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Freezing of Aqueous-Alcohol Nanodroplets
TONG SUN, Barbara Wyslouzil, The Ohio State University
Abstract Number: 200
Working Group: Aerosol Physics
Abstract
The freezing of ice in supercooled aqueous droplets is a phenomenon frequently encountered in nature and industry. Since water is not necessarily pure, understanding the role of other species in enhancing or suppressing ice nucleation is important. Amphiphilic alcohols are particularly interesting due to their increasing prevalence in the atmosphere, and their propensity to be highly enriched at the surface. Thus, the temperature at which aqueous-alcohol nanodroplet freeze will be influenced both by the presence of the alcohol and its distribution throughout the droplet.
In the current study, we generate aqueous-alcohol aerosols in a supersonic nozzle with overall mole fractions of the short-chain alcohols n-propanol and n-pentanol up to 0.08, and examine how the freezing process changes relative to that of pure water. Condensation and freezing are characterized by two complementary techniques, pressure trace measurements and Fourier transform infrared spectroscopy. When an alcohol is present condensation occurs earlier in the expansion, largely due to the decrease in interfacial free energy of the binary critical clusters relative to the pure water critical clusters. FTIR results suggest that when 12 nm particles freeze, the tetrahedral structure of the ice core is well preserved for overall alcohol mole fractions up to 0.031 for n-propanol and 0.043 for n-pentanol. For n-propanol, ice nucleation is systematically delayed and the rate is gradually reduced as the alcohol mol fraction increases. In contrast, the freezing kinetics are not altered for n-pentanol until mole fractions reach ~0.03. For each alcohol we estimated the partitioning of alcohol between the surface and the interior or bulk as a function of overall composition. For nanodroplets with comparable bulk concentrations, n-pentanol perturbs the ice freezing process more than n-propanol, consistent with the longer alkyl chain length of the C5 alcohol.