The effect of hydroxyl functional groups and molar mass on the viscosity of non-crystalline organic and organic–water particles

Abstract

Abstract. The viscosities of three polyols and three saccharides, all in the non-crystalline state, have been studied. Two of the polyols (2-methyl-1,4-butanediol and 1,2,3-butanetriol) were studied under dry conditions, the third (1,2,3,4-butanetetrol) was studied as a function of relative humidity (RH), including under dry conditions, and the saccharides (glucose, raffinose, and maltohexaose) were studied as a function of RH. The mean viscosities of the polyols under dry conditions range from 1.5 × 10−1 to 3.7 × 101 Pa s, with the highest viscosity being that of the tetrol. Using a combination of data determined experimentally here and literature data for alkanes, alcohols, and polyols with a C3 to C6 carbon backbone, we show (1) there is a near-linear relationship between log10 (viscosity) and the number of hydroxyl groups in the molecule, (2) that on average the addition of one OH group increases the viscosity by a factor of approximately 22 to 45, (3) the sensitivity of viscosity to the addition of one OH group is not a strong function of the number of OH functional groups already present in the molecule up to three OH groups, and (4) higher sensitivities are observed when the molecule has more than three OH groups. Viscosities reported here for 1,2,3,4-butanetetrol particles are lower than previously reported measurements using aerosol optical tweezers, and additional studies are required to resolve these discrepancies. For saccharide particles at 30 % RH, viscosity increases by approximately 2–5 orders of magnitude as molar mass increases from 180 to 342 g mol−1, and at 80 % RH, viscosity increases by approximately 4–5 orders of magnitude as molar mass increases from 180 to 991 g mol−1. These results suggest oligomerization of highly oxidized compounds in atmospheric secondary organic aerosol (SOA) could lead to large increases in viscosity, and may be at least partially responsible for the high viscosities observed in some SOA. Finally, two quantitative structure–property relationship models (Sastri and Rao, 1992; Marrero-Morejón and Pardillo-Fontdevila, 2000) were used to predict the viscosity of alkanes, alcohols, and polyols with a C3–C6 carbon backbone. Both models show reasonably good agreement with measured viscosities for the alkanes, alcohols, and polyols studied here except for the case of a hexol, the viscosity of which is underpredicted by 1–3 orders of magnitude by each of the models.

Predicting the glass transition temperature and viscosity of secondary organic material using molecular composition

“…Recently, Rothfuss and Petters (2017a) showed an approximately linear relationship between the number of OH groups and T g for compounds with up to eight OH groups. Grayson et al (2017) showed that the addition of hydroxyl functional groups increases viscosity, a conclusion supported by both the experimental data and quantitative structure-property relationship model. The correlation between T g and the number of carbon atoms is consistent with the free volume theory, in which molecular motion is restricted by the difference between the space required for a molecule to vibrate versus the space in which the molecule resides (i.e., the free volume; White and Lipson, 2016).…”

Effect of dry or wet substrate deposition on the organic volume fraction of core–shell aerosol particles

“…Typically existing as submicrometer-sized aerosols with inorganic core encased by organic shell (core-shell), their heterogeneous mixing states derive directly from the complex variety of organic, inorganic, and biological species in the SSML and ocean water (Cochran et al, 2017;Ault et al, 2013). Nascent SSAs affect the Earth's climate and atmosphere through radiative forcing, directly by scattering and absorbing solar radiation, and indirectly by acting as cloud condensation or ice nuclei (Lee et al, 2017b;Haywood and Boucher, 2000;Jacobson, 2001); however, their chemical and biological complexity that control the mixing state hinder our ability to accurately predict their climate cooling abilities (Lee et al, 2017a, b). Uncertainty in the mixing state, even with the knowledge of the chemical composition, may produce erroneous predictions of cloud activation from individual particles (Ault and Axson, 2017).…”

Supplementary material to "Predicting the glass transition temperature and viscosity of secondary organic material using molecular composition"

“…3a. κ was set to 0.1 based on field and laboratory measurements (Gunthe et al, 2009;Lambe et al, 2011b;Pajunoja et al, 2014;Petters et al, 2017) and k GT was assumed to be 2.5 (Zobrist et al, 2008;Koop et al, 2011). Using these parameters, the predicted viscosities match the magnitude and the RHdependence of the measured viscosity of α-pinene and isoprene SOA.…”