Mikhail A. Anisimov, University of Maryland
We are also investigating the thermodynamic behavior of supercooled aqueous solutions in light of the second-critical-point hypothesis. In particular we have examined experimental data for aqueous solutions of glycerol and NaCl, intending to extend our group’s equation of state for supercooled water, which includes a liquid-liquid phase transition and a liquid-liquid critical point, to solutions. This equation of state was published in 2012 by V. Holten and M. A. Anisimov. Archer and Carter, of the National Institute for Science and Technology, have published data showing that the increase in the isobaric heat capacity upon supercooling moves to lower temperatures and is suppressed in magnitude as NaCl is added. At mole fraction 0.1, no increase is observed. We find this to be in agreement with a well-developed body of theoretical work on mixtures. When response functions such as isobaric heat capacity and isothermal compressibility are measured at constant concentration (as they nearly always are in practice) rather than at constant chemical potential, the near-critical behavior of these response functions changes: response functions that had diverged strongly in the pure fluid diverge only weakly, and those that had diverged weakly remain finite. An equation of state based on this body of theory gives an excellent quantitative match to Archer and Carter’s Data. A graph showing this behavior is the Table of Contents image for this report. We are also in the process of examining data on aqueous solutions of glycerol and extending the two-state model to that system. In a similar fashion as for NaCl, we will present an extension of Holten and Anisimov’s equation of state to this system. Additionally, a previous shows a phase transition in deeply supercooled aqueous solutions of glycerol. There exist thermodynamic data on supercooled water-glycerol systems, and Murata and Tanaka have observed a phase transition in supercooled aqueous solutions of glycerol. The transition that they report is from one homogeneous phase to another; however, we propose that for experiments at constant concentration, there must also exist a two-phase region. We will offer an estimate of the size and position of this region in terms of temperature, pressure, and concentration of solute. Within a month we intend to submit a manuscript that will include this research to the Journal of Physical Chemistry B.
One student, John Biddle, was supported by the PRF grant for the academic year 2012-2013 and the summer of 2013. He received his Master of Science degree in physics from the University of Maryland, College Park in May 2013 and advanced to candidacy as a Ph. D. student in physics in May 2013. The research described above will be part of his Ph. D. dissertation.