Freezing and glass transitions upon cooling and warming and ice/freeze-concentration-solution morphology of emulsified aqueous citric acid.
Although freeze-induced phase separation and the ice/FCS (freeze-concentration solution) morphology of aqueous solutions play an important role in fields ranging from life sciences and biotechnology to geophysics and high-altitude ice clouds, their understanding is far from complete. Herein, using differential scanning calorimetry (DSC) and optical cryo-microscope (OC-M), we have studied the freezing and glass transition behavior and the ice/FCS morphology of emulsified 10-60wt% CA (citric acid) solutions in the temperature region of ∼308and153K. We have obtained a lot of new result which are understandable and unclear. The most essential understandable results are as follows: (i) similar to bulk CA/H2O, emulsified CA/H2O also freezes upon cooling and warming and (ii) the ice/FCS morphology of frozen drops smaller than ∼3-4μm is less ramified than that of frozen bulk solutions. Unclear results, among others, are as follows: (i) in contrast to bulk solutions, which produce one freezing event, emulsified CA/H2O produces two freezing events and (ii) in emulsions, drop concentration is not uniform. Our results demonstrate that DSC thermograms and OC-M images/movies are mutually supplementary and allow us to extract important information which cannot be gained when DSC and OC-M techniques are used alone.
Laboratory of Polymer Chemistry, Department of Chemistry, P.O. Box 55, University of Helsinki, FI-00014, Finland; Department of Physics, P.O. Box 48, University of Helsinki, FI-00014, Finland. Electronic address: anatoli.bogdan@helsinki.fi.
Anatoli
A
Bogdan
Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92093-0356, United States.
Mario J
MJ
Molina
Laboratory of Polymer Chemistry, Department of Chemistry, P.O. Box 55, University of Helsinki, FI-00014, Finland.
Heikki
H
Tenhu
D002152:Calorimetry, Differential Scanning; D002626:Chemistry, Pharmaceutical / Q000379:methods*; D019343:Citric Acid / Q000737:chemistry*; D020285:Cryoelectron Microscopy; D004655:Emulsions; D005612:Freeze Drying; D005615:Freezing; D005898:Glass; D007053:Ice; D012996:Solutions; D013696:Temperature; D014867:Water / Q000737:chemistry
D004655:Emulsions; D007053:Ice; D012996:Solutions; D014867:Water; D019343:Citric Acid
DSC; Emulsified aqueous citric acid; Freeze-concentrated solution (FCS); Freezing; Glass transition; Lyophilization; Optical cryo-microscope (OC-M)