Water Microdroplets Research Articles

Kinetic approach to the analysis of the flow around a levitating microdroplet

Two-dimensional direct Monte Carlo simulation of the flow around an evaporating water microdroplet in the air atmosphere above a dry or liquid surface has been carried out. We consider a cylindrical droplet 1 μm in diameter at a distance of 1 μm above the surface. The temperature of the droplet is taken equal to 65°C and 80°C. A significant decrease in the air concentration between the droplet and the liquid film surface is observed. The features of water vapor flow separately from the droplet surface and from the liquid film surface are considered. It is shown that, under certain conditions, condensation on the lower side and evaporation on the upper side on the droplet can occur simultaneously.

Catalyst-Free Decarboxylative Amination of Carboxylic Acids in Water Microdroplets.

Previous studies have shown that hydroxyl radicals can be formed at the water-gas surface of water microdroplets. We report the use of in situ generated hydroxyl radicals to carry out an organic transformation in one step, namely, the formation of anilines from aryl acids as well as both ammonia and primary/secondary amines via decarboxylation. Benzoic acids and amines are dissolved in water, and the solution is sprayed to form microdroplets whose chemical contents are analyzed mass spectrometrically. All intermediates and products are determined using mass spectrometry (MS) as well as in some cases tandem mass spectrometry (MS2). These results support the following reaction mechanism: NR2OH, formed via reaction of the amine with •OH, reacts with benzoic acid to form an isocyanate via a Lossen rearrangement. Hydrolysis followed by liberation of CO2 then delivers the aniline product. Notably, the scope of this transformation includes a variety of amines and aromatic acids and enables their conversion into aniline and N-substituted anilines, all in a single step. Additionally, this reaction occurs at room temperature and does not require metal catalysts or organic solvents.

Nanoreactors in action for a durable microactuator using spontaneous combustion of gases in nanobubbles

A number of recent studies report enhancement of chemical reactions on water microdroplets or inside nanobubbles in water. This finding promises exciting applications, although the mechanism of the reaction acceleration is still not clear. Specifically, the spontaneous combustion of hydrogen and oxygen in nanobubbles opens the way to fabricate truly microscopic engines. An example is an electrochemical membrane actuator with all three dimensions in the micrometer range. The actuator is driven by short voltage pulses of alternating polarity, which generate only nanobubbles. The device operation is, however, restricted by a fast degradation of the electrodes related to a high current density. Here it is demonstrated that the actuator with ruthenium electrodes does not show signs of degradation in the long-term operation. It is the only material able to withstand the extreme conditions of the alternating polarity electrolysis. This property is due to combination of a high mechanical hardness and metallic conductivity of ruthenium oxide. The actuator combines two features considered impossible: on-water catalysis and combustion in a microscopic volume. It provides an exceptional opportunity to drive autonomous microdevices especially for medical or biological applications.

Spontaneous Formation of Hydrogen Peroxide in Water Microdroplets.

There is accumulating evidence that many chemical reactions are accelerated by several orders of magnitude in micrometer-sized aqueous or organic liquid droplets compared to their corresponding bulk liquid phase. However, the molecular origin of the enhanced rates remains unclear as in the case of spontaneous appearance of 1 μM hydrogen peroxide in water microdroplets. In this Letter, we consider the range of ionization energies and whether interfacial electric fields of a microdroplet can feasibly overcome the high energy step from hydroxide ions (OH-) to hydroxyl radicals (OH•) in a primary H2O2 mechanism. We find that the vertical ionization energies (VIEs) of partially solvated OH- ions are greatly lowered relative to the average VIE in the bulk liquid, unlike the case of the Cl- anion which shows no reduction in the VIEs regardless of solvation environment. Overall reduced hydrogen-bonding and undercoordination of OH- are structural features that are more readily present at the air-water interface, where the energy scale for ionization can be matched by statistically probable electric field values.

Direct C(sp3)-N Bond Formation between Toluene and Amine in Water Microdroplets.

Unlike the inertness of bulk water, water microdroplets exhibit some remarkable reactivities. We report that water microdroplets can directly produce stable C7H7+ cations (a combination of benzylic and tropylium cations) from toluene and other substrates at room temperature with a positive voltage (+4 kV) applied to the droplet spray source. The C7H7+ cation and the benzyl radical (C6H5CH2·) are both generated via hydroxyl radicals at the water-gas interface of the microdroplets. The C7H7+ signal is observed directly by mass spectrometry. Dissolved amines (primary, secondary, and tertiary) in the microdroplets can react with both C7H7+ and C6H5CH2· to form the corresponding alkyl C(sp3)-N coupling products in one step, which cannot be achieved in bulk water or other solvents. The products were identified using tandem mass spectrometry (MS2) and 1H NMR spectroscopy. Notably, the direct C(sp3)-N bond formation products were obtained in the absence of a catalyst. In the presence of a radical scavenger, the mass spectra of the C(sp3)-N coupling products are strongly suppressed, which supports the hypothesis that this reaction is driven by hydroxyl radicals generated in the water microdroplets. Taken together, these results show that water microdroplets provide a new method for direct one-step C(sp3)-N bond formation without the need for a metal catalyst.

Cover Feature: Acid and Base Water Coexists in a Micro‐Structured Ionic Liquid and Catalyzes Organic Reactions in One‐Pot (ChemCatChem 19/2022)

The Cover Feature artistically shows that when 5 wt% acid water is added to a conventional ionic liquid, such as 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]), with a simple pipette, water microdroplets of ∼1 μm spontaneously form at room temperature. In their Research Article, A. Leyva-Pérez and co-workers describe that the same effect occurs when basic water is added, and also when both acidic and basic water are added sequentially. The microdroplets form without mixing. The microstructured acid/base liquid system performs as a catalyst/solvent system for one-pot organic reactions, such as the rearrangement of stilbene oxide, one-pot ketal hydrolysis/Knoevenagel condensation, or Pd/Cu-catalysed one-pot Sonogashira coupling/silane deprotection/Au-catalysed alkyne hydration. This extremely simple acid/base system is a rare example of co-existing simple acids (i.e., HCl) and bases (i.e., KOH). More information can be found in the Research Article by A. Leyva-Pérez and co-workers.

Spraying Water Microdroplets Containing 1,2,3-Triazole Converts Carbon Dioxide into Formic Acid.

We report the use of 1,2,3-triazole (Tz)-containing water microdroplets for gas-phase carbon dioxide (CO2) reduction at room temperature. Using a coaxial sonic spraying setup, the CO2 can be efficiently captured by Tz and converted to formic acid (HCOOH; FA) at the gas-liquid interface (GLI). A mass spectrometer operated in negative ion mode monitors the capture of CO2 to form the bicarbonate anion (HCO3-) and conversion to form the formate anion (HCOO-). Varied FA species were successfully identified by MS/MS experiments including the formate monomer ([FA - H]-, m/z 45), the dimer ([2FA - H]-, m/z 91; [2FA + Na - 2H]-, m/z 113), the trimer ([3FA - H]-, m/z 137), and some other adducts (such as [FA - H + H2CO3]-, m/z 107; [2FA + Na - 2H + Tz]-, m/z 182). The reaction conditions were systematically optimized to make the maximum conversion yield reach over 80% with an FA concentration of approximately 71 ± 3.1 μM. The mechanism for the reaction is speculated to be that Tz donates the proton and the hydroxide (OH-) at the GLI, resulting in a stepwise yield of electrons to reduce gas-phase CO2 to FA.

Water-Tree Characteristics and Its Mechanical Mechanism of Crosslinked Polyethylene Grafted with Polar-Group Molecules.

In order to restrain electric-stress impacts of water micro-droplets in insulation defects under alternating current (AC) electric fields in crosslinked polyethylene (XLPE) material, the present study represents chemical graft modifications of introducing chloroacetic acid allyl ester (CAAE) and maleic anhydride (MAH) individually as two specific polar-group molecules into XLPE material with peroxide melting approach. The accelerated water-tree aging experiments are implemented by means of a water-blade electrode to measure the improved water resistance and the affording mechanism of the graft-modified XLPE material in reference to benchmark XLPE. Melting–crystallization process, dynamic viscoelasticity and stress-strain characteristics are tested utilizing differential scanning calorimeter (DSC), dynamic thermomechanical analyzer (DMA) and electronic tension machine, respectively. Water-tree morphology is observed for various aging times to evaluate dimension characteristics in water-tree developing processes. Monte Carlo molecular simulations are performed to calculate free-energy, thermodynamic phase diagram, interaction parameter and mixing energy of binary mixing systems consisting of CAAE or MAH and water molecules to evaluate their thermodynamic miscibility. Water-tree experiments indicate that water-tree resistance to XLPE can be significantly improved by grafting CAAE or MAH, as indicated by reducing the characteristic length of water-trees from 120 to 80 μm. Heterogeneous nucleation centers of polyethylene crystallization are rendered by the grafted polar-group molecules to ameliorate crystalline microstructures, as manifested by crystallinity increment from 33.5 to 36.2, which favors improving water-tree resistance and mechanical performances. The highly hydrophilic nature of CAAE can evidently inhibit water molecules from aggregating into water micro-droplets in amorphous regions between crystal lamellae, thus acquiring a significant promotion in water-tree resistance of CAAE-modified XLPE. In contrast, the grafted MAH molecules can enhance van der Waals forces between polyethylene molecular chains in amorphous regions much greater than the grafted CAAE and simultaneously act as more efficient crystallization nucleation centers to ameliorate crystalline microstructures of XLPE, resulting in a greater improvement (relaxation peak magnitude increases by >10%) of mechanical toughness in amorphous phase, which primarily accounts for water-tree resistance promotion.

High Electric Field on Water Microdroplets Catalyzes Spontaneous and Ultrafast Oxidative C-H/N-H Cross-Coupling.

Oxidative C-H/N-H cross-coupling has emerged as an atom-economical method for the construction of C-N bonds. Conventional oxidative C-H/N-H coupling requires at least one of the following: high temperatures, strong oxidizers, transition metal catalysts, organic solvents, light, and electrochemical cells. In this study, by merely spraying the water solutions of the substrates into microdroplets at room temperature, we show a series of oxidative C-H/N-H coupling products that are strikingly produced in a spontaneous and ultrafast manner. The reactions are accelerated by six orders of magnitude compared to the same reactions in the bulk. It has been previously proposed by fluorescence microscopy and theory that the spontaneously generated electric field at the microdroplets peripheries can be in the ∼109 V/m range. Based on mass spectrometric analysis of key radical intermediates, we opine that the ultrahigh electric field catalytically oxidizes the substrates by removing an electron, which further promotes C/N coupling. Taken together, we anticipate that microdroplet chemistry will be an avenue rich in green opportunities of constructing C-heteroatom bonds.

Acid and Base Water Coexists in a Micro‐Structured Ionic Liquid and Catalyzes Organic Reactions in One‐Pot

AbstractThe most paradigmatic example of antagonist chemical species are protons and hydroxyls, and their coexistence in a liquid system is of high interest. Here we show that commercial ionic liquids stabilize microdroplets of water with pH<0 and pH>14, concomitantly and without any additive, and that this micro‐structured liquid medium catalyzes organic reactions in one‐pot.

Capture of Hydroxyl Radicals by Hydronium Cations in Water Microdroplets

AbstractDespite 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)+.

Capture of Hydroxyl Radicals by Hydronium Cations in Water Microdroplets.

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 H2 O2 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 (H4 O2 )+ that is essentially OH⋅-H3 O+ , 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 (H4 O2 )+ should be attributed to reactions with OH⋅-H3 O+ rather than with the water dimer cation (H2 O-OH2 )+ .

Tuning Ice Nucleation by Mussel-Adhesive Inspired Polyelectrolytes: The Role of Hydrogen Bonding

Tuning Ice Nucleation by Mussel-Adhesive Inspired Polyelectrolytes: The Role of Hydrogen Bonding

Pore-scale processes in tertiary low salinity waterflooding in a carbonate rock: Micro-dispersions, water film growth, and wettability change

HypothesisThe wettability change from oil-wet towards more water-wet conditions by injecting diluted brine can improve oil recovery from reservoir rocks, known as low salinity waterflooding. We investigated the underlying pore-scale mechanisms of this process to determine if improved recovery was associated with a change in local contact angle, and if additional displacement was facilitated by the formation of micro-dispersions of water in oil and water film swelling. ExperimentsX-ray imaging and high-pressure and temperature flow apparatus were used to investigate and compare high and low salinity waterflooding in a carbonate rock sample. The sample was placed in contact with crude oil to obtain an initial wetting state found in hydrocarbon reservoirs. High salinity brine was then injected at increasing flow rates followed by low salinity brine injection using the same procedure. FindingsDevelopment of water micro-droplets within the oil phase and detachment of oil layers from the rock surface were observed after low salinity waterflooding. During high salinity waterflooding, contact angles showed insignificant changes from the initial value of 115°, while the mean curvature and local capillary pressure values remained negative, consistent with oil-wet conditions. However, with low salinity, the decrease in contact angle to 102° and the shift in the mean curvature and capillary pressure to positive values indicate a wettability change. Overall, our analysis captured the in situ mechanisms and processes associated with the low salinity effect and ultimate increase in oil recovery.

Fabrication of microstructured poly(3-hydroxybutyrate) films with controlled surface topography

The possibility of fabrication of microstructured poly-3-hydroxybutyrate films by self-assembly water microdroplets technique, using artificial templates and polymer inverse emulsions has been studied. It has been established that self-assembly water microdroplets technique allows forming ordered microstructures of poly-3-hydroxybutyrate with a hexagonal arrangement of cells with an adjustable diameter from 1 to 4 цт. It has been shown that application of inverse emulsions of poly-3-hydroxybutyrate allows us to fabricate porous films with a pore size in the range from 0.4 to 3 ^m, while the structure of the films and the pore size can be controlled by changing the polymer concentration in the dispersion medium and the volume ratio of the phases. Using spin-coating technique and artificial templates, it is possible to obtain poly-3-hydroxybutyrate microstructured replicas, which are characterized by a high degree of uniformity and the absence of defective areas. It has been shown that the formed microstructured poly-3-hydroxybutyrate films with controlled surface topography are promising for use as scaffolds for stem cells.

Capturing Reactive Carbanions by Microdroplets.

Carbanions appear in many organic or biological reactions as fleeting intermediates, prohibiting direct observation or spectroscopic measurement. An aqueous environment is known to rapidly annihilate a carbanion species, reducing its lifetime to as short as picoseconds. We report that aqueous microdroplets can capture and stabilize reactive carbanion intermediates isolated from four classic organic reactions, aldol and Knoevenagel condensations, alkyne alkylation, and the Reimer-Tiemann reaction, enabling the detection of their carbanion intermediates by desorption electrospray ionization mass spectrometry. This is accomplished in real time of the reaction, allowing new insights into reaction mechanisms to be obtained. The efficacy of microdroplets in capturing such elusive species was examined by varying the solvent and the microdroplet negative charge density. We observed that microdroplets composed of water-methanol outperform other solvents, such as pure water, in capturing carbanions, which is in contrast to the earlier report that presented the highest performance of pure water microdroplets in capturing carbocations. We offer some mechanistic insights to explain the discriminatory behavior of these two oppositely charged species in microdroplets.

Sprayed Water Microdroplets Are Able to Generate Hydrogen Peroxide Spontaneously.

Ultrapure N2 gas was bubbled through water, and the humidified output containing undetectable concentrations of ozone filled a closed chamber in which 18 MΩ-cm water was sprayed through a silica capillary to form microdroplets. Analysis of the collected microdroplets by NMR spectroscopy showed the presence of hydrogen peroxide at a concentration level ranging from 0.3 to 1.5 μM depending on the flow conditions. This was confirmed using a spectrofluorometric assay. We suggest that this finding establishes that when sprayed to form microdroplets, water has the ability to produce hydrogen peroxide by itself. When the N2 gas is replaced by compressed air or O2 gas, the concentration of hydrogen peroxide is found to increase, indicating that gas-surface interactions with O2 in aqueous microdroplets promote the formation of hydrogen peroxide.

Revisiting the Effect of the Air-Water Interface of Ultrasonically Atomized Water Microdroplets on H2O2 Formation.

Studying chemical processes at the air-water interface is always challenging. A recent report claimed that H2O2 was formed spontaneously on the surface of condensed water microdroplets. However, a newer report concluded that the detected H2O2 in the previous report could originate in part from the water vapor source that involved ultrasonic atomization of liquid water. Here, this phenomenon is reinvestigated regarding the influence of ultrasonic cavitation, surface modification of droplets, and solutes in the bulk liquid on H2O2 production. When the droplet surfaces were modified by surfactants, H2O2 production did not change, whereas adding gases or inorganic compounds to the bulk solution caused significant changes in H2O2 production. These results confirm that H2O2 formation originates from cavitation in bulk solutions. It is concluded that the air-water interface of water microdroplets itself does not generate H2O2.

Sprayed water microdroplets containing dissolved pyridine spontaneously generate pyridyl anions.

The anion of pyridine, C5H5N−, has been thought to be short lived in the gas phase and was only previously observed indirectly. In the condensed phase, C5H5N− is known to be stabilized by solvation with other molecules. We provide in this study striking results for the formation of isolated C5H5N− from microdroplets of water containing dissolved pyridine observed in the negative ion mass spectrum. The gas-phase lifetime of C5H5N− is estimated to be at least 50 ms, which is much longer than previously thought. The generated C5H5N− captured CO2 molecules to form a stable (Py-CO2)− complex, further confirming the existence of C5H5N−. We propose that the high electric field at the air–water interface of a microdroplet helps OH− to transfer an electron to pyridine to form C5H5N− and the hydroxyl radical •OH. Oxidation products of the Py reacting with •OH are also observed in the mass spectrum recorded in positive mode, which further supports this mechanism. The present study pushes the limits of the reducing and oxidizing power of water microdroplets to a new level, emphasizing how different the behavior of microdroplets can be from bulk water. We also note that the easy formation of C5H5N− in water microdroplets presents a green chemistry way to synthesize value-added chemicals.

Optimizing Coaxial Sonic Spray Geometry for Generating Water Microdroplets.

Sonic spray creates a stream of neutral and charged microdroplets without application of voltage, heating, laser irradiation, or corona discharge. The solvent of interest flows through an inner capillary (usually constructed of fused silica) that is surrounded by an outer stainless-steel tube through which a nebulizing gas flows under pressure. This technique has been widely used as the interface in mass spectrometric studies for chemical analysis and for understanding microdroplet chemistry. We have used light scattering to characterize the size distribution and density for water microdroplets as a function of several parameters, such as water quality, water flow rate, nebulizing gas pressure, and sonic sprayer geometry. We find that the size distribution of the microdroplets, which is critical to many applications, depends most sensitively on the distance between the inner and outer capillary outlets and the gas flow pressure. The best performance as measured by the smallness of the microdroplet diameters is obtained when the gas flow pressure is the highest and there is no separation distance, d, between the two capillary outlets. In addition, at d = 0 mm, the microdroplet diameter distribution is nearly independent of the water flow rate, indicating that studies under these conditions can be scaled up.