions

Speed of sound, viscosity and Raman spectra of aqueous calcium nitrate and cadmium nitrate solutions were measured as functions of molality and temperature. The isentropic compressibility isotherms for both systems cross over in a narrow molality region. In comparison with Ca(NO3)2(aq) solutions, Cd(NO3)2(aq) solutions have lower isentropic compressibilities due to a lower charge to radius ratio. The observed Raman spectral changes in the ν3 (≈1400 cm−1) and ν4 (≈700 cm−1) modes with an increase in molality suggest that the symmetry of NO3− changes from D3h to C2v, and solvent-separated and/or solvent-shared ion pairs are formed in both systems. The results from plotting electrical conductivity versus shear relaxation time also imply that the influence of the solvent-separated and/or solvent-shared ion pairs begins ≈2.0 mol-kg−1 for these systems. The larger Δν values for the ν3 mode for Cd(NO3)2(aq) solutions indicate stronger solvent-separated and/or solvent-shared ion pairs formation in comparison to Ca(NO3)2(aq) solutions.