Mary Jane Shultz

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.

Proton order in the ice crystal surface.

The physics of the ice crystal surface and its interaction with adsorbates are not only of fundamental interest but also of considerable importance to terrestrial and planetary chemistry. Yet the atomic-level structure of even the pristine ice surface at low temperature is still far from well understood. This computational study focuses on the pattern of dangling H and dangling O (lone pairs) atoms at the basal ice surface. Dangling atoms serve as binding sites for adsorbates capable of hydrogen- and electrostatic bonding. Extension of the well known orientational disorder (“proton disorder”) of bulk crystal ice to the surface would naturally suggest a disordered dangling atom pattern; however, extensive computer simulations employing two different empirical potentials indicate significant free energy preference for a striped phase with alternating rows of dangling H and dangling O atoms, as suggested long ago by Fletcher [Fletcher NH (1992) Philos Mag 66:109–115]. The presence of striped phase domains within the basal surface is consistent with the hitherto unexplained minor fractional peaks in the helium diffraction pattern observed 10 years ago. Compared with the disordered model, the striped model yields improved agreement between computations and experimental ppp-polarized sum frequency generation spectra.

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.

Water: a responsive small molecule.

Unique among small molecules, water forms a nearly tetrahedral yet flexible hydrogen-bond network. In addition to its flexibility, this network is dynamic: bonds are formed or broken on a picosecond time scale. These unique features make probing the local structure of water challenging. Despite the challenges, there is intense interest in developing a picture of the local water structure due to waters fundamental importance in many fields of chemistry. Understanding changes in the local network structure of water near solutes likely holds the key to unlock problems from analyzing parameters that determine the three dimensional structure of proteins to modeling the fate of volatile materials released into the atmosphere. Pictures of the local structure of water are heavily influenced by what is known about the structure of ice. In hexagonal I(h) ice, the most stable form of solid water under ordinary conditions, water has an equal number of donor and acceptor bonds; a kind of symmetry. This symmetric tetrahedral coordination is only approximately preserved in the liquid. The most obvious manifestation of this altered tetrahedral bonding is the greater density in the liquid compared with the solid. Formation of an interface or addition of solutes further modifies the local bonding in water. Because the O-H stretching frequency is sensitive to the environment, vibrational spectroscopy provides an excellent probe for the hydrogen-bond structure in water. In this Account, we examine both local interactions between water and small solutes and longer range interactions at the aqueous surface. Locally, the results suggest that water is not a symmetric donor or acceptor, but rather has a propensity to act as an acceptor. In interactions with hydrocarbons, action is centered at the water oxygen. For soluble inorganic salts, interaction is greater with the cation than the anion. The vibrational spectrum of the surface of salt solutions is altered compared with that of neat water. Studies of local salt-water interactions suggest that the picture of the local water structure and the ion distribution at the surface deduced from the surface vibrational spectrum should encompass both ions of the salt.

The single-crystal, basal face of ice Ih investigated with sum frequency generation

Sum frequency generation spectroscopy has been used to investigate the hydrogen-bonded region of single-crystal, hexagonal ice in the temperature range of 113-178 K. The temperature and polarization dependences of the signal are used in conjunction with a recent theoretical model to suggest an interpretation of the bluest and reddest of the hydrogen-bonded peaks. The reddest feature is associated with strong hydrogen bonding; the dynamic polarizability of this feature is primarily parallel to the surface. It is assigned to a cooperative motion among the companion to the free-OH and four-coordinate oscillators hydrogen bonded to dangling lone-pair molecules on the surface. The bluest hydrogen-bonded feature is similarly assigned to a cooperative motion of the OH stretch of dangling lone-pair molecules and of four-coordinate molecules in the lower half bilayer that are hydrogen bonded to free-OH molecules. Reconstruction induced strain is present at as low as 113 K. These results provide a richer picture of the ice surface than has heretofore been possible.

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.

Experimental and theoretical evidence for bilayer-by-bilayer surface melting of crystalline ice

Significance Over 150 years ago, Faraday discovered the presence of a water layer on ice below the bulk melting temperature. This layer is important for surface chemistry and glacier sliding close to subfreezing conditions. The nature and thickness of this quasi-liquid layer has remained controversial. By combining experimental and simulated surface-specific vibrational spectroscopy, the thickness of this quasi-liquid layer is shown to change in a noncontinuous, stepwise fashion around 257 K. Below this temperature, the first bilayer is already molten; the second bilayer melts at this transition temperature. The blue shift in the vibrational response of the outermost water molecules accompanying the transition reveals a weakening of the hydrogen bond network upon an increase of the water layer thickness. On the surface of water ice, a quasi-liquid layer (QLL) has been extensively reported at temperatures below its bulk melting point at 273 K. Approaching the bulk melting temperature from below, the thickness of the QLL is known to increase. To elucidate the precise temperature variation of the QLL, and its nature, we investigate the surface melting of hexagonal ice by combining noncontact, surface-specific vibrational sum frequency generation (SFG) spectroscopy and spectra calculated from molecular dynamics simulations. Using SFG, we probe the outermost water layers of distinct single crystalline ice faces at different temperatures. For the basal face, a stepwise, sudden weakening of the hydrogen-bonded structure of the outermost water layers occurs at 257 K. The spectral calculations from the molecular dynamics simulations reproduce the experimental findings; this allows us to interpret our experimental findings in terms of a stepwise change from one to two molten bilayers at the transition temperature.