Mole Ratio Dependence of the Mutual Deliquescence Relative

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Mole Ratio Dependence of the Mutual Deliquescence Relative Humidity of Aqueous Salts of Atmospheric Importance Bryant N Fong, James T. Kennon, and Hashim M. Ali J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b02706 • Publication Date (Web): 03 May 2016 Downloaded from http://pubs.acs.org on May 7, 2016

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The Journal of Physical Chemistry

Mole Ratio Dependence of the Mutual Deliquescence Relative Humidity of Aqueous Salts of Atmospheric Importance

Bryant N. Fong, James T. Kennon, Hashim M. Ali* Department of Chemistry and Physics, Arkansas State University, Jonesboro, AR 72404 Keywords: ATR-FTIR, Mutual deliquescence, mixed particles systems, molar ratio *Corresponding Author

Email: [email protected] Phone: 1 870-972-3215

Fax: 1 870-972-3089

Present Address: Department of Chemistry and Physics, Arkansas State University, PO Box 419 State University, AR 72467

ACS Paragon Plus Environment

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ABSTRACT

The response of the mutual deliquescence relative humidity (MDRH) of several mixed salt systems to changes in mole ratio is presented here. The MDRH of NH4Cl-NaCl, NH4Cl(NH4)2SO4 and, for the first time the NaCl-NaBr systems was acquired as a function of mole ratio. These changes were studied using Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy. The MDRH of 1:1 salt mixtures was consistently found to be lower than the individual deliquescence relative humidity (iDRH) of the NH4Cl-NaCl and the NH4Cl-(NH4)2SO4. The exception was the MDRH of the NaCl-NaBr system, which was found to be higher than the iDRH of NaBr particles, but lower than the iDRH of NaCl particles. When the mole ratio of the mixed system was varied, the MDRH of the mixtures showed a slight dependence on the mole ratio.


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INTRODUCTION According to the 2013 Intergovernmental Panel on Climate Change (IPCC) report, uncertainty remains about aerosol reactions and their role in climate radiative forcing.1 The uncertainty stems from the lack of details on several quantifiable atmospheric processes like their optical and hygroscopic properties under different atmospheric conditions. These conditions determine a particle’s chemical reactivity, and optical behavior. Salt particles, for example, are hygroscopic in nature and exhibit chemical and physical changes as a function of relative humidity (RH). The extent to which atmospheric particles reflect or absorb radiation depends on their size, chemical composition, and physical state. For example, studies have shown that the light scattering efficiency of a wet ammonium sulfate particles at high RH is fifteen times greater compared to a dry particle.2,

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Other studies find that wet aerosols have a decreased light

scattering efficiency resulting from changes in diameter and refractive index.4,

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These

contradictory views have created confusion in the role of aerosol mixtures in climate chemistry, leading to uncertainty about their contributions in global climate model (GCM) predictions, making it one of the most outstanding issues for current and future research.6 Experimental data are still needed to improve our understanding of aerosol chemistry and to use these data to validate these models and improve their predictive capabilities. In terms of mixed aerosol particles, their characteristics and response to atmospheric variables like water vapor (as humidity) are still limited to theories created a decade ago and new data is needed to improve these theories.2 Crystalline salts absorb water at a specific RH, referred to as the deliquescence relative humidity (DRH) to form saturated solution droplets. Several field measurements have found that atmospheric particles such as mineral dust exhibit


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hydrophilic behaviors and influence the DRH of other particles in the atmosphere.7 For example, Montmorillonite (a type of mineral dust) has been found to decrease the mutual deliquescence relative humidity (MDRH) of ammonium sulfate ((NH4)2SO4) particles and sodium chloride (NaCl) particles when compared to the individual salts due to ion-molecule interactions in the lattice structure. 8 Tang studied the phase change behavior of NaCl-KCl mixed particles at 298 K, and found that the mixed system deliquesces at an MDRH value of 73.8%, a value lower than the iDRH of NaCl (75%) and KCl (84%).9 These theoretical studies and more have shown that mixed particles hygroscopicity, an important contributor to the overall optical characteristics of multicomponent aerosols, is still a complex and relatively misunderstood area. In a mixed system, the lowest humidity where the two components will mutually go into solution is determined by the ratio of the iDRH of the individual salt component. The ratio of the two components is the eutonic composition. Mixed compositions will take up some water at DRH of the eutonic composition until RH is increased to MDRH of the mixture of interest and the system fully deliquesces with no solids remaining. Several models have been put forward to predict the hygroscopic behavior of mixed aerosols. The most widely used model is the Extended Aerosol Inorganic Model (E-AIM), which is used to model the deliquescence behaviors of various (1:1) multicomponent solutions of various inorganic electrolytes and organic molecules at 298.15 K.10 The E-AIM model predicts that the MDRH of a mixed electrolyte system will be lowered with the addition of a second electrolyte due to changes in the activity coefficient and ionic interactions. Our experiment results will be compared with the results of the E-AIM model III (E-AIM-III) which predicts thermodynamics of the H+- NH4+- Na+- SO42−- NO3−- Cl−- H2O system. The model does not


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predict the MDRH of mixed systems beyond 1:1 mole ration, and does not have results for the NaCl-NaBr. Both of these studies and results are presented here for the first time. In this study, water uptake on three mixed atmospherically relevant systems was analyzed with Attenuated Total Reflectance–Fourier Transform-Infrared (ATR-FTIR) spectroscopy. ATR-FTIR spectroscopy has previously been used to analyze the individual deliquescence and efflorescence properties of single component inorganic salts including sodium chloride (NaCl), ammonium nitrate (NH4NO3) and ammonium sulfate ((NH4)2SO4) and other salts.11-17 Water uptake on a variety of mole ratios (0.25, 0.33, 0.5, 0.67, and 0.75) of mixed salts was investigated to understand the changes in MDRH values when mixtures have different amount of mole ratio in the system. The three multi-ion systems studied here include sodium chlorideammonium chloride (NaCl-NH4Cl), ammonium chloride-ammonium sulfate (NH4Cl-(NH4)2SO4) and sodium chloride-sodium bromide (NaCl-NaBr) systems.

These mixtures were chosen

because of their prevalence in the atmosphere with NaCl and NaBr being significant components in sea salts, while the ammonium salts originate from fertilizers and soil aerosolization.

EXPERIMENTAL AND THEORETICAL METHODS Ammonium chloride (Certified ACS) and sodium bromide (ACS) were purchased from Fisher Scientific.

Reagent grade sodium chloride was purchased from Carolina Biological

Supply, while ammonium sulfate (certified ACS) was purchased from Spectrum Quality Products. Deliquescence studies were carried out in a modified commercial ATR apparatus with a horizontal liquid crystal mount attachment (Pike Technologies) and a Zinc Selenide (ZnSe) crystal as the Infrared Reflective Element (IRE). The bottom portion and optics attachment of the liquid cell were used as-is from Pike Technologies, while the top portion of the liquid cell was attached to a Micro-Environmental Chamber (MEC) which accommodates humidified air


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flow and a temperature/RH sensor. The chamber is held to the crystal mount with eight screws, and sealed with a Teflon O-ring. The MEC allows control of RH conditions for real time measurement of phase transitions.

A complete diagram of the MEC can be found in

Schuttlefield et al. 2007.11 A single beam Thermo-Nicolet 8700 FTIR system with a liquid- nitrogen- cooled Mercury Cadmium Telluride (MCT) detector and KBr beam splitter recorded all infrared spectra. A commercial air dryer (Kaeser, KADW series) purged the FTIR spectrometer optics and internal sample compartment from carbon dioxide and water vapor as well as provided air for the flow system. Normally 100 scans were averaged at a 4 cm-1 instrument resolution over an 800 – 4000 cm-1 spectral range over 60 seconds. All ATR-FTIR spectra were referenced to a clean ZnSe crystal under dry air conditions (RH 3000 cm-1 are not typical of liquid water, because of the presence of Br- and Cl- ions, two of the highly polarizable halide anions, which have been shown to be able to disturb the interfacial water structure. It has been reported

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that large

polarizable halide ions like Br- have strong surface propensity resulting in the formation of coordinated complexes of bromine with water at low RH. This leads to a change in the structure and dynamics of water molecules at the water-air interface in the presence of these highly polarizable ions. Non polarizable ions or even large multiply charged ions like nitrates (NO3-) or sulfates (SO42-) have a low surface propensity and are excluded from the water-air interface and therefore do not interfere with the surface interfacial water network. The water peaks in the presence of these large charges ions show the expected broad characteristic of bulk aqueous phase water, as seen in Figure 1 A and B. MDRH analysis of 1:1 mole ratio systems


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To characterize the MDRH value of the each aqueous system, the OH stretching region (3300 cm-1 to 2600 cm-1) of water (υ H2O) was plotted as a function of RH and is shown as black squares in Figure 2. The iDRH of the individual salts is also presented for comparison to the MDRH which is also identified in the figure for clarity. The MDRH value is identified as the lower of two RH values that show a significant RH difference between consecutive RH values, taken from the largest jump in the area under the υ (H2O) peak. In the NH4Cl-NaCl-system, there is little or no change observed in the IR spectra between RH values less than 1% until close to an RH value of 65% (Figure 2A). The sharp increase in water content observed at 65.8 ± 2.1 % is taken as the MDRH of this system. After deliquescence, the droplet becomes less saturated, continuing to absorb water and grow in size. The experimental MDRH acquired is close to the value predicted by the E-AIM model III of 69.5% for the NH4Cl-NaCl system. The approximate 3.7% RH difference in experimental and model prediction could be the results of experimental factors (like particle size, RH measurement variability) that are not accounted for in the model. The experimental MDRH of the NH4Cl-NaCl-system at 65.8 ± 2.1 % RH is lower than the iDRH of the salts (75% and 77%, for NaCl and NH4Cl, respectively) showing that presence of the other salt in the system has led to a lowering of the MDRH value. The lowering of the MDRH compared to the individual DRH values has been seen before in other mixed systems investigated in literature. For example, the MDRH of a 1:1 NH4Cl-NaCl system was found to be 68 % RH by measuring the vapor pressure above the saturated solution.20 Chang and Lee found the MDRH of NaCl-Na2SO4 mixed system to be around 74%, a value lower than the iDRH of NaCl (76%) and Na2SO4 (84%).22 They attributed the lowering of MDRH values of the mixtures to mass change effects leading to a lower MDRH point. Ge et al. also found similar lowering in the MDRH of the (NH4)2SO4-NH4NO3 system.23 They found the


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MDRH of this system to be 60%, which is lower than the iDRH of (NH4)2SO4 (80%) and NH4NO3 (62%). They also attributed mass change effects that contributed to the lowering of the MDRH values. Therefore, the lowering of the MDRH of our system can also be attributed to the mass change effects that have been elaborated and summarized in literature.8, 18, 22 The integrated absorbance of the NH4Cl-(NH4)2SO4 system follows similar trends observed in the NH4Cl-NaCl system. There are small changes in absorbance of water at low relative humidities until a sudden increase in water uptake at 70.1 ± 0.3% RH. The MDRH predicted by the model at 71.9% RH and observed experimentally at 70.1 % RH is lower than the iDRH of the salts at 77% and 81%, for NH4Cl and (NH4)2SO4, respectively.24 Again, similar to above, mass change effects are attributed with lowering the MDRH of the mixed aerosol. For the NaCl-NaBr system, there were no studies that report the MDRH of the 1:1 mixture. The uptake of water in the NaCl-NaBr system shows two water absorption behavior (Figure 2C) as the RH in increased. At relative humidities lower than 40% there are small changes in the integrated absorbance of the OH stretch as the values stay constant. Between 40%-70%, a gradual increase in the integrated area of the OH peak occurs. It is also observed that two separate abrupt changes in the water uptake can be identified, one at 45% RH and another at 67% RH. The first abrupt change is assigned to the deliquescences of the eutonic component (a mixture of solid NaBr and other aqueous species). As the RH is increased, most of the solids in the systems change into a homogeneous aqueous phase with complete deliquescences of the mixture occurring at 67.3 ± 1.2% RH. The value of 45% is close to the iDRH of the NaBr salts as reported in literature.24 This shows that the NaBr particles deliquesce first as their iDRH point is reached. At this stage, some of the NaCl particles have not deliquesced, existing as dry particles surrounded by the eutonic


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composition consisting of the deliquesced NaBr droplets. As the RH is increased, these species remain unchanged until the solution reaches close to 67.3 % RH, where another abrupt change in water uptake is observed.

Since NaCl particles deliquesce at 75%, the change in RH

characteristic at 67.3 % indicates that at this point, all the remaining solid particles in the system have now deliquesced into an aqueous solution and this value was taken at the MDRH of the NaCl-NaBr system. The MDRH value acquired here is lower than the iDRH of NaCl, as expected from thermodynamic modeling. Unlike NaCl, the deliquescences behavior of NaBr salts (and their mixed components) is not widely known. It has been reported that ion-solvent interactions can have a significant impact on the uptake and spectral signature of these salts with water.25 Since both Br- and Cl- are both highly polarizable, they interact with water more strongly and show spectral characteristics (shifts, or extra bands) that are different than most binary salts. These characteristics were observed in Figure 1C, and the changes in these ion-solvent interactions manifest themselves as changes in the integrated absorbance as a function of RH (Figure. 2C). The different behavior of the NaCl-NaBr system, with an MDRH value higher than the iDRH of NaBr, but lower than the iDRH of NaCl could be due to ionic interactions that lead to the presence of undissolved solid particles past the iDRH of NaBr, which lead to the lowering of the overall MDRH of the system. It has been shown that the addition of another electrolyte in a solution tends to affect the ionic activity of the solute, which in turn affects the deliquescence of the individual components.26 These ionic interactions, in some cases, can result in a “salting out” effect, where the mixing of two solutes can induce the precipitation of one of the solutes or create the presence of undissolved solids beyond the iDRH. This “salting out” effect may keep


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some of the NaBr particles past their iDRH and cause the lowering of the iDRH of NaCl to a MDRH of the system to around 67.3%. For all the two case studies above where model comparisons can be made, the 1:1 MDRH values between experiment and model are relatively close to each other. However, minor variations and the difference between the model prediction and experimental data suggest that other factors (like particle size, shape, mixing RH measurement etc.) contribute to particle water absorption that the model does not take into account. Variation of mole ratio on MDRH values Aerosol particles change in composition as they age in the atmosphere due to mixing and chemical reactions. This ageing leads to mixed components that have different mole ratio composition. The influence of RH on these mixed aerosols with varying mole ratio is investigated here and is shown in Figure 3. Each data point in the figure comes from an average of three measurements. The relative mole solute fraction is defined as X = mA/(mA+ mB) where A and B represent the different salts in the system. The MDRH of the NH4Cl-NaCl system across mole ratio for 0.25, 0.33, 0.5, 0.67 and 0.75 mole ratios were 65.1, 65.9, 65.8, 64.1 and 65.8 % RH respectively and are plotted in Figure 3A. As can be seen, the values are close to the MDRH of the 1:1 (0.5) mole ratio system of 65.8% (the dashed line in the figure), indicating that the MDRH of the NH4Cl-NaCl has a slight dependence on mole ratio. Changing the mole ratio of the NH4Cl-(NH4)2SO4 system also shows slight variation on the MDRH of the mixtures (Figure 3B) as the values for 0.25, 0.33, 0.5, 0.67 and 0.75 mole ratios were found to be 69.5, 69.3, 70.1, 70.4 and 70.4 respectively, as plotted in Figure 3B. These values are lower than the iDRH of NH4Cl and (NH4)2SO4 , at 75% and 81% respectively, but close to the MDRH of the 1:1 (0.5) mole ratio system of 70.1 ± 0.3% (the dashed line in the


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figure). This slight dependence in MDRH as a function of mole ratio has been observed before when the salts of (NH4)2SO4 were mixed with small molecular weight organic acids. The Wiedensohler group observed a slight shift in the MDRH to lower RH and smoothing of deliquescence behavior when the organic mass fraction was varied.27 The lower hygroscopic growth was explained by neutralization of gas-phase ammonia and/or association with dicarboxylic acid cations. The MDRH of ammonium sulfate and glutaric acid mixtures has also been reported to be slightly less than the iDRH of pure ammonium sulfate.28

For the NaBr-NaCl system, no study has been found in literature that investigated the deliquescence of the 1:1 mixture, and the variation of the MDRH with mole ratio. The MDRH as function of mole ratio of 0.25, 0.33, 0.5, 0.67 and 0.75 mole ratios were measured at 69.1, 68.8, 67.3, 65.7, and 62.3, respectively, as seen in Figure 3C. These value varies slightly but are still close to the MDRH of the 1:1 systems value of 67.3 ± 1.2 % RH. This unexpected behavior of the NaCl-NaBr system could be due to the ionic interactions proposed earlier leading to formation of hydrated salts in the system, or even a reaction between the two salts to form another complex (solid) salt. It has also been shown before that water can interact with these halide ions to form stable clusters during the whole solubility range of these salts.29,30 At higher RH, the clusters are surrounded by water molecules, and as seen in Figure 1C, the spectra changes to broader peaks corresponding to more dilute solution when the salt particles deliquesce. Unfortunately, our system (ATR-FTIR) is not sensitive enough to be able to identify these complexes but their presence can be inferred from the appearance of the two stage transitions points and the unexpected behavior of the MDRH of the NaCl-NaBr system, with an MDRH value between the iDRH values of the NaCl and NaBr (as opposed to a value lower the iDRH as seen in the other salts).


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CONCLUSIONS The MDRH of particle mixtures of atmospheric relevance was studied as a function of mole ratio by ATR-FTIR spectroscopy. The MDRH values NH4Cl-NaCl and NH4Cl-(NH4)2SO4 systems were consistently found to be lower than the iDRH of the individual salts. For the NaClNaBr system, the MDRH of the system was found to be between the iDRH of the two salts, presumably due to the influence of the NaBr salt, which have a low iDRH, leading to an earlier deliquescence point. Mole ratio variations, although still leading to a lower MDRH value, show slight dependence on the MDRH of the salt mixtures when compared over the whole mole ratio range. When the experimental results were compared with the E-AIM calculations, they closely matched model calculations (with the exception of the NaCl-NaBr system), showing that we can use the E-AIM model adequately to predict MDRH of simple binary inorganic salts of atmospheric importance. ATMOSPHERIC IMPLICATIONS There still exists considerable uncertainty on the role of aerosols in influencing the climate and how to incorporate these contributions into global climate models (GCMs). Accurate experimental data on these behaviors is still needed to validate mathematical and thermodynamic calculations used in these GCMs. In our study, the MDRH values of 1:1 mixed salts systems acquired experimentally from spectroscopic measurements agreed (within experimental error) with those predicted by an aerosol model, the Extended Aerosol Inorganics Model (E-AIM). The addition of the NaCl-NaBr data, which is not present in literature, and for the E-AIM model does not predict, is hoped to improve on our knowledge of the deliquescent behavior of these relevant salts.

This agreement between experiment and calculation gives confidence for


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modelers to be able to use these values to enhance the predictive capabilities of larger GCMs and improve our understanding of the influence of aerosols to the climate. ACKNOWLEDGEMENT The authors would like to acknowledge Dr. Benjamin Rougeau at Arkansas State University for his continued support to our research. Heather Southe and Lindsey Martin analyzed many of the aerosol mixtures. Funding was provided by the RISE program in 20102011 (NSF grant #REU-0552608) for summer research 2010 and 2011 and Arkansas State University faculty research awards.

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Table 1: MDRH of 1:1 mole ratio of the mixed systems studied here compared to E-AIM III predicted results and literature values Salt System

Individual DRH*

MDRH of 1:1 systems MDRH of 1:1 systems from E-AIM model III from our experiments

NaCl NH4Cl

75 77

69.5

65.8 ± 2.1 (68)**

(NH4)2SO4

81

71.9

70.1 ± 0.3 (71)**

NH4Cl

77

NaCl NaBr

75 45

N/A

67.3 ± 1.2

* iDRH values from Martin, 2000 **Values in parenthesis are from Adam and Merz, 1929.


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Figure 1: FTIR-ATR spectra of 1:1 mole ratio systems at representative relative humidities labeled (A) NH4Cl-NaCl, at RH values labelled 1, 2, 3 and 4 (1.2, 70.9, 75.4 and 79.9% respectively) B) NH4Cl(NH4)2SO4 at RH values labelled 1, 2, 3 and 4 (1.8, 70.1, 75.7 and 82.7% respectively) (C) NaCl-NaBr at RH values labelled 1, 2, 3 and 4 (50.9, 69.7, 73.6 and 82.49% respectively).

Figure 2: Comparison between individual DRH (iDRH) values of the salts and their Mutual DRH (MDRH) at 1:1 mole ratio of (A) NH4Cl-NaCl, B) NH4Cl-(NH4)2SO4 (C) NaCl-NaBr

Figure 3: MDRH (black line/squares) values as a function of mole ratio. The dotted line indicates the MDRH of the 1:1 (0.5) mole ratio systems. At 0, the salt is entirely composed of salt 2 (denominator in mole fraction), while at one, the salt is entirely composed of salt 1 (numerator in mole fraction). Error bars are one standard deviation of the mean of three replicate measurements. (A) NH4Cl-NaCl, B) NH4Cl(NH4)2SO4 (C) NaCl-NaBr.


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Table of Contents Graphic


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68x54mm (288 x 288 DPI)


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Figure 1: FTIR-ATR spectra of 1:1 mole ratio systems at representative relative humidities labeled (A) NH4Cl-NaCl, at RH values labelled 1, 2, 3 and 4 (1.2, 70.9, 75.4 and 79.9% respectively) B) NH4Cl(NH4)2SO4 at RH values labelled 1, 2, 3 and 4 (1.8, 70.1, 75.7 and 82.7% respectively) (C) NaCl-NaBr at RH values labelled 1, 2, 3 and 4 (50.9, 69.7, 73.6 and 82.49% respectively). 249x89mm (150 x 150 DPI)



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Figure 2: Comparison between individual DRH (iDRH) values of the salts and their Mutual DRH (MDRH) at 1:1 mole ratio of (A) NH4Cl-NaCl, B) NH4Cl-(NH4)2SO4 (C) NaCl-NaBr 268x88mm (150 x 150 DPI)


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Figure 3: MDRH (black line/squares) values as a function of mole ratio. The dotted line indicates the MDRH of the 1:1 (0.5) mole ratio systems. At 0, the salt is entirely composed of salt 2 (denominator in mole fraction), while at one, the salt is entirely composed of salt 1 (numerator in mole fraction). Error bars are one standard deviation of the mean of three replicate measurements. (A) NH4Cl-NaCl, B) NH4Cl-(NH4)2SO4 (C) NaCl-NaBr. 313x95mm (150 x 150 DPI)


Mole Ratio Dependence of the Mutual Deliquescence Relative

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