Effects of salt concentration on the structure and vibrational sum frequency generation spectra of liquid/vapor interfaces of aqueous solutions of metal nitrates

Abstract

We have investigated the effects of varying salt concentrations on the structure of the liquid/vapor interfaces of aqueous solutions of NaNO₃, Mg(NO₃)₂, and Ca(NO₃)₂ salts using molecular dynamics simulations and vibrational sum frequency generation (VSFG) spectral calculations. The current study reveals a weak interfacial propensity of the nitrate ions and formation of an ionic double-layer at the interfaces. The tetrahedral hydrogen bond network is disrupted more by ions in the bulk phase compared to the interface, with the extent of disruption increasing with concentration. The VSFG spectra show three peaks: a positive peak in the range of 3000–3550 cm⁻¹ arising from O–H groups hydrogen bonded to water, a negative peak in the range of 3550–3650 cm⁻¹ due to O–H groups hydrogen bonded to the oxygen atoms of the nitrate ions, and a positive peak at around 3750 cm⁻¹ corresponding to free O–H groups. The positive peak intensity (3000–3550 cm⁻¹) follows the order Mg(NO₃)₂ > Ca(NO₃)₂ > NaNO₃ and the intensity for the systems with divalent cations increases with salt concentration. The electric field generated by the ionic double layers in the Mg(NO₃)₂ systems is higher because of higher charge density of Mg²⁺ ions, and with concentration, the strength of the electric field increases further. The intensity of the negative peak (3550–3650 cm⁻¹) increases with the increasing concentration of the salts as the number of O–H groups hydrogen bonded to the oxygen atoms of the nitrate ions increases. The intensity of the positive peak at ∼3750 cm⁻¹ does not show any significant change with changes in the salt concentration.