High-Speed Imaging and Electrochemical Studies during the Freezing of Supercooled Aqueous Droplets

In der vorliegenden Arbeit werden Wassertropfen mit einem Durchmesser von etwa 2-3 mm akustisch levitiert oder zwischen sehr dünnen Drahtringen positioniert, um Randeffekte zu vermeiden, und um bis zu 24 K unterkühlt. Weil die Schmelzenthalpie nicht schnell genug an die Umgebung abgegeben werden kann und nur teilweise im Tropfen gespeichert werden kann, spaltet sich der Gefrierprozess in zwei Phasen auf. Per Hochgeschwindigkeitskamera wird in einem für diese Arbeiten entwickelten Kühlsystem in unterkühlten Wassertropfen ein Eiswachstum mit konstanter, schneller Geschwindigkeit beobachtet. Sie liegt in der Größenordnung von 0,1 m/s und wächst mit steigender Unterkühlung zunächst linear bis quadratisch an. Dagegen wird für stark unterkühlte Tropfen eine Tendenz zu einem Geschwindigkeitsmaximum beobachtet. Es wird ein neues Modell zur Beschreibung der Gefriergeschwindigkeit vorgestellt, welches in gutem Einklang mit den experimentellen Befunden steht. Um die komplexen Wärmeproduktions- und -transportprozesse beim Gefrieren der Tropfen zu erfassen, wurden zwei Modelle im Rahmen von FEM-Simulationen entwickelt und damit die Evolution der Verteilungen von Wärme und dendritischem Eis für viele Kombinationen von Radius, Unterkühlung, Gefriergeschwindigkeit und relativer Tropfengeschwindigkeit berechnet. Es wird gezeigt, dass erst für sehr kleine Tropfen die Oberfläche groß genug ist, um einen signifikanten Anteil der Schmelzwärme an die Umgebung abzugeben und der kritische Radius, der ein einstufiges Gefrieren ermöglicht, zwischen 0,1 und 1 Mikrometern zu erwarten und hauptsächlich von der Unterkühlung sowie der Gefriergeschwindigkeit abhängig ist. Diese Grenze liegt innerhalb der Größenverteilung der unterkühlten Tropfen in der Atmosphäre. Als ein wesentliches Ergebnis dieser Arbeit werden erstmalig jeweils ein elektrischer Effekt für die erste und die zweite Gefrierstufe an unterkühlten Tropfen beobachtet und untersucht. Der Effekt in der ersten Gefrierstufe zeigt einen charakteristischen Doppelpeak mit einer Amplitude von bis zu 3 V, ist abhängig von der Art der Ionen und der Unterkühlung. Während der Effekt für höhere Konzentrationen abrupt einbricht, verschwindet er nicht mit abnehmender Ionenkonzentration. Der beobachtete Effekt der zweiten Gefrierstufe ähnelt in seiner Gestalt dem bekannten Workman-Reynolds-Effekt und ist in seiner Stärke und Polarität von der vorausgehenden dendritischen Phase, insbesondere von ihrer Gefrierrichtung, abhängig.In this work water droplets with a diameter of about 2-3 mm are levitated acoustically or positioned between thin wire loops to minimize wall effects and cooled down up to 249 K. Because the heat cannot be released to the environment quickly enough and can only be partially stored in the system, the freezing process splits in two stages. In this work fast and constant freezing speeds of supercooled water droplets are measured with a high-speed camera in a newly developed cooling chamber. The ice grows roughly planar through the droplet. Furthermore, details of the dendritic structure are noted in some cases. The freezing speed is in the order of 0.1 m/s and increases with supercooling linearly to quadratically, but for the strongest supercoolings the freezing speed tends to reach a maximum. Based on the theory of Wilson and Frenkel a new model is presented, which predicts the freezing speed as a function of supercooling under consideration of the dendritic freezing stage and is in good agreement with the experimental data. Two new finite element models are have been developed to unravel the complex heat production and transfer processes during the whole freezing of the supercooled droplets. So, the evolution of heat and ice portion are computed for many combinations of droplet radii, supercoolings, freezing speeds and relative droplet speeds. It is shown, that only for very small droplets a significant portion of the freezing enthalpy is released to the environment. As a further important result the critical radius, which allows a one step freezing, is estimated to exist between 0.1 and 1 micrometers. This critical radius depends mainly on the supercooling temperature and the freezing speed and meets well the size distribution of droplets in the atmosphere. Further results are the observation of two electric effects during both freezing steps. The effect in the first step shows a characteristic double peak with an amplitude of 3 V and depends on sort and concentration of the ions as well as on the supercooling. Whereas the effect vanishes for high concentrations, it persist to exist even for very low concentrations in contrast to the Workman-Reynolds-freezing-potential. The characteristics of this effect in the second freezing step is similar to the WRFP and the polarity as well as the strength of the effect are depending on the direction of proceeding dendritic freezing step. The findings are relevant in particular for atmospheric physics and chemistry

Imaging Temperature and Thickness of Thin Planar Liquid Water Jets in Vacuum

We present spatially resolved measurements of the temperature of a flat liquid water microjet for varying pressures, from vacuum to 100% relative humidity. The entire jet surface is probed in a single shot by a high-resolution infrared camera. Obtained 2D images are substantially influenced by the temperature of the apparatus on the opposite side of the IR camera; a protocol to correct for the thermal background radiation is presented. In vacuum, we observe cooling rates due to water evaporation on the order of 105 K/s. For our system, this corresponds to a temperature decrease of approximately 15 K between upstream and downstream positions of the flowing leaf. Making reasonable assumptions on the absorption of the thermal background radiation in the flatjet we can extend our analysis to infer a thickness map. For a reference system our value for the thickness is in good agreement with the one reported from white light interferometry.Comment: The following article has been submitted to Structural Dynamics. After it is published, it will be found at Lin

Imaging temperature and thickness of thin planar liquid water jets in vacuum

We present spatially resolved measurements of the temperature of a flat liquid water microjet for varying ambient pressures, from vacuum to 100% relative humidity. The entire jet surface is probed in a single shot by a high-resolution infrared camera. Obtained 2D images are substantially influenced by the temperature of the apparatus on the opposite side of the infrared camera; a protocol to correct for the thermal background radiation is presented. In vacuum, we observe cooling rates due to water evaporation on the order of 105 K/s. For our system, this corresponds to a temperature decrease in approximately 15 K between upstream and downstream positions of the flowing leaf. Making reasonable assumptions on the absorption of the thermal background radiation in the flatjet, we can extend our analysis to infer a thickness map. For a reference system, our value for the thickness is in good agreement with the one reported from white light interferometry

Observation of Intermolecular Coulombic Decay and Shake-up Satellites in Liquid Ammonia

We report the first nitrogen 1s Auger–Meitner electron spectrum from a liquid ammonia microjet at a temperature of ~223 K (–50 °C) and compare it with the simultaneously measured spectrum for gas-phase ammonia. The spectra from both phases are interpreted with the assis- tance of high-level electronic structure and ab initio molecular dynamics calculations. In addition to the regular Auger–Meitner-electron features, we observe electron emission at kinetic energies of 374–388 eV, above the leading Auger–Meitner peak (3a12). Based on the electronic structure calculations, we assign this peak to a shake-up satellite in the gas phase, i.e., Auger–Meitner emission from an intermediate state with additional valence excitation present. The high-energy contribution is significantly enhanced in the liquid phase. We consider various mechanisms contributing to this feature. First, in analogy with other hydrogen-bonded liquids (noticeably water), the high-energy signal may be a signature for an ultrafast proton transfer taking place before the electronic decay (proton transfer mediated charge separation). The ab initio dynamical calculations show, however, that such a process is much slower than electronic decay and is, thus, very unlikely. Next, we consider a non-local version of the Auger–Meitner decay, the Intermolecular Coulombic Decay. The electronic structure calculations support an important contribution of this purely electronic mechanism. Finally, we discuss a non-local enhancement of the shake-up processes

The solvation shell probed by resonant intermolecular Coulombic decay

Molecules involved in solvation shells have properties differing from those of the bulk solvent, which can in turn affect reactivity. Among key properties of these molecules are their nature and electronic structure. Widely used tools to characterize this type of property are X-ray-based spectroscopies, which, however, usually lack the capability to selectively probe the solvation-shell molecules. A class of X-ray triggered “non-local” processes has the recognized potential to provide this selectivity. Intermolecular Coulombic decay (ICD) and related processes involve neighbouring molecules in the decay of the X-ray-excited target, and are thus naturally sensitive to its immediate environment. Applying electron spectroscopy to aqueous solutions, we explore the resonant flavours of ICD and demonstrate how it can inform on the first solvation shell of excited solvated cations. One particular ICD process turns out to be a potent marker of the formation of ion pairs. Another gives a direct access to the electron binding energies of the water molecules in the first solvation shell, a quantity previously elusive to direct measurements. The resonant nature of the processes makes them readily measurable, providing powerful new spectroscopic tools

Uptake of Ammonia by Ice Surfaces at Atmospheric Temperatures

We present an ambient pressure X-ray photoelectron spectroscopy investigation of the adsorption of ammonia on ice over the temperature range of -23 °C to -50 °C. Previous flow tube studies showed significant uptake of ammonia at these temperatures to ice, which was linked to the incorporation of ammonium into the ice crystal lattice. Our present investigation shows a significant uptake of ammonia to the ice interface, with ammonia concentrations that exceed those measured in past studies for the case of bulk snow ice and samples. We also have indication that some of the ammonia is protonated at the ice surface and thus adsorbed there as ammonium ions. The impact of high ammonia concentrations at the air-ice interface on the surface chemistry of ice clouds is discussed. The present study lays the groundwork for investigating the reaction of adsorbed ammonia with other trace gases in the atmosphere, which is demonstrated on the example of a proof-of-principle experiment of its interaction with acetic acid

Hydrolysis of oligosaccharides over solid acid catalysts: a review

Mild fractionation/pretreatment processes are becoming the most preferred choices for biomass processing within the biorefinery framework. To further explore their advantages, new developments are needed, especially to increase the extent of the hydrolysis of poly- and oligosaccharides. A possible way forward is the use of solid acid catalysts that may overcome many current drawbacks of other common methods. In this Review, the advantages and limitations of the use of heterogeneous catalysis for the main groups of solid acid catalysts (zeolites, resins, carbon materials, clays, silicas, and other oxides) and their relation to the hydrolysis of model soluble disaccharides and soluble poly- and oligosaccharides are presented and discussed. Special attention is given to the hydrolysis of hemicelluloses and hemicellulose-derived saccharides into monosaccharides, the impact on process performance of potential catalyst poisons originating from biomass and biomass hydrolysates (e.g., proteins, mineral ions, etc.). The data clearly point out the need for studying hemicelluloses in natura rather than in model compound solutions that do not retain the relevant factors influencing process performance. Furthermore, the desirable traits that solid acid catalysts must possess for the efficient hemicellulose hydrolysis are also presented and discussed with regard to the design of new catalysts

Surface accumulation and acid-base equilibrium of phenol at the liquid-vapor interface

We have investigated the surfactant properties of phenol in aqueous solution as a function of pH and bulk concentration using liquid-jet photoelectron spectroscopy (LJ-PES) and surface tension measurements. The emphasis of this work is on the determination of the Gibbs free energy of adsorption and surface excess of phenol and its conjugate base phenolate at the bulk pKa (9.99), which can be determined for each species using photoelectron spectroscopy. These values are compared to those obtained in measurements well below and well above the pKa, where pure phenol or phenolate, respectively, are the dominant species, and where the Gibbs free energy of adsorption determined from surface tension and LJ-PES data are in excellent agreement. At the bulk pKa the surface-sensitive LJ-PES measurements show a deviation of the expected phenol/phenolate ratio in favor of phenol, i.e., an apparent upward shift of the pKa* at the surface. In addition, the Gibbs free energies of adsorption determined by LJ-PES at the bulk pKa for phenol and phenolate deviate from those observed for the pure solutions. We discuss these observations in view of the different surface propensity of phenol and phenolate as well as potential cooperative interactions between them in the near-surface region

Correction: Photoelectron angular distributions as sensitive probes of surfactant layer structure at the liquid-vapor interface

Correction for 'Photoelectron angular distributions as sensitive probes of surfactant layer structure at the liquid-vapor interface' by Rémi Dupuy et al., Phys. Chem. Chem. Phys., 2022, 24, 4796-4808, https://doi.org/10.1039/D1CP05621B. The authors would like to correct a typographical error on page 4798, right column, second paragraph of subsection 2.4, where the ε value of the Lennard-Jones potential used for the Na+ ions was reported with an incorrect unit as 0.01814 kcal mol-1. The correct unit is kJ mol-1. The full correct sentence should thus read: "For sodium cations, we used a Lennard-Jones (LJ) potential with ε = 0.01814 kJ mol-1 and σ = 3.206 Å." The correct value of ε = 0.01814 kJ mol-1 was used in all calculations, and thus the results of the calculations and the conclusions based on them are not affected by this error. The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers

Interaction of ions and surfactants at the seawater-air interface

The interface of the oceans and aqueous aerosols with air drives many important physical and chemical processes in the environment, including the uptake of CO2 by the oceans. Transport across and reactions at the ocean–air boundary are in large part determined by the chemical composition of the interface, i.e., the first few nanometers into the ocean. The main constituents of the interface, besides water molecules, are dissolved ions and amphiphilic surfactants, which are ubiquitous in nature. We have used a combination of surface tension measurements and liquid-jet X-ray photoelectron spectroscopy to investigate model seawater solutions at realistic ocean-water ion concentrations in the absence and in the presence of model surfactants. Our investigations provide a quantitative picture of the enhancement or reduction of the concentration of ions due to the presence of charged surfactants at the interface. We have also directly determined the concentration of surfactants at the interface, which is related to the ionic strength of the solution (i.e., the “salting out” effect). Our results show that the interaction of ions and surfactants can strongly change the concentration of both classes of species at aqueous solution–air interfaces, with direct consequences for heterogeneous reactions as well as gas uptake and release at ocean–air interfaces