Articles | Volume 17, issue 13
https://doi.org/10.5194/acp-17-8509-2017
https://doi.org/10.5194/acp-17-8509-2017
Research article
 | 
13 Jul 2017
Research article |  | 13 Jul 2017

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

James W. Grayson, Erin Evoy, Mijung Song, Yangxi Chu, Adrian Maclean, Allena Nguyen, Mary Alice Upshur, Marzieh Ebrahimi, Chak K. Chan, Franz M. Geiger, Regan J. Thomson, and Allan K. Bertram

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.

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Short summary
The viscosities of four polyols and three saccharides mixed with water were determined. The results from the polyol studies suggest viscosity increases by 1–2 orders of magnitude with the addition of an OH functional group to a carbon backbone. The results from the saccharide studies suggest that the viscosity of highly oxidized compounds is strongly dependent on molar mass and oligomerization of highly oxidized compounds in atmospheric SOM could lead to large increases in viscosity.
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