Water Structure at the Air−Aqueous Interface of Divalent Cation and Nitrate Solutions

Abstract

The water surface structure of aqueous magnesium, calcium, and strontium nitrate solutions with six to seven water molecules on average solvating each ion was investigated using vibrational sum frequency generation (VSFG) spectroscopy. Raman (polarized) and infrared spectroscopies were used for understanding solvation effects. Infrared reflection spectra were analyzed to further understand the VSFG data. The VSFG spectral changes indicate that the divalent countercation species play a key role in the surface perturbation of the water. In addition, the data show that the solvated ions, and possibly their ion pairs, approach the aqueous surface. The identity of the divalent cation may cause a difference in the concentration gradient near the surface, thereby increasing the VSFG-active region, which then indicates an increase in the interfacial depth. The interface becomes thickened with Mg(2+) < Ca(2+) < Sr(2+). The free OH orientation measured from the surface normal from the salt solutions changes from 33 (from neat water) to approximately 60 degrees .

Environmental Chemistry at Vapor/Water Interfaces: Insights from Vibrational Sum Frequency Generation Spectroscopy

“…The influence of molecular anions such as NO 3 − , SO 4 2− , and CO 3 2− on the hydrogen-bonding structure of water has also been the focus of many VSFG studies (14,44,48,49,53,57,96). An understanding of the VSFG results for polyatomic anion-containing solutions in the water hydrogen-bonding region proves more elusive than that for the halide salt solutions.…”

“…Beyond examining the water structure at the vapor/neat water interface, there is much interest in elucidating molecular behavior via VSFG at the vapor/water interface for a wide variety of chemical systems. With perspective toward understanding the role that tropospheric aqueous aerosols play in atmospheric chemistry, among other applications, VSFG has been applied to study the behavior that inorganic ions (25,(44)(45)(46)(47)(48)(49)(50)(51)(52)(53)(54)(55)(56)(57), small molecular solutes (25,(58)(59)(60)(61)(62)(63)(64)(65)(66)(67)(68)(69), and lipids (32, 35,[70][71][72][73][74][75][76][77][78][79][80][81][82][83][84][85][86][87][88]…”

Surface Electric Fields of Aqueous Solutions of NH4NO3, Mg(NO3)2, NaNO3, and LiNO3: Implications for Atmospheric Aerosol Chemistry

“…18 There have been no other VSFG studies on nitrate salts other than NaNO 3 except that from Allen and co-workers of aqueous Mg(NO 3 ) 2 , Ca(NO 3 ) 2 , and Sr(NO 3 ) 2 . 27 These nitrate-mode and hydrogenbonding studies suggested that NO 3 − ions, and their corresponding ion pairs, approach the aqueous surface and that interfacial ion pairing between NO 3 − and divalent cations (Mg 2+ , Ca 2+ , Sr 2+ ) followed the trend of increasing inversely with the cation charge density. 55 In the work presented here, selected nitrate salt solutions (LiNO 3 , NaNO 3 , NH 4 NO 3 , and Mg(NO 3 ) 2 ) and their interfacial ion distributions are investigated using VSFG and HD-VSFG spectroscopy.…”

Influence of aqueous ionic strength upon liquid:liquid interfacial structure and microsolvation

“…The specific study of aqueous nitrate at the vapor interface has received much attention because it is abundant in the atmosphere and involved in a variety of reactions therein. [1,2] Second harmonic generations (SHG) and vibrational sum frequency generation (VSFG) have been instrumental in developing an understanding of the relationship between ion concentration (between 1 M and 6.8 M) [3][4][5][6][7][8][9] and the meso-scale interfacial properties in the liquid:vapor system (e.g. surface tension, interfacial width) as well as the orientation of interfacial nitrate relative to the bulk liquid.…”