Spectroscopic Studies of Solvated Hydrogen and Hydroxide Ions at Aqueous Surfaces

Abstract

Measuring the molecular properties of the surface of acidic and basic aqueous solutions is essential to understanding a wide range of important biological, chemical, and environmental processes on our planet. In the present studies, vibrational sum-frequency spectroscopy (VSFS) is employed in combination with isotopic dilution experiments at the vapor/water interface to elucidate the interfacial water structure as the pH is varied with HCl and NaOH. In acidic solutions, solvated proton species are seen throughout the interfacial region, and they alter the hydrogen bonding between water molecules in ways that reflect their depth in the interfacial region. At the higher frequencies of the OH stretch region, there is spectral evidence for solvated proton species residing in the topmost layers of the interfacial region. As reported in previous VSF studies, more strongly bound solvated proton species are observed at lower OH stretching frequencies. The solvated proton species that have stronger hydrogen bonding are similar in structure to those found in bulk acid solutions and likely reside somewhat deeper in the interfacial region. There is also evidence of OH stretching from solvated protons and relatively strong hydrogen bonding in the solvation sphere that is similar to other solvated ions. In contrast, water molecules solvating OH(-) ions show relatively weak hydrogen bonding and significantly less interfacial order. VSF spectra are acquired under multiple polarizations to provide crucial information for the interpretation of the spectra and for the determination of interfacial structure.

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.…”

“…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).…”

“…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). 7 It was believed that at high pH, OH -anions might have emerged at the interface with enough concentration to significantly distort the interfacial hydrogen-bonding network of water.…”

“…7 It was believed that at high pH, OH -anions might have emerged at the interface with enough concentration to significantly distort the interfacial hydrogen-bonding network of water. 7 Salt solutions have also been investigated. Results showed that ions of small size like to stay in the bulk, while negative ions with large radius, such as Br -and I -prefer to stay close to the surface.…”