On the structure of water at the aqueous/air interface

Vibrational sum frequency spectroscopy (VSFS) and molecular dynamics (MD) simulations were used in concert to investigate the molecular structure and hydrogen bonding of the air/water interface. MD simulations were performed with a variety of water models. The results indicated that only the upper most two layers of water molecules are ordered in this system. There is a strong preference to have the top layer arranged such that the OH moiety points upward into the air. This orientational preference arises from two factors that involve the maximization of the number of hydrogen bonds formed and the minimization of partial charge that is exposed. Specifically, the lone pairs from oxygen are less likely to face into the air compared with the OH moiety because this would expose more partial charge and, therefore, be unfavorable on enthalpic grounds. The two-layer interfacial water structure model implies that there should be four distinct types of OH stretches for this system. Namely, one directs upward and another points downward in each layer. Interestingly, VSFS experiments revealed the presence of four OH stretch region peaks at 3117, 3222, 3448, and 3696 cm^-1. The phases of the 3117 and 3696 cm^-1 resonances carried a positive sign, which indicates that these features arise from OH groups with protons facing upward toward the air. The other two resonances emanate from OH groups with protons facing downward toward the bulk aqueous solution. On the basis of this, we assign the 3117 cm^-1 peak to the OH moiety from a water molecule in the second layer, which is hydrogen bonded upward toward the top layer. On the other hand, the peak at 3222 cm^-1 should arise from water molecules in the top layer with the OH moiety facing downward to hydrogen bond to the second layer. The 3448 cm^-1 peak arises from hydrogen bonding between water molecules in the second layer and the more disordered water molecules of the bulk liquid. Finally, the peak at 3696 cm^-1 is assigned to the free OH moiety pointing upward in the top layer.