Chemistry of ice: Migration of ions and gases by directional freezing of water

Differences in the mechanical properties of intestinal tissue based on preservation freezing duration and temperature

In this study, tissue samples were stress tested to determine if freezing duration and temperature alter their mechanical properties. Tissue samples taken from the small intestine of pigs were assigned to 5 groups: fresh tissue, -28.9 °C for 7 days, -62.2 °C for 7 days, -28.9 °C for 30 days, and -62.2 °C for 30 days. Tissue was stored in PBS for the assigned temperature and duration until testing occurred with the exception of fresh tissue which was tested at sample collection. Before testing, samples were thawed in a room temperature bath, and the thickness was measured. Samples were then mounted in a biaxial test system using four anchoring rakes. Each sample was pulled to a strain of 0.2 with the corresponding forces recorded. This cycle of relaxation to 0.2 strain was repeated 5 times per sample. The thickness and force values were used to find the first Piola-Kirchhoff stress experienced at 0.2 strain and the strain energy. The average stress values in the circumferential direction were: fresh tissue: 22.3 ± 9.85 kPa; -28.9 °C for 7 days: 37.8 ± 14.1 kPa; -62.2 °C for 7 days: 46.5 ± 19.0 kPa; -28.9 °C for 30 days: 46.4 ± 22.7 kPa; -62.2 °C for 30 days: 40.1 ± 19.5 kPa. The stress and strain energy values of frozen tissue were statistically higher than the fresh tissue, although no statistical difference was found by varying duration or temperature. Based on this result, we determined that freezing tissue at any of the tested temperatures or durations increases the stiffness of the thawed tissue. This possibly occurs due to the directional formation of ice, which increases ion concentrations and glycosaminoglycan (GAG) interactions near collagen fibrils.

Molecular simulation of the confined crystallization of ice in cement nanopore

Freezing of water under nanoconfinement exhibits physical peculiarities with respect to the bulk water. However, experimental observations are extremely challenging at this scale, which limits our understanding of the effect of confinement on water properties upon freezing. In this study, we use molecular dynamic simulations to investigate how confinement affects the kinetics of growth of ice and the thermodynamic equilibrium of ice-liquid coexistence. TIP4P/Ice water model and CSH-FF model were applied to simulate ice crystallization in a confined cement system at temperatures down to 220 K. We adapted an interface detection algorithm and reparameterized the CHILL/CHILL+ algorithm to capture ice growth. The confinement leads to a shift of the maximum growth rate of ice to a higher temperature than for bulk water. Both the confinement and surface impurities contribute to slowing down the ice growth. For the ice-liquid coexistence at equilibrium, we derive a formulation of Thomson's equation adapted to statistical physics quantities accessible by molecular simulation, and we show that this adapted equation predicts accurately the melting line of bulk and confined ice Ih as a function of pressure. The confinement decreases systematically the melting temperature of ice of about 5 K compared with bulk ice Ih. A premelted water film about 1 nm thick is observed between the solid wall and ice, and its thickness is found to decrease continuously as temperature is lowered. We note that the surface impurities are key to the formation of the premelted water nanofilm when the temperature is lower than 250 K.

The migration law of magnesium ions during freezing and melting processes

To explore the migration law of magnesium ions (Mg²⁺) during freezing and melting processes, laboratory simulation experiments involving freezing and melting were carried out to investigate the influence of ice thickness, freezing temperature, initial concentration, and initial pH on the distribution of Mg²⁺ in the ice-water system. The distribution coefficient "K" (the ratio of the Mg²⁺ concentration in the ice layer to the Mg²⁺ concentration in the water layer under ice) was used to characterize the migration ability of Mg²⁺. The results showed that during the freezing process, the concentration distribution of Mg²⁺ in the ice and water two-phase system was as follows: ice layer < water before freezing < water layer under ice; in other words, it migrated from ice layer to the water layer under ice. "K" decreased with increasing ice thickness, freezing temperature, initial concentration, and initial pH; the higher the ice thickness, freezing temperature, initial concentration, and initial pH were, the higher the migration efficiency of Mg²⁺ into the water layer under ice was. During the melting process, Mg²⁺ was released in large amounts (50-60%) at the initial stage (0-25%) and in small amounts (25-100%) uniformly in the middle and later periods. According to the change of Mg²⁺ concentration in ice melt water, an exponential model was established to predict Mg²⁺ concentration in ice melt period. The migration law of Mg²⁺during the freezing and melting process was explained by using first principles. Graphical abstract.

3D printed receptacle with diffuser membrane for manipulating pressurized air and water

Water research is one of many fields where fused filament fabrication 3D printing offers the freedom of customization and the inclusion of commercial components. We present our 330 mL 3D printed laboratory receptacle that has been customized to control pressurized air and liquid in one body. During our tests, water has been stored without loss, and batches were frozen whilst circulating and diffusing air through the liquid. The printing has been optimized with the slicer software and gcode editing, letting the fused filament 3D printer to build a diffuser membrane surrounded by air-tight walls. Detailed construction instructions are given, including piping and control board for operation. According to the functionality of the receptacle and the aeration system, the solutions have been found durable and low-cost. Disadvantages are the time invested in creating customized code, and certain limitations of the membrane itself.

Effects of fulvic acid and fulvic ions on Escherichia coli survival in river under repeated freeze-thaw cycles

The effects of fulvic acid (FA) and ions on mesophilic pathogenic bacteria survival under freeze-thaw (FT) stress in natural water and its resistant mechanisms are rarely understood. Therefore, survival patterns of Escherichia coli in river water added with various concentrations of FA or FA-ion under FT stress were studied in this work. Meanwhile, cell surface hydrophobicity (CSH), unit activities of superoxide dismutase (SOD) and catalase (CAT) were determined and Escherichia coli morphologies were observed to explore the bacterial resistant mechanisms against FT stress. The results demonstrated that FT cycles significantly reduced bacterial quantities as sampling time, i.e. freeze-thaw cycle time increased. And the biggest reducing rate was observed after the first FT cycle in every system. Ttd values, time needed to reach detection limit under FT stress decreased under FT stress as FA was added into water, while the changes of ttd values were quite complicated when FA and various ions existed together. Generally, the ttd values of FA-cation systems exceeded that of FA system except FA-Ca²⁺ systems, but it was opposite for FA-anion systems. CSH was heightened after FT cycles and reached peak value at last sampling time in every system. Mechanical constraint from extracellular ice crystals and high CSH induced bacterial aggregation, which protect inner cells of aggregation from extracellular ice crystals. And the unit activities of SOD were significantly higher than those of CAT. Unit activities of SOD and CAT in large part of tested systems increased with sampling time under FT stress, which reduced reactive oxygen species produced from repeated FT cycles. Thus, these could improve the resistance of Escherichia coli to freeze-thaw stress and promote their survival. This work explored the survival pattern and strategy of Escherichia coli in natural water under FT stress.

The Effect of Sulfuric Acid Concentration on the Physical and Electrochemical Properties of Vanadyl Solutions

The effects of sulfuric acid concentration in VO2+ solutions were investigated via electrochemical methods and electron paramagnetic resonance. The viscosity of solutions containing 0.01 M VOSO4 in 0.1–7.0 M H2SO4 was measured. Diffusion coefficients were independently measured via electrochemical methods and electron paramagnetic resonance (EPR), with excellent agreement between the techniques employed and literature values. Analysis of cyclic voltammograms suggest the oxidation of VO2+ to VO2+ is quasi-reversible at high H2SO4 concentrations (>5 mol/L), and approaching irreversible at lower H2SO4 concentrations. Further analysis reveals a likely electrochemical/chemical (EC) mechanism where the H2SO4 facilitates the electrochemical step but hinders the chemical step. Fundamental insights of VO2+/H2SO4 solutions can lead to a more comprehensive understanding of the concentration effects in electrolyte solutions.

Water and Salt Migration with Phase Change in Saline Soil during Freezing and Thawing Processes

Water and salt transfer coupled with phase change may cause serious damage to engineering structures in saline soil regions. In this study, the migration of water and salt in silty clay collected from the Qinghai-Tibet Plateau is explored experimentally and numerically during freezing and thawing processes. The results revealed that there are significant differences in the variations of liquid water content and solution concentration for different initial salt contents, due to salt crystallization and dissolution. The temperature-induced water migration is determined by the soil properties, which can be well explained by the thermodynamics of mass transfer. The amount of salt migrated upward during cooling is slightly larger than that transported downward in the warming period, implying that salt may be accumulated in the surface soil after a large number of circulations and finally result in soil salinization.