Quantifying water diffusion in high-viscosity and glassy aqueous solutions using a Raman isotope tracer method

Abstract

Abstract. Recent research suggests that under certain temperature and relative humidity conditions atmospheric aerosol may be present in the form of a glassy solid. In order to understand the impacts that this may have on aerosolcloud interactions and atmospheric chemistry, knowledge of water diffusion within such aerosol particles is required. Here, a method is described in which Raman spectroscopy is used to observe D 2 O diffusion in high-viscosity aqueous solutions, enabling a quantitative assessment of water diffusion coefficients, D water , as a function of relative humidity. Results for sucrose solutions compare well with literature data at 23.5 ± 0.3 • C, and demonstrate that water diffusion is slow (D water ∼ 5 × 10 −17 m 2 s −1 ), but not arrested, just below the glass transition at a water activity of 0.2. Room temperature water diffusion coefficients are also presented for aqueous levoglucosan and an aqueous mixture of raffinose, dicarboxylic acids and ammonium sulphate: at low humidity, diffusion is retarded but still occurs on millisecond to second timescales in atmospherically relevant-sized particles. The effect of gel formation on diffusion in magnesium sulfate solutions is shown to be markedly different from the gradual decrease in diffusion coefficients of highly viscous liquids. We show that using the Stokes-Einstein equation to determine diffusion timescales from viscosity leads to values which are more than 5 orders of magnitude too big, which emphasises the need to make measurements of diffusion coefficients. In addition, comparison of bounce fraction data for levoglucosan with measured diffusion data reveals that even when particles bounce the diffusion timescales for water are a fraction of a second for a 100 nm particle. This suggests a high bounce fraction does not necessarily indicate retarded water diffusion.

Diffusivity measurements of volatile organics in levitated viscous aerosol particles

“…PEG-4 (this study, 19 • C) is shown red. The black data point represents sucrose as calculated from the parametrization of Price et al (2014). All data points correspond to values of x suc between 0.38 and 0.41.…”

“…Comparison between experimental diffusivities and Stokes-Einstein prediction for molecules of various hydrodynamic radii r H at room temperature. The diffusivity of water is shown in light blue and was taken from the parametrizations of Zobrist et al (2011) and Price et al (2014). The data points for fluorescein, rhodamine 6G and calcein, shown in green, were taken from Chenyakin et al (2017).…”

“…Measured water diffusivities may exceed Stokes-Einstein predictions by several orders of magnitude (Power et al, 2013;Price et al, 2014). Several bulk and levitated aerosol particle techniques, based on isotopic tracers, fluorescence and mass transport measurements, have emerged to measure water diffusivity in highly viscous systems (Zobrist et al, 2011;Price et al, 2014;Lienhard et al, 2015;Davies and Wilson, 2016;Marshall et al, 2016). Few studies have addressed the diffusion of larger molecules (Champion et al, 1997;Price et al, 2016;Chenyakin et al, 2017) such as VOCs.…”

“…However, substantial decoupling of diffusivity and viscosity has been observed in various systems for temperatures close to or below the glass transition temperature T g (Corti et al, 2008a, b). Measured water diffusivities may exceed Stokes-Einstein predictions by several orders of magnitude (Power et al, 2013;Price et al, 2014). Several bulk and levitated aerosol particle techniques, based on isotopic tracers, fluorescence and mass transport measurements, have emerged to measure water diffusivity in highly viscous systems (Zobrist et al, 2011;Price et al, 2014;Lienhard et al, 2015;Davies and Wilson, 2016;Marshall et al, 2016).…”

Dynamic viscosity mapping of the oxidation of squalene aerosol particles

“…An aerosol's viscous state also has direct implications on the particle morphology, phase behaviour, optical properties, aging processes and lifetime. At the same time, only few direct measurement methods are available for monitoring such properties 1, [6][7][8][9][10][11][12][13][14][15][16] . Recently, we have demonstrated that viscosity of model organic aerosols, such as oleic acid droplets, and secondary organic aerosols (SOA) produced from the oxidation of myrcene or -pinene can be measured and imaged in a spatially and time-resolved manner using Fluorescence Lifetime Imaging Microscopy (FLIM) of molecular rotors 17 .…”