Heather C. Allen

Interfacial Water Structure Associated with Phospholipid Membranes Studied by Phase-Sensitive Vibrational Sum Frequency Generation Spectroscopy

Phase-sensitive vibrational sum frequency generation is employed to investigate the water structure at phospholipid/water interfaces. Interfacial water molecules are oriented preferentially by the electrostatic potential imposed by the phospholipids and have, on average, their dipole pointing toward the phospholipid tails for all phospholipids studied, dipalmitoyl phosphocholine (DPPC), dipalmitoyl phosphoethanolamine (DPPE), dipalmitoyl phosphate (DPPA), dipalmitoyl phosphoglycerol (DPPG), and dipalmitoyl phospho-l-serine (DPPS). Zwitterionic DPPC and DPPE reveal weaker water orienting capability relative to net negative DPPA, DPPG, and DPPS. Binding of calcium cations to the lipid phosphate group reduces ordering of the water molecules.

Environmental Chemistry at Vapor/Water Interfaces: Insights from Vibrational Sum Frequency Generation Spectroscopy

The chemistry that occurs at surfaces has been an intense area of study for many years owing to its complexity and importance in describing a wide range of physical phenomena. The vapor/water interface is particularly interesting from an environmental chemistry perspective as this surface plays host to a wide range of chemistries that influence atmospheric and geochemical interactions. The application of vibrational sum frequency generation (VSFG), an inherently surface-specific, even-order nonlinear optical spectroscopy, enables the direct interrogation of various vapor/aqueous interfaces to elucidate the behavior and reaction of chemical species within the surface regime. In this review we discuss the application of VSFG to the study of a variety of atmospherically important systems at the vapor/aqueous interface. Chemical systems presented include inorganic ionic solutions prevalent in aqueous marine aerosols, small molecular solutes, and long-chain fatty acids relevant to fat-coated aerosols. The ability of VSFG to probe both the organization and reactions that may occur for these systems is highlighted. A future perspective toward the application of VSFG to the study of environmental interfaces is also provided.

Solvation of Magnesium Dication: Molecular Dynamics Simulation and Vibrational Spectroscopic Study of Magnesium Chloride in Aqueous Solutions

Magnesium dication plays many significant roles in biochemistry. While it is available to the environment from both ocean waters and mineral salts on land, its roles in environmental and atmospheric chemistry are still relatively unknown. Several pieces of experimental evidence suggest that contact ion pairing may not exist at ambient conditions in solutions of magnesium chloride up to saturation concentrations. This is not typical of most ions. There has been disagreement in the molecular dynamics literature concerning the existence of ion pairing in magnesium chloride solutions. Using a force field developed during this study, we show that contact ion pairing is not energetically favorable. Additionally, we present a concentration-dependent Raman spectroscopic study of the Mg-O(water) hexaaquo stretch that clearly supports the absence of ion pairing in MgCl(2) solutions, although a transition occurring in the spectrum between 0.06x and 0.09x suggests a change in solution structure. Finally, we compare experimental and calculated observables to validate our force field as well as two other commonly used magnesium force fields, and in the process show that ion pairing of magnesium clearly is not observed at higher concentrations in aqueous solutions of magnesium chloride, independent of the choice of magnesium force field, although some force fields give better agreement to experimental results than others.

Shedding light on water structure at air-aqueous interfaces: ions, lipids, and hydration

An account is given of the current state of understanding of aqueous salt, acid, and lipid/water surfaces, interfacial depth, and molecular organization within the air-solution interfacial region. Water structure, hydration, surface propensity of solutes, and surface organization are discussed. In this perspective, vibrational sum frequency generation spectroscopic studies of aqueous surfaces are interpreted. Comment on future directions within the field of aqueous surface structure is provided.

Ionic Binding of Na+ versus K+ to the Carboxylic Acid Headgroup of Palmitic Acid Monolayers Studied by Vibrational Sum Frequency Generation Spectroscopy†

Ionic binding of alkali ions Na(+) and K(+) to the carboxylic acid headgroups of fatty acid monolayers is studied as a proxy toward understanding the fundamental chemistry in cell biology. In this study, we used broad-bandwidth sum frequency generation (BBSFG) vibrational spectroscopy to investigate the ionic binding event that leads to deprotonation and complex formation of fatty acid headgroups. Palmitic acid (C(15)H(31)COOH) exists as a monolayer on aqueous surfaces. Surface vibrational stretch modes of palmitic acid from 1400 cm(-1) to 3700 cm(-1) were observed (nu(s)-COO(-), nu-C horizontal lineO, nu-C-H, nu-O-H of -COOH, free OH). Palmitic acid is mostly protonated at the aqueous surface at neutral pH (approximately 6). However, various degrees of deprotonation are initiated by the introduction of Na(+) and K(+) that results in the complexation of K(+):COO(-) and solvent separated Na(+):COO(-). Evidence in several spectral regions indicates that K(+) exhibits stronger ionic binding affinity to the carboxylate anion relative to Na(+).

Na+ and Ca2+ Effect on the Hydration and Orientation of the Phosphate Group of DPPC at Air−Water and Air−Hydrated Silica Interfaces

Hydration and orientation of the phosphate group of dipalmitoylphosphatidylcholine (DPPC) monolayers in the liquid-expanded (LE) phase and the liquid-condensed (LC) phase in the presence of sodium ions and calcium ions was investigated with vibrational sum frequency generation (SFG) spectroscopy at the air-aqueous interface in conjunction with surface pressure measurements. In the LE phase, both sodium and calcium affect the phosphate group hydration. In the LC phase, however, sodium ions affect the phosphate hydration subtly, while calcium ions cause a marked dehydration. Silica-supported DPPC monolayers prepared by the Langmuir-Blodgett method reveal similar hydration behavior relative to that observed in the corresponding aqueous subphase for the case of water and in the presence of sodium ions. However, in the presence of calcium ions the phosphate group dehydration is greater than that from the corresponding purely aqueous CaCl(2) subphase. The average tilt angles from the surface normal of the PO(2)(-) group of DPPC monolayers on the water surface and on the silica substrate calculated from SFG data are found to be 59 degrees +/- 3 degrees and 72 degrees +/- 5 degrees , respectively. Orientation of the phosphate group is additionally affected by the presence of ions. These findings show that extrapolation of results obtained from model membranes from liquid surfaces to solid supports may not be warranted since there are differences in headgroup organization on the two subphases.

Organization of Water and Atmospherically Relevant Ions and Solutes: Vibrational Sum Frequency Spectroscopy at the Vapor/Liquid and Liquid/Solid Interfaces

The nature of waters hydrogen-bonding network is a vital influence on the chemistry that occurs at interfaces, but a complete understanding of interfacial water has proven elusive. Even-order nonlinear optical spectroscopies, such as vibrational sum frequency generation (VSFG) spectroscopy and heterodyne detected phase-sensitive sum frequency generation (PS-SFG) spectroscopy, are inherently surface specific. With the advent of advances in these spectroscopic techniques, researchers can now explore many long-standing questions about the dynamics and structures present at the vapor-water and water-solid interfaces. Of special interest to the atmospheric chemistry community is the accommodation of ions and solutes by waters hydrogen-bonding network. A better understanding of how ions and solutes behave in hydrogen-bonded water has afforded a fresh perspective of aqueous aerosols, because the interactions involved therein drive phenomena such as the hydrolysis of atmospheric chemical species. In this Account, we present work from our laboratory focusing on applying VSFG and the recently developed PS-SFG techniques to probe the perturbation of waters hydrogen-bonding network at the vapor-water interface by a variety of ions and solutes. We also present very recent results from our laboratory on the direct observation of the adsorption of ions at the water-CaF(2) interface. We begin by discussing the influence of ions and solutes on interfacial water structure. Results for halide salts and the acid analogs on interfacial water structure are shown to be quite different, as would be expected from differences in surface tension measurements that have been known for a long time. Also examined are systems with the largely polarizable molecular anions nitrate (NO(3)(-)), sulfate (SO(4)(2-)), carbonate (CO(3)(2-)), and bicarbonate (HCO(3)(-)).These systems feature more complicated influences on interfacial water structure than halide-containing solutions; however, our conventional VSFG results for both nitrate and sulfate solutions are in agreement with recent PS-SFG results and molecular dynamics simulations. We also discuss recent PS-SFG work on carbonate and bicarbonate systems in which the accommodation of the bicarbonate ion at the vapor-water interface is in stark contrast to the carbonate results. Perturbation of interfacial water by solutes is examined for solutions of dimethyl sulfoxide and methylsulfonic acid. PS-SFG results for these systems are striking: they illustrate the dramatic changes that interfacial water molecules undergo in the presence of solutes that are not observed with conventional VSFG. Finally, we discuss direct sulfate ion adsorption for the aqueous sodium sulfate-CaF(2) interface, with the goal of elucidating water behavior at this surface.

Binding of Mg2+ and Ca2+ to Palmitic Acid and Deprotonation of the COOH Headgroup Studied by Vibrational Sum Frequency Generation Spectroscopy

At the air/liquid interface, cation binding specificity of alkaline earth cations, Mg(2+) and Ca(2+), with the biologically relevant ligand carboxylate (COO(-)) using vibrational sum frequency generation spectroscopy is reported. The empirical evidence strongly supports that the ionic binding strength is much stronger for Ca(2+) to COO(-) than that for Mg(2+). We conclude that at a near-neutral pH, the mechanism that governs Ca(2+) binding to COO(-) is accompanied by commensurate deprotonation of the carboxyl headgroup. In addition, surface molecular structure and ion concentration influence the cation binding behavior at the air/liquid interface. In a 0.1 M Ca(2+)(aq) solution, Ca(2+) initially favors forming ionic complexes in a 2:1 bridging configuration (2Ca(2+):1COO(-)) but 1:1 chelating bidentate complexes (1Ca(2+):1COO(-)) gradually emerge as secondary species as the system reaches equilibrium. As the Ca(2+) concentration rises to 0.3 M, the primary complexed species exists in the 2:1 bridging configuration. Unlike Ca(2+), Mg(2+) at 0.1 and 0.3 M favors a solvent-separated ionic complex with COO(-).

Effects of Sodium Chloride Particles, Ozone, UV, and Relative Humidity on Atmospheric Corrosion of Silver

The corrosion of Ag contaminated with NaCl particles in gaseous environments containing humidity and ozone was investigated. In particular, the effects of relative humidity and UV light illumination were quantitatively analyzed using a coulometric reduction technique. The atmospheric corrosion of Ag was greatly accelerated in the presence of ozone and UV light. Unlike bare Ag i.e., with no NaCl particles on the surface, Ag with NaCl exhibited fast corrosion even in the dark, with no UV in the presence of ozone. Samples exposed to different outdoor environments and samples exposed in a salt spray chamber were studied for comparison. Ag corroded at extremely low rates in a salt spray chamber partly because of the combined absence of light and oxidizing agents such as ozone.

Interfacial water structure and effects of Mg2+ and Ca2+ binding to the COOH headgroup of a palmitic acid monolayer studied by sum frequency spectroscopy.

The interfacial hydrogen-bonding network that uniquely exists in between a palmitic acid (PA) monolayer and the underneath surface water molecules was studied using vibrational sum frequency generation (VSFG) spectroscopy. Perturbations due to cation binding of Mg(2+) and Ca(2+) were identified. The polar ordering of the interfacial water molecules under the influence of the surface field of dissociated PA headgroups was observed. Only a fraction of PA molecules are deprotonated at the air/water interface with a neat water subphase, yet the submonolayer concentration of negatively charged PA headgroups induces considerable polar ordering on the interfacial water molecules relative to the neat water surface without the PA film. Upon addition of calcium and magnesium chloride salts to the subphase of the PA monolayer, the extent of polar ordering of the interfacial water molecules was reduced. Ca(2+) was observed to have the greater impact on the interfacial hydrogen-bonding network relative to Mg(2+), consistent with the greater binding affinity of Ca(2+) toward the carboxylate group relative to Mg(2+) and thereby modifying the interfacial charge. At high-salt concentrations, the already disrupted hydrogen-bonding network reorganizes and reverts to its original hydrogen-bonding structure as that which appeared at the neat salt solution surface without a PA monolayer.