Evaporation Kinetics and Final Particle Morphology of Multicomponent Salt Solution Droplets

Abstract

In both nature and industry, aerosol droplets contain complex mixtures of solutes, which in many cases include multiple inorganic components. Understanding the drying kinetics of these droplets and the impact on resultant particle morphology is essential for a variety of applications including improving inhalable drugs, mitigating disease transmission, and developing more accurate climate models. However, the previous literature has only focused on the relationship between drying kinetics and particle morphology for aerosol droplets containing a single nonvolatile component. Here we investigate the drying kinetics of NaCl-(NH₄)₂SO₄, NaCl-NH₄NO₃, and NaCl-CaCl₂ mixed salt aqueous aerosol droplets (25-35 μm radius) and the resulting morphology and composition of the dried microparticles. A comparative kinetics electrodynamic balance was used to measure evaporation profiles for each mixed salt aerosol at a range of relative humidities (RH) (0-50% RH); measurements of the evaporation kinetics are shown to be consistent with predictions from the "Single Aerosol Drying Kinetics and Trajectories" model. Populations of the mixed salt droplets were dried in a falling droplet column under different RH conditions and imaged using scanning electron microscopy to observe the impact of the drying kinetics on the morphology. Energy dispersive spectroscopy was used in tandem to obtain atomic maps and view the impact of drying kinetics on the composition of the resultant particles. It has been shown that the relationship between drying kinetics and dry particle morphology in mixed salt solution droplets is compositionally dependent and determined by the predominant salts that crystallize (i.e., (NH₄)₂SO₄, Na₂SO₄, or NaCl). The degree of homogeneity in composition throughout the particle microstructure is dependent on the drying rate.

Keywords: aerosol; chloride; crystallization; drying; single-droplet; sulfate.

Figure 1

(a) Evaporation profiles for aqueous NaCl (0.02 MFS) droplets at varying RHs and 293.0 K compared with evaporation profiles modeled by SADKAT. Crystallization occurs at the time point indicated by a star, where the average a_w was calculated to be 0.381 ± 0.016 and the estimated dry particle size was 5.57 ± 0.04 μm. (b) SEM image of a NaCl microparticle dried at 0% RH, 294 K. (c) SEM image of a NaCl microparticle dried at 40% RH, 294 K.

Figure 2

Degree of circularity for populations of dried particles at varying RHs. 0% RH = orange, 20% RH = green, 30% RH = gray, 40% RH = blue. (a) NaCl, (b) NaCl-(NH₄)₂SO₄ (X_NaCl = 0.75), (c) NaCl-(NH₄)₂SO₄ (X_NaCl = 0.50), and (d) (NH₄)₂SO₄.

Figure 3

(a) Evaporation profiles for aqueous (NH₄)₂SO₄ (0.02 MFS) droplets at varying RH and 293.2K compared with evaporation profiles modeled by SADKAT. Crystallization occurs at the time point indicated by a star, where the average a_w was calculated to be 0.424 ± 0.028% and the estimated dry particle size was 5.51 ± 0.01 μm. (b) SEM image of a (NH₄)₂SO₄ microparticle dried at 0% RH, 294 K. (c) SEM image of a (NH₄)₂SO₄ microparticle dried at 20% RH, 294 K.

Figure 4

(a) Evaporation profiles for aqueous NaCl:(NH₄)₂SO₄ (X_NaCl = 0.25, 0.02 MFS) droplets at varying RHs and 291.5 K compared with evaporation profiles modeled by SADKAT. The star points indicate the point of crystallization in the evaporation process, where the average a_w was calculated to be 0.360 ± 0.021 and the estimated dry particle size was 6.29 ± 0.01 μm. (b) Normalized radius squared plots for the data shown in panel a. (c) SEM image of a NaCl:(NH₄)₂SO₄ microparticle dried at 0% RH, 294 K. (d) SEM image of a NaCl:(NH₄)₂SO₄ microparticle dried at 20% RH, 294 K.

Figure 5

EDX mapping of a NaCl:(NH₄)₂SO₄ (X_NaCl = 0.25, 0.02 MFS) microparticle dried at 0% RH and 294 K. Elements mapped are (a) chlorine, (b) nitrogen, (c) sodium, and (d) sulfur.

Figure 6

(a) Evaporation profiles for aqueous NaCl:(NH₄)₂SO₄ (X_NaCl = 0.5, 0.02 MFS) droplets at varying RHs and 293.6 K compared with evaporation profiles modeled by SADKAT. The star points indicate the point of crystallization in the evaporation process, where the average a_w was calculated to be 0.356 ± 0.032 and the estimated dry particle size was 5.11 ± 0.04 μm. (b) Normalized radius squared plots for the data shown in panel a. (c) SEM image of a NaCl:(NH₄)₂SO₄ microparticle dried at 0% RH, 294 K. (d) SEM image of a NaCl:(NH₄)₂SO₄ microparticle dried at 20% RH, 294 K.

Figure 7

Mass fraction of the salts predicted by E-AIM to be present in the dried particle at the efflorescence point with SEM images for morphological comparison. Purple = (NH₄)₂SO₄, orange = NH₄Cl, yellow = Na₂SO₄, and blue = NaCl. The RH input for E-AIM was obtained by calculating the a_w from the evaporation profiles (see Figures 1, 2, 3, 5, and 7). The change in droplet size from the kinetic data allows for an MFS at the efflorescence point to be calculated; this can be related to an a_w using the “a_w vs MFS parametrizations” (Table S1).

Figure 8

(a) Evaporation profiles for aqueous NaCl:(NH₄)₂SO₄ (X_NaCl = 0.75, 0.02 MFS) droplets at varying RHs and 293.5 K compared with evaporation profiles modeled by SADKAT. The star points indicate the point of crystallization in the evaporation process, where the average a_w was calculated to be 0.415 ± 0.010 and the estimated dry particle size was 7.19 ± 0.07 μm. (b) Normalized radius squared plots for the data shown in panel a. (c) SEM image of NaCl:(NH₄)₂SO₄ microparticles dried at 0% RH, 294 K. (d) SEM image of NaCl:(NH₄)₂SO₄ microparticles dried at 30% RH, 294 K.

Figure 9

(a) Evaporation profiles for aqueous NaCl:NH₄NO₃ (X_NaCl = 0.50, 0.02 MFS) droplets at varying RHs and 293.4 K compared with evaporation profiles modeled by SADKAT. The star points indicate the point of crystallization in the evaporation process. (b) Normalized radius squared plots for the data shown in panel a. (c) SEM image of a NaCl:NH₄NO₃ microparticle dried at 0% RH, 294 K. (d) SEM image of a NaCl:NH₄NO₃ microparticle dried at 20% RH, 294 K.

Figure 10

EDS mapping of NaCl:NH₄NO₃ (X_NaCl = 0.50, 0.02 MFS) microparticles dried at (a) 0% RH, (b) 20% RH, and (c) 40% RH. Elements mapped are chlorine in red, nitrogen in yellow, and sodium in blue.

Figure 11

(a) Evaporation profiles for aqueous NaCl:CaCl₂ (X_NaCl = 0.65, 0.1 MFS) droplets at varying RHs and 294.0 K compared with evaporation profiles modeled by SADKAT. The triangle points indicate the equilibration of the particle radius in the evaporation process. (b) Normalized radius squared plots for the data shown in panel a. (c) SEM image of a NaCl:CaCl₂ microparticle dried at 0% RH, 294 K. (d) SEM image of a NaCl:CaCl₂ microparticle dried at 40% RH, 294 K.