π-Hydrogen Bonding of Aromatics on the Surface of Aerosols: Insights from Ab Initio and Molecular Dynamics Simulation

Revealing the Impacts of Chemical Complexity on Submicrometer Sea Spray Aerosol Morphology

Sea spray aerosol (SSA) ejected through bursting bubbles at the ocean surface is a complex mixture of salts and organic species. Submicrometer SSA particles have long atmospheric lifetimes and play a critical role in the climate system. Composition impacts their ability to form marine clouds, yet their cloud-forming potential is difficult to study due to their small size. Here, we use large-scale molecular dynamics (MD) simulations as a "computational microscope" to provide never-before-seen views of 40 nm model aerosol particles and their molecular morphologies. We investigate how increasing chemical complexity impacts the distribution of organic material throughout individual particles for a range of organic constituents with varying chemical properties. Our simulations show that common organic marine surfactants readily partition between both the surface and interior of the aerosol, indicating that nascent SSA may be more heterogeneous than traditional morphological models suggest. We support our computational observations of SSA surface heterogeneity with Brewster angle microscopy on model interfaces. These observations indicate that increased chemical complexity in submicrometer SSA leads to a reduced surface coverage by marine organics, which may facilitate water uptake in the atmosphere. Our work thus establishes large-scale MD simulations as a novel technique for interrogating aerosols at the single-particle level.

Review of air-water interface adsorption and reactions between trace gaseous organic and oxidant compounds

The surface chemistry of the atmospheric aerosol through homogeneous and heterogeneous catalytic reactions in the bulk water and the air-water surface is reviewed. Water plays a critical role as a substrate or an actual reactant in atmospheric reactions. The atmospheric aerosol differs in shape and surface area. Many gaseous reactive species and oxidants react at the air-water surface. Different thermodynamic methods to estimate partitioning coefficients are explored. The Gibbs free energy is reduced when reactant gaseous species react with oxidant at the air-water surface; this phenomenon is explained using examples. Langmuir-Hinshelwood reaction mechanism to quantify the heterogeneous reaction rate at the air-water interface is discussed. Critical comparisons of various sampling techniques used to analyze adsorption and reaction at the water surface are presented. The heterogeneous reaction rate at the air-water surface is significantly higher than in the bulk water phase due to a cage effect, higher rate of reactions, and lower Gibbs free energy of adsorption.

Quantitative relationship between the structures and properties of VOCs and SOA formation on the surfaces of acidic aerosol particles

In this research, all the efforts, based on a series of molecular dynamics simulations on the interfacial process between VOC-contaminated air and acidic sulfate, were made to find how the structures and properties of VOCs are related to the formation of SOAs. The experimental fractional aerosol coefficients (FACs) were used to quantify the SOA formation and 14 VOC species were chosen based on the atmosphere inventory and the FAC magnitude. Finally, the quantitative relationship (QR) was found through the FAC as a function of the two variables the total valid interactions (Tg) and the diffusion coefficient (D), with R square 0.94. Meanwhile, the effect of water was explored and the QR was proved to be rational and reliable. The QR not only explained the SOA formation capacity of VOCs, but could also predict the SOA formation of new molecules.

Impact of Adsorbed Water on the Interaction of Limonene with Hydroxylated SiO2: Implications of π-Hydrogen Bonding for Surfaces in Humid Environments

The indoor environment is a dynamic one with many variables impacting indoor air quality and indoor air chemistry. These include relative humidity (RH) and the presence of different surfaces. Although it has been suggested that the indoor concentrations of gas-phase compounds increase at higher relative humidity, because of displacement of these compounds from indoor surfaces, little is known from a molecular perspective about how RH and adsorbed water impact the adsorption of indoor relevant organic compounds such as limonene with indoor relevant surfaces. Herein, we investigate the effects of RH on the adsorption of limonene, a hydrophobic molecule, on hydroxylated SiO₂ surfaces, a model for glass surfaces. Experimental data using infrared spectroscopy to directly measure limonene adsorption are combined with both force field-based molecular dynamics (MD) and ab initio molecular dynamics (AIMD) simulations to understand the competitive interactions between limonene, water, and the SiO₂ surface. The spectroscopic data provide evidence that adsorbed limonene is not completely displaced by adsorbed water, even at high RH (∼80%) when the water layer coverage is close to three monolayers (MLs). These experimental data are supported by AIMD and MD simulations, which indicate that limonene is present at the adsorbed water interface but displaced from direct interactions with SiO₂. This study shows that although some limonene can desorb from the surface, even at the highest RH, more than half the limonene remains adsorbed on the surface that can undergo continued surface reactivity. A complex network of π-hydrogen bonds, water-water hydrogen bonds, and SiO₂-water hydrogen bonds explains these interactions at the air/adsorbed water/SiO₂ interface that hold the hydrophobic limonene molecule at the interface. Importantly, these interactions are most likely present for a range of other systems involving organic compounds and solid surfaces at ambient relative humidity and may be important in a range of scientific areas, from sensor development to cultural heritage science.

Hidden Isolated OH at the Charged Hydrophobic Interface Revealed by Two-Dimensional Heterodyne-Detected VSFG Spectroscopy

Water around hydrophobic groups mediates hydrophobic interactions that play key roles in many chemical and biological processes. Thus, the molecular-level elucidation of the properties of water in the vicinity of hydrophobic groups is important. We report on the structure and dynamics of water at two oppositely charged hydrophobic ion/water interfaces, that is, the tetraphenylborate-ion (TPB^- )/water and tetraphenylarsonium-ion (TPA⁺ )/water interfaces, which are clarified by two-dimensional heterodyne-detected vibrational sum-frequency generation (2D HD-VSFG) spectroscopy. The obtained 2D HD-VSFG spectra of the anionic TPB^- interface reveal the existence of distinct π-hydrogen bonded OH groups in addition to the usual hydrogen-bonded OH groups, which are hidden in the steady-state spectrum. In contrast, 2D HD-VSFG spectra of the cationic TPA⁺ interface only show the presence of usual hydrogen-bonded OH groups. The present study demonstrates that the sign of the interfacial charge governs the structure and dynamics of water molecules that face the hydrophobic region.

A blob model to parameterize polymer hole free volumes and solute diffusion

Solute diffusion in solid polymers has tremendous applications in packaging, reservoir, and biomedical technologies but remains poorly understood. Diffusion of non-entangled linear solutes with chemically identical patterns (blobs) deviates dramatically in polymers in the solid-state (α_lin > 1, Macromolecules 2013, 46, 874) from their behaviors in the molten state (α_lin = 1, Macromolecules, 2007, 40, 3970). This work uses the scale invariance of the diffusivities, D, of linear probes D(N·M_blob + M_anchor,T,T_g) = N^{-α_lin(T,T_g)}D(M_blob + M_anchor,T,T_g) comprising N identical blobs of mass M_blob and possibly one different terminal pattern (anchor of mass M_anchor) to evaluate the amounts of hole-free volume in seven polymers (aliphatic, semi-aromatic and aromatic) over a broad range of temperatures (-70 K ≤T-T_g≤ 160 K). The new parameterization of the concept of hole-free volumes opens the application of the free-volume theory (FVT) developed by Vrentas and Duda to practically any polymer, regardless of the availability of free-volume parameters. The quality of the estimations was tested with various probes including n-alkanes, 1-alcohols, n-alkyl acetates, and n-alkylbenzene. The effects of enthalpic and entropic effects of the blobs and the anchor were analyzed and quantified. Blind validation of the reformulated FVT was tested successfully by predicting from first principles the diffusivities of water and toluene in amorphous polyethylene terephthalate from 4 °C to 180 °C and in various other polymers. The new blob model would open the rational design of additives with controlled diffusivities in thermoplastics.

Understanding the properties of methanesulfinic acid at the air-water interface

Methanesulfinic acid (MSIA), an organic sulfur compound, is mainly produced in the oxidation process of dimethyl sulfide in the atmosphere. The properties of MSIA at the air-water interface were studied using molecular dynamics (MD) simulations. The result shows that the lowest system free energy is located at the interface. Because the free energy difference between the interface and water phase is 3.2 kJ mol^-1, the MSIA molecule can easily get out of the free energy well and travel to water phase by the thermal motion, leading to only a 21% probability of its occurrence at the interface. The MSIA molecule tends to tilt at the interface with the sulfino group (-S(O)-OH) pointing toward the water phase. The feature of hydration status at the air-water interface may be favorable to the heterogeneous oxidation of MSIA.

Extensive H-atom abstraction from benzoate by OH-radicals at the air-water interface

Much is known about OH-radical chemistry in the gas-phase and bulk water. Important atmospheric and biological processes, however, involve little investigated OH-radical reactions at aqueous interfaces with hydrophobic media. Here, we report the online mass-specific identification of the products and intermediates generated on the surface of aqueous (H₂O, D₂O) benzoate-h5 and -d5 microjets by ∼8 ns ˙OH(g) pulses in air at 1 atm. Isotopic labeling lets us unambiguously identify the phenylperoxyl radicals that ensue H-abstraction from the aromatic ring and establish a lower bound (>26%) to this process as it takes place in the interfacial water nanolayers probed by our experiments. The significant extent of H-abstraction vs. its negligible contribution both in the gas-phase and bulk water underscores the unique properties of the air-water interface as a reaction medium. The enhancement of H-atom abstraction in interfacial water is ascribed, in part, to the relative destabilization of a more polar transition state for OH-radical addition vs. H-abstraction due to incomplete hydration at the low water densities prevalent therein.

Chain-propagation, chain-transfer, and hydride-abstraction by cyclic carbocations on water surfaces

New mechanisms for the growth and increase in complexity of atmospheric aerosol particles are elucidated. The present findings will also be useful for interfacial polymer/oligomer synthesis.