The present work deals with a water-ionic salt system, H2O-IS, (the water concentration is 30 wt%, IS consists of 90 mol% ethylammonium nitrate, EAN, and 10 mol% triethylammonium methanesulfonate, TEAMS). In our DSC experiments, the temperature interval studied was from +25 to -150 °C, and three different cooling/heating rates: 20, 10, and 5 °C per minute were used. First, the salt free of water was analyzed. It was found that the pure salt undergoes a glass transition at -90 °C, which is independent of the cooling rate. Second, for H2O-IS, regular crystallization of water was observed at -65 °C when the cooling rate of 5 °C per minute was used. For 10 and 20 °C per minute, on cooling the heat flow stays almost constant from room temperature down to -80 °C, then exhibits an exothermic peak centered at -105 °C, and then becomes constant again down to -150 °C. On subsequent heating, the heat flow exhibits an endothermic peak, which resembles that observed on cooling in the same temperature interval. In contrast to the cooling part, a sharp exothermic peak centered around -70 °C and an endothermic peak at -40 °C are observed upon further heating. In order to better understand the transitions centered at -105 °C, the following DSC experiment was performed. First, the sample was cooled at 10 °C/min from 25 °C to -150 °C, followed by heating to -85 °C. The sample was then kept isothermal for 10 minutes to allow the water to undergo cold crystallization. The subsequent cooling to -150 °C did not exhibit the exothermic peak at -105 °C indicating that the water underwent crystallization. This experiment proves that the phase transition, which is centered at -105 °C and observed when the sample is continuously cooled to -150 °C, is water crystallization. In order to complete the understanding of the phase transitions, Raman spectroscopy experiments were performed in the same temperature interval. Comparison of the data corresponding to the water-ionic salt system to that of pure water indicates that the peak at -105 °C does correspond to water crystallization. The Raman spectra also indicate that the water crystallites obtained are very small, which is consistent with their melting at -100 °C. In conclusion, our combined DSC and Raman experiments provided the data to resolve the dilemma of whether the transition centered at -105 °C is a liquid-liquid phase transition or water crystallization.

DSC and Raman spectroscopy study of phase transitions and aggregate state of constituents in a water: Ionic salt system

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.