Steve Baldelli

Sum frequency generation spectroscopy of the aqueous interface: Ionic and soluble molecular solutions

The liquid interface of aqueous solutions is of central importance to numerous phenomena from cloud processing of combustion generated oxides to corrosion degradation of structural materials to transport across cell membranes. Despite the importance of this interface, little molecular-level information was known about it prior to the last decade-and-a-half. Molecular-level information is important not only for a fundamental understanding of processes at interfaces, but also for predicting methods for diminishing deleterious effects. Recently, the non-linear spectroscopic method, sum frequency generation (SFG), has been applied to the investigation of the structure of the liquid interface. This review focuses on the liquid-air interface of aqueous solutions containing soluble, ionic species - H2SO4, HNO3, HCl, alkali sulphates and bisulphates, NaCl and NaNO3 - as well as soluble molecular species-glycerol, sulphuric acid and ammonia. Ionic materials influence the structure of water at the interface through an electric double layer which arises from the differential distribution of anions and cations near the interface. Due to the extreme size of the proton, the strongest field is generated by acidic materials. As the concentration of these ionic materials increases, ion pairs form diminishing the strength of the double layer. This enables the ion-pair complex to penetrate to the interface and either displace water or bind it into hydrated complexes. Soluble materials of lower surface tension partition to the interface and either displace water from the interface or bind water into hydrated complexes. In particular, the conjectured ammonia-water complex on aqueous solutions is observed and it is determined to tilt 34-38 from the normal.

Surface enhanced sum frequency generation of carbon monoxide adsorbed on platinum nanoparticle arrays

Sum frequency generation (SFG) vibrational spectroscopy is used to study the adsorption of CO at ∼1 atm pressure on Pt nanoparticle arrays and Pt thin films. The SFG signal of CO adsorbed on platinum particles of 45 nm diameter is ∼10 000 times larger than from CO on smooth Pt films. The large enhancement is explained by plasmon resonance and Maxwell–Garnett theory. The Pt arrays are prepared using electron beam lithography to produce particles with uniform spacing and sizes on an oxidized Si(100) wafer. Further, as the Pt coverage increases the SFG signal shows a polarization dependence that is explained considering the dielectric properties of a metal film on a dielectric surface. In addition, SFG permits investigation of the CO adsorbed on the particles at ∼1 atm, which is not possible with most surface analytical techniques, that will allow for the study of the reaction of small molecules on surfaces relevant in heterogeneous catalysis.

The Structure of Water on HCl Solutions studied with Sum Frequency Generation

Abstract The influence of dissolved HCl on the surface of water has been investigated with sum frequency generation (SFG) spectroscopy. Ions in solution cause water on the surface to be reoriented relative to pure water, with hydrogen atoms directed toward the bulk solution. There is no signal due to molecular HCl suggesting that oriented water molecules, not molecular HCl, dominate the surface. A model is proposed to account for the reported SFG results as well as surface tension and surface potential measurements. The result suggests that application of the Gibbs equation to determine surface excess may need to be reevaluated.

FIRST SPECTROSCOPIC EVIDENCE FOR MOLECULAR HCL ON A LIQUID SURFACE WITH SUM FREQUENCY GENERATION

Sum frequency generation spectroscopy has been used to obtain the vibrational spectrum of HCl on the surface of a liquid. HCl was studied on the surface of 96 wt % H2SO4, 12 M HCl solution, liquid HCl and glass, of which only liquid HCl produces a resonant signal. Implications for the form of HCl on surfaces and the reactions in the atmosphere are discussed.Sum frequency generation spectroscopy has been used to obtain the vibrational spectrum of HCl on the surface of a liquid. HCl was studied on the surface of 96 wt % H2SO4, 12 M HCl solution, liquid HCl and glass, of which only liquid HCl produces a resonant signal. Implications for the form of HCl on surfaces and the reactions in the atmosphere are discussed.

Ammonia-Water Complexes on the Surface of Aqueous Solutions Observed with Sum Frequency Generation

Sum frequency generation (SFG) is used to obtain the vibrational spectrum of aqueous ammonia solution. The SF spectrum of concentrated solutions is dominated by the N–H symmetric stretch (ν1) at 3312 cm−1 and a weaker deformation mode (2ν4) at 3200 cm−1. The free OH peak of water is suppressed, compared to pure water, and the hydrogen-bonded region (3000–3450 cm−1) has interference in the presence of ammonia molecules at the interface. In dilute solution, ν1 is less intense and the free OH peak of water appears. These observations confirm the existence of an ammonia–water complex at the liquid/vapor interface.

Sum frequency generation by water on supercooled H2SO4/H2O liquid solutions at stratospheric temperature

Abstract Sum frequency generation spectra indicate that the surface water of liquid sulfuric acid solutions varies as a function of concentration, but not with temperature. At 0.1 x H 2 SO 4 (where x =mole fraction, 38 wt%), subsurface ionic species orient surface water molecules. The surface of 0.2 x H 2 SO 4 (58 wt%) solutions, however, features H 2 SO 4 /H 2 O complexes both at 273 K and supercooled at 216 K. The results support a picture of stratospheric chemistry in which sulfuric acid aerosols are coated with hydrogen-bonded water/sulfuric acid complexes.

Sum frequency generation investigation of water at the surface of H2O/H2SO4 and H2O/Cs2SO4 binary systems

Abstract The vibrational structure of water at the air/solution interface of an ionic solution has been obtained for the first time. Using vibrational sum frequency generation it is determined that ions in solution have a large orientational effect on the structure of the surface water. Electrolytic solutions, ionic in nature, cause water to be oriented into a more regular hydrogen-bonded network through an electric double layer at the interface. In electrolytic solutions where molecular or associated H 2 SO 4 or Cs 2 SO 4 species dominate, the surface water molecules are bound into hydrate complexes. These effects are explained using hard soft acid base (HSAB) theory.

Water Confined at the Liquid-Air Interface

The liquid interface of aqueous solutions is of central importance to numerous phenomena from cloud processing of combustion generated oxides to corrosion degradation of structural materials to transport across cell membranes. Recently, the nonlinear spectroscopic method, sum frequency generation (SFG), has been applied to investigate the structure of liquid interfaces and alteration of that structure by materials in solution. This chapter focuses on two categories of materials in solution: inorganic ionic materials that are nonvolatile — H2SO4, HNO3, alkali sulfates and bisulfates, NaCl, and NaNO3 — and soluble molecules that are volatile — HCl and NH3. Ionic materials influence the structure of water at the interface through an electric double layer that arises from the differential distribution of anions and cations near the interface. Two models for the effect of the double layer are discussed. Soluble molecular materials of lower surface tension partition to the interface and displace surface water molecules. Ammonia is a rather unique probe of water at the surface. At low concentrations, ammonia merely docks to the dangling-OH groups. At intermediate concentrations, the surface changes little as the bulk concentration increases and at higher concentrations, ammonia blankets the surface and displaces water at the surface.