Ultrasonic studies on ion-solvent interactions in aqueous mixed electrolytes

Ion-solvent interactions in aqueous solutions of mixed electrolytes, namely (NaCl + NaBr), (NaCl + NaI) and (NaBr + NaI), have been investigated by studying their effect on the temperature of the sound velocity maximum (TSVM) of water (Tw). The ultrasonic velocity as a function of temperature over a range of 5°C either side of the TSVM at intervals of [asymptotically equal to] 2°C was determined using a single-crystal variable-path interferometer working at 3MHz with an accuracy ±0.003%. The accuracy in fixing the TSVM is ±02°C. The structural contribution to the shift in Tw of pure water due to the addition of mixed electrolytes, ΔTstr, has been found to be negative and increasing with concentration of the mixed electrolytes. At any given concentration, the ΔTstr of (NaCl + NaBr) > (NaCl + NaI) > (NaBr + NaI). This indicates that the structure promoting nature of Na+ is balanced and subdued by the presence of halide ions in the order (Br- + I-) > (Cl- + I-) > (Cl- + Br-), consistent with the behaviour of halide ions in disrupting the hydrogen-bonded structure of water, which is in the order I- > Br- > Cl-. The results are also explained as due to the reduction in the structure breaking efficiency of Cl-, Br- and I- in the presence of Na+ in aqueous mixed electrolyte solutions.