Supercooled Sulfuric Acid Research Articles

The relationship between charge and molecular dynamics in viscous acid hydrates.

Oscillatory shear rheology has been employed to access the structural rearrangements of deeply supercooled sulfuric acid tetrahydrate (SA4H) and phosphoric acid monohydrate, the latter in protonated (PA1H) and deuterated (PA1D) forms. Their viscoelastic responses are analyzed in relation to their previously investigated electric conductivity. The comparison of the also presently reported dielectric response of deuterated sulfuric acid tetrahydrate (SA4D) and that of its protonated analog SA4H reveals an absence of isotope effects for the charge transport in this hydrate. This finding clearly contrasts with the situation known for PA1H and PA1D. Our analyses also demonstrate that the conductivity relaxation profiles of acid hydrides closely resemble those exhibited by classical ionic electrolytes, even though the charge transport in phosphoric acid hydrates is dominated by proton transfer processes. At variance with this dielectric simplicity, the viscoelastic responses of these materials depend on their structural compositions. While SA4H displays a "simple liquid"-like viscoelastic behavior, the mechanical responses of PA1H and PA1D are more complex, revealing relaxation modes, which are faster than their ubiquitous structural rearrangements. Interestingly, the characteristic rates of these fast mechanical relaxations agree well with the characteristic frequencies of the charge rearrangements probed in the dielectric investigations, suggesting appearance of a proton transfer in mechanical relaxation of phosphoric acid hydrates. These findings open the exciting perspective of exploiting shear rheology to access not only the dynamics of the matrix but also that of the charge carriers in highly viscous decoupled conductors.

Effects of variation in coagulation and photochemistry parameters on the particle size distributions in the Venus clouds

This paper explores the effects that variation in the coalescence efficiency of the Venus cloud particles can have on the structure of the Venus cloud. It is motivated by the acknowledgment of uncertainties in the measured parameters—and the assumptions made to account for them—that define our present knowledge of the particle characteristics. Specifically, we explore the consequence of allowing the coalescence efficiency of supercooled sulfuric acid in the upper clouds to tend to zero. This produces a cloud that occasionally exhibits an enhancement of small particles at altitude (similar to the upper hazes observed by Pioneer Venus and subsequently shown to be somewhat transient). This simulated cloud occasionally exhibits a rapid growth of particle size near cloud base, exhibiting characteristics similar to those seen in the controversial Mode 3 particles. These results demonstrate that a subset of the variations observed as near-infrared opacity variations in the lower and middle clouds of Venus can be explained by microphysical, in addition to dynamical, variations. Furthermore, the existence of a population of particles exhibiting less efficient coalescence efficiencies would support the likelihood of conditions suitable for charge exchange, hence lightning, in the Venus clouds. We recommend future laboratory studies on the coalescence properties of sulfuric acid under the range of conditions experienced in the Venus clouds. We also recommend future in situ measurements to better characterize the properties of the cloud particles themselves, especially composition and particle habits (shapes).

Molecular Dynamics Study of Water Transfer at Supercooled Sulfuric Acid Solution Surface Covered with Butanol

The evaporation and condensation mechanisms of water through a butanol film on sulfuric acid solution are elucidated by molecular dynamics simulation. A previous experiment by Nathanson et al. reported the mass accommodation coefficient α to be almost unity, whereas MD simulation of water scattering on the butanol film on water predicted a value of α significantly smaller than unity. This discrepancy is elucidated by considering the protonated butanol at the sulfuric acid solution surface, which roughens the surface layer, and the low temperature at the supercooled condition.

Molecular Dynamics Simulations of Water Transport through Butanol Films

It is well-known that well-formed monolayers of long-chain alcohols on water dramatically reduce the rate of water evaporation. Recent experiments, however, have shown that the short-chain surfactant butanol has a negligible effect on the rate of water evaporation from a solution of supercooled sulfuric acid. We used molecular dynamics simulations to examine the condensation of water through a layer of butanol coating water. From those simulations, we observed a reduction in the rate of water condensation by a factor of 3 which would imply a similar reduction in the rate of evaporation. It was observed that a vapor molecule would condense only if it formed a long-lived hydrogen bond with another molecule and that there was no dependence on the initial velocity of the vapor molecule. The ability of the surfactant to participate in hydrogen bonding was found to facilitate the condensation of water into the bulk.

Surfactant Control of Gas Transport and Reactions at the Surface of Sulfuric Acid

Aerosol particles in the atmosphere are tiny chemical reactors that catalyze numerous reactions, including the conversion of benign gases into ozone-destroying ones. In the lower stratosphere, these particles are often supercooled mixtures of water and sulfuric acid. The different species present at the surface of these droplets (H(2)O, H(3)O(+), HSO(4)(-), H(2)SO(4), and SO(4)(2-)) stand at the "gas-liquid frontier"; as the first to be struck by impinging molecules, these species provide the initial environment for solvation and reaction. Furthermore, aerosol particles may contain a wide range of organic molecules, some of which migrate to the surface and coat the droplet. How do ambient gases dissolve in the droplet if it is coated with an organic layer? At one extreme, monolayer films of insoluble, long-chain alcohols can dramatically reduce gas transport, packing so tightly at the surface of water that they impede water evaporation by factors of 10,000 or more. Shorter chain surfactants are expected to pack less tightly, but we wondered whether these incomplete monolayers also block gas transport and whether this system could serve as a model for understanding the surfaces of atmospheric aerosol particles. To address these questions, our research focuses on small, soluble surfactants such as butanol and hexanol dissolved in supercooled sulfuric acid. These amphiphilic molecules spontaneously segregate to the surface and coat the acid but only to a degree. Gas-liquid scattering experiments reveal that these porous films behave in surprisingly diverse ways: they can impose a barrier (to N(2)O(5) hydrolysis), be "invisible" (to water evaporation), or even enhance gas uptake (of HCl). The transition from obstacle to catalyst can be traced to specific interactions between the surfactant and each gas. For example, the hydrolysis of N(2)O(5) may be impeded because of its large size and because alcohol molecules that straddle the interface limit contact between N(2)O(5) and its H(3)O(+) and H(2)O reaction partners. However, these same alcohol molecules assist HCl dissociation because the alcohol OH groups provide extra interfacial protonation sites. Interestingly, butanol does not impede water evaporation, in part because the butyl chains pack much more loosely than insoluble, long-chain surfactants. Through these investigations, we hope to gain insight into the mechanisms by which surfactants on sulfuric acid and other aqueous solutions affect transport and reactivity at the gas-liquid interface.

Collisions of noble gases with supercooled sulfuric acid–water solutions

The collisions of hyperthermal noble gases (He, Ne, Ar, Kr, Xe) with supercooled binary sulfuric acid-water mixtures (57-77 wt%) were explored in the temperature range between 210 and 240 K. The experiments were performed by directing a molecular beam of the respective gases onto a continuously renewed liquid surface and monitoring the velocity of the scattered molecules by mass spectrometry. Depending on the initial translational energies and molecular masses, we observe both inelastic scattering from the surface as well as thermalization followed by subsequent desorption. The experiments indicate that the repulsive momentum transfer in the inelastic scattering channel increases with increasing mass of the impinging gas, while it is only weakly affected by the initial velocities. The final energy of the thermally desorbing atoms can always be approximated by a Maxwell-Boltzmann distribution equal to the liquid bulk phase temperature. The influence of the binary composition of the liquid phase is only noticeable in the case of Ne, whilst this dependence diminishes for gases with molecular masses >or=40 amu. The probability of thermalisation relative to inelastic scattering increases with the bulk phase temperature, independent of the molecular masses of the colliding gas. In contrast, the fractional energy transfer during collision does not increase with temperature, except for Neon. These results can be interpreted in the model framework of hard-sphere collisions of noble gases with the surface, during which water and sulfuric acid molecules interact independently with the impinging gas.

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 of Water and Uptake of HCl and HBr through Hexanol Films at the Surface of Supercooled Sulfuric Acid

Vacuum evaporation and molecular beam scattering experiments have been used to monitor the loss of water and dissolution of HCl and HBr in deuterated sulfuric acid at 213 K containing 0 to 100 mM hexanol. The addition of 1-hexanol to the acid creates a surface film of hexyl species. This film becomes more compact with decreasing acidity, ranging from approximately 62% to approximately 68% of maximum packing on 68 to 56 wt % D(2)SO(4), respectively. D(2)O evaporation from 68 wt % acid remains unaltered by the hexyl film, where it is most porous, but is impeded by approximately 20% from 56 and 60 wt % acid. H --> D exchange experiments further indicate that the hexyl film on 68 wt % acid enhances conversion of HCl and HBr into DCl and DBr, which is interpreted as an increase in HCl and HBr entry into the bulk acid. For this permeable hexyl film, the hydroxyl groups of surface hexanol molecules may assist uptake by providing extra sites for HCl and HBr hydrogen bonding and dissociation. In contrast, HCl --> DCl exchange in 60 wt % D(2)SO(4) at first rises with hexyl surface coverage but then drops back to the bare acid value as the hexyl species pack more tightly. HCl entry is actually diminished by the hexyl film on 56 wt % acid, where the film is most compact. These experiments reveal a transition from a porous hexanol film on 68 wt % sulfuric acid that enhances HCl and HBr uptake to one on 56 wt % acid that slightly impedes HCl and D(2)O transport.

Surfactant Control of Gas Uptake: Effect of Butanol Films on HCl and HBr Entry into Supercooled Sulfuric Acid

The entry of HCl into 60-68 wt % D(2)SO(4) and HBr into 68 wt % acid containing 0-0.18 M 1-butanol was monitored by measuring the fractions of impinging HCl and HBr molecules that desorb as DCl and DBr after undergoing H --> D exchange within the deuterated acid. The addition of 0.18 M butanol to the acid creates butyl films that reach approximately 80% surface coverage at 213 K. Surprisingly, this butyl film does not impede exchange but instead enhances it: the HCl --> DCl exchange fractions increase from 0.52 to 0.74 for 60 wt % D(2)SO(4) and from 0.14 to 0.27 for 68 wt % D(2)SO(4). HBr --> DBr exchange increases even more sharply, rising from 0.22 to 0.65 for 68 wt % D(2)SO(4). We demonstrate that this enhanced exchange corresponds to enhanced uptake into the butyl-coated acid for HBr and infer this equivalence for HCl. In contrast, the entry probability of the basic molecule CF(3)CH(2)OH exceeds 0.85 at all acid concentrations and is only slightly diminished by the butyl film. The OD groups of surface butanol molecules may assist entry by providing extra interfacial protonation sites for HCl and HBr dissociation. The experiments suggest that short-chain surfactants in sulfuric acid aerosols do not hinder heterogeneous reactions of HCl or HBr with other solute species.

Evaporation of Water through Butanol Films at the Surface of Supercooled Sulfuric Acid

The evaporation of water was monitored from 60, 64, and 68 wt % D(2)SO(4) at 213 K containing 0-0.18 M 1-butanol. Measurements were performed in vacuum using a mass spectrometer to record the velocities and relative fluxes of the desorbing D(2)O. In addition, the surface activity of butanol in the acid was characterized by hyperthermal argon atom scattering in conjunction with surface tension and butanol evaporation measurements. The segregated butyl species reach surface concentrations of approximately 4 x 10(14) cm(-2) (approximately 80% surface coverage) at 0.18 M bulk concentration. We find that the butyl films do not impede the evaporation of D(2)O from the acid to within the 5% uncertainty of the measurements. This result implies that small, soluble surfactants such as butanol form porous films that will not alter the growth or shrinkage of supercooled sulfuric acid droplets in the atmosphere.

Molecular beam studies of gas-liquid interfaces.

Molecular beam scattering experiments provide a way to disentangle the elementary steps involved in energy transfer and chemical reactions between gases and liquids. After surveying the history and recent progress in this field, we review studies of the kinematics of gas-liquid collisions and proton exchange of HCl, DCl, and HBr with supercooled sulfuric acid and liquid glycerol. These experiments help to clarify the role of the surface region in controlling trapping and interfacial- and bulk-phase reactions.

Reaction and desorption of HCl and HBr following collisions with supercooled sulfuric acid

Gaseous HCl and HBr react with sulfuric acid at rates that depend strongly on acid concentration over a range of acidities typical of stratospheric aerosols. We monitor the competition between immediate desorption and H→D exchange after HCl and HBr thermalize upon collision with the surface of deuterated sulfuric acid: the exchange probabilities decrease from 0.7 to 0.1 (HCl) and 0.9 to 0.2 (HBr) as the acid concentration is increased from 55 to 70 wt% D2SO4 at 213 K. These measurements imply that HCl and HBr desorb faster than they dissociate at higher acidities, impeding the formation of Cl− and Br− in more acidic aerosols. Residence time measurements of HCl molecules that do enter the acid, however, indicate that HCl is more soluble than expected at high sulfuric acid concentrations.

Uptake Dynamics and Diffusion of HCl in Sulfuric Acid Solution Measured in Single Levitated Microdroplets

The uptake dynamics of HCl by single sulfuric acid microdroplets under stratospheric conditions is investigated. The droplets are typically 30−70 μm in diameter and weakly charged so that they can be stored in an electrodynamic trap. The gas uptake of the droplets is monitored either by measuring their size by angle resolved Mie- scattering patterns or by electrostatic balancing of the droplets in the trap. At low temperatures and high sulfuric acid concentration (T < 190 K for 48 wt % H2SO4 and T < 195 K for 56 wt % H2SO4, respectively), liquid-phase diffusion inside the droplet is the rate-limiting step in the overall uptake process. In this regime, the diffusion coefficients Dliq of HCl in supercooled sulfuric acid solutions are found to increase strongly with temperature and H2O concentration. The results are discussed with respect to diffusion models that have been proposed recently. In contrast, at higher temperatures and lower sulfuric acid concentrations (30−40 wt % H2SO4, 185−207 K) gas-phase diffusion with subsequent accommodation/dissolution at the liquid surface determines the observed uptake velocity. A new method to deduce accommodation coefficients α is proposed.

Interactions of HBr, HCl, and HOBr with supercooled sulfuric acid solutions of stratospheric composition

The time‐dependent uptakes of HBr and HCl by supercooled solutions of sulfuric acid have been studied using a low‐temperature flow tube coupled to a mass spectrometer. From these experiments, typical values for the quantity H*D1/2 (where H* is the effective Henry's law constant and D is the liquid phase diffusion coefficient) have been measured to be 1100 and 3.3 (M/atm)(cm2/s)1/2 for HBr and HCl, respectively, on 59.6 wt% sulfuric acid at 210 K. By estimating values for the liquid phase diffusion coefficient, these measurements indicate that HBr is over 2 orders of magnitude more soluble in atmospheric sulfate aerosols than HCl. Similarly, HD1/2 for HOBr on 69.8 wt% sulfuric acid at 228 K has been measured to be 20 (M/atm)(cm2/s)1/2, implying that HOBr, too, is highly soluble in sulfuric acid solutions. By exposing sulfuric acid films simultaneously to HOBr and either HCl or HBr, the liquid phase reactions HOBr + HCl → BrCl + H2O and HOBr + HBr → Br2 + H2O have been studied. The results indicate that the HOBr/HCl heterogeneous reaction may play an important role in the activation of stratospheric chlorine under volcanic aerosol conditions.

Viscosity of Supercooled Sulfuric Acid Solutions

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTViscosity of Supercooled Sulfuric Acid SolutionsLeah R. Williams and Forrest S. LongCite this: J. Phys. Chem. 1995, 99, 11, 3748–3751Publication Date (Print):March 1, 1995Publication History Published online1 May 2002Published inissue 1 March 1995https://doi.org/10.1021/j100011a050RIGHTS & PERMISSIONSArticle Views364Altmetric-Citations62LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (391 KB) Get e-Alerts

Supercooled Sulfuric Acid Droplets: Perturbed Stratospheric Chemistry in Early Winter

AbstractDuring the onset of polar winter in the stratosphere, before the formation of nitric acid ice clouds (referred to as polar stratospheric clouds, or PSCs), sulfate aerosol particles may catalyze heterogeneous chemical reactions that redistribute chlorine from reservoir species to more active forms. This activated chlorine can lead to ozone destruction in the fall and early winter, prior to the formation of PSCs, at middle and high latitudes over the winter half‐year in both hemispheres. During periods of slow cooling, a significant fraction of ambient chlorine nitrate (ClONO2) may be processed into active chlorine through its reactions with water and HCl at the surfaces of dilute supercooled sulfuric acid droplets. This mechanism for chlorine activation may explain the occurrence of widespread regions of reduced ozone abundance in the Northern Hemisphere at high‐ and middle‐latitudes outside of the polar vortex. Following major volcanic eruptions, a large increase in the surface area of sulfate particles could lead to significant repartitioning of chlorine species via the reaction of ClONO2 with H2O, even under midlatitude conditions, which may explain in part the detection of ozone reductions in volcanic clouds. Chemical transformation of other compounds may also be accelerated by the presence of dilute supercooled sulfate aerosols in the stratosphere, including the reaction of HOCl with HCl, and the hydrolysis of COF2 to HF; however, neither of these latter processes appear to have first‐order significance for the chemistry of the stratosphere.