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

Abstract

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, ε^E, excess over the additive contributions of solute and solvent. ε^E thus contains information about the effect of inter-molecular interactions in the ν₂ + ν₃ spectrum. The spectrum of ε^E 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, ε^E_salt. From the behaviour of ε^E_salt 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 x_salt (mole fraction of salt) dependence of ε^E_salt, we suggest that there is an ion-pairing in x_salt > 0.032, 0.036, and 0.04 for NaCl, NaBr and NaI respectively. The positive band of ε^E at 5123 cm⁻¹ is attributed to a modestly organized hydrogen bond network of H₂O (or liquid-likeness), and the x_salt dependence of ε^E_salt indicated a qualitative difference in the effect of Cl⁻ from those of Br⁻ and I⁻. Namely, the values of ε^E_salt stay constant for Cl⁻ but those for Br⁻ and I⁻ decrease smoothly on increasing the salt mole fraction. The mole fraction dependence of ε^E_salt at the 4873 cm⁻¹ band, due to ice-likeness in H₂O, shows a subtle difference between Cl⁻ and (Br⁻, I⁻) pair.