Continuous measurements of the water activities of aqueous droplets of water-soluble organic compounds

Water-soluble organic compounds have recently received much attention because of their ability to absorb water and affect the radiation balance and the climate. Partly because of their relatively high volatility, thermodynamic data on water-soluble organic compounds are scarce. Recently, we have developed a method based on the scanning electrodynamic balance (SEDB) that enables the measurement of water activity data of evaporating droplets within an hour, which can potentially be used to measure volatile species. This paper demonstrates the use of the SEDB to study the hygroscopic growth of selected atmospheric species, including semivolatile organic species with vapor pressure up to 1 × 10-4 mmHg. We also measured the water activities, the crystallization relative humidity, and the deliquescence relative humidity (DRH) of aqueous solutions of maleic acid and glutaric acid. The DRHs of maleic acid and glutaric acid are in general agreement with the literature, except that glutaric acid shows a small delay in the completion of deliquescence due to mass-transfer limitation. The water activities of equal molar mixtures of maleic acid and malic acid and of malonic acid and glutaric acid were also measured. The Zdanovskii-Stokes-Robinson (ZSR) predictions agree well with the measurements of the mixtures. The UNIFAC (UNIQUAC functional group activity coefficients) predictions, using the modified functional group interaction parameters of COOH, OH, and H2O derived from our earlier measurements of the water activities of aqueous droplets of a list of dicarboxylic and multi-functional acids, are also in agreement with the mixture data.