Different freezing behavior of millimeter- and micrometer-scaled (NH₄)₂SO₄/H₂O droplets

J Phys Condens Matter. 2011 Jan 26;23(3):035103. doi: 10.1088/0953-8984/23/3/035103. Epub 2010 Dec 16.

Abstract

Although the freezing of aqueous solutions is important for nature and different branches of science and freeze-applications, our understanding of the freezing process is not complete. For example, numerous measurements of micrometer-scaled (NH(4))(2)SO(4)/H(2)O droplets report one freezing event below the eutectic point. However, measurements of larger millimeter-scaled droplets reveal two freezing events: the freezing out of ice and subsequent freezing of a residual freeze-concentrated solution. To resolve this apparent contradiction we performed numerous calorimetric measurements which indicate that the freezing of a residual solution of millimeter-scaled 5-38 wt% (NH(4))(2)SO(4) droplets occurs mainly between ∼ 210 and 225 K. We also find that micrometer-scaled droplets produce one freezing event which is within or in the vicinity of the ∼ 210-225 K region. This fact and the analysis of thermograms suggest that the residual solution of micrometer-scaled droplets may partly crystallize simultaneously with ice and partly transform to glass at T(g)≈172 K. Our results suggest for the first time that the size of (NH(4))(2)SO(4)/H(2)O droplets may affect the number of freezing events below the eutectic point.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Ammonium Sulfate / chemistry*
  • Calorimetry, Differential Scanning
  • Freezing*
  • Transition Temperature
  • Water / chemistry*

Substances

  • Water
  • Ammonium Sulfate