Efflorescence Relative Humidity Values Research Articles

Evaluating Leaf Wettability and Salt Hygroscopicity as Drivers for Foliar Absorption.

The objective of this study was to evaluate the rate of foliar absorption of magnesium (Mg) salts with different deliquescence and efflorescence relative humidity values (DRH and ERH, also known as point of deliquescence (POD) and point of efflorescence (POE), respectively) when supplied to leaves of model plants with different wettability properties. For this purpose, a greenhouse pot experiment was conducted with lettuce (very wettable), broccoli (highly unwettable) and leek (highly unwettable). Foliar sprays contained 0.1% surfactant plus 100 mM Mg supplied as MgCl2·6H2O, Mg(NO3)2·6H2O or MgSO4·7H2O. Leaf Mg concentrations were determined 1 and 7 days after foliar application. Anion concentrations were also measured in lettuce where a significant foliar Mg absorption was detected. Leaf wettability, leaf surface free energy and fertilizer drop deposit appearance onto the foliage were assessed. It is concluded that despite including a surfactant in the spray formulation, leaf wettability plays a major role in foliar Mg absorption.

Development of the low-temperature hygroscopicity tandem differential mobility analyzer (Low-T HTDMA) and its application to (NH4)2SO4 and NaCl particles

Influence of water vapor on aerosol particles depends on both relative humidity (RH) and temperature (T). Both temperature and RH should simultaneously be regulated to investigate physical properties of aerosol particles. We developed a low-temperature hygroscopicity tandem differential mobility analyzer (low-T HTDMA), which can be operated in the range of −20 °C ≤ T ≤ 22.5 °C (room temperature), and 0% < RH ≤ 90% (T > 0 °C) or slightly lower than ice saturation RH (T ≤ −10 °C). Temperature was controlled using a chest freezer, refrigerated liquid circulators, fans, and heat exchangers. The employment of heat exchangers allowed the operation of the instrument without immersing the whole setup into liquid coolant. Aerosol particles were humidified using a Shirasu porous glass (SPG) humidifier to facilitate efficient humidification. The performance of the low-T HTDMA was tested using (NH4)2SO4 and NaCl particles. The hygroscopic growth factors of these particles agreed well with previous studies. Deliquescence RH (DRH) of both (NH4)2SO4 and NaCl particles also agreed well with the literature. Compared with the exiting data, efflorescence RH (ERH) measured by the low-T HTDMA exhibited minimal temperature dependence. For instance, the ERH values were 42% (T = −20 °C), 42% (T = −10 °C), and 43% (T = 22.5 °C) for NaCl particles. We suggest that further independent laboratory studies will need to be conducted for experimentally constraining thermodynamic properties of atmospherically important chemical species for T < 0 °C.

Corrosion Behavior of 304 Stainless Steel Exposed to a Simulated Salt Lake Atmosphere

The corrosion behavior of stainless steel exposed to a simulated salt lake atmosphere has been investigated by analyzing the evolution of surface morphologies and corrosion products, the initiation and development of pits, and the electrochemical characteristics. The results indicated that (Mg6Fe2(OH)16(CO3)(H2O)4.5)0.25, a layered double hydroxide, has been detected for the first time in the corrosion products formed on stainless steel exposed to a simulated salt lake atmosphere. The specimens exposed to MgCl2 deposit conditions were corroded more severely than those exposed to NaCl deposit conditions, which was attributed to the differences in the deliquescence relative humidity and efflorescence relative humidity values of MgCl2 and NaCl. In addition, a special corrosion morphology consisting of a concentric circle of yellowish material was observed on the specimens exposed to MgCl2 deposit conditions, which was attributed to the formation of Mg(OH)2, inhibiting the diffusion and migration of OH− ions to the anode region. The maximum pit depth followed a power function with respect to corrosion time. The corrosion mechanism of stainless steel exposed to a simulated salt lake atmosphere is also discussed.

Freezing of perchlorate and chloride brines under Mars-relevant conditions

Perchlorate and chloride salts on Mars could readily absorb water vapor and deliquesce into aqueous solutions. The deliquescence relative humidity (RH) as well as the efflorescence (recrystallization) RH of several Mars-relevant salts are now well known; however, the conditions that could cause a brine to freeze are not well established. It is often assumed that ice formation will occur whenever the saturation with respect to ice, Sice, of the system is greater than or equal to unity; however, ice nucleation is often hindered due to a kinetic barrier. For ice to form, a critical cluster of the ice crystal must first be achieved, often requiring Sice>1. Here we use a Raman microscope and an environmental cell to examine the RH and temperature conditions required for Mg(ClO4)2 and MgCl2 brines to freeze into ice. By examining the salt phase present both optically and spectrally under different low temperature conditions, it is found that both salts exhibit Sice values much greater than unity, meaning that supersaturation readily occurs and brines can persist under conditions previously thought to lead to freezing. The RH range of ice formation for Mg(ClO4)2 from 218 to 245K is 83–95%, respectively, corresponding to Sice=1.30–1.54. The RH of ice formation for MgCl2 ranges from 80 to 100% for temperatures between 221 and 252K, corresponding to Sice=1.30–1.35. In addition to ice nucleation, the deliquescence and efflorescence relative humidity values for MgCl2 were determined. Two hydrates for MgCl2 were observed, and exhibited different deliquescence relative humidity (DRH) values. The DRH for MgCl2·4H2O was found to be 12.8±0.3% at 243K with slightly increasing DRH as temperature decreased. The DRH for MgCl2·6H2O was found to be 31.3±0.6% at 242K with little temperature dependence. The DRH of MgCl2·6H2O was measured below the previously reported eutectic, 240K, suggesting that the eutectic might be incorrect or that there is a different relevant hydration state. The aqueous solutions of MgCl2 recrystallized to the tetrahydrate at low RH in the range of 3–9% RH at temperatures 265–235K. Together, the ice nucleation, DRH, and efflorescence relative humidity (ERH) results show that Mg(ClO4)2 and MgCl2 brines on present day Mars may have the ability to exist for up to 2h longer than previously predicted.

Determination of interfacial parameters of a soluble particle in a nonideal solution from measured deliquescence and efflorescence humidities

Abstract. In order to study the growth/shrinking of a hygroscopic nanoparticle during hydration/dehydration in an atmosphere of water vapour, we have employed a thermodynamic approach proposed by Shchekin et al. (2008). This approach uses the mechanic and thermodynamic concept of disjoining pressure of thin films and allows, among others, the prediction of the humidity growth factor of both (i) a homogeneous solution droplet with completely dissolved residual core and (ii) a heterogeneous solution droplet with partially dissolved residual core as a function of the ambient relative humidity. For application to a nanometric sodium chloride particle we have extended the original approach by (i) considering the nonideality of the solution through the dependence of molecular volumes of the solvent and solute molecules and the solute and solvent activities on the solution concentration, (ii) deriving an equation for the estimation of the efflorescence properties of a homogeneous solution droplet, and (iii) combining the empirical power law fittings for the size dependence of the deliquescence and efflorescence relative humidity values by Biskos et al. (2006a). It was demonstrated how the solution/solute interface energy and the correlation length of a thin solution film can be determined from a combination of experimentally determinable efflorescence and deliquescence humidities with the present calculus. The solution/solute interface energy was found to be in close agreement with some previous values reported in the literature, while it strongly differs from data of some other sources. The calculated deliquescence humidity shows a low sensitivity to the choice of the numerical value for the film correlation length. The estimated film correlation length of 1 nm for a nanometric sodium chloride particle with dry particle radius of 5 nm was found to be reconcilable with available a priori estimates of the correlation length from the literature when the measurement uncertainty of the deliquescence humidity is considered. Considering the combination of an extensive calculus, a comprehensive set of thermophysical constraints, and independent measurements of the deliquescence and efflorescence humidities as functions of dry particle radius, the obtained values of the solution/solute interface energy and the correlation length are in close agreement with previous estimations. The humidification of sodium chloride particles in the initial hydration stages was found to be very sensitive to the specification of the disjoining pressure. The enhancement of the wettability of the particle surface leads to an earlier onset of hygroscopic growth.

Formation of aqueous solutions on Mars via deliquescence of chloride–perchlorate binary mixtures

Perchlorate salts, known to exist on Mars, can readily absorb water vapor and deliquesce into aqueous solutions even at low temperatures. The multiple soluble ionic species, such as chloride salts, present in the Martian subsurface may affect this deliquescence. Here we study the deliquescence (solid to aqueous transition) and efflorescence (aqueous to solid transition) of three perchlorate/chloride mixtures: KClO4/KCl at 253 K, NaClO4/NaCl at 243 and 253 K, and Mg(ClO4)2/MgCl2 at 243 and 253 K. A Raman microscope with an environmental cell was used to monitor the phase transitions of internally mixed ClO−4/Cl− particles as a function of the perchlorate mole fraction. The eutonic relative humidity (where deliquescence begins to occur regardless of ClO−4 mole fraction), deliquescence relative humidity (DRH, where complete deliquescence occurs), and efflorescence relative humidity (ERH) were measured for several perchlorate mole ratios for each cation system. At the temperatures studied, the eutonic relative humidity was measured to be 28% RH for Mg(ClO4)2/MgCl2 mixtures, 38% RH for NaClO4/NaCl mixtures, and 82% RH for KClO4/KCl mixtures. The DRH depends on the perchlorate mole ratio, but is below the DRH of the least deliquescent (highest DRH) pure salt. When humidity is lowered around an aqueous salt mixture, we find that efflorescence occurs at an RH below the DRH due to the kinetic inhibition of crystallization. The ERH values of the salt solutions were as low as 5% RH for Mg(ClO4)2/MgCl2 mixtures, as low as 13% RH for NaClO4/NaCl mixtures, and as low as 66% RH for KClO4/KCl mixtures. The low eutonic RH values for the Na+ and Mg2+ perchlorate/chloride mixtures are important: wherever Mg(ClO4)2 and MgCl2 or NaClO4 and NaCl coexist at the temperatures studied, mixtures will contain a stable aqueous phase above 28 or 38% RH, respectively, regardless of the perchlorate mole fraction. This liquid water may persist until 5 or 13% RH, respectively.

Laboratory studies of perchlorate phase transitions: Support for metastable aqueous perchlorate solutions on Mars

Perchlorate salts, recently discovered on Mars, are known to readily absorb water vapor from the atmosphere and deliquesce into the aqueous phase at room temperature. Here we study the deliquescence (crystalline solid to liquid transition) and efflorescence (liquid to crystalline solid transition) of perchlorate salts at low temperatures relevant to Mars. A Raman microscope and environmental cell were used to determine the deliquescence relative humidity (DRH) and efflorescence relative humidity (ERH) of NaClO 4 and Mg(ClO 4) 2 as a function of temperature and hydration state. We find that the deliquescence of anhydrous NaClO 4 is only slightly dependent on temperature and occurs at ~ 38% RH. The DRH of NaClO 4·H 2O increases with decreasing temperature from 51% at 273 K to 64% at 228 K. The DRH of Mg(ClO 4) 2·6H 2O also increases with decreasing temperature from 42% at 273 K to 64% at 223 K. The efflorescence of both NaClO 4 and Mg(ClO 4) 2 salt solutions occurs at a lower RH than deliquescence due to the kinetic inhibition of crystallization. For all temperatures studied, the ERH values of NaClO 4 and Mg(ClO 4) 2 are 13% and 19%, respectively. These results indicate perchlorate salts can exist as metastable, supersaturated solutions over a wide range of RH and temperature conditions. Summer diurnal temperature and relative humidity cycles at low latitudes on Mars could allow the surface salts to be aqueous for several hours per day.

Efflorescence of Ammonium Sulfate and Coated Ammonium Sulfate Particles: Evidence for Surface Nucleation

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.

Nanosize Effect on the Deliquescence and the Efflorescence of Sodium Chloride Particles

The deliquescence and efflorescence relative humidity values of 6- to 60-nm NaCl particles were measured using a tandem nano-Differential Mobility Analyzer. The deliquescence relative humidity (DRH) increased when the dry particle mobility diameter decreased below approximately 40 nm. The efflorescence relative humidity (ERH) similarly increased. For example, the DRH and ERH of 6-nm particles were 87% and 53%, respectively, compared to 75% and 45% for particles larger than 40 nm. Power law fits describing the nanosize effect are: DRH(d m) = 213 d m −1.6+ 76 and ERH(d m) = 213 d m −1.65+ 44, which are calibrated for 6 < d m < 60 nm with less than 1% RH uncertainty and where d m is the dry particle mobility diameter (nm). Two independent methods were used to generate the aerosol particles, namely by vaporizing and condensing granular sodium chloride and by electrospraying a high-purity sodium chloride aqueous solution, to investigate possible effects of impurities on the results. The DRH and ERH values were...

Phase Transitions of Single Salt Particles Studied Using a Transmission Electron Microscope with an Environmental Cell

The hygroscopic behavior of 0.1 to 4 μ m NaBr, CsCl, NaCl, (NH4)2SO4, and KBr particles were monitored using a transmission electron microscope (TEM) equipped with an environmental cell into which gases can be introduced. This instrument, commonly called an environmental transmission electron microscope or ETEM, allowed us to observe phase transitions and behavior of small particles at relative humidities between 0 and 100%. We used deliquescence relative humidity and efflorescence relative humidity values from the literature for each salt to calibrate the relative humidity in the environmental cell. Using our methodology, we reliably and accurately measured the phase transitions and hygroscopic behavior of inorganic particles with the ETEM.