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1.
Proc Natl Acad Sci U S A ; 116(17): 8184-8189, 2019 04 23.
Artigo em Inglês | MEDLINE | ID: mdl-30948638

RESUMO

Ice nucleation in the atmosphere influences cloud properties, altering precipitation and the radiative balance, ultimately regulating Earth's climate. An accepted ice nucleation pathway, known as deposition nucleation, assumes a direct transition of water from the vapor to the ice phase, without an intermediate liquid phase. However, studies have shown that nucleation occurs through a liquid phase in porous particles with narrow cracks or surface imperfections where the condensation of liquid below water saturation can occur, questioning the validity of deposition nucleation. We show that deposition nucleation cannot explain the strongly enhanced ice nucleation efficiency of porous compared with nonporous particles at temperatures below -40 °C and the absence of ice nucleation below water saturation at -35 °C. Using classical nucleation theory (CNT) and molecular dynamics simulations (MDS), we show that a network of closely spaced pores is necessary to overcome the barrier for macroscopic ice-crystal growth from narrow cylindrical pores. In the absence of pores, CNT predicts that the nucleation barrier is insurmountable, consistent with the absence of ice formation in MDS. Our results confirm that pore condensation and freezing (PCF), i.e., a mechanism of ice formation that proceeds via liquid water condensation in pores, is a dominant pathway for atmospheric ice nucleation below water saturation. We conclude that the ice nucleation activity of particles in the cirrus regime is determined by the porosity and wettability of pores. PCF represents a mechanism by which porous particles like dust could impact cloud radiative forcing and, thus, the climate via ice cloud formation.

2.
Sci Rep ; 7(1): 16634, 2017 11 30.
Artigo em Inglês | MEDLINE | ID: mdl-29192142

RESUMO

Homogeneous ice nucleation needs supercooling of more than 35 K to become effective. When pressure is applied to water, the melting and the freezing points both decrease. Conversely, melting and freezing temperatures increase under negative pressure, i.e. when water is stretched. This study presents an extrapolation of homogeneous ice nucleation temperatures from positive to negative pressures as a basis for further exploration of ice nucleation under negative pressure. It predicts that increasing negative pressure at temperatures below about 262 K eventually results in homogeneous ice nucleation while at warmer temperature homogeneous cavitation, i. e. bubble nucleation, dominates. Negative pressure occurs locally and briefly when water is stretched due to mechanical shock, sonic waves, or fragmentation. The occurrence of such transient negative pressure should suffice to trigger homogeneous ice nucleation at large supercooling in the absence of ice-nucleating surfaces. In addition, negative pressure can act together with ice-inducing surfaces to enhance their intrinsic ice nucleation efficiency. Dynamic ice nucleation can be used to improve properties and uniformity of frozen products by applying ultrasonic fields and might also be relevant for the freezing of large drops in rainclouds.

4.
Faraday Discuss ; 165: 289-316, 2013.
Artigo em Inglês | MEDLINE | ID: mdl-24601008

RESUMO

Despite major progress in the understanding of properties of tropospheric aerosol particles, it remains challenging to understand their physical state and morphology. To obtain more detailed knowledge of the phases, phase transitions and morphologies of internally mixed organic/inorganic aerosol particles, we evaluated liquid-liquid phase separation (LLPS), deliquescence relative humidity (DRH) and efflorescence relative humidity (ERH) of 33 organic/ammonium sulfate (AS)/H2O systems from our own and literature data. The organic fraction consists of single compounds or mixtures with up to ten aliphatic and/or aromatic components with carboxylic acid, hydroxyl, carbonyl, ether, and ester functionalities, covering O : C ratios between 0.29 and 1.33. Thirteen out of these 33 systems did not show LLPS for any of the studied organic-to-inorganic mixing ratios, sixteen underwent LLPS showing core-shell morphology, and four showed both core-shell and partially engulfed configurations depending on the organic-to-inorganic ratio and RH. In all cases the organic fractions of the systems with partially engulfed configurations consisted of dicarboxylic acids. AS in mixed organic/AS/H2O particles deliquesced between 70 and 84% RH. AS effloresced below 58% RH or remained in a one-liquid-phase state. AS in droplets with LLPS always showed efflorescence with ERH between 30 and 50% RH, providing clear evidence that the presence of LLPS facilitates AS efflorescence. Spreading coefficients of the organic-rich phase on the AS-rich phase for systems containing polyethylene glycol 400 (PEG-400) and a mixture of dicarboxylic acids are in agreement with the optically observed morphologies of droplets deposited on the hydrophobic substrate. Analysis of high resolution elastic Mie resonance spectra allowed the detection of LLPS for single levitated droplets consisting of PEG-400/AS/ H2O, whereas LLPS was difficult to detect in (2-methylglutaric acid + 3-methylglutaric acid + 2,2-dimethylsuccinic acid)/AS/H2O. Measured Mie spectra of PEG-400/AS/H2O at 93.5% and at 80.9% RH agreed with computed Mie spectra for a homogeneous and a core-shell configuration, respectively, confirming the results obtained from droplets deposited on a hydrophobic substrate. Based on the presented evidence, we therefore consider the core-shell morphology to be the prevalent configuration of liquid-liquid-phase-separated tropospheric organic/AS/H2O particles.

5.
Chem Soc Rev ; 41(19): 6631-62, 2012 Oct 07.
Artigo em Inglês | MEDLINE | ID: mdl-22739756

RESUMO

The complex interplay of processes that govern the size, composition, phase and morphology of aerosol particles in the atmosphere is challenging to understand and model. Measurements on single aerosol particles (2 to 100 µm in diameter) held in electrodynamic, optical and acoustic traps or deposited on a surface can allow the individual processes to be studied in isolation under controlled laboratory conditions. In particular, measurements can now be made of particle size with unprecedented accuracy (sub-nanometre) and over a wide range of timescales (spanning from milliseconds to many days). The physical state of a particle can be unambiguously identified and its composition and phase can be resolved with a high degree of spatial resolution. In this review, we describe the advances made in our understanding of aerosol properties and processes from measurements made of phase behaviour, hygroscopic growth, morphology, vapour pressure and the kinetics of water transport for single particles. We also show that studies of the oxidative aging of single particles, although limited in number, can allow the interplay of these properties to be investigated. We conclude by considering the contributions that single particle measurements can continue to make to our understanding of the properties and processes occurring in atmospheric aerosol.

6.
Phys Chem Chem Phys ; 13(8): 3514-26, 2011 Feb 28.
Artigo em Inglês | MEDLINE | ID: mdl-21229162

RESUMO

We present measurements of water uptake and release by single micrometre-sized aqueous sucrose particles. The experiments were performed in an electrodynamic balance where the particles can be stored contact-free in a temperature and humidity controlled chamber for several days. Aqueous sucrose particles react to a change in ambient humidity by absorbing/desorbing water from the gas phase. This water absorption (desorption) results in an increasing (decreasing) droplet size and a decreasing (increasing) solute concentration. Optical techniques were employed to follow minute changes of the droplet's size, with a sensitivity of 0.2 nm, as a result of changes in temperature or humidity. We exposed several particles either to humidity cycles (between ∼2% and 90%) at 291 K or to constant relative humidity and temperature conditions over long periods of time (up to several days) at temperatures ranging from 203 to 291 K. In doing so, a retarded water uptake and release at low relative humidities and/or low temperatures was observed. Under the conditions studied here, the kinetics of this water absorption/desorption process is controlled entirely by liquid-phase diffusion of water molecules. Hence, it is possible to derive the translational diffusion coefficient of water molecules, D(H(2)O,) from these data by simulating the growth or shrinkage of a particle with a liquid-phase diffusion model. Values for D(H(2)O)-values as low as 10(-24) m(2) s(-1) are determined using data at temperatures down to 203 K deep in the glassy state. From the experiment and modelling we can infer strong concentration gradients within a single particle including a glassy skin in the outer shells of the particle. Such glassy skins practically isolate the liquid core of a particle from the surrounding gas phase, resulting in extremely long equilibration times for such particles, caused by the strongly non-linear relationship between concentration and D(H(2)O). We present a new parameterization of D(H(2)O) that facilitates describing the stability of aqueous food and pharmaceutical formulations in the glassy state, the processing of amorphous aerosol particles in spray-drying technology, and the suppression of heterogeneous chemical reactions in glassy atmospheric aerosol particles.


Assuntos
Sacarose/química , Água/química , Difusão , Umidade , Modelos Teóricos , Transição de Fase , Temperatura
7.
J Phys Chem A ; 114(35): 9486-95, 2010 Sep 09.
Artigo em Inglês | MEDLINE | ID: mdl-20712361

RESUMO

Using optical microscopy, we investigated the efflorescence of ammonium sulfate (AS) in aqueous AS and in aqueous 1:1 and 8:1 (by dry weight) poly(ethylene glycol)-400 (PEG-400)/AS particles deposited on a hydrophobically coated slide. Aqueous PEG-400/AS particles exposed to decreasing relative humidity (RH) exhibit a liquid-liquid phase separation below approximately 90% RH with the PEG-400-rich phase surrounding the aqueous AS inner phase. Pure aqueous AS particles effloresced in the RH range from 36.3% to 43.7%, in agreement with literature data (31-48% RH). In contrast, aqueous 1:1 (by dry weight) PEG-400/AS particles with diameters of the AS phase from 7.2 to 19.2 mum effloresced between 26.8% and 33.9% RH and aqueous 8:1 (by dry weight) PEG-400/AS particles with diameters of the AS phase from 1.8 to 7.3 mum between 24.3% and 29.3% RH. Such low efflorescence relative humidity (ERH) values have never been reached before for AS particles of this size range. We show that these unprecedented low ERHs of AS in PEG-400/AS particles could not possibly be explained by the presence of low amounts of PEG-400 in the aqueous AS phase, by a potential inhibition of water evaporation via anomalously slow diffusion through the PEG coating, or by different time scales between various experimental techniques. High-speed photography of the efflorescence process allowed the development of the AS crystallization fronts within the particles to be monitored with millisecond time resolution. The nucleation sites were inferred from the initial crystal growth sites. Analysis of the probability distribution of initial sites of 31 and 19 efflorescence events for pure AS and 1:1 (by dry weight) PEG-400/AS particles, respectively, showed that the particle volume can be excluded as the preferred nucleation site in the case of pure AS particles. For aqueous 1:1 (by dry weight) PEG-400/AS particles preferential AS nucleation in the PEG phase and at the PEG/AS/substrate contact line can be excluded. On the basis of this probability analysis of efflorescence events together with the AS ERH values of pure aqueous AS and aqueous PEG-400/AS particles aforementioned, we suggest that in pure aqueous AS particles nucleation starts at the surface of the particles and attribute the lower ERH values observed for aqueous PEG-400/AS particles to the suppression of the surface-induced nucleation process. Our results suggest that surface-induced nucleation is likely to also occur during the efflorescence of atmospheric AS aerosol particles, possibly constituting the dominating nucleation pathway.


Assuntos
Sulfato de Amônio/química , Materiais Revestidos Biocompatíveis/química , Polietilenoglicóis/química , Umidade , Tamanho da Partícula , Propriedades de Superfície , Água/química
8.
J Phys Chem A ; 113(41): 10966-78, 2009 Oct 15.
Artigo em Inglês | MEDLINE | ID: mdl-19775109

RESUMO

Currently, the physical state of mixed organic/inorganic aerosol particles is not well characterized, largely because of the still unclear chemical composition of the organic fraction and of its properties with respect to mixing with the inorganic fraction. To obtain insight in the possible phases and phase transitions of such aerosol particles, we investigated the ternary poly(ethylene glycol)-400/ammonium sulfate/water system as a representative model system with partially immiscible constituents. For this purpose, we used optical microscopy and micro-Raman spectroscopy on micrometer-sized particles deposited on a hydrophobically coated substrate. The particles show liquid-liquid phase separations both upon decreasing (approximately 90-85%) and increasing (during ammonium sulfate deliquescence) relative humidities. In dependence upon the organic-to-inorganic ratio, OIR (i.e., poly(ethylene glycol)-400 to ammonium sulfate dry mass), phase separation is observed to occur by fundamentally different mechanisms, namely, nucleation-and-growth (OIR = 8:1 to 2:1), spinodal decomposition (OIR = 1.5:1 to 1:1.5) and growth of a second phase at the surface of the particle (OIR = 1:2 to 1:8). For each of these mechanisms, after completion of the phase separation, the resulting morphology of the particles is an aqueous ammonium sulfate inner phase surrounded by a mainly poly(ethylene glycol)-400 containing outer phase. We depict the various physical states of the ternary system in the relative humidity/composition phase diagram, constructed from bulk data and single particle measurements. Given the complex chemical composition of the organic fraction in tropospheric aerosols, it is expected that repulsive forces between the organic and inorganic aerosol constituents exist and that liquid-liquid phase separations commonly occur. The presence of liquid-liquid phase separations may change the partitioning of semivolatile species between the gas and the condensed phase, whereas the predominantly organic shell is likely to influence heterogeneous chemical reactions, such as N(2)O(5) hydrolysis.

9.
Science ; 314(5804): 1399-402, 2006 Dec 01.
Artigo em Inglês | MEDLINE | ID: mdl-17138887
10.
J Phys Chem A ; 110(5): 1881-93, 2006 Feb 09.
Artigo em Inglês | MEDLINE | ID: mdl-16451021

RESUMO

A correct description of the aerosol's phases is required to determine its gas/particle partitioning, its reactivity and its water uptake and release. In this study, we investigate organic/electrolyte interactions of ammonium sulfate, nitrate and sodium chloride with substances containing carboxylic acids (COOH) and hydroxyl (OH) functional groups. As organic model compounds, we chose polyols with different OH/CHn (n = 0-3) ratios-namely, glycerol, 1,4-butanediol, and 1,2-hexanediol-as well as PEG 400 and a mixture of dicarboxylic acids consisting of malic, malonic, maleic, glutaric, and methylsuccinic acid. Bulk solubility and water activity measurements of these model systems together with a survey of literature data showed that NaCl is a salting-out agent for alcohols and organic acids whereas ammonium nitrate and sulfate exhibited salting-in and salting-out tendencies depending on the nature and number of functional groups as well as on the concentration of the solution. All investigated salts induce a liquid-liquid phase separation in the 1,2-hexanediol/water system. Considering the composition of the tropospheric aerosol, such phase separations might indeed occur frequently when particles in the atmosphere are exposed to varying relative humidity. To complement the bulk experiments, we investigated single particles consisting of ammonium sulfate and dicarboxylic acids as well as of ammonium sulfate and PEG 400 in an electrodynamic balance. Whereas the relative humidities of total deliquescence as well as the water uptake and release of the fully deliquesced particles are in good agreement with the bulk results and represent thermodynamic equilibrium, the water uptake before full deliquescence shows significant deviations. These deviations may be caused by morphological effects.

11.
Opt Express ; 14(15): 6951-62, 2006 Jul 24.
Artigo em Inglês | MEDLINE | ID: mdl-19516879

RESUMO

We report a novel and fairly simple optical technique for sizing and measuring the evaporation rates of aqueous solution aerosol particles. A ball-lens LED with high degree of spatial coherence is used as a "white" light source to excite the Morphology Dependent Resonance (MDR) spectra of a microdroplet levitated in an electrodynamic balance (EDB). The spectra are recorded by an Optical Multichannel Analyzer. We show that very low vapor pressures of substances in aqueous solution particles can be measured for different temperatures and relative humidities (hence for different concentrations). As an application we measured the vapor pressure and the enthalpy of vaporization of aqueous malonic acid, a substance of interest for atmospheric science.

12.
J Pharm Sci ; 92(1): 149-60, 2003 Jan.
Artigo em Inglês | MEDLINE | ID: mdl-12486691

RESUMO

Knowledge and control of the polymorphic phase of chemical compounds are important aspects of drug development in the pharmaceutical industry. We report herein in situ and real-time Raman spectroscopic polymorphic analysis of optically trapped microcrystals in a microliter volume format. The system studied in particular was the recrystallization of carbamazepine (CBZ) in methanol. Raman spectrometry enabled noninvasive measurement of the amount of dissolved CBZ in a sample as well as polymorphic characterization, whereas exclusive recrystallization of either CBZ form I or CBZ form III from saturated solutions was achieved by specific selection of sample cell cooling profiles. Additionally, using a microcell versus a macroscopic volume gives the advantage of reaching equilibrium much faster while using little compound quantity. We demonstrate that laser Raman spectral polymorphic analysis in a microliter cell is a potentially viable screening platform for polymorphic analysis and could lead to a new high throughput method for polymorph screening.


Assuntos
Soluções Farmacêuticas/análise , Tecnologia Farmacêutica/métodos , Cristalização , Lasers , Soluções Farmacêuticas/química , Análise Espectral Raman/instrumentação , Análise Espectral Raman/métodos , Tecnologia Farmacêutica/instrumentação
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