The interfaces of neat water and aqueous solutions play a prominent role in many technological processes and in the environment. Examples of aqueous interfaces are ultrathin water films that cover most hydrophilic surfaces under ambient relative humidities, the liquid/solid interface which drives many electrochemical reactions, and the liquid/vapor interface, which governs the uptake and release of trace gases by the oceans and cloud droplets. In this article we review some of the recent experimental and theoretical advances in our knowledge of the properties of aqueous interfaces and discuss open questions and gaps in our understanding.

Water at Interfaces

Silica is the most abundant metal oxide and the main component of the Earth’s crust. Its behavior in contact with water plays a critical role in a variety of geochemical and environmental processes. Despite its key role, the details of the aqueous silica interface at the microscopic molecular level are still elusive. Here we provide such a detailed understanding of the molecular behavior of the silica–water interface, using density functional theory based molecular dynamics (DFTMD) simulations, where a consistent treatment of the electronic structure of solvent and surface is provided. We have calculated the acidity of the silanol groups at the interface directly from the DFTMD simulations, without any fitting of parameters to the experimental data. We find two types of silanol groups at the surface of quartz: out-of-plane silanols with a strong acidic character (pKa = 5.6), which consequently results in the formation of strong and short hydrogen bonds with water molecules at the interface, and in-plane s...

The Silica-Water Interface: How the Silanols Determine the Surface Acidity and Modulate the Water Properties.

2 Aerosol 3 Patricia K. Quinn,*,† Douglas B. Collins,‡ Vicki H. Grassian,‡,§ Kimberly A. Prather,‡ 4 and Timothy S. Bates 5 †Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, Seattle, Washington 98115, United 6 States 7 ‡Center for Aerosol Impacts on Climate and the Environment, University of California at San Diego, La Jolla, California 92024, 8 United States 9 Department of Chemistry, University of Iowa, Iowa City, Iowa 52242, United States 10 Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Seattle, Washington 98105, United States 1

Chemistry and related properties of freshly emitted sea spray aerosol.

The importance of water in atmospheric and environmental chemistry initiated recent studies with results documenting catalysis, suppression and anti-catalysis of thermal and photochemical reactions due to hydrogen bonding of reagents with water. Water, even one water molecule in binary complexes, has been shown by quantum chemistry to stabilize the transition state and lower its energy. However, new results underscore the need to evaluate the relative competing rates between reaction and dissipation to elucidate the role of water in chemistry. Water clusters have been used successfully as models for reactions in gas-phase, in aqueous condensed phases and at aqueous surfaces. Opportunities for experimental and theoretical chemical physics to make fundamental new discoveries abound. Work in this field is timely given the importance of water in atmospheric and environmental chemistry.

Perspective: Water cluster mediated atmospheric chemistry

Vibrational sum-frequency generation (VSFG) spectroscopy is a powerful tool to study interfaces. Recently, multiplex heterodyne-detected VSFG (HD-VSFG) has been developed, which enables the direct measurement of complex second-order nonlinear susceptibility [χ(2)]. HD-VSFG has remarkable advantages over conventional VSFG. For example, the imaginary part of χ(2) [Imχ(2)] obtained with this interferometric technique is the direct counterpart to the infrared [Imχ(1)] and Raman [Imχ(3)] spectra in the bulk, and it is free from the spectral deformation inevitable in conventional VSFG [|χ(2)|2] spectra. The Imχ(2) signal is obtained with a sign that contains unambiguous information about the up/down orientation of interfacial molecules. Furthermore, HD-VSFG can be straightforwardly extended to time-resolved measurements when combined with photoexcitation. In this review, we describe the present status of experiments and applications of multiplex HD-VSFG spectroscopy, in particular with regard to the orientation...

Structure and Dynamics of Interfacial Water Studied by Heterodyne-Detected Vibrational Sum-Frequency Generation

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.

Interfacial water structure associated with phospholipid membranes studied by phase-sensitive vibrational sum frequency generation spectroscopy.

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.

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

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 CaCl2 subphase. The average tilt angles from the surface normal of the PO2− gr...

Na(+) and Ca(2+) effect on the hydration and orientation of the phosphate group of DPPC at air-water and air-hydrated silica interfaces

The interaction between dimethylsulfoxide (DMSO) and phospholipid monolayers with different polar headgroups was studied using “in situ” Brewster angle microscopy (BAM) coupled to a Langmuir trough. For a 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) monolayer, DMSO was shown to significantly impact the structure of the liquid expanded (LE) and gaseous phases. The domains reorganized to much larger domain structures. Domains in the liquid condensed (LC) phase were formed on the DMSO-containing subphase at the mean molecular area where only gaseous and LE phases were previously observed on the pure water subphase. These results clearly demonstrate the condensing and caging effect of DMSO molecules on the DPPC monolayer. Similar effects were found on dipalmitoyl phosphatidyl ethanolamine, glycerol, and serine phospholipids, indicating that the condensing and caging effect is not dependent upon the phospholipid headgroup structure. The DMSO-induced condensing and caging effect is the molecular mechanism...

Reorganization and caging of DPPC, DPPE, DPPG, and DPPS monolayers caused by dimethylsulfoxide observed using Brewster angle microscopy.

We present a direct comparison of phase sensitive sum-frequency generation experiments with phase reconstruction obtained by the maximum entropy method. We show that both methods lead to the same complex spectrum. Furthermore, we discuss the strengths and weaknesses of each of these methods, analyzing possible sources of experimental and analytical errors. A simulation program for maximum entropy phase reconstruction is available at: http://lbp.epfl.ch/.

Xiangke Chen

Papers

Interfacial water structure associated with phospholipid membranes studied by phase-sensitive vibrational sum frequency generation spectroscopy.

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

Na(+) and Ca(2+) effect on the hydration and orientation of the phosphate group of DPPC at air-water and air-hydrated silica interfaces

Reorganization and caging of DPPC, DPPE, DPPG, and DPPS monolayers caused by dimethylsulfoxide observed using Brewster angle microscopy.

Direct comparison of phase-sensitive vibrational sum frequency generation with maximum entropy method: case study of water.