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At the air–water interface, many reactions are accelerated, sometimes by several orders of magnitude. This phenomenon has proved to be particularly important in water microdroplets, where the spontaneous oxidation of many species stable in bulk has been experimentally demonstrated. Different theories have been proposed to explain this finding, but it is currently believed that the role of interfacial electric fields is key. In this work, we have carried out a quantum chemistry study aimed at shedding some light on this question. We have studied two prototypical processes in which a hydroxide anion transfers its excess electron to either the water environment or a dioxygen molecule. To model the interface, we use a cluster of 21 water molecules immersed in an electric field, and we examine the energetics of the studied reactions as a function of field magnitude. Our results reveal that electric fields close to those estimated for the neat air–water interface (∼0.15 V ⋅ Å−1) have a moderate effect on the reaction energetics and that much stronger fields (>1 V ⋅ Å−1) are required to get spontaneous electron transfer. Therefore, the study suggests that additional factors such as an excess charge in microdroplets need to be considered for explaining the experimental observations. [ABSTRACT FROM AUTHOR]

Water microdroplets have been demonstrated to exhibit extraordinary chemical behaviors, including the abilities to accelerate chemical reactions by several orders of magnitude and to trigger reactions that cannot occur in bulk water. One of the most striking examples is the spontaneous generation of hydroxyl radical from hydroxide ions. Alcohols and alkoxide ions, being structurally similar to water and hydroxide ions, might exhibit similar behavior on microdroplets. Here, we report the spontaneous generation of alkoxide radicals from alcohols (RCH2OH) in aqueous microdroplets through quantum chemical calculations, quantum mechanical/molecular mechanical (QM/MM) molecular dynamics (MD) simulations, ab initio MD simulations, and mass spectrometry. Our results show that an electric field (EF) on the order of 10−1 V/Å and partial solvation at the air‐water interface jointly promote the dissociation of RCH2OH into RCH2O− and H3O+ ions. QM/MM MD simulations indicate that RCH2O− can be ionized to produce RCH2O⋅ radicals on the microdroplet surface. Furthermore, partial solvation and the EF collaboratively catalyze the isomerization of the RCH2O⋅ radical into a more stable tautomer, R⋅CHOH. This study highlights the molecular mechanisms underlying the widespread generation of radicals at the microdroplet surface and provides insights into the importance of fundamental alcohol chemistry in the atmosphere. [ABSTRACT FROM AUTHOR]

The efficient reduction of CO2 into valuable products is a challenging task in an international context marked by the climate change crisis and the need to move away from fossil fuels. Recently, the use of water microdroplets has emerged as an interesting reaction media where many redox processes which do not occur in conventional solutions take place spontaneously. Indeed, several experimental studies in microdroplets have already been devoted to study the reduction of CO2 with promising results. The increased reactivity in microdroplets is thought to be linked to unique electrostatic solvation effects at the air‐water interface. In the present work, we report a theoretical investigation on this issue for CO2 using first‐principles molecular dynamics simulations. We show that CO2 is stabilized at the interface, where it can accumulate, and that compared to bulk water solution, its electron capture ability is larger. Our results suggest that reduction of CO2 might be easier in interface‐rich systems such as water microdroplets, which is in line with early experimental data and indicate directions for future laboratory studies. The effect of other relevant factors which could play a role in CO2 reduction potential is discussed. [ABSTRACT FROM AUTHOR]

We present a catalyst‐free route for the reduction of carbon dioxide integrated with the formation of a carbon‐carbon bond at the air/water interface of negatively charged aqueous microdroplets, at ambient temperature. The reactions proceed through carbanion generation at the α‐carbon of a ketone followed by nucleophilic addition to CO2. Online mass spectrometry reveals that the product is an α‐ketoacid. Several factors, such as the concentration of the reagents, pressure of CO2 gas, and distance traveled by the droplets, control the kinetics of the reaction. Theoretical calculations suggest that water in the microdroplets facilitates this unusual chemistry. Furthermore, such a microdroplet strategy has been extended to seven different ketones. This work demonstrates a green pathway for the reduction of CO2 to useful carboxylated organic products. [ABSTRACT FROM AUTHOR]

Pore‐network models are powerful tools for studying particle transport in complex porous media, and investigating the role of interfaces in their fate. The first step in simulating particle transport using pore‐network models is to quantitatively describe particle transport in a single pore, and obtain relationships between pore‐averaged deposition rate coefficients and various pore‐scale parameters. So, in this study, a three‐dimensional (3D) mathematical model is developed to simulate the transport and retention of nanoparticles within a single partially saturated pore with an angular cross‐section. The model accounts for particle deposition at solid‐water interfaces (SWIs), air‐water interfaces (AWIs), and air‐water‐solid (AWS) contact regions. We provide a novel formulation for particle diffusive transport from AWI to AWS, where particles are assumed to be retained irreversibly by capillary forces. The model involves 12 dimensionless parameters representing various physicochemical conditions. The 3D model results are averaged over the pore cross‐section and then fitted to breakthrough curves from one‐dimensional (1D) advection‐dispersion‐sorption equations with three‐site kinetics to estimate 1D‐averaged deposition rate coefficients at interfaces. We find that half‐corner angle, particle size, radius of curvature of AWI, and mean flow velocity have a significant effect on those coefficients. In contrast, chemical parameters such as ionic strength and surface potentials of particles and interfaces have negligible effects. AWS is found to be the major retention site for particles, especially for hydrophobic particles. We develop algebraic relationships between 1D‐averaged deposition rate coefficients at interfaces vis‐à‐vis various pore‐scale parameters. These relationships are needed for pore‐network models to upscale nanoparticle transport to continuum scale. Key Points: A novel formulation for particle diffusive transport from air‐water interface to contact regionParameters describing geometry, flow, and hydrophobic energy govern particle retention at interfacesDeveloped algebraic relationships between 1D‐averaged deposition rate coefficients at interfaces vis‐à‐vis various pore‐scale parameters [ABSTRACT FROM AUTHOR]

Interfaces, the boundary that separates two or more chemical compositions and/or phases of matter, alters basic chemical and physical properties including the thermodynamics of selectivity, transition states, and pathways of chemical reactions, nucleation events and phase growth, and kinetic barriers and mechanisms for mass transport and heat transport. While progress has been made in advancing more interface‐sensitive experimental approaches, their interpretation requires new theoretical methods and models that in turn can further elaborate on the microscopic physics that make interfacial chemistry so unique compared to the bulk phase. In this review, we describe some of the most recent theoretical efforts in modeling interfaces, and what has been learned about the transport and chemical transformations that occur at the air–liquid and solid–liquid interfaces. This article is categorized under:Structure and Mechanism > Reaction Mechanisms and CatalysisStructure and Mechanism > Computational Materials ScienceSoftware > Quantum ChemistrySoftware > Simulation Methods [ABSTRACT FROM AUTHOR]

The Sequential Quantum Mechanics/Molecular Mechanics scheme has been enacted to perform a systematic investigation of the polarizability (α) and first hyperpolarizability (β) responses at the water–vacuum interface. After performing classical molecular dynamics simulations to provide snapshots of the structures, quantum chemistry calculations of the linear and nonlinear optical responses have been performed for clusters of five water molecules at the time‐dependent DFT level in combination with different embedding schemes, ranging from point charges to polarizable point charges, with and without local field effects. When going from the bulk to the interface, the main observations of these calculations encompass i) a modest increase of the average polarizability but an increase by about a factor of two of its anisotropy, ii) an increase by about 20 % of the βHRS response, accompanied by a small increase of its depolarization ratio, and iii) a net increase of the component of the β tensor normal to the interface (βzzz) as well as of β//. Globally, the interfacial effects on β are localized at the first molecular layer while they are observed up to the fourth molecular layer on α. [ABSTRACT FROM AUTHOR]

Per‐ and polyfluoroalkyl substances (PFASs), also known as "forever chemicals," are manmade chemicals that have been increasingly detected in various geological settings since the early 2000s. The soil and subsurface environments are the geological media commonly affected by PFAS. We conducted a comprehensive review of peer‐reviewed articles published from 2010 through 2022 concerning the fate and transport of PFAS in subsurface environments. This review is organized into different subsections, covering the basics of PFAS properties and how they affect the occurrence, fate, and transport of PFAS, the fundamental processes affecting subsurface transport and fate of PFAS, and mathematical models for describing and predicting PFAS transport behaviors. Mechanisms governing PFAS transport in the subsurface environment, including the sorption of PFAS at the air‐water interface, solid‐water interface, and nonaqueous phase liquids‐water interface, were explored in detail. Challenges and future research priorities are identified to better mitigate the global challenges of PFAS contamination. Plain Language Summary: Per‐ and polyfluoroalkyl substances (PFASs) are a group of manmade chemicals used in a multitude of applications and commercial products (clothing, cookware, cosmetics, personal care products, fire extinguishers, etc.). Their widespread use has, however, lead to increasing release and distribution in terrestrial and aquatic ecosystems. PFAS, due to their unique properties, are very persistent in the environment as they do not degrade easily. Unfortunately, they are also toxic to organisms, including humans. In this review, we discuss the omnipresence of PFAS in water and soil and explain how PFAS move through subsurface media. There are thousands of different PFAS compounds, and only few of them have been studied in more detail and their transport behavior is known. PFAS tend to accumulate at the air‐water interface in the vadose zone. Generally, PFAS with long carbon chains are retained in soils more strongly than short‐chain compounds. Negatively charged PFAS are more mobile in soils than neutral and positively charged PFAS, and thus can readily pollute groundwater. Nonetheless, due to their persistence and slow desorption, even more strongly sorbed PFAS can be a sustained source for groundwater pollution. We finally identify challenges and future research needs to address the emerging threat of PFAS contamination of the environment. [ABSTRACT FROM AUTHOR]

Despite the high stability of bulk water, water microdroplets possess strikingly different properties, such as the presence of hydroxyl radicals (OH⋅) at the air–water interface. Previous studies exhibited the recombination of OH⋅ into H2O2 molecules and the capture of OH⋅ by oxidizing other molecules. By spraying pure water microdroplets into a mass spectrometer, we detected OH⋅ in the form of (H4O2)+ that is essentially OH⋅−H3O+, a hydroxyl radical combined with a hydronium cation through hydrogen bonding. We also successfully captured it with two OH⋅ scavengers, caffeine and melatonin, and key oxidation radical intermediates that bear important mechanistic information were seen. It is suggested that some previous reactions involving (H4O2)+ should be attributed to reactions with OH⋅−H3O+ rather than with the water dimer cation (H2O−OH2)+. [ABSTRACT FROM AUTHOR]

Linear dyes are molecular mimics of dipole antennas that receive UV–vis light. In this work the assembly of linear dyes via the Langmuir technique to achieve uniform dye alignment for optically anisotropic molecular dipole antenna arrays is presented. The molecular orientations in these arrays are quantified from Langmuir isotherms, topography data, and from polarization‐ and angle‐dependent UV–vis transmission spectra. It is achieved the smallest angles by which the transition dipole moment orientation deviates from vertical alignment (16°–30°) in the antenna arrays that have been reported in literature so far. The resulting maximum absorption contrast between grazing and vertical incidence amounts to 75%. This high optical anisotropy enables application as anisotropic receiver arrays in optical communication, as optical layers for privacy applications, or other applications building on dichroic dye layers. [ABSTRACT FROM AUTHOR]

Fenton or Fenton‐like reactions are ubiquitous in nature, and the hydroxyl radicals (·OH) generated in these reactions are accountable for a plethora of oxidation processes both in the environment and in vivo. Among these oxidation reactions, lipid oxidation initiated by ·OH radicals has long been oversimplified as a peroxidation mechanism, but in reality, it is a highly complicated process that can result in a large variety of products. Using the unique field‐induced droplet ionization mass spectrometry (FIDI‐MS) methodology that is capable of selective sampling of amphiphilic molecules that reside at the air–water interface, here, we show distinct mechanisms from the ultraviolet (UV)‐enhanced Fenton oxidations of two phospholipids, POPC and POPG, even though these two lipids possess the same functional groups that are vulnerable to ·OH attack. We postulate that it is the different packing densities that determine the permeability of ambient NO molecules into the monolayers, resulting in highly distinct reaction pathways and products. We anticipate that this work will be a wake‐up call that the lipid peroxidation mechanism is sometimes taken for granted and that lipid oxidation can be subtly affected by various factors that deserves deeper investigations. [ABSTRACT FROM AUTHOR]

Understanding the hydrological processes of colloids within the karst vadose zone is vital to the security of karst groundwater and providing appropriate paleohydrological explanations of colloid‐facilitated metals in speleothem. This study addresses the mobilization mechanisms driving colloidal organic matter (COM) transport in the karst vadose zone using a 15‐year long monthly monitoring dataset from a cave drip point (HS4) in Heshang Cave, Qingjiang Valley, China. Variations in COM concentrations were reported as the fluorescence difference values of raw and filtered (<0.22 μm) samples at an excitation wavelength of 320 nm and emission wavelength of ~400 nm. A fluorescence humification index (HIX) lower than 0.8 and an autochthonous index (BIX) higher than 1.2 indicated that the origin of COM was mainly from the karst vadose zone, rather than the soil zone. The COM concentration varied from 0.001 to 0.038 Raman Unit (RU), with evident seasonal fluctuations. Rising limbs for COM values occurred prior to rising limbs within a dripwater hydrograph; moreover, the COM peak values corresponding to the beginning of the increasing hydrograph generally suggested that the mobilization of COM reflected the movement of the air–water interface (AWI) in the karst vadose zone rather than rainfall intensity or flow velocity. COM peak values were positively correlated with the antecedent drying duration and negatively correlated with HIX values. These phenomena may be explained by the increased amount of organic matter that was aggregated and absorbed on the surface of carbonate in the karst vadose zone during a longer drying duration. Moreover, the longer drying duration was also beneficial to autochthonous biological activity, which subsequently decreased the HIX value of the organic matter in the karst vadose zone. The movement of AWI and the drying duration are both controlled by the outside weather conditions. This study is therefore conducive to evaluating the security of karst groundwater in response to climate change, and challenges prevailing paleoclimate interpretations of colloid‐facilitated metal abundance timeseries reported from speleothems. [ABSTRACT FROM AUTHOR]

The oxidation of antioxidants by oxidizers imposes great challenges to both living organisms and the food industry. Here we show that the host–guest complexation of the carefully designed, positively charged, amphiphilic guanidinocalix[5]arene pentadodecyl ether (GC5A‐12C) and negatively charged oleic acid (OA), a well‐known cell membrane antioxidant, prevents the oxidation of the complex monolayers at the air–water interface from two potent oxidizers hydroxyl radicals (OH) and singlet delta oxygen (SDO). OH is generated from the gas phase and attacks from the top of the monolayer, while SDO is generated inside the monolayer and attacks amphiphiles from a lateral direction. Field‐induced droplet ionization mass spectrometry results have demonstrated that the host–guest complexation achieves steric shielding and prevents both types of oxidation as a result of the tight and "sleeved in" physical arrangement, rather than the chemical reactivity, of the complexes. [ABSTRACT FROM AUTHOR]

Sea surface microlayers (SMLs) play an important role in the transport and fate of surfactants from both anthropogenic and biogenic sources. The formation of sea spray aerosols (SSAs) at the SML has been recognized as a critical pathway for the entry of surfactants into the atmosphere. Perfluorocarboxylic acids (PFCAs) have become increasingly concerning as a class of emerging organic pollutants, having high enrichment in surface seawaters due to their remarkable surface activity. Using artificial seawater coated by extensively existed surfactants, including stearyl alcohol (C18OH) and stearic acid (SA) as simplified models of the SML and fresh SSAs, the interfacial behaviors of soluble and insoluble PFCAs were examined. Information about lateral packing and chain conformation of films was obtained by means of Langmuir trough and infrared reflection‐absorption spectroscopy (IRRAS). Perfluorooctanoic acid (PFOA) dissolved in aqueous subphases was found to be incorporated into lipid monolayers via alterations in surface pressure‐area (π‐A) isotherms. With negligible solubility, perfluorotetradecanoic acid (PFTA) formed condensed mixed monolayers with other lipids at the air‐water interface. The mixed monolayers of C18OH/PFTA and SA/PFTA were expanded by the addition of sea salts into the pure water subphase. Compared with the π‐A isotherms of SA/PFTA, decreased molecular areas were observed for those of C18OH/PFTA. This research suggests that PFCAs can be incorporated into air‐water interfaces by acting as film‐forming materials. The introduction of PFCAs will alter the surface properties and thus the atmospheric fate and behavior of SSAs and SMLs. Key Points: Both soluble and insoluble perfluorocarboxylic acids can be incorporated into air‐water interfaces by acting as film‐forming materialsIntermolecular interactions between fluorinated and nonfluorinated alkyl chains can affect interfacial molecular arrangementsAlterations in surface packing warrant further discussion on implications for atmospheric processes from aqueous interfaces [ABSTRACT FROM AUTHOR]

In this study, we report a novel air–water interfacial self‐crystallization (AWISC) method for scalable depositing continuous metal–organic framework (MOF) layers on modification‐free polyvinylidene fluoride (PVDF) hollow fibers. Through importing MOF precursors into porous hollow fiber substrates with outer diameters of 1.2 mm and evaporating aqueous solutions under mild conditions, the metal ions and linkers close to solution surface can be concentrated firstly, thus the crystallization of MOFs will preferentially occur at interface of air and liquid precursors. The formed crystals can block off the pores of substrates to form defect‐free MOF membranes. The prepared ZIF‐8 membranes exhibit superior performance in molecular separation, with high rejections of 94.1 ∼ 99.5% for small molecules (molecular weight: 320 ∼ 800 Da) and large permeance up to 50 L m−2 h−1 bar−1. Moreover, by combining AWISC and microfluidic processing, the high‐performance ZIF‐8 hollow fiber membranes with long length of 30 cm can be easily fabricated in scalability. [ABSTRACT FROM AUTHOR]

Molecular functions depend on conformations and motions of the corresponding molecular species. An air–water interface is a suitable asymmetric field for the control of molecular conformations and motions under a small applied force. In this work, double‐paddled binuclear PtII complexes containing pyrazole rings linked by alkyl spacers were synthesized and their orientations and emission properties dynamically manipulated at the air–water interface. The complexes emerge from water with concurrent variation of interface orientation of the planes of the PtII complexes from perpendicular to parallel during mechanical compression suggesting a unique 'submarine emission'. Phosphorescence of the complexes is quenched at the air–water interface prior to monolayer formation with intensities subsequently rapidly increasing during monolayer compression. These results indicate that asymmetric reactions and motions might be controlled by applying mechanical force at the air–water interface. [ABSTRACT FROM AUTHOR]

Per‐ and polyfluoroalkyl substances (PFAS) are emerging contaminants of critical concern. As surfactants, PFAS tend to accumulate at air‐water interfaces and may stay in the vadose zone for long times before contaminating groundwater. Yet not well understood, the extent of retention in the vadose zone has critical implications for risk management and remediation strategies. We present the first mathematical model that accounts for surfactant‐induced flow and solid‐phase and air‐water interfacial adsorption. We apply the model to simulate PFOS (a PFAS compound of primary concern) transport in the vadose zone at a model fire‐training area site impacted by aqueous film‐forming foam (AFFF). Air‐water interfacial adsorption is shown to have a significant impact—amplified by the low water content due to gravity drainage—total retardation factors range from 233 to 1,355 for the sand and 146 to 792 for the soil used in the study. The simulations illustrate it can take several decades or longer for PFOS to reach groundwater. Counterintuitively, the lower water content in the sand—due to stronger drainage and weaker capillary retention—leads to retardation factors greater than for the soil. Also, most PFOS is adsorbed at air‐water interfaces with only 1–2% in the aqueous phase. The implications include (1) fine‐texture materials could have lower retardation factors than sand due to higher retained water content, (2) soil PFAS concentrations are likely to be orders of magnitude higher than those in groundwater at source zones. Both implications are consistent with recent field observations at hundreds of AFFF‐impacted sites.Key Points: We develop a mathematical model for the transport and retention of PFAS in the vadose zoneAir‐water interfacial adsorption leads to retardation factors of several hundreds or greaterRetardation factors for fine‐texture media can be lower than those for sand due to greater retained water content [ABSTRACT FROM AUTHOR]

BACKGROUND: The formulation of a biodegradable carrier which effectively concentrates microorganisms on air–water interfaces is proposed. This avoids the dispersion of bacteria into the bulk liquid phase and at the same time prevents their sedimentation. This formulation can be used in biocontrol and bioremediation treatments where the target is at the position of the air–water interface, as in the case of the treatment of rice diseases caused by Sclerotium oryzae and Rhizoctonia complex. The carrier is an oil‐in‐water (O/W) emulsion which contains lecithin and chitosan in both phases at different proportions. In a stable formulation, bacteria that are adsorbed onto the surface of oil droplets are carried with them and flowed upward to the air–water interface, due to buoyancy forces. RESULTS: When using the biodegradable carrier, it is possible to recover at least 15‐fold more bacteria from the air–water interface than in the case of using the aqueous formulation. CONCLUSION: The emulsion O/W is applied to the surface by dripping, resulting in a homogeneous two‐dimensional film distribution. With this application device, the number of bacteria at the air–water interface is significantly increased. © 2019 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

The atmospheric role of photochemical processes involving NO2 beyond its dissociation limit (398 nm) is controversial. Recent experiments have confirmed that excited NO2* beyond 420 nm reacts with water according to NO2*+H2O→HONO+OH. However, the estimated kinetic constant for this process in the gas phase is quite small (k≈10−15–3.4×10−14 cm3 molecule−1 s−1) suggesting minor atmospheric implications of the formed radicals. In this work, ab initio molecular dynamics simulations of NO2 adsorbed at the air–water interface reveal that the OH production rate increases by about 2 orders of magnitude with respect to gas phase, attaining ozone reference values for NO2 concentrations corresponding to slightly polluted rural areas. This finding substantiates the argument that chemistry on clouds can be an additional source of OH radicals in the troposphere and suggests directions for future laboratory experimental studies. [ABSTRACT FROM AUTHOR]

Summary: Although high‐resolution single‐particle cryo‐electron microscopy (cryo‐EM) is now producing a rapid stream of breakthroughs in structural biology, it nevertheless remains the case that the preparation of suitable frozen‐hydrated samples on electron microscopy grids is often quite challenging. Purified samples that are intact and structurally homogeneous – while still in the test tube – may not necessarily survive the standard methods of making extremely thin, aqueous films on grids. As a result, it is often necessary to try a variety of experimental conditions before finally finding an approach that is optimal for the specimen at hand. Here, we summarize some of our collective experiences to date in optimizing sample preparation, in the hope that doing so will be useful to others, especially those new to the field. We also hope that an open discussion of these common challenges will encourage the development of more generally applicable methodology. Our collective experiences span a diverse range of biochemical samples and most of the commonly used variations in how grids are currently prepared. Unfortunately, none of the currently used optimization methods can be said, in advance, to be the one that ultimately will work when a project first begins. Nevertheless, there are some preferred first steps to explore when facing specific problems that can be more generally recommended, based on our experience and that of many others in the cryo‐EM field. [ABSTRACT FROM AUTHOR]

The gas‐phase reaction of organic acids with SO3 has been recognized as essential in promoting aerosol‐particle formation. However, at the air–water interface, this reaction is much less understood. We performed systematic Born–Oppenheimer molecular dynamics (BOMD) simulations to study the reaction of various organic acids with SO3 on a water droplet. The results show that with the involvement of interfacial water molecules, organic acids can react with SO3 and form the ion pair of sulfuric‐carboxylic anhydride and hydronium. This mechanism is in contrast to the gas‐phase reaction mechanisms in which the organic acid either serves as a catalyst for the reaction between SO3 and H2O or reacts with SO3 directly. The distinct reaction at the water surface has important atmospheric implications, for example, promoting water condensation, uptaking atmospheric condesation species, and incorporating "SO42−" into organic species in aerosol particles. Therefore, this reaction, typically occurring within a few picoseconds, provides another pathway towards aerosol formation. [ABSTRACT FROM AUTHOR]

This study describes the size distribution and concentration of particles expelled by a portable, 3-L ultrasonic humidifier. The ultrasonic humidifier was filled with waters of varying mineral content and hardness. Aerosol size distributions were measured during 8 hours of humidifier operation in a typical bedroom. Humidifiers produced approximately 1.22 × 1010-2.50 × 1010 airborne particles per milliliter of water consumed, resulting in airborne particle concentrations of 3.01-5.91 × 104 #/cm3, with modes ranging between 109 and 322 nm in diameter. The emission rate of particles varied by water type from 1.02 × 109 to 2.27 × 109 #/s. Lower mineral waters produced fewer, smaller particles when compared to higher mineral waters. Chemical analyses of particles collected with a cascade impactor indicated that the minerals in emitted particles had the same relative mineral concentrations as the fill water. Our results demonstrate that ultrasonic humidifiers should be considered a source of inhalation exposure to minerals dissolved in water, and that the magnitude of exposure to inhalable particles will vary with water quality. [ABSTRACT FROM AUTHOR]

The surface tension of the air-water interface increases upon addition of inorganic salts, implying a negative surface excess of ionic species. Most acids, however, induce a decrease in surface tension, indicating a positive surface excess of hydrated protons. In combination with the apparent negative charge at pure air-water interfaces derived from electrokinetic experiments, this experimental observation has been a source of intense debate since the mid-19th century. Herein, we calculate surface tensions and ionic surface propensities at air-water interfaces from classical, thermodynamically consistent molecular dynamics simulations. The surface tensions of NaOH, HCl, and NaCl solutions show outstanding quantitative agreement with experiment. Of the studied ions, only H3O+ adsorbs to the air-water interface. The adsorption is explained by the deep potential well caused by the orientation of the H3O+ dipole in the interfacial electric field, which is confirmed by ab initio simulations. [ABSTRACT FROM AUTHOR]

Graphene oxide (GO) can be enriched at the air-water interface by the adsorption of surfactant molecules to the surfaces of the GO sheets. The synergism between the surfactant and GO is shown to be responsible for the improved interfacial performance of the composite through a subtle balance of surface charge and surface activity. The use of a photoaddressable surfactant provides a unique probe for investigating the fundamental mechanisms that control adsorption, by inducing spatiotemporal modulation of the surfactant properties by irradiation with light of certain wavelengths. Tensiometry measurements uncover the interfacial activity of the materials, whereas X-ray reflectivity serves to independently determine the interfacial structure and composition. The ratio between the surfactant and GO appears to be the key factor controlling adsorption, with pH and salt offering additional finer control of interfacial properties. This synergism between GO sheets and a surface active small molecule surfactant is utilized to stabilize oil-in-water emulsions with unprecedented effectiveness. [ABSTRACT FROM AUTHOR]

Abstract: A simple one‐step method of preparing freestanding polypyrrole (PPy) and p‐toluenesulfonic acid doped PPy (PTSA‐PPy) films has been developed. This novel method provides a simple and general route for preparing rough surfaced hydrophobic PPy films having many potential applications. We have shown the effective removal of pollutant by these films from aqueous medium. [ABSTRACT FROM AUTHOR]

Air-water interfaces play an important role in unsaturated porous media, giving rise to phenomena like capillarity. Less recognized and understood are interactions of colloids with the air-water interface in porous media and the implications of these interactions for fate and transport of colloids. In this review, we discuss how colloids, both suspended in the aqueous phase and attached at pore walls, interact with air-water interfaces in porous media. We discuss the theory of colloid/air-water interface interactions, based on the different forces acting between colloids and the air-water interface (DLVO, hydrophobic, capillary forces) and based on thermodynamic considerations (Gibbs free energy). Subsurface colloids are usually electrostatically repelled from the air-water interface because most subsurface colloids and the air-water are negatively charged. However, hydrophobic interactions can lead to attraction to the air-water interface. When colloids are at the air-water interface, capillary forces are usually dominant over other forces. Moving air-water interfaces are effective in mobilizing and transporting colloids from surfaces. Thermodynamic considerations show that, for a colloid, the air-water interface is the favored state as compared with the suspension phase, except for hydrophilic colloids in the nanometer size range. Experimental evidence indicates that colloid mobilization in soils often occurs through macropores, although matrix transport is also prevalent in absence of macropores. Moving air-water interfaces, e.g., occurring during infiltration, imbibition, or drainage, have been shown to scour colloids from surfaces and translocate colloids. Colloids can also be pinned to surfaces by thin water films and capillary menisci at the air-water-solid interface line, causing colloid retention and immobilization. Air-water interfaces thus can both mobilize or immobilize colloids in porous media, depending on hydrodynamics and colloid and surface chemistry. [ABSTRACT FROM AUTHOR]

Criegee intermediates have implications as key intermediates in atmospheric, organic, and enzymatic reactions. However, their chemistry in aqueous environments is relatively unexplored. Herein, Born-Oppenheimer molecular dynamics (BOMD) simulations examine the dynamic behavior of syn- and anti-CH3CHOO at the air-water interface. They show that unlike the simplest Criegee intermediate (CH2OO), both syn- and anti-CH3CHOO remain inert towards reaction with water. The unexpected high stability of C2 Criegee intermediates is due to the presence of a hydrophobic methyl substituent on the Criegee carbon that lowers the proton transfer ability and inhibits the formation of a pre-reaction complex for the Criegee-water reaction. The simulation of the larger Criegee intermediates, (CH3)2COO, syn- and anti-CH2C(CH3)C(H)OO on the water droplet surface suggests that strongly hydrophobic substituents determine the reactivity of Criegee intermediates at the air-water interface. [ABSTRACT FROM AUTHOR]

The dynamic interfacial properties and dilational rheology of gemini sulfonate surfactant (SGS) and its mixtures with quaternary ammonium bromides (DTAB, CTAB) at the air-water interface were investigated using drop shape analysis. Results suggest that the adsorption process of these surfactants is diffusion-controlled at dilute concentrations, whereas the adsorption mechanism gradually shifts to a mixed kinetic-diffusion control with increasing surfactant concentration. The mixed surfactant system possesses the best surface activity when the molar ratios of SGS/DTAB and SGS/CTAB mixtures are 9:10. The formation of catanionic complexes shields the electrostatic repulsion between surfactant molecules and lowers the electrostatic adsorption barrier. Therefore, SGS/DTAB and SGS/CTAB mixtures exhibit higher adsorption rates than either component alone. The effects of oscillating frequency and surfactant concentration on the surface dilational properties of SGS, DTAB, CTAB, SGS/DTAB, and SGS/CTAB mixtures were also determined. As the oscillating frequency increases, the dilational elasticity of these surfactants gradually increases. The dilational elasticity peaks at a certain concentration, which is less than the critical micelle concentration (CMC). Results show that the dilational elasticity of SGS/DTAB and SGS/CTAB mixtures is higher than that of either component, resulting from the formation of a denser monomolecular adsorption layer at the air-water interface. Our study provides a basis for understanding the interaction mechanism of catanionic surfactant mixtures containing Gemini surfactant at the air-water interface. [ABSTRACT FROM AUTHOR]

Keywords: air-water interface; embedding effects; first hyperpolarizability; second harmonic generation; sequential MD-then-QM approach EN air-water interface embedding effects first hyperpolarizability second harmonic generation sequential MD-then-QM approach 1 1 1 01/31/23 20230101 NES 230101 B The Front Cover b shows the interfacial selectivity of second harmonic generation at the water-vacuum interface, which is targeted in this work. Air-water interface, embedding effects, first hyperpolarizability, second harmonic generation, sequential MD-then-QM approach Front Cover: Investigation of the Second Harmonic Generation at the Water-Vacuum Interface by Using Multi-Scale Modeling Methods (ChemistryOpen 1/2023). [Extracted from the article]

The influence of intermediate degradation products on the hydrolytic degradation of poly[(rac-lactide)-co-glycolide] (PLGA, 50 mol% lactide) at the air-water interface is investigated using the Langmuir film balance. For that purpose, PLGA bulk samples were degraded in aqueous solution for different time periods to generate different sized fragments with varying water solubility. After dissolution in chloroform the water-insoluble degradation products are able to form Langmuir monolayers with an increased elasticity modulus by decreasing molecular weights. Watersoluble degradation products of PLGA were found to be surface active and form an adsorption layer at the air- water interface, which can be further compressed, and revealing a different adsorption behavior in dependence on their composition. The obtained results imply that measured surface area reduction as it is determined in Langmuir monolayer degradation (LMD) experiment is restricted by the effect of the surface activity of adsorbed oligomer fragments during the degradation process. The surface activity of the formed degradation products makes it difficult to derive degradation mechanism from a common LMD experiment. To elucidate the mechanism in detail, dimers, trimers and longer oligomers with known composition and end-groups have to be investigated. [ABSTRACT FROM AUTHOR]

Protein unfolding at an air-water interface has been demonstrated such that the X-ray reflectivity can be measured with an acquisition time of 1 s using a recently developed simultaneous multiple-angle-wavelength-dispersive X-ray reflectometer. This has enabled the electron density profile of the adsorbed protein molecules to be obtained in real time. A globular protein, lysozyme, adsorbed at the air-water interface is found to unfold into a flat shape within 1 s. [ABSTRACT FROM AUTHOR]

Recent experiments in our laboratory have shown that the probability of gaseous HNO3 deprotonation on the surface of water is dramatically enhanced by anions. Herein, we report a quantum chemical study of how a HNO3 molecule transfers its proton upon approaching water clusters containing or not a chloride ion. We find that HNO3 always binds to the outermost water molecules both via donating and accepting hydrogen-bonds, but the free energy barrier for subsequent proton transfer into the clusters is greatly reduced in the presence of Cl−. As the dissociation of HNO3 embedded in water clusters is barrierless, we infer that interfacial proton transfer to water is hindered by the cost of creating a cavity for NO3−. Our findings suggest that nearby anions catalyze HNO3 dissociation by preorganizing interfacial water and drawing the proton-away from the incipient [H+---NO3−] close ion-pairs generated at the interface. This catalytic mechanism would operate in the 1 mM Cl− range (1 Cl− in ∼5.5 × 104 water molecules) covered by our experiments if weakly adsorbed HNO3 were able to explore extended surface domains before desorbing or diffusing (undissociated) into bulk water. © 2012 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

The article examines the effect of anionic non-surface active protein-polysaccharides (PS) on the foaming and interfacial characteristics of milk whey proteins (MWP) and whey protein isolate (WPI). The influence of the PS is evaluated while the ionic strength of the aqueous media is stabilized. The dynamic properties of the adsorbed films of the WPI and MWP are correlated to the foaming characteristics of the biopolymer systems.

Capillary forces acting at the air-water interface play an important role in colloid fate and transport in subsurface porous media. We quantified capillary forces between different particles (sphere, cylinder, cube, disk, sheet, and natural mineral particles) and a moving air-water interface. The particles had different sizes and contact angles (ranging from 14° to 121°). Theoretical calculations using the Young-Laplace equation were used to support and generalize the experimental data. When the air-water interface moved over the particles, there were strong capillary forces acting on the particles in the direction of the moving interface. The measured maximum capillary forces were similar to those calculated by the Young-Laplace equation. The larger the contact angles and the larger the particle size, the stronger were the capillary forces. Particles with irregular shape and sharp edges experienced greater forces than smooth particles. Generalization of the results indicates that capillary forces exerted by a moving air-water interface can readily exceed attractive Derjaguin-Landau-Verwey-Overbeek (DLVO) and gravity forces for typical subsurface particles, and a moving air-water interface is therefore an effective mechanism for mobilization of particles in porous media. Particles in the colloidal size range are particularly susceptible for mobilization by a moving air-water interface. [ABSTRACT FROM AUTHOR]

The article reviews current understanding of scalar exchange processes at gas-liquid interfaces. Progress being made in the understanding of turbulence near solid boundaries like wall turbulence is discussed. It will focus on the type of simple gas-liquid systems with a continuous interface between the fluids, like the air-water interface of surface waters, the gas-liquid interface of stirred tanks or the steam-water interface of film condensing equipment. Much has been learned using direct numerical simulations (DNS) and experimental studies using particle imaging velocimetry (PIV), both will be discussed in the article.

Dynamics of adsorption and viscoelasticity of biopolymers (β-lactoglobulin (β-lg) + polysaccharides (PS)) at 20 °C and pH 7 have been studied. Protein concentration in the bulk phase was 0.1 wt %, and the concentration of polysaccharides (xanthan gum, λ-carrageenan, and propylenglicol alginate with different degrees of esterification and viscosity) was varied from 0.1% to 0.5 wt %. The results reveal a significant effect of surface-active and non surface-active polysaccharides on the dynamics of the formation and viscoelasticity of adsorbed films at the air-water interface. The rate of diffusion of the biopolymers increased in the mixed systems, but the effect was more significant at the highest concentration of polysaccharide (0.5 wt %). The rate of rearrangement of the adsorbed films decreased in the presence of polysaccharides as compared to the protein film. Competitive adsorption, complexation and limited thermodynamic incompatibility between β-lactoglobulin and polysaccharide would explain the observed effects. [ABSTRACT FROM AUTHOR]

The relationships among air-water interfacial area, capillary pressure, and water saturation were investigated for a sandy, natural porous medium. Air-water interfacial areas as a function of water saturation were measured using two methods, gas phase partitioning tracer tests and synchrotron X-ray microtomography. The tracer test method provides a measure of effective total (capillary and film) interfacial area, whereas microtomography can be used to determine both capillary-associated and total areas (the latter is the focus of this study). Air-water interfacial areas determined with both methods increased continuously with decreasing water saturation. The areas measured with the tracer test method were significantly larger than those obtained from microtomography. The maximum values measured with the tracer test method approached the N2/BET-measured specific solid surface area, whereas the maximum values measured by microtomography approached the smooth-sphere-calculated specific solid surface area. The interfacial area-saturation data were combined with capillary pressure-saturation data obtained from water drainage experiments to examine the relationship between total air-water interfacial area and capillary pressure. Air-water interfacial area was observed to increase monotonically with increasing capillary pressure and then to plateau at values that correspond to areas associated with residual water saturation. These results are consistent with previously reported theoretically and computationally based analyses of functional relationships between total nonwetting-wetting interfacial area and capillary pressure. [ABSTRACT FROM AUTHOR]

Background: Air bubbles had been shown to remove particles and bacteria from surfaces, but they had not yet been studied regarding the removal of mature biofilm from a surface. Methods: Streptococcus mutans were grown as a biofilm on glass coverslips and were exposed to a fluid stream with or without bubbles. Three parameters (stream velocity, gas fraction, and bubble size) were varied in the bubble stream to determine which conditions best remove the biofilm. Results: At low velocities bubbles enhance biofilm removal compared with the liquid alone. Stream conditions that were shown to be the most effective in removing biofilm were large bubbles at low gas fractions. Conclusions: These results suggest that flowing bubble streams may be a desirable feature to incorporate into oral hygiene products to remove accumulated biofilms such as dental plaque. [ABSTRACT FROM AUTHOR]

In this study the relation between the ability of protein self-association and the surface properties at air-water interfaces is investigated using a combination of spectroscopic techniques. Three forms of chicken egg ovalbumin were obtained with different self-associating behavior: native ovalbumin, heat-treated ov-albumin-being a cluster of 12-16 predominantly noncovalently bound proteins, and succinylated ovalbu-min, as a form with diminished aggregation properties due to increased electrostatic repulsion. While the bulk diffusion of aggregated protein is clearly slower compared to monomeric protein, the efficiency of transport to the interface is increased, just like the efficiency of sticking to rather than bouncing from the interface. On a timescale of hours, the aggregated protein dissociates and adopts a conformation comparable to that of native protein adsorbed to the interface. The exerted surface pressure is higher for aggregated material, most probably because the deformability of the particle is smaller. Aggregated protein has a lower ability to desorb from the interface upon compression of the surface layer, resulting in a steadily increasing surface pressure upon reducing the available area for the surface layer. This observation is opposite to what is observed for succinylated protein that may desorb more easily and thereby suppresses the buildup of a surface pressure. Generally, this work demonstrates that modulating the ability of proteins to self-associate offers a tool to control the rheological properties of interfaces. [ABSTRACT FROM AUTHOR]

We present results from pore-scale observations of colloid transport in an unsaturated physical micromodel. The experiments were conducted separately using three different sizes of carboxylate polystyrene latex spheres and Bacteriophage MS2 virus. The main focus was to investigate the pore-scale transport processes of colloids as they interact with the air-water interface (AWI) of trapped air bubbles in unsaturated porous media, as well as the release of colloids during imbibition. The colloids travel through the water phase but are attracted to the AWI by either collision or attractive forces and are accumulated at the AWI almost irreversibly, until the dissolution of the air bubble reduces or eliminates the AWI. Once the air bubbles are near the end of the dissolution process, the colloids can be transported by advective liquid flow, as colloidal clusters. The clusters can then attach to other AWI down-gradient or be trapped in pore throats that would have allowed them to pass through individually. We also observed small air bubbles with attached colloids that traveled through the porous medium during the gas dissolution process. We used Derjaguin-Landau-Verwey-Overbeek (DLVO) theory to help explain the observed results. The strength of the force that holds the colloids at the AWI was estimated, assuming that the capillary force is the major force that holds the colloids at the AWI. Our calculations indicate that the forces that hold the colloids at the AWI are larger than the energy barrier between the colloids. Therefore it is quite likely that the clusters of colloids are formed by the colloids attached at the AWI as they move closer at the end of the bubble dissolution process. Coagulation at the AWI may increase the overall filtration for colloids transported through the vadose zone. Just as important, colloids trapped in the AWI might be quite mobile when the air bubbles are released at the end of the dissolution process, resulting in increased breakthrough. These pore-scale mechanisms are likely to play a significant role in the macroscopic transport of colloids in unsaturated porous media. [ABSTRACT FROM AUTHOR]

A gas-phase miscible-displacement method, using decane as an interfacial tracer, was used to measure air-water interfacial areas for a sand with water contents ranging from ∼2% to 20%. The expected trend of decreasing interfacial areas with increasing water contents was observed. The maximum estimated interfacial area of 19,500 cm−1 appears reasonable given it is smaller than the measured surface area of the porous medium (60,888 cm−1). Comparison of the experimental data presented herein with literature data provided further insight into the characterization of the air-water interface in unsaturated porous media. Specifically, comparison of interfacial areas measured using gas-phase versus aqueous-phase methods indicates that the gas-phase method generally yields larger interfacial areas than the aqueous-phase methods, even when accounting for differences in water content and physical properties of the porous media. The observations are consistent with proposed differences in interfacial accessibility of the aqueous- and gas-phase tracers. Evaluation of the data in light of functional interfacial domains, described herein, yields the hypothesis that aqueous interfacial tracers measure primarily air-water interfaces formed by 'capillary water,' while gas-phase tracers measure air-water interfaces formed by both capillary and surface-adsorbed (film) water. The gas- and aqueous-phase methods may each provide interfacial area information that is more relevant to specific problems of interest. For example, gas-phase interfacial area measurements may be most relevant to contaminant transport in unsaturated systems, where retention at the air-water interface may be significant. Conversely, the aqueous-phase methods may yield information with direct bearing on multiphase flow processes that are dominated by capillary-phase behavior. [ABSTRACT FROM AUTHOR]

We have studied the behavior of monopalmitin and monoolein monolayers, spread at the air-aqueous phase interface, as a function of temperature and surface pressure. The subphases were aqueous ethanol solutions at 0.5 and 1 mol/L. The structural characteristics of these films at interface were deduced from the π- A isotherms, as measured with an automated Langmuir-type film balance. The monolayer structure and stability were functions of hydrocarbon chain length and the presence of a double bond. Generally, the factors that decreased monolayer stability produced transformations toward configurations with more expanded structures. Changes in the subphase composition had a direct influence on the monolayer molecular structure. This study showed the existence of interactions between film and ethanol molecules at the interface. As a consequence of these interactions, a contraction in the monolayer structure was observed. The magnitude of interactions between monoglyceride and ethanol molecules at the interface depends on the surface pressure, temperature, and surface composition. Stronger film-substrate interactions produced changes in monolayer stability. Relationships between film elasticity and structural characteristics are also discussed. [ABSTRACT FROM AUTHOR]

Interfacial properties (surface tension, σ, and critical micelle concentration, CMC) of aqueous solutions of Tween 20 (polyoxyethylene sorbitan monolaurate) and/or bovine serum albumin (BSA) were evaluated. Temperature, Tween 20 concentration in the aqueous phase, BSA/Tween 20 ratio, and aqueous phase composition [water, ethanol (0.5, 1.0, and 2.5 M), and sucrose (0.5 M)] were the variables studied. The CMC of Tween 20 was determined by surface tension measurements (Wilhelmy plate method). The existence of BSA-Tween 20 interactions was deduced from surface tension measurements. The results show that the effect of temperature on CMC depends on the aqueous phase composition, but the σ value at CMC, σCMC, does decrease as temperature is increased. The CMC and σCMC values also depend on the aqueous phase composition. In aqueous ethanol solutions, the CMC increases, but σCMC decreases. However, in sucrose aqueous solutions, the CMC decreases, but there is no significant effect on σCMC. The BSA-Tween 20 interactions at the interface depend on both Tween 20 concentration ( C) and solute in the bulk phase. In water and aqueous solutions of ethanol and sucrose, σ values decrease in the presence of protein at CCMC. This is an indication that the interfacial characteristics of the mixed film are determined by either the protein or the lipid at the higher and lower protein/lipid ratio, respectively. In the intermediate region, the existence of BSA-Tween 20 interactions dominates the interfacial characteristics of mixed films. [ABSTRACT FROM AUTHOR]

The surface activity of bovine serum albumin (BSA) in water and aqueous solutions of ethanol (0.5, 1.0, and 2.0 M) and sucrose (0.5 and 1.0 M) has been investigated over a range of protein concentrations (5–1.10−5, % w/w). The surface tension data were determined by the Wilhelmy plate method. Surface data at low protein concentrations indicate a low surface activity, which rises to a plateau as the monolayer is saturated at higher protein concentrations. The protein concentration and surface tension at the plateau depend on the aqueous phase composition. The effect of aqueous phase composition on BSA-lipid interactions has been investigated by spreading an insoluble lipid (monostearin or monoolein) on a film of BSA previously adsorbed on the interface. The existence of protein-lipid interactions depends on the protein/lipid ratio. The surface activity of mixed BSA-lipid films is determined by the lipid because the surface pressure of the mixed film is the same as the lipid equilibrium spreading pressure, and the monolayer is not saturated by BSA. However, the surface activity of mixed BSA-lipid films is determined by BSA as the monolayer is saturated by the protein. [ABSTRACT FROM AUTHOR]

Air–Water Interface In their Communication on page 15846; ff., D. J. Bonthuis et al. describe the calculation of surface tensions and ionic surface propensities at air–water interfaces by molecular dynamics simulations. [ABSTRACT FROM AUTHOR]