Visualization data on the freezing process of micrometer-scaled aqueous citric acid drops

Abstract

The visualization data (8 movies) presented in this article are related to the research article entitled “Freezing and glass transitions upon cooling and warming and ice/freeze-concentration-solution morphology of emulsified aqueous citric acid” (A. Bogdan, M.J. Molina, H. Tenhu, 2016) [1]. The movies recorded in-situ with optical cryo-miscroscopy (OC-M) demonstrate for the first time freezing processes that occur during the cooling and subsequent warming of emulsified micrometer-scaled aqueous citric acid (CA) drops. The movies are made publicly available to enable critical or extended analyzes.

Physical Chemistry of the Freezing Process of Atmospheric Aqueous Drops

“…The arrow in Figure e shows the spot of heterogeneous ice nucleation from which a spherulitic IF developed. All images in Figure indicate that in frozen formulations, the ice phase is not a population of isolated ice crystals surrounded by a FCS, as previously believed, but a continuous ramified IF entangled with a FCS. − In lyophilization/freeze-drying, freezing is the first and most important step. Finding the means to control the pace of FIPS, and, consequently, the morphology of IF/FCS is important for the optimization of time- and energy-consumed lyophilization, which is widely used in pharmaceutics, biotechnology, food industry, tissue engineering, etc. − The IF/FCS morphology of frozen formulations controls the duration of lyophilization and, consequently, the quality attributes of lyophilized products (drugs, foods, etc.…”

“…The onset of freezing is observed as an abrupt black flash in films thicker than ∼100 μm (images in Figure , parts a and b), and an abrupt darkening in ∼10–15 μm-thick films (images in Figure , parts c and d). The abrupt black flash and darkening are due to light scattering from a rapidly forming complex and ramified spherulitic ice framework (IF), entangled with a FCS. − The very fast freezing process displayed in Movies SM4–SM7 indicates that ice nucleation and subsequent freezing/ice-growth belong to only one physical process, and, consequently, ice nucleation should be considered together with the entire freezing process. On the other hand, images a–d do not provide information whether the ice nucleation event took place in a uniform solution or in a pure water domain within solutions.…”

“…In this work, we investigated the freezing behavior of (i) emulsified pure water and emulsified H 2 SO 4 /H 2 O solutions, (ii) micrometer-scaled pure water drops nebulized on a silicon (Si) wafer, and (iii) bulk inorganic and organic solutions relevant to atmospheric and pharmaceutical sciences. Details about solution preparation and emulsification procedure have been described elsewhere. − For the in situ visualization of freezing process and the ice/FCS morphology of frozen solutions we used the ́2-dimensionaĺ solution approach described earlier, employing an Olympus BX51 optical cryo-microscope (OC-M) equipped with a Linkam cold stage and Linksys32 temperature control and video capture software. − To this end we placed bulk pure water, solution and emulsion samples between a microscope glass slide and a cover glass. We also investigated the freezing behavior of bulk pure water and solutions samples (∼6 mg) calorimetrically using a Mettler Toledo DSC 822.…”

“…FIPS and the ice/FCS morphology of frozen solutions is readily observable. OC-M images and movies obtained in situ during the freezing of aqueous solutions were presented in our previous work. ,− FIPS occurs independently of whether the freezing of aqueous solutions has been initiated by homogeneous ,− ,, or by heterogeneous ice nucleation. Existing experimental techniques do not distinguish whether FIPS occurs already during ice nucleation in a uniform solution, or an ice nucleation event takes place in pure water domains , within solutions.…”