B. Zobrist

Heterogeneous ice nucleation in aqueous solutions: the role of water activity.

Heterogeneous ice nucleation experiments have been performed with four different ice nuclei (IN), namely nonadecanol, silica, silver iodide and Arizona test dust. All IN are either immersed in the droplets or located at the droplets surface. The IN were exposed to various aqueous solutions, which consist of (NH4)2SO4, H2SO4, MgCl2, NaCl, LiCl, Ca(NO3)2, K2CO3, CH3COONa, ethylene glycol, glycerol, malonic acid, PEG300 or a NaCl/malonic acid mixture. Freezing was studied using a differential scanning calorimeter and a cold finger cell. The results show that the heterogeneous ice freezing temperatures decrease with increasing solute concentration; however, the magnitude of this effect is solute dependent. In contrast, when the results are analyzed in terms of the solution water activity a very consistent behavior emerges: heterogeneous ice nucleation temperatures for all four IN converge each onto a single line, irrespective of the nature of the solute. We find that a constant offset with respect to the ice melting point curve, Deltaaw,het, can describe the observed freezing temperatures for each IN. Such a behavior is well-known for homogeneous ice nucleation from supercooled liquid droplets and has led to the development of water-activity-based ice nucleation theory. The large variety of investigated solutes together with different general types of ice nuclei studied (monolayers, ionic crystals, covalently bound network-forming compounds, and a mixture of chemically different crystallites) underlines the general applicability of water-activity-based ice nucleation theory also for heterogeneous ice nucleation in the immersion mode. Finally, the ice nucleation efficiencies of the various IN, as well as the atmospheric implication of the developed parametrization are discussed.

Ice nucleation in aqueous solutions of poly ethylene glycol with different molar mass

Homogeneous ice nucleation was investigated in aqueous solutions of poly[ethylene glycol] (PEG) with a molar mass between 300 and 6000 g mol−1. Experiments were performed with a differential scanning calorimeter using emulsified aqueous PEG solutions with concentrations of 0–44 wt %. Equilibrium phase transition temperatures are determined and discussed, in particular the simultaneous occurrence of metastable and stable eutectic temperatures. The observed homogeneous freezing temperatures of ice reveal a continuous increase in the supercooling of PEG solutions with increasing molar mass of the PEG. The freezing behavior was investigated within the framework of water-activity-based ice nucleation theory. The latter predicts that homogeneous ice nucleation in aqueous solutions is independent of the nature of the solute, but depends only on the water activity of the solution. Water activity data of various PEG solutions in the stable and supercooled range were compared. It was found that the water activity o...

Inhibition of efflorescence in mixed organic–inorganic particles at temperatures less than 250 K

It is now well recognized that mixed organic-inorganic particles are abundant in the atmosphere. While there have been numerous studies of efflorescence of mixed organic-inorganic particles close to 293 K, there are only a few at temperatures less than 273 K. Understanding the efflorescence properties of these particles at temperatures less than 273 K could be especially important for predicting ice nucleation in the upper troposphere. We studied the efflorescence properties of mixed citric acid-ammonium sulfate particles as a function of temperature to better understand the efflorescence properties of mixed organic-inorganic particles in the middle and upper troposphere. Our data for 293 K illustrate that the addition of citric acid decreases the ERH of ammonium sulfate, which is consistent with the trends observed with other systems containing highly oxygenated organic compounds. At low temperatures the trend is qualitatively the same, but efflorescence can be inhibited by smaller concentrations of citric acid. For example at temperatures <250 K an organic mass/(organic mass + sulfate mass) of only 0.33 is needed to inhibit efflorescence of ammonium sulfate. In the upper troposphere the organic mass/(organic mass + sulfate mass) can often be larger than this value. As a result, particles in the upper troposphere may be more likely to remain in the liquid state than previously thought and solid ammonium sulfate may be less likely to participate in heterogeneous ice nucleation in the upper troposphere. Additional studies are required on other model organic systems.

Experimental evidence for excess entropy discontinuities in glass-forming solutions

Glass transition temperatures T(g) are investigated in aqueous binary and multi-component solutions consisting of citric acid, calcium nitrate (Ca(NO(3))(2)), malonic acid, raffinose, and ammonium bisulfate (NH(4)HSO(4)) using a differential scanning calorimeter. Based on measured glass transition temperatures of binary aqueous mixtures and fitted binary coefficients, the T(g) of multi-component systems can be predicted using mixing rules. However, the experimentally observed T(g) in multi-component solutions show considerable deviations from two theoretical approaches considered. The deviations from these predictions are explained in terms of the molar excess mixing entropy difference between the supercooled liquid and glassy state at T(g). The multi-component mixtures involve contributions to these excess mixing entropies that the mixing rules do not take into account.

Response to “Comment on ‘Experimental evidence for excess entropy discontinuities in glass-forming solutions”’ [J. Chem. Phys. 139, 047101 (2013)]

In their comment, Bogdan and Loerting1 (hereafter called BoLo) question the validity of the experimental data of Lienhard et al. (hereafter called LZZKP) concerning the glass transition temperatures (T_g) of binary aqueous citric acid and aqueous malonic acid solutions. BoLo present own measurements and find disagreements between their results and the results published by LZZKP. In this reply, we show calorimetric thermograms from which the results published by LZZKP are derived and discuss why BoLo’s criticisms are unjustified. Below, we address each of the four claims.