Effects of Cations on the Structure, Dynamics and Vibrational Sum Frequency Generation Spectroscopy of Liquid/Vapor Interfaces of Aqueous Solutions of Monovalent and Divalent Metal Nitrates

Abstract

We have employed molecular dynamics (MD) simulations and theoretical vibrational sum frequency generation spectroscopy (VSFG) to investigate the structure and interactions of water and ions at liquid/vapor interfaces of aqueous solutions of monovalent and divalent metal nitrates of NaNO₃, Mg(NO₃)₂ and Ca(NO₃)₂. Structural properties, such as the number density profiles, average number of hydrogen bonds, slab radial distribution functions (SRDFs), and tetrahedral order parameter, are calculated to investigate the structure of water in bulk and at the interfaces. The ion pairing tendency is determined in terms of cation-anion SRDF and is found to be following the order Mg(NO₃)₂ < Ca(NO₃)₂ < NaNO₃ both at the interfacial and bulk regions. An ionic double layer is found to be formed at the interface. A weak propensity of anions is found to be at the interface, while the cations are found to be present below the interfacial region. Three prominent features are observed in the VSFG spectrum of the liquid/vapor interfaces of aqueous metal nitrate solutions: a free (dangling) O-H peak at 3750 cm^-1, a peak at 3589 cm^-1 due to O-H groups that are hydrogen bonded to nitrates, and a broad peak at 3200--3500 cm^-1 due to O-H modes that are hydrogen bonded to water. The charge density of cations affects the intensity of the 3200--3500 cm^-1 peak. The presence of ions is found to have very little effect on the position and intensity of the dangling peak as compared to that for neat water/vapor interface. The ionic double layers generate electric fields that reorganize and reorient the water molecules towards vapor. This upward reorientation of water leads to a positive region in the VSFG spectrum of O-H modes which are hydrogen bonded to water in contrast to that of neat water/vapor interface. The O-H groups hydrogen bonded to the nitrates are mostly oriented downwards and the strength of such hydrogen bonds is found to be weaker than those hydrogen bonded to water.