Oxide- and Silicate-Water Interfaces and Their Roles in Technology and the Environment.

Interfacial reactions drive all elemental cycling on Earth and play pivotal roles in human activities such as agriculture, water purification, energy production and storage, environmental contaminant remediation, and nuclear waste repository management. The onset of the 21st century marked the beginning of a more detailed understanding of mineral aqueous interfaces enabled by advances in techniques that use tunable high-flux focused ultrafast laser and X-ray sources to provide near-atomic measurement resolution, as well as by nanofabrication approaches that enable transmission electron microscopy in a liquid cell. This leap into atomic- and nanometer-scale measurements has uncovered scale-dependent phenomena whose reaction thermodynamics, kinetics, and pathways deviate from previous observations made on larger systems. A second key advance is new experimental evidence for what scientists hypothesized but could not test previously, namely, interfacial chemical reactions are frequently driven by "anomalies" or "non-idealities" such as defects, nanoconfinement, and other nontypical chemical structures. Third, progress in computational chemistry has yielded new insights that allow a move beyond simple schematics, leading to a molecular model of these complex interfaces. In combination with surface-sensitive measurements, we have gained knowledge of the interfacial structure and dynamics, including the underlying solid surface and the immediately adjacent water and aqueous ions, enabling a better definition of what constitutes the oxide- and silicate-water interfaces. This critical review discusses how science progresses from understanding ideal solid-water interfaces to more realistic systems, focusing on accomplishments in the last 20 years and identifying challenges and future opportunities for the community to address. We anticipate that the next 20 years will focus on understanding and predicting dynamic transient and reactive structures over greater spatial and temporal ranges as well as systems of greater structural and chemical complexity. Closer collaborations of theoretical and experimental experts across disciplines will continue to be critical to achieving this great aspiration.

NaCl, MgCl2, and AlCl3 Surface Coverages on Fused Silica and Adsorption Free Energies at pH 4 from Nonlinear Optics.

We employ amplitude- and phase-resolved second harmonic generation experiments to probe interactions of fused silica:aqueous interfaces with Al3+, Mg2+, and Na+ cations at pH 4 and as a function of metal cation concentration. We quantify the second-order nonlinear susceptibility and the total interfacial potential in the presence and absence of a 10 mM screening electrolyte to understand the influence of charge screening on cation adsorption. Strong cation:surface interactions are observed in the absence of the screening electrolyte. The total potential is then employed to estimate the total number of absorbed cations cm-2. The contributions to the total potential from the bound and mobile charges were separated using Gouy-Chapman-Stern model estimates. All three cations bind fully reversibly, indicating physisorption as the mode of interaction. Of the isotherm models tested, the Kd adsorption model fits the data with binding constants of 3-30 and ∼300 mol-1 for the low (<0.1 mM) and high (0.1-3 mM) concentration regimes, corresponding to adsorption free energies of -13 to -18 and -24 kJ mol-1 at room temperature, respectively. The maximum surface coverages are around 1013 cations cm-2, matching the number of deprotonated silanol groups on silica at pH 4. Clear signs of decoupled Stern and diffuse layer nonlinear optical responses are observed and found to be cation-specific.

Large Gas-Phase Source of Esters and Other Accretion Products in the Atmosphere.

Dimeric accretion products have been observed both in atmospheric aerosol particles and in the gas phase. With their low volatilities, they are key contributors to the formation of new aerosol particles, acting as seeds for more volatile organic vapors to partition onto. Many particle-phase accretion products have been identified as esters. Various gas- and particle-phase formation pathways have been suggested for them, yet evidence remains inconclusive. In contrast, peroxide accretion products have been shown to form via gas-phase peroxy radical (RO2) cross reactions. Here, we show that these reactions can also be a major source of esters and other types of accretion products. We studied α-pinene ozonolysis using state-of-the-art chemical ionization mass spectrometry together with different isotopic labeling approaches and quantum chemical calculations, finding strong evidence for fast radical isomerization before accretion. Specifically, this isomerization seems to happen within the intermediate complex of two alkoxy (RO) radicals, which generally determines the branching of all RO2-RO2 reactions. Accretion products are formed when the radicals in the complex recombine. We found that RO with suitable structures can undergo extremely rapid C-C ß scissions before recombination, often resulting in ester products. We also found evidence of this previously overlooked RO2-RO2 reaction pathway forming alkyl accretion products and speculate that some earlier peroxide identifications may in fact be hemiacetals or ethers. Our findings help answer several outstanding questions on the sources of accretion products in organic aerosol and bridge our knowledge of the gas phase formation and particle phase detection of accretion products. As esters are inherently more stable than peroxides, this also impacts their further reactivity in the aerosol.

Organic synthesis in the study of terpene-derived oxidation products in the atmosphere.

Covering: 1997 up to 2022Volatile biogenic terpenes involved in the formation of secondary organic aerosol (SOA) particles participate in rich atmospheric chemistry that impacts numerous aspects of the earth's complex climate system. Despite the importance of these species, understanding their fate in the atmosphere and determining their atmospherically-relevant properties has been limited by the availability of authentic standards and probe molecules. Advances in synthetic organic chemistry directly aimed at answering these questions have, however, led to exciting discoveries at the interface of chemistry and atmospheric science. Herein we provide a review of the literature regarding the synthesis of commercially unavailable authentic standards used to analyze the composition, properties, and mechanisms of SOA particles in the atmosphere.

Quantification of Stern Layer Water Molecules, Total Potentials, and Energy Densities at Fused Silica:Water Interfaces for Adsorbed Alkali Chlorides, CTAB, PFOA, and PFAS.

We have employed amplitude- and phase-resolved second-harmonic generation spectroscopy to investigate ion-specific effects of monovalent cations at the fused silica:water interface maintained under acidic, neutral, and alkaline conditions. We find a negligible dependence of the total potential (as negative as -400 mV at pH 14), the second-order nonlinear susceptibility (as large as 1.5 × 10-21 m2 V-1 at pH 14), the number of Stern layer water molecules (1 × 1015 cm-2 at pH 5.8), and the energy associated with water alignment upon going from neutral to high pH (ca. -24 kJ mol-1 to -48 kJ mol-1 at pH 13 and 14, close to the cohesive energy of liquid water but smaller than that of ice) on chlorides of the alkali series (M+ = Li+, Na+, K+, Rb+, and Cs+). Attempts are presented to provide estimates for the molecular hyperpolarizability of the cations and anions in the Stern layer at high pH, which arrive at ca. 20-fold larger values for αtotal ions(2) = αM+(2) + αOH-(2) + αCl-(2) when compared to water's molecular hyperpolarizability estimate from theory and point to a sizable contribution of deprotonated silanol groups at high pH. In contrast to the alkali series, a pronounced dependence of the total potential and the second-order nonlinear susceptibility on monovalent cationic (cetrimonium bromide, CTAB) and anionic (perfluorooctanoic and perfluorooctanesulfonic acid, PFOA and PFOS) surfactants was quantifiable. Our findings are consistent with a low surface coverage of the alkali cations and a high surface coverage of the surfactants. Moreover, they underscore the important contribution of Stern layer water molecules to the total potential and second-order nonlinear susceptibility. Finally, they demonstrate the applicability of heterodyne-detected second-harmonic generation spectroscopy for identifying perfluorinated acids at mineral:water interfaces.

Water Structure in the Electrical Double Layer and the Contributions to the Total Interfacial Potential at Different Surface Charge Densities.

The electric double layer governs the processes of all charged surfaces in aqueous solutions; however, elucidating the structure of the water molecules is challenging for even the most advanced spectroscopic techniques. Here, we present the individual Stern layer and diffuse layer OH stretching spectra at the silica/water interface in the presence of NaCl over a wide pH range using a combination of vibrational sum frequency generation spectroscopy, heterodyned second harmonic generation, and streaming potential measurements. We find that the Stern layer water molecules and diffuse layer water molecules respond differently to pH changes: unlike the diffuse layer, whose water molecules remain net-oriented in one direction, water molecules in the Stern layer flip their net orientation as the solution pH is reduced from basic to acidic. We obtain an experimental estimate of the non-Gouy-Chapman (Stern) potential contribution to the total potential drop across the insulator/electrolyte interface and discuss it in the context of dipolar, quadrupolar, and higher order potential contributions that vary with the observed changes in the net orientation of water in the Stern layer. Our findings show that a purely Gouy-Chapman (Stern) view is insufficient to accurately describe the electrical double layer of aqueous interfaces.

Energy conversion via metal nanolayers.

Current approaches for electric power generation from nanoscale conducting or semiconducting layers in contact with moving aqueous droplets are promising as they show efficiencies of around 30%, yet even the most successful ones pose challenges regarding fabrication and scaling. Here, we report stable, all-inorganic single-element structures synthesized in a single step that generate electrical current when alternating salinity gradients flow along its surface in a liquid flow cell. Nanolayers of iron, vanadium, or nickel, 10 to 30 nm thin, produce open-circuit potentials of several tens of millivolt and current densities of several microA cm-2 at aqueous flow velocities of just a few cm s-1 The principle of operation is strongly sensitive to charge-carrier motion in the thermal oxide nanooverlayer that forms spontaneously in air and then self-terminates. Indeed, experiments suggest a role for intraoxide electron transfer for Fe, V, and Ni nanolayers, as their thermal oxides contain several metal-oxidation states, whereas controls using Al or Cr nanolayers, which self-terminate with oxides that are redox inactive under the experimental conditions, exhibit dramatically diminished performance. The nanolayers are shown to generate electrical current in various modes of application with moving liquids, including sliding liquid droplets, salinity gradients in a flowing liquid, and in the oscillatory motion of a liquid without a salinity gradient.

Molecular Chirality and Cloud Activation Potentials of Dimeric α-Pinene Oxidation Products.

The surface activity of ten atmospherically relevant α-pinene-derived dimers having varying terminal functional groups and backbone stereochemistry is reported. We find ∼10% differences in surface activity between diastereomers of the same dimer, demonstrating that surface activity depends upon backbone stereochemistry. Octanol-water (KOW) and octanol-ammonium sulfate partitioning coefficient (KOAS) measurements of our standards align well with the surface activity measurements, with the more surface-active dimers exhibiting increased hydrophobicity. Our findings establish a link between molecular chirality and cloud activation potential of secondary organic aerosol particles. Given the diurnal variations in enantiomeric excess of biogenic emissions, possible contributions of such a link to biosphere:atmosphere feedbacks as well as aerosol particle viscosity and phase separation are discussed.

Divalent Ion Specific Outcomes on Stern Layer Structure and Total Surface Potential at the Silica:Water Interface.

The second-order nonlinear susceptibility, χ(2), in the Stern layer and the total interfacial potential drop, Φ(0)tot, across the oxide:water interface are estimated from SHG amplitude and phase measurements for divalent cations (Mg2+, Ca2+, Sr2+, and Ba2+) at the silica:water interface at pH 5.8 and various ionic strengths. We find that interfacial structure and total potential depend strongly on ion valency. We observe statistically significant differences between the experimentally determined χ(2) value for NaCl and that of the alkali earth series but smaller differences between ions of the same valency in that series. These differences are particularly pronounced at intermediate salt concentrations, which we attribute to the influence of hydration structure in the Stern layer. Furthermore, we corroborate the differences by examining the effects of anion substitution (SO42- for Cl-). Finally, we identify that hysteresis in measuring the reversibility of ion adsorption and desorption at fused silica in forward and reverse titrations manifests itself both in Stern layer structure and in total interfacial potential for some of the salts, most notably for CaCl2 and MgSO4 but less so for BaCl2 and NaCl.

In Situ Ni2+ Stain for Liposome Imaging by Liquid-Cell Transmission Electron Microscopy.

Solvated soft matter, both biological and synthetic, can now be imaged in liquids using liquid-cell transmission electron microscopy (LCTEM). However, such systems are usually composed solely of organic molecules (low Z elements) producing low contrast in TEM, especially within thick liquid films. We aimed to visualize liposomes by LCTEM rather than requiring cryogenic TEM (cryoTEM). This is achieved here by imaging in the presence of aqueous metal salt solutions. The increase in scattering cross-section by the cation gives a staining effect that develops in situ, which could be captured by real space TEM and verified by in situ energy dispersive x-ray spectroscopy (EDS). We identified beam-induced staining as a time-dependent process that enhances contrast to otherwise low contrast materials. We describe the development of this imaging method and identify conditions leading to exceptionally low electron doses for morphology visualization of unilamellar vesicles before beam-induced damage propagates.

Partially (resp. fully) reversible adsorption of monoterpenes (resp. alkanes and cycloalkanes) to fused silica.

This work compares the extent of reversibility and the thermodynamics of adsorption (Kads, ΔG°ads) of room-temperature vapors of common environmentally relevant monoterpenes (α-pinene, ß-pinene, limonene, and 3-carene) and industrially relevant cyclic and acyclic non-terpene hydrocarbons (cyclohexane, hexane, octane, and cyclooctane) to fused silica surfaces. Vibrational sum frequency generation spectroscopy carried out in the C-H stretching region shows negligible surface coverage-dependent changes in the molecular orientation of all species surveyed except for cyclohexane. The group of monoterpenes studied here distinctly exhibits partially reversible adsorption to fused silica surfaces compared to the group of non-terpene hydrocarbons, demonstrating a link between molecular structure and adsorption thermodynamics. The standard Gibbs free energy of adsorption is nonlinearly correlated with the equilibrium vapor pressure of the compounds surveyed.

Single-component supported lipid bilayers probed using broadband nonlinear optics.

Broadband SFG spectroscopy is shown to offer considerable advantages over scanning systems in terms of signal-to-noise ratios when probing well-formed single-component supported lipid bilayers formed from zwitterionic lipids with PC headgroups. The SFG spectra obtained from bilayers formed from DOPC, POPC, DLPC, DMPC, DPPC and DSPC show a common peak at ∼2980 cm-1, which is subject to interference between the C-H and the O-H stretches from the aqueous phase, while membranes having transition temperatures above the laboratory temperature produce SFG spectra with at least two additional peaks, one at ∼2920 cm-1 and another at ∼2880 cm-1. The results validate spectroscopic and structural data from SFG experiments utilizing asymmetric bilayers in which one leaflet differs from the other in the extent of deuteration. Differences in H2O-D2O exchange experiments reveal that the lineshapes of the broadband SFG spectra are significantly influenced by interference from OH oscillators in the aqueous phase, even when those oscillators are not probed by the incident infrared light in our broadband setup. In the absence of spectral interference from the OH stretches of the solvent, the alkyl chain terminal methyl group of the bilayer is found to be tilted at an angle of 15° to 35° from the surface normal.

Second-Order Vibrational Lineshapes from the Air/Water Interface.

We explore by means of modeling how absorptive-dispersive mixing between the second- and third-order terms modifies the imaginary χtotal(2) responses from air/water interfaces under conditions of varying charge densities and ionic strength. To do so, we use published Im(χ(2)) and χ(3) spectra of the neat air/water interface that were obtained either from computations or experiments. We find that the χtotal(2) spectral lineshapes corresponding to experimentally measured spectra contain significant contributions from both interfacial χ(2) and bulk χ(3) terms at interfacial charge densities equivalent to less than 0.005% of a monolayer of water molecules, especially in the 3100 to 3300 cm-1 frequency region. Additionally, the role of short-range static dipole potentials is examined under conditions mimicking brine. Our results indicate that surface potentials, if indeed present at the air/water interface, manifest themselves spectroscopically in the tightly bonded H-bond network observable in the 3200 cm-1 frequency range.

Relative permittivity in the electrical double layer from nonlinear optics.

Second harmonic generation (SHG) spectroscopy has been applied to probe the fused silica/water interface at pH 7 and the uncharged 11¯02 sapphire/water interface at pH 5.2 in contact with aqueous solutions of NaCl, NaBr, NaI, KCl, RbCl, and CsCl as low as several 10 µM. For ionic strengths up to about 0.1 mM, the SHG responses were observed to increase, reversibly for all salts surveyed, when compared to the condition of zero salt added. Further increases in the salt concentration led to monotonic decreases in the SHG response. The SHG increases followed by decreases are found to be consistent with recent reports of phase interference and phase matching in nonlinear optics. By varying the relative permittivity employed in common mean field theories used to describe electrical double layers and by comparing our results to available literature data, we find that models recapitulating the experimental observations are the ones in which (1) the relative permittivity of the diffuse layer is that of bulk water, with other possible values as low as 30, (2) the surface charge density varies with salt concentration, and (3) the charge in the Stern layer or its thickness varies with salt concentration. We also note that the experimental data exhibit sensitivity depending on whether the salt concentration is increased from low to high values or decreased from high to low values, which, however, is not borne out in the fits, at least within the current uncertainties associated with the model point estimates.

Quantifying the Electrostatics of Polycation-Lipid Bilayer Interactions.

Mechanistic insight into how polycations disrupt and cross cell membranes is needed for understanding and controlling polycation-membrane interactions, yet such information is surprisingly difficult to obtain at the molecular level. We use second harmonic and vibrational sum frequency generation spectroscopies along with quartz crystal microbalance with dissipation monitoring and computer simulations to quantify the interaction of poly(allylamine) hydrochloride (PAH) and its monomeric precursor allylamine hydrochloride (AH) with lipid bilayers. We find PAH adsorption to be reversible and nondisruptive to the bilayer under the conditions of our experiments. With an observed free adsorption energy of -52.7 ± 0.6 kJ/mol, PAH adsorption was found to be surprisingly less favorable relative to AH (-14.6 ± 0.4 kJ/mol) when considering a simple additive model. By experimentally quantifying the number of adsorbates and the average amount of charge carried by each adsorbate, we find that the PAH is associated with only 70% of the positive charges it could hold while the AH remains mostly charged while attached to the membrane. Simulations indicate that PAH pulls in condensed counterions from solution to avoid charge-repulsion along its backbone and with other PAH molecules to attach to, and completely cover, the bilayer surface. In addition, computations indicate that the amine groups shift their pKa values due to the confined environment upon adsorption to the surface. Our results provide experimental constraints for theoretical calculations, which yield atomistic views of the structures that are formed when polycations interact with lipid membranes that will be important for predicting polycation-membrane interactions.

Highly Oxygenated Multifunctional Compounds in α-Pinene Secondary Organic Aerosol.

Highly oxygenated multifunctional organic compounds (HOMs) originating from biogenic emissions constitute a widespread source of organic aerosols in the pristine atmosphere. However, the molecular forms in which HOMs are present in the condensed phase upon gas-particle partitioning remain unclear. In this study, we show that highly oxygenated molecules that contain multiple peroxide functionalities are readily cationized by the attachment of Na+ during electrospray ionization operated in the positive ion mode. With this method, we present the first identification of HOMs characterized as C8-10H12-18O4-9 monomers and C16-20H24-36O8-14 dimers in α-pinene derived secondary organic aerosol (SOA). Simultaneous detection of these molecules in the gas phase provides direct evidence for their gas-to-particle conversion. Molecular properties of particulate HOMs generated from ozonolysis and OH oxidation of unsubstituted (C10H16) and deuterated (C10H13D3) α-pinene are investigated using coupled ion mobility spectrometry with mass spectrometry. The systematic shift in the mass of monomers in the deuterated system is consistent with the decomposition of isomeric vinylhydroperoxides to release vinoxy radical isotopologues, the precursors to a sequence of autoxidation reactions that ultimately yield HOMs in the gas phase. The remarkable difference observed in the dimer abundance under O3- versus OH-dominant environments underlines the competition between intramolecular hydrogen migration of peroxy radicals and their bimolecular termination reactions. Our results provide new and direct molecular-level information for a key component needed for achieving carbon mass closure of α-pinene SOA.

Unanticipated Stickiness of α-Pinene.

The adsorption of α-pinene to solid surfaces is an important primary step during the chemical conversion of this common terpene over mesoporous materials, as well as during the formation of atmospheric aerosols. We provide evidence of tight and loose physisorbed states of α-pinene bound on amorphous SiO2 as determined by their adsorption entropy, enthalpy, and binding free energies characterized by computational modeling and vibrational sum frequency generation (SFG) spectroscopy. We find that adsorption is partially (40-60%) irreversible over days at 294-342 K and 1 ATM total pressure of helium, which is supported by molecular dynamics (MD) simulations. The distribution of α-pinene orientation remains invariant with temperature and partial pressure of α-pinene. Using the Redlich-Peterson adsorption model in conjunction with a van't Hoff analysis of adsorption isotherms recorded for up to 2.6 Torr α-pinene in 1 ATM total pressure of helium, we obtain ΔS°ads, ΔH°ads, and ΔG°ads values of -57 (±7) J mol-1 K-1, -39 (±2) kJ mol-1, and -22 (±5) kJ mol-1, respectively, associated with the reversibly bound population of α-pinene. These values are in good agreement with density functional theory (DFT)-corrected force field calculations based on configurational sampling from MD simulations. Our findings are expected to have direct implications on the conversion of terpenes by silica-based catalysts and for the synthesis of secondary organic aerosol (SOA) in atmospheric chambers and flow tubes.