Visualization of freezing process in situ upon cooling and warming of aqueous solutions

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.