Spectrum of excess partial molar absorptivity. Part II: a near infrared spectroscopic study of aqueous Na-halides

Our earlier thermodynamic studies suggested that F− and Cl− form hydration shells with the hydration number 14 ± 2 and 2.3 ± 0.6, respectively, and leave the bulk H2O away from hydration shells unperturbed. Br− and I−, on the other hand, form hydrogen bonds directly with the momentarily existing hydrogen bond network of H2O, and retard the degree of entropy–volume cross fluctuation inherent in liquid H2O. The effect of the latter is stronger for I− than Br−. Here we seek additional information about this qualitative difference between Cl− and (Br− and I−) pair by near infrared (NIR) spectroscopy. We analyze the ν2 + ν3 band of H2O in the range 4600–5500 cm−1 of aqueous solutions of NaCl, NaBr and NaI, by a new approach. From observed absorbance, we calculate excess molar absorptivity, εE, excess over the additive contributions of solute and solvent. εE thus contains information about the effect of inter-molecular interactions in the ν2 + ν3 spectrum. The spectrum of εE shows three bands; two negative ones at 5263 and 4873 cm−1, and the positive band at 5123 cm−1. We then define and calculate the excess partial molar absorptivity of each salt, εEsalt. From the behaviour of εEsalt we suggest that the negative band at 5263 cm−1 represents free H2O without much hydrogen bonding under the influence of local electric field of ions. Furthermore, from a sudden change in the xsalt (mole fraction of salt) dependence of εEsalt, we suggest that there is an ion-pairing in xsalt > 0.032, 0.036, and 0.04 for NaCl, NaBr and NaI respectively. The positive band of εE at 5123 cm−1 is attributed to a modestly organized hydrogen bond network of H2O (or liquid-likeness), and the xsalt dependence of εEsalt indicated a qualitative difference in the effect of Cl− from those of Br− and I−. Namely, the values of εEsalt stay constant for Cl− but those for Br− and I− decrease smoothly on increasing the salt mole fraction. The mole fraction dependence of εEsalt at the 4873 cm−1 band, due to ice-likeness in H2O, shows a subtle difference between Cl− and (Br−, I−) pair.