Rise in the pH of an unfrozen solution in ice due to the presence of NaCl and promotion of decomposition of gallic acids owing to a change in the pH

Oxidative decomposition of gallic acid occurs in alkaline solutions but hardly arises in acidic solutions. We have found that the addition of sodium chloride promotes the decomposition of gallic acid caused by freezing even under neutral and acidic conditions. Even at pH 4.5, gallic acid was decomposed by freezing in the presence of NaCl; however, in the absence of NaCl, it was hardly decomposed by freezing at pH lower than 7. Chloride ions are more easily incorporated in ice than sodium ions when the NaCl solution is frozen. The unfrozen solution in ice becomes positively charged, and as a result, protons transfer from the unfrozen solution to the ice. We measured the pH in the unfrozen solution which coexists with single-crystal ice formed from a 5 mmol dm(-3) NaCl solution and determined the pH to be 8.6 at equilibrium with CO(2) of 380 ppm or 11.3 in the absence of CO(2) compared to pH 5.6 in the original solution. From the model calculation performed for gallic acid solution in the presence of 5 mmol dm(-3) NaCl, it can be estimated that the amount of OH(-) transferred from the ice to the solution corresponds to 1.26 x 10(-5) mol dm(-3). The amount of OH(-) transferred is concentrated into the unfrozen solution and affects the pH of the unfrozen solution. Therefore, the pH in an unfrozen gallic acid solution in ice becomes alkaline, and the decomposition of gallic acid proceeds. It is expected that other base-catalyzed reactions in weakly acidic solutions also proceed by freezing in the presence of NaCl without the need for any alkaline reagents.     

Accelerating Electrochemical Reactions in a Voltage-Controlled Interfacial Microreactor

Microdroplet chemistry is attracting increasing attention for accelerated reactions at the solution–air interface. We report herein a voltage-controlled interfacial microreactor that enables acceleration of electrochemical reactions which are not observed in bulk or conventional electrochemical cells. The microreactor is formed at the interface of the Taylor cone in an electrospray emitter with a large orifice, thus allowing continuous contact of the electrode and the reactants at/near the interface. As a proof-of-concept, electrooxidative C−H/N−H coupling and electrooxidation of benzyl alcohol were shown to be accelerated by more than an order of magnitude as compared to the corresponding bulk reactions. The new electrochemical microreactor has unique features that allow i) voltage-controlled acceleration of electrochemical reactions by voltage-dependent formation of the interfacial microreactor; ii) “reversible” electrochemical derivatization; and iii) in situ mechanistic study and capture of key radical intermediates when coupled with mass spectrometry.     

Time-resolving analysis of cryotropic gelation of water/poly(vinyl alcohol) solutions via small-angle neutron scattering

The structural transformations occurring in initially homogeneous aqueous solutions of poly(vinyl alcohol) (PVA) through application of freezing (-13 degrees C) and thawing (20 degrees C) cycles is investigated by time resolving small-angle neutron scattering (SANS). These measurements indicate that formation of gels of complex hierarchical structure arises from occurrence of different elementary processes, involving different length and time scales. The fastest process that could be detected by our measurements during the first cryotropic treatment consists of the crystallization of the solvent. However, solvent crystallization is incomplete, and an unfrozen liquid microphase more concentrated in PVA than the initial solution is also formed. Crystallization of PVA takes place inside the unfrozen liquid microphase and is slowed down because of formation of a microgel fraction. Water crystallization takes place in the early 10 min of the treatment of the solution at subzero temperatures, and although below 0 degrees C the PVA solutions used for preparation of cryogels should be below the spinodal curve, occurrence of liquid-liquid phase separation could not be detected in our experiments. Upon thawing, ice crystals melt, and transparent gels are obtained that become opaque in approximately 200 min, due to a slow and progressive increase of the size of microheterogeneities (dilute and dense regions) imprinted during the fast freezing by the crystallization of water. During the permanence of these gels at room temperature (for hours), the presence of a high content of water (higher than 85% by mass) prevents further crystallization of PVA. Crystallization of PVA, in turn, is resumed by freezing the gels at subzero temperatures, after water crystallization and consequent formation of an unfrozen microphase. The kinetic parameters of PVA crystallization during the permanence of these gels at subzero temperatures are the same shown by PVA during the first freezing step of the solutions.     

Acceleration of ammonium nitrite denitrification by freezing: determination of activation energy from the temperature of maximum reaction rate

A reaction of ammonium nitrite in ice was investigated. Upon freezing, some nitrite is oxidized by dissolved oxygen and some nitrite reacts with ammonium to produce nitrogen and water in a denitrification reaction. The former reaction was accelerated only during freezing, and the latter one was accelerated even after the whole sample was frozen. The denitrification reaction proceeded at very low concentration in ice, which were conditions under which the reaction would not proceed in solution. The nitrogen production increased linearly with increasing initial concentration of ammonium nitrite. The concentration factor in the unfrozen solution in ice was estimated to be 50.6 when the initial concentration was 0.5 mmol dm(-3), as obtained from comparison of reaction rates in solution and in ice. A new method for determination of the activation energy is proposed that gives a value of 53 to 61 kJ mol(-1) for denitrification. The reaction order of the denitrification process is also determined using our method, and it is concluded to follow third-order kinetics.     

Comparison of Photochemical Reactions of m-Cresol in Aqueous Solution and in Ice

We compared the photochemical reaction of m-cresol containing OH precursors such as H₂O₂, NO₂^- and NO₃^- in aqueous solution with those in ice. The results show that the conversion rate of m-cresol in aqueous solution was higher than that in ice. H₂O₂, NO₂^- and NO₃^- all accelerated the photoconversion of m-cresol in both aqueous solution and ice. The photochemical reactions of m-cresol obeys the first order kinetics equation. According to the photoproducts identified by GC-MS, we proposed that hydroxylation and nitration reactions occurred in both aqueous solution and ice. Coupling reaction was common in ice, however, in aqueous solution it was found only in UV system. Our results suggest that the photochemical reactions of m-cresol were different in aqueous solution and in ice.     

Calorimetric and relaxation properties of xylitol-water mixtures

We present the first broadband dielectric spectroscopy (BDS) and differential scanning calorimetry study of supercooled xylitol-water mixtures in the whole concentration range and in wide frequency (10(-2)-10(6) Hz) and temperature (120-365 K) ranges. The calorimetric glass transition, T(g), decreases from 247 K for pure xylitol to about 181 K at a water concentration of approximately 37 wt. %. At water concentrations in the range 29-35 wt. % a plentiful calorimetric behaviour is observed. In addition to the glass transition, almost simultaneous crystallization and melting events occurring around 230-240 K. At higher water concentrations ice is formed during cooling and the glass transition temperature increases to a steady value of about 200 K for all higher water concentrations. This T(g) corresponds to an unfrozen xylitol-water solution containing 20 wt. % water. In addition to the true glass transition we also observed a glass transition-like feature at 220 K for all the ice containing samples. However, this feature is more likely due to ice dissolution [A. Inaba and O. Andersson, Thermochim. Acta, 461, 44 (2007)]. In the case of the BDS measurements the presence of water clearly has an effect on both the cooperative α-relaxation and the secondary β-relaxation. The α-relaxation shows a non-Arrhenius temperature dependence and becomes faster with increasing concentration of water. The fragility of the solutions, determined by the temperature dependence of the α-relaxation close to the dynamic glass transition, decreases with increasing water content up to about 26 wt. % water, where ice starts to form. This decrease in fragility with increasing water content is most likely caused by the increasing density of hydrogen bonds, forming a network-like structure in the deeply supercooled regime. The intensity of the secondary β-relaxation of xylitol decreases noticeably already at a water content of 2 wt. %, and at a water content above 5 wt. % it has been replaced by a considerably stronger water (w) relaxation at about the same frequency. However, the similarities in time scale and activation energy between the w-relaxation and the β-relaxation of xylitol at water contents below 13 wt. % suggest that the w-relaxation is governed, in some way, by the β-relaxation of xylitol, since clusters of water molecules are rare at these water concentrations. At higher water concentrations the intensity and relaxation rate of the w-relaxation increase rapidly with increasing water content (up to the concentration where ice starts to form), most likely due to a rapid increase of small water clusters where an increasing number of water molecules interacting with other water molecules.     

Number Density of Liquid Inclusions Formed in Frozen Aqueous Electrolyte

Frozen aqueous chlorides (≤50 mM ) are characterized by using confocal fluorescence microscopy and small angel X‐ray scattering (SAXS). The former method allows us to determine the size of a liquid inclusion formed in the ice matrix at temperatures above the eutectic point of the system (t_eu). Isolated liquid inclusions of a uniform size are formed when the temperature of a frozen electrolyte increases past t_eu. The size of the liquid inclusions depends on the observation temperature as well as on the concentration (c_salt) and type of salt dissolved in the original unfrozen solution. However, the number density of liquid inclusions is almost constant and independent of these experimental parameters, particularly when an electrolyte is frozen in liquid nitrogen. Salt accumulation can then occur at the imperfections of the ice crystals. The occurrence probability of the imperfections is independent of the nature of an incorporated salt. The amount of a salt confined in each inclusion ranges from 7 to 240 fmol, depending on c_salt. SAXS measurements provide information on the size of individual salt crystals formed at temperatures below t_eu. The radius of gyration of a salt crystal ranges from 2 to 2.8 nm, and does not depend significantly on c_salt. Thus, each inclusion is formed from 10⁶–10⁹ nanocrystals, which can act as seeds. When doped ice is prepared at higher temperatures, for example ?16 °C, the isolation of liquid inclusions is not sufficient and coalescence occurs more easily upon an increase in temperature or c_salt. However, when c_salt is lower than 10 mM , the number density of liquid inclusions is almost constant, irrespective of the freezing temperature.     

Organic Reactions in Microdroplets: Reaction Acceleration Revealed by Mass Spectrometry

The striking finding that reaction acceleration occurs in confined‐volume solutions sets up an apparent conundrum: Microdroplets formed by spray ionization can be used to monitor the course of bulk‐phase reactions and also to accelerate reactions between the reagents in such a reaction. This Minireview introduces droplet and thin‐film acceleration phenomena and summarizes recent methods applied to study accelerated reactions in confined‐volume, high‐surface‐area solutions. Conditions that dictate either simple monitoring or acceleration are reconciled in the occurrence of discontinuous and complete desolvation as the endpoint of droplet evolution. The contrasting features of microdroplet and bulk‐solution reactions are described together with possible mechanisms that drive reaction acceleration in microdroplets. Current applications of droplet microreactors are noted as is reaction acceleration in confined volumes and possible future scale‐up.     

Crystalline ice growth on Pt(111) and Pd(111): nonwetting growth on a hydrophobic water monolayer

The growth of crystalline ice films on Pt(111) and Pd(111) is investigated using temperature programed desorption of the water films and of rare gases adsorbed on the water films. The water monolayer wets both Pt(111) and Pd(111) at all temperatures investigated [e.g., 20-155 K for Pt(111)]. However, crystalline ice films grown at higher temperatures (e.g., T>135 K) do not wet the monolayer. Similar results are obtained for crystalline ice films of D2O and H2O. Amorphous water films, which initially wet the surface, crystallize and dewet, exposing the water monolayer when they are annealed at higher temperatures. Thinner films crystallize and dewet at lower temperatures than thicker films. For samples sputtered with energetic Xe atoms to prepare ice crystallites surrounded by bare Pt(111), subsequent annealing of the films causes water molecules to diffuse off the ice crystallites to reform the water monolayer. A simple model suggests that, for crystalline films grown at high temperatures, the ice crystallites are initially widely separated with typical distances between crystallites of approximately 14 nm or more. The experimental results are consistent with recent theory and experiments suggesting that the molecules in the water monolayer form a surface with no dangling OH bonds or lone pair electrons, giving rise to a hydrophobic water monolayer on both Pt(111) and Pd(111).     

Accelerating effect of silica on the indicator reaction o-dianisidine-H(2)O(2)

Reaction of oxidation of o-dianisidine (o-D) with H(2)O(2) which is widely used in catalytic methods of analysis in solution has been conducted on silica plates for thin-layer chromatography. The rate of the reaction catalyzed by model compounds (p-toluenesulphonyl chloride, methyl benzoate, benzoic acid, and acrylamide) is noticeably higher on silica than in solution in comparable conditions. The degree of acceleration varies depending on the catalyst and is more pronounced at its lower concentrations. By use of p-toluenesulphonyl chloride determination as an example it has been shown that the accelerating effect of silica enables to decrease the detection limit down to 0.07 nmol cm(-2) (as compared with 4 nmol.cm(-2) in solution); the accuracy is not diminished. It is concluded that catalytic indicator reactions on solid supports may represent high interest for analytical chemists.     

Formation of low-temperature cirrus from H2SO4/H2O aerosol droplets

We present experimental results obtained with a differential scanning calorimeter (DSC) that indicate the small ice particles in low-temperature cirrus clouds are not completely solid but rather coated with an unfrozen H2SO4/H2O overlayer. Our results provide a new look on the formation, development, and microphysical properties of low-temperature cirrus clouds.     

Uptake measurements of ethanol on ice surfaces and on supercooled aqueous solutions doped with nitric acid between 213 and 243 K

Uptake of ethanol either on pure frozen ice surfaces or supercooled solutions doped with HNO3 (0.63 and 2.49 wt %) has been investigated using a coated wall flow tube coupled to a mass spectrometric detection. The experiments were conducted over the temperature range of 213-243 K. Uptake of ethanol on these surfaces was always found to be totally reversible whatever were the experimental conditions. The number of ethanol molecules adsorbed per surface unit was conventionally plotted as a function of ethanol concentration in the gas phase and subsequently analyzed using Langmuir's model. The amount of ethanol molecules taken up on nitric acid doped-ice surfaces was found to increase largely with increasing nitric acid concentrations. For example at 223 K, and for an ethanol gas-phase concentration of 1x10(13) molecules cm3, the number of adsorbed molecules are (in units of molecules cm-2): approximately 1.3x10(14) on pure ice; approximately 1.4x10(15) on ice doped with HNO3 0.63 wt %; approximately 7.5x10(15) on ice doped with HNO3, 2.49 wt %, i.e. 60 times larger than on pure ice. Since, according to the shape of the isotherms, the adsorption did not proceed beyond monolayer coverage, the enormous increase of ethanol uptake was explained by considering its dissolution in either a supercooled liquid layer (T<230 K) or a liquid solution (T>230 K). The formation of both was indeed favored by the presence of the HNO3. Our experimental results suggest that the amount of ethanol dissolved in such supercooled solutions follows Henry's law and that the Henry's law constants at low temperatures, i.e., 223-243 K, can be estimated by extrapolation from higher temperatures. Such supercooled solutions which exist in the troposphere either in deep convective clouds or in mixed clouds for temperature above 233 K, might be responsible for the scavenging of large amounts of soluble species, such as nitric and sulfuric acids, oxygenated VOCs including alcohols, carboxylic acids, and formaldehyde.     

Diffusion of HDO in pure and acid-doped ice films

In these experiments, a few bilayers of D(2)O were vapor-deposited on a pure crystalline H(2)O ice film or an ice film doped with a small amount of HCl. Upon deposition, H/D isotopic exchange quickly converted the D(2)O layer into an HDO-rich mixture layer. Infrared absorption spectroscopy followed the changes of the HDO from the initial HDO mixture layer to HDO isolated in the H(2)O ice film. This was possible because isolated HDO in H(2)O ice has a unique, sharp peak in the O-D stretch region that can be distinguished from the broad peak due to the initial HDO mixture layer. The absorbance of isolated HDO displayed first-order kinetics and was attributed to diffusion of HDO from the HDO-rich mixture layer into the underlying H(2)O ice film. While negligible diffusion was observed for pure ice films and for ice films with HCl concentrations up to 1 x 10(-4) mole fraction, diffusion of HDO occurred for higher concentrations of (2-20) x 10(-4) mole fraction HCl with a concentration-independent rate constant. The diffusion under these conditions followed Arrhenius behavior for T = 135-145 K yielding E(a) = 25 +/- 5 kJ/mol. The mechanism for the HDO diffusion involves either (i) molecular self-diffusion or (ii) long-range H/D diffusion by a series of multiple proton hop and orientational turn steps. While these spectroscopic results compare favorably with recent studies of molecular self-diffusion in low-temperature ice films, the diffusion results from all the ice film studies at low temperatures (ca. T < 170 K) differ from earlier bulk ice studies at higher temperatures (ca. T > 220 K). A comparison and discussion of the various diffusion studies are included in this report.     

Real-time high-resolution X-ray imaging and nuclear magnetic resonance study of the hydration of pure and Na-doped C3A in the presence of sulfates

This study details the differences in real-time hydration between pure tricalcium aluminate (cubic C(3)A or 3CaO·Al(2)O(3)) and Na-doped tricalcium aluminate (orthorhombic C(3)A or Na(2)Ca(8)Al(6)O(18)), in aqueous solutions containing sulfate ions. Pure phases were synthesized in the laboratory to develop an independent benchmark for the reactions, meaning that their reactions during hydration in a simulated early age cement pore solution (saturated with respect to gypsum and lime) were able to be isolated. Because the rate of this reaction is extremely rapid, most microscopy methods are not adequate to study the early phases of the reactions in the early stages. Here, a high-resolution full-field soft X-ray imaging technique operating in the X-ray water window, combined with solution analysis by (27)Al nuclear magnetic resonance (NMR) spectroscopy, was used to capture information regarding the mechanism of C(3)A hydration during the early stages. There are differences in the hydration mechanism between the two types of C(3)A, which are also dependent on the concentration of sulfate ions in the solution. The reactions with cubic C(3)A (pure) seem to be more influenced by higher concentrations of sulfate ions, forming smaller ettringite needles at a slower pace than the orthorhombic C(3)A (Na-doped) sample. The rate of release of aluminate species into the solution phase is also accelerated by Na doping.     

Infrared optical constants of highly diluted sulfuric acid solution droplets at cirrus temperatures

Complex refractive indices for supercooled sulfuric acid solution droplets in the mid-infrared spectral regime (wavenumber range 6000-800 cm(-1)) have been retrieved for acid concentrations ranging from 33 to 10 wt % H2SO4 at temperatures between 235 and 230 K, from 36 to 15 wt % H2SO4 at temperatures between 225 and 219 K, and from 37 to 20 wt % H2SO4 at temperatures between 211 and 205 K. The optical constants were derived with a Mie inversion technique from measured H2SO4/H2O aerosol extinction spectra that were recorded during controlled expansion cooling experiments in the large coolable aerosol chamber AIDA of Forschungszentrum Karlsruhe. The new data sets cover a range of atmospherically relevant temperatures and compositions in the binary sulfuric acid/water system for which infrared refractive indices have not been published so far, namely, the regime when supercooled H2SO4/H2O solution droplets at T < 235 K are subjected to an environment that is supersaturated with respect to the ice phase. With increasing ice supersaturation, the H2SO4/H2O aerosol particles will continuously dilute by the uptake of water vapor from the gas phase until freezing of the solution droplets eventually occurs when the acid concentration has dropped below a critical, temperature-dependent threshold value. With the aid of the new measurements, the homogeneous freezing process of supercooled H2SO4/H2O solution droplets at cirrus temperatures can be quantitatively analyzed by means of Fourier transform infrared spectroscopy, thereby overcoming a major drawback from previous studies: the need to use complex refractive indices that were measured at temperatures well above 235 K to deduce the composition of the low-concentrated H2SO4/H2O aerosol particles. As in the case of the complex refractive indices for sulfuric acid solutions with acid concentrations greater than 37 wt % H2SO4, the new low-temperature optical constants for highly diluted droplets also reveal significant temperature-induced spectral variations in comparison with the refractive indices for higher temperatures, which are associated with a change in the equilibrium between sulfate and bisulfate ions.     

Evaporation loss of dissolved volatile substances from ice surfaces

Volatile acidic solutes were used to make dilute solutions, which were frozen by various methods. The concentration of solutes and the pH of the samples were measured before and after being frozen. When the sample solution is frozen from the bottom to the top, solutes are concentrated into the unfrozen solution (i.e., the upper part of the sample) due to the freeze concentration effect. Thereafter, concentrated anions combine with protons to form acids, and the amount of acids in the unfrozen solution increase as the ice formation progresses. At the end of freezing, the acid is saturated at the ice surface, and if the formed acid is volatile, then evaporation occurs. Frozen solutions were allowed to stand below 0 degrees C, where evaporation rates were obtained in the following order: formate > acetate > propionate > n-butyrate > chloride > nitrate. Except for nitrate, evaporation rates were enough to take place in frozen water of the natural environment (e.g., ice crystal, graupel, snow crystal, and frozen droplets). The relationship between the evaporation rate of volatile acids and their physical properties demonstrate that the evaporation rates of weak acids are faster than those of strong acids, and the evaporation rates among weak acids are the same as the volatility of weak acids.     

Dynamic and sub-ambient thermal transition relationships in water–sucrose solutions

This work was undertaken to investigate thermal and dynamic transitions observed in the temperature range close to the bulk ice melting temperature in sucrose solutions. Measurements of thermal (differential calorimetry) and dynamic (neutron scattering) properties were compared in order to give a physical interpretation of the thermal transitions observed during the thawing of amorphous sucrose solutions. In fact, the freezing of biological material leads to the distinction between different pools of water: bulk water which becomes ice after freezing, unfrozen water trapped in the glassy matrix or close to the interface of solutes can be considered, and finally freezable confined water with a lower melting point than bulk water and with properties depending on both the ice presence and the microstructure of the material. The transition temperatures such as glass transition or melting are dependent on the freezing protocol used and examples of annealing effects are presented, in order to underline the necessity of a good temperature control during freezing for the study of biological material with freezable water.     

Production of oxygen by electronically induced dissociations in ice

A solid-state chemical model is given for the production of O2 by electronic excitation of ice, a process that occurs on icy bodies in the outer solar system. Based on a review of the relevant available laboratory data, we propose that a trapped oxygen atom-water complex is the principal precursor for the formation of molecular oxygen in low-temperature ice at low fluences. Oxygen formation then occurs through direct excitation of this complex or by its reaction with a freshly produced, nonthermal O from an another excitation event. We describe a model for the latter process that includes competition with precursor destruction and the effect of sample structure. This allows us to put the ultraviolet photon, low-energy electron, and fast-ion experiments on a common footing for the first time. The formation of the trapped oxygen atom precursor is favored by the preferential loss of molecular hydrogen and is quenched by reactions with mobile H. The presence of impurity scavengers can limit the trapping of O, leading to the formation of oxygen-rich molecules in ice. Rate equations that include these reactions are given and integrated to obtain an analytic approximation for describing the experimental results on the production and loss of molecular oxygen from ice samples. In the proposed model, the loss rate varies, roughly, inversely with solid-state defect density at low temperatures, leading to a yield that increases with increasing temperature as observed. Cross sections obtained from fits of the model to laboratory data are evaluated in light of the proposed solid-state chemistry.     

States of water in poly(vinyl alcohol) hydrogels

Poly(vinyl alcohol) (PVA) hydrogels with various water contents were prepared from 10 wt% aqueous solutions of mixtures of PVA and anionic poly(styrene sulfonic acid) sodium salt (NaPSS) by casting, dehydrating, and then extracting NaPSS. The existence of three types of ice were suggested by differential scanning calorimetry (DSC) measurements for every frozen hydrogel. In the frozen hydrogels the states of water, except unfrozen water, were ice of free water and disordered ice crystals. The mobility and activation energy for motion of water molecules in unfrozen hydrogels were investigated by using pulsed nuclear magnetic resonance (PNMR) measurements. It was concluded that there are two states of water in the unfrozen hydrogel, i.e. unfrozen water and disordered water which is mainly formed in narrow apertures in the hydrogel. The discrepancy between the DSC and PNMR measurements was explained by a structural transformation during cooling.     

Influence of sodium chloride on the melting of ice and crystallization and dissociation of CCl3F hydrate in water in oil emulsion

Trichlorofluoromethane (CCl₃F) and water form clathrate hydrate which melts at 8.5 °C under atmospheric pressure. By DSC and X rays analysis we could distinguish between hydrate and ice formed in emulsion containing NaCl and show that quantity of hydrate formed and its dissociation temperature are dependent on solution concentration. The equilibrium curve hydrate-NaCl solution is displaced towards higher temperatures with respect to corresponding ice curve. Consequently solid–liquid equilibrium can not be established in presence of both solids. Growth of hydrate crystals at the expense of ice was evidenced. Role of salt in hydrate growth and ice melting is examined.