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

The water surface structure of aqueous magnesium, calcium, and strontium nitrate solutions with six to seven ion was investigated using vibrational sum frequency generation water molecules on average solvating each (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 Mg2+ < Sr2+. The free OH orientation measured from the surface normal from the salt solutions changes from 33 (from neat water) to similar to 60 degrees.