Probing the Water Uptake and Phase State of Individual Sucrose Nanoparticles Using Atomic Force Microscopy

Abstract

The effects of atmospheric aerosols on the climate and atmosphere of Earth can vary significantly depending upon their properties, including size, morphology, and phase state, all of which are influenced by varying relative humidity (RH) in the atmosphere. A significant fraction of atmospheric aerosols is below 100 nm in size. However, as a result of size limitations of conventional experimental techniques, how the particle-to-particle variability of the phase state of aerosols influences atmospheric processes is poorly understood. To address this issue, the atomic force microscopy (AFM) methodology that was previously established for sub-micrometer aerosols is extended to measure the water uptake and identify the phase state of individual sucrose nanoparticles. Quantified growth factors (GFs) of individual sucrose nanoparticles up to 60% RH were lower than expected values observed on the sub-micrometer sucrose particles. The effect could be attributed to the semisolid sucrose nanoparticle restructuring on a substrate. At RH > 60%, sucrose nanoparticles are liquid and GFs overlap well with the sub-micrometer particles and theoretical predictions. This suggests that quantification of GFs of nanoparticles may be inaccurate for the RH range where particles are semisolid but becomes accurate at elevated RH where particles are liquid. Despite this, however, the identified phase states of the nanoparticles were comparable to their sub-micrometer counterparts. The identified phase transitions between solid and semisolid and between semisolid and liquid for sucrose were at ∼18 and 60% RH, which are equivalent to viscosities of 10 11.2 and 10 2.5 Pa s, respectively. This work demonstrates that measurements of the phase state using AFM are applicable to nanosized particles, even when the substrate alters the shape of semisolid nanoparticles and alters the GF.

Complex Hygroscopic Behavior of Ambient Aerosol Particles Revealed by a Piezoelectric Technique

“…Phase transitions occurring at lower RH values encompass various phenomena such as liquid−liquid phase separation, efflorescence, and the formation of highly viscous and semisolid amorphous states like glasses and gels. 64,65 Thus, an enhancement in the knowledge about subsaturated water uptake characteristics is crucial for accurately quantifying the radiative forcing effects of ambient aerosol particles in the atmosphere. 39,66,67 The strong size dependence of κ m also indicated varying chemical compositions for the different size ranges measured in this study.…”

Quantifying the Viscosity of Individual Submicrometer Semisolid Particles Using Atomic Force Microscopy

“…Sucrose, glucose, and raffinose aerosols were generated with a constant output atomizer (TSI, Model 3076) from 1 mM aqueous solutions. The aerosol stream was mixed with wet air at a constant rate of 20 L/min to achieve ∼80% RH in the mixing chamber and then aerosols were deposited by impaction using a Micro Orifice Uniform Deposit Impactor (MOUDI; MSP, Inc., Model 110). ,,, Particles were deposited on hydrophobically coated (Rain-X) silicon wafers (Ted Pella Inc., part no. 16008) placed on the MOUDI stage 7, which corresponds to an aerodynamic diameter range of 0.3–0.56 μm.…”

“…For force measurements, at least 5 repeated force-vertical piezo displacement curves with a typical maximum applied loading force of 20 nN were collected at each selected RH ranging from ∼10% to 60%. The equilibration time after each change in RH was approximately 10–15 min to ensure the particles are in thermodynamic equilibrium with surrounding water vapor at a particular RH. ,, For each sample at a particular RH, the approach data in the contact region, where the tip is indenting into the particle, were used to determine the particle viscosity at the corresponding RH. The viscosity values were determined for each saccharide system at a particular RH, with each value reported as an average and two standard deviations.…”

“…The equilibration time after each change in RH was approximately 10–15 min to ensure the particles are in thermodynamic equilibrium with surrounding water vapor at a particular RH. 3 , 4 , 41 For each sample at a particular RH, the approach data in the contact region, where the tip is indenting into the particle, were used to determine the particle viscosity at the corresponding RH. The viscosity values were determined for each saccharide system at a particular RH, with each value reported as an average and two standard deviations.…”

“…Atmospheric aerosols play an important role in radiative forcing either directly by scattering, reflecting, or absorbing solar radiation or indirectly by acting as cloud condensation nuclei (CCN) or ice nucleating particles (INPs) to facilitate cloud formation. − These climate-relevant aerosol effects depend on the physicochemical properties of individual aerosols, such as morphology, mixing state, size, composition, phase state and viscosity, − all in turn can vary depending on the surrounding relative humidity (RH) and temperature. − In particular, the phase state of aerosols (i.e., solid, semisolid, and liquid) is important as it can regulate reactivity with atmospheric gases and control their CCN and water uptake behavior, and their ability to act as INPs. − ,,− …”