title: Visualization of freezing process in situ upon cooling and warming of aqueous solutions.
abstract: The freezing of aqueous solutions and reciprocal distribution of ice and a freeze-concentrated solution (FCS) are poorly understood in spite of their importance in fields ranging from biotechnology and life sciences to geophysics and climate change. Using an optical cryo-microscope and differential scanning calorimetry, we demonstrate that upon cooling of citric acid and sucrose solutions a fast freezing process results in a continuous ice framework (IF) and two freeze-concentrated solution regions of different concentrations, FCS1 and FCS2. The FCS1 is maximally freeze-concentrated and interweaves with IF. The less concentrated FCS2 envelops the entire IF/FCS1. We find that upon further cooling, the FCS1 transforms to glass, whereas the slow freezing of FCS2 continues until it is terminated by a FCS2-glass transition. We observe the resumed slow freezing of FCS2 upon subsequent warming. The net thermal effect of the resumed freezing and a reverse glass-FCS1 transition produces the Ttr2-transition which before has only been observed upon warming of frozen hydrocarbon solutions and which nature has remained misunderstood for decades.
authors.affiliation: 1] Institute of Physical Chemistry, University of Innsbruck, Innrain 80-82, A-6020, Innsbruck, Austria [2] Laboratory of Polymer Chemistry, Department of Chemistry, University of Helsinki, P.O. Box 55, FIN-00014, Helsinki, Finland [3] Department of Physical Sciences, University of Helsinki, P.O. Box 64, FI-00014, Helsinki, Finland.
authors.forename: Anatoli
authors.initials: A
authors.lastname: Bogdan
authors.affiliation: Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92093-0356, USA.
authors.forename: Mario J
authors.initials: MJ
authors.lastname: Molina
authors.affiliation: Laboratory of Polymer Chemistry, Department of Chemistry, University of Helsinki, P.O. Box 55, FIN-00014, Helsinki, Finland.
authors.forename: Heikki
authors.initials: H
authors.lastname: Tenhu
authors.affiliation: Institute of Physical Chemistry, University of Innsbruck, Innrain 80-82, A-6020, Innsbruck, Austria.
authors.forename: Erminald
authors.initials: E
authors.lastname: Bertel
authors.affiliation: Faculty of Medicine, University of Helsinki, P.O. Box 63, FIN-00014, Helsinki, Finland.
authors.forename: Natalia
authors.initials: N
authors.lastname: Bogdan
authors.affiliation: Institute of Physical Chemistry, University of Innsbruck, Innrain 80-82, A-6020, Innsbruck, Austria.
authors.forename: Thomas
authors.initials: T
authors.lastname: Loerting