Effects of molecular weight and temperature on liquid-liquid phase separation in particles containing organic species and inorganic salts

Atmospheric particles containing organic species and inorganic salts may undergo liquid-liquid phase separation when the relative humidity varies between high and low values. To better understand the parameters that affect liquid-liquid phase separation in atmospheric particles, we studied the effects of molecular weight and temperature on liquid-liquid phase separation in particles containing one organic species mixed with either ammonium sulfate or ammonium bisulfate. In the molecular-weight-dependent studies, we measured liquid-liquid phase separation relative humidity (SRH) in particles containing ammonium sulfate and organic species with large molecular weights (up to 1153 Da). These results were combined with recent studies of liquid-liquid phase separation in the literature to assess if molecular weight is a useful parameter for predicting SRH. The combined results, which include results from 33 different particle types, illustrate that SRH does not depend strongly on molecular weight (i.e., a clear relationship between molecular weight and SRH was not observed). In the temperature-dependent studies, we measured liquid-liquid phase separation in particles containing ammonium sulfate mixed with 20 different organic species at 244 +/- 1 K, 263 +/- 1 K, and 278 +/- 1 K; a few particles were also studied at 290 +/- 1 K. These new results were combined with previous measurements of the same particle types at 290 +/- 1 K. The combined SRH data illustrate that for the organic-ammonium sulfate particles studied, the SRH does not depend strongly on temperature. At most the SRH varied by 9.7% as the temperature varied from 290 to 244 K. The high SRH values (>65%) in these experiments may explain the lack of temperature dependence. Since water is a plasticizer, high relative humidities can lead to high water contents, low viscosities, and high diffusion rates in the particles. For these cases, unless the temperature is very low, liquid-liquid phase separation is not expected to be kinetically inhibited. The occurrence of liquid-liquid phase separation and SRH did depend strongly on temperature over the range of 290-244K for particles containing alpha,4-dihydroxy-3-methoxybenzeneacetic acid mixed with ammonium bisulfate. For this particle type, a combination of low temperatures and low water content likely favored kinetic inhabitation of the liquid-liquid phase separation by slow diffusion rates in highly viscous particles. The combined results suggest that liquid-liquid phase separation is likely a common occurrence in atmospheric particles at temperatures from 244-290 K, although particles that do not undergo liquid-liquid phase separation are also likely common.