Long distance ion-water interactions in aqueous sulfate nanodrops persist to ambient temperatures in the upper atmosphere

Dynamics of Rayleigh Fission Processes in ∼100 nm Charged Aqueous Nanodrops

Fission of micron-size charged droplets has been observed using optical methods, but little is known about fission dynamics and breakup of smaller nanosize droplets that are important in a variety of natural and industrial processes. Here, spontaneous fission of individual aqueous nanodrops formed by electrospray is investigated using charge detection mass spectrometry. Fission processes ranging from formation of just two progeny droplets in 2 ms to production of dozens of progeny droplets over 100+ ms are observed for nanodrops that are charged above the Rayleigh limit. These results indicate that Rayleigh fission is a continuum of processes that produce progeny droplets that vary widely in charge, mass, and number.

The role of analyte concentration in accelerated reaction rates in evaporating droplets

Accelerated reactions in microdroplets have been reported for a wide range of reactions with some microdroplet reactions occurring over a million times faster than the same reaction in bulk solution. Unique chemistry at the air-water interface has been implicated as a primary factor for accelerated reaction rates, but the role of analyte concentration in evaporating droplets has not been as well studied. Here, theta-glass electrospray emitters and mass spectrometry are used to rapidly mix two solutions on the low to sub-microsecond time scale and produce aqueous nanodrops with different sizes and lifetimes. We demonstrate that for a simple bimolecular reaction where surface chemistry does not appear to play a role, reaction rate acceleration factors are between 10² and 10⁷ for different initial solution concentrations, and these values do not depend on nanodrop size. A rate acceleration factor of 10⁷ is among the highest reported and can be attributed to concentration of analyte molecules, initially far apart in dilute solution, but brought into close proximity in the nanodrop through evaporation of solvent from the nanodrops prior to ion formation. These data indicate that analyte concentration phenomenon is a significant factor in reaction acceleration where droplet volume throughout the experiment is not carefully controlled.

An evaluation of the capacitive behavior of supercapacitors as a function of the radius of cations using simulations with a constant potential method

We report on the atomistic molecular dynamics, applying the constant potential method to determine the structural and electrostatic interactions at the electrode-electrolyte interface of electrochemical supercapacitors as a function of the cation radius (Cs⁺, Rb⁺, K⁺, Na⁺, Li⁺). We find that the electrical double layer is susceptible to the size, hydration layer volume, and cations' mobility and analyzed them. Besides, the transient potential shows an increase in magnitude and length as a function of the monocation size, i.e., Cs⁺ > Rb⁺ > K⁺ > Na⁺ > Li⁺. On the other hand, the charge distribution along the electrode surface is less uniform for large monocations. Nonetheless, the difference is not observed as a function of the radius of the cation for the integral capacitance. Our results are comparable to studies that employed the fixed charge method for treating such systems.

Manifolds of low energy structures for a magic number of hydrated sulfate: SO42-(H2O)24

Low energy structures of SO₄^2-(H₂O)₂₄ have been obtained using a combination of classical molecular dynamics simulations and refinement of structures and energies by quantum chemical calculations. Extensive exploration of the potential energy surface led to a number of low-energy structures, confirmed by accurate calibration calculations. An overall analysis of this large set was made after devising appropriate structural descriptors such as the numbers of cycles and their combinations. Low energy structures bear common motifs, the most prominent being fused cycles involving alternatively four and six water molecules. The latter adopt specific conformations which ensure the appropriate surface curvature to form a closed cage without dangling O-H bonds and at the same time provide 12-coordination of the sulfate ion. A prominent feature to take into account is isomerism via inversion of hydrogen bond orientations along cycles. This generates large families of ca. 100 isomers for this cluster size, spanning energy windows of 10-30 kJ mol^-1. This relatively ignored isomerism must be taken into account to identify reliably the lowest energy minima. The overall picture is that the magic number cluster SO₄^2-(H₂O)₂₄ does not correspond to formation of a single, remarkable structure, but rather to a manifold of structural families with similar stabilities. Extensive calculations on isomerization mechanisms within a family indicate that large barriers are associated to direct inversion of hydrogen bond networks. Possible implications of these results for magic number clusters of other anions are discussed.

Direct observation of ion emission from charged aqueous nanodrops: effects on gaseous macromolecular charging

Mechanistic information about how gaseous ions are formed from charged droplets has been difficult to establish because direct observation of nanodrops in a size range relevant to gaseous macromolecular ion formation by optical or traditional mass spectrometry methods is challenging owing to their small size and heterogeneity. Here, the mass and charge of individual aqueous nanodrops between 1-10 MDa (15-32 nm diameter) with ∼50-300 charges are dynamically monitored for 1 s using charge detection mass spectrometry. Discrete losses of minimally solvated singly charged ions occur, marking the first direct observation of ion emission from aqueous nanodrops in late stages of droplet evaporation relevant to macromolecular ion formation in native mass spectrometry. Nanodrop charge depends on the identity of constituent ions, with pure water nanodrops charged slightly above the Rayleigh limit and aqueous solutions containing alkali metal ions charged progressively below the Rayleigh limit with increasing cation size. MS2 capsid ions (∼3.5 MDa; ∼27 nm diameter) are more highly charged from aqueous ammonium acetate than from its biochemically preferred, 100 mM NaCl/10 mM Na phosphate solution, consistent with ion emission reducing the nanodrop and resulting capsid charge. The extent of charging indicates that the capsid partially collapses inside the nanodrops prior to the charging and formation of the dehydrated gaseous ions. These results demonstrate that ion emission can affect macromolecular charging and that conformational changes to macromolecular structure can occur in nanodrops prior to the formation of naked gaseous ions.

Cryogenic ion trap vibrational spectroscopy of the microhydrated sulfate dianions SO42-(H2O)3-8

Infrared photodissociation spectra of the D2-tagged microhydrated sulfate dianions with three to eight water molecules are presented over a broad spectral range that covers the OH stretching and H2O bending modes of the solvent molecules at higher energies, the sulfate stretching modes of the solute at intermediate energies and the intermolecular solute librational modes at the lowest energies. A low ion temperature combined with messenger-tagging ensures well-resolved vibrational spectra that allow for structure assignments based on a comparison to harmonic and anharmonic IR spectra from density functional theory (DFT) calculations. DFT ab initio molecular dynamics simulations are required to disentangle the broad and complex spectral signatures of microhydrated sulfate dianions in the OH stretching region and to identify systematic trends in the correlation of the strength and evolution of the solute-solvent and solvent-solvent interactions with cluster size. The onset for the formation of the second solvation shell is observed for n = 8.

Effects of Temperature on Cs+(H2O)20 Clathrate Structure

Clusters consisting of 20 water molecules and a single cesium ion are especially stable due to their clathrate structure that is composed exclusively of three-coordinate water molecules. Clathrate stability was investigated using infrared photodissociation (IRPD) spectroscopy in the free-OH stretching region (∼3600-3800 cm^-1) at ion cell temperatures between 135 and 355 K. At 275 K and colder, IRPD spectra of Cs⁺(H₂O)₂₀ have just one acceptor-acceptor-donor band. At higher temperatures, a higher-energy acceptor-donor band emerges and grows in intensity. Non-clathrate Na⁺(H₂O)₂₀ structures contain both of these bands, which do not change significantly in intensity over the temperature range. These results indicate a rapid onset in the conversion from clathrate to non-clathrate structures with temperature and suggest that some clathrate population remains even at the highest temperatures investigated. These results provide new insights into the role of entropy in clathrate stability.

Infrared Spectroscopy of Size-Selected Hydrated Carbon Dioxide Radical Anions CO2 .- (H2 O)n (n=2-61) in the C-O Stretch Region

Understanding the intrinsic properties of the hydrated carbon dioxide radical anions CO2 .- (H2 O)n is relevant for electrochemical carbon dioxide functionalization. CO2 .- (H2 O)n (n=2-61) is investigated by using infrared action spectroscopy in the 1150-2220 cm-1 region in an ICR (ion cyclotron resonance) cell cooled to T=80 K. The spectra show an absorption band around 1280 cm-1 , which is assigned to the symmetric C-O stretching vibration νs . It blueshifts with increasing cluster size, reaching the bulk value, within the experimental linewidth, for n=20. The antisymmetric C-O vibration νas is strongly coupled with the water bending mode ν2 , causing a broad feature at approximately 1650 cm-1 . For larger clusters, an additional broad and weak band appears above 1900 cm-1 similar to bulk water, which is assigned to a combination band of water bending and libration modes. Quantum chemical calculations provide insight into the interaction of CO2 .- with the hydrogen-bonding network.