Observation of Hydronium Ions at the Air−Aqueous Acid Interface:  Vibrational Spectroscopic Studies of Aqueous HCl, HBr, and HI

Abstract

The air−liquid interface of aqueous hydrogen-halide solutions is examined using vibrational sum frequency generation spectroscopy. Infrared and Raman spectroscopies are used to compare the effects of the ions on the water structure in the bulk solutions to that of the interface. The addition of HCl, HBr, and HI to water is found to cause a significant disruption in the hydrogen-bonding network at the air−liquid interface, similar to that which is observed for sodium halides. However, a convolution of additional effects are observed for acids at the air−aqueous interface: interfacial H3O+ (and H5O2 +) which gives rise to surface potential changes, an increase in interfacial depth from interfacial concentration gradients, and a decrease in the number of dangling OH bonds relative to the neat water surface. Additionally, and moreover, sum frequency spectra reveal a surface proton continuum at frequencies below 3000 cm-1, indicating that hydronium ions (and Zundel ions) exist at the air−aqueous interface.

Interfacial Structures of Acidic and Basic Aqueous Solutions

“…5,6,16 Using SFVS, several research groups have successfully obtained vibrational spectra of water/vapor interfaces of acid, basic and salt solutions. [5][6][7][8]17,18 In all cases, the SFVS spectrum appears to roughly consist of three spectral features in the OH stretch region: a relatively sharp peak at 3700 cm -1 attributed to the dangling OH protruding at the surface, and two broad bands at ~3200 and ~3450 cm -1 from bonded OH often labeled as ice-like and liquid-like bands, respectively. 16 When the acid concentration in water was sufficiently high (pH ≤ 2 with HCl), strong enhancement of both ice-like and liquid-like bands were observed.…”

“…1,2 They have been the topics of extensive theoretical and experimental studies in recent years, [3][4][5][6][7][8] but good understanding of the water interfacial structure at the molecular level is still lacking. The most concerned question is whether H + and OH -ions would emerge at the interface and affect the interfacial hydrogen-bonding network of water.…”

“…3,[9][10][11][12] The details, such as the depth profile of ion concentrations, depend very much on the molecular model and interaction potentials assumed. [9][10][11][12] Experiments carried out with scanning tunneling microscopy (STM) 13 , X-ray spectroscopy 14,15 , and attenuated total reflection (ATR) 8 have been used to verify the theoretical prediction. They however suffer from either the water surface being too dynamic or the techniques not being sufficiently surface-specific.…”

“…16 When the acid concentration in water was sufficiently high (pH ≤ 2 with HCl), strong enhancement of both ice-like and liquid-like bands were observed. 7,8,11,18 Appearance of hydronium ions at the surface was used to explain the enhancement. 7,8,11 In basic solution, only the liquid-like band displayed slight changes when pH is sufficiently high (pH≥13).…”

Orientational Motions of Vibrational Chromophores in Molecules at the Air/Water Interface with Time-Resolved Sum Frequency Generation

“…the dynamics of rotation in the interfacial plane differ from the out-of-plane orientational motions. [15][16][17][18] Second harmonic generation (SHG) and sum frequency generation (SFG), [11][12][13][14][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] as second-order spectroscopies, are interface specific, including buried interfaces that are accessible to light. Second harmonic generation (SHG) depends primarily on the electronic properties of the interfacial molecules whereas SFG is sensitive to molecular vibrations when one of the incident beams is in the infrared.…”