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-miscroscope 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, FCS(1) and FCS(2). The FCS(1) is maximally freeze-concentrated and interweaves with IF. The less concentrated FCS(2) envelops the entire IF/FCS(1). We find that upon further cooling, the FCS(1) transforms to glass, whereas the slow freezing of FCS(2) continues until it is terminated by a FCS(2)-glass transition. We observe the resumed slow freezing of FCS(2) upon subsequent warming. The net thermal effect of the resumed freezing and a reverse glass-FCS(1) transition produces the T(tr2)-transition which before has only been observed upon warming of frozen hydrocarbon solutions and which nature has remained misunderstood for decades.