Satoshi Takeya

In Situ Observation of CO2 Hydrate by X‐ray Diffraction

Abstract: In situ observations of CO2 hydrate growth using high‐energy X‐rays were done at high‐pressures and the growth rates of CO2 hydrate from ice particles were measured. Assuming a model of diffusion through CO2 hydrate layers, interdiffusion coefficients of CO2 and H2O molecules were calculated between 233 K and 272.5 K. The diffusion coefficient at 263 K was 7.4 × 10−16 m2/s, and the activation energy of diffusion was 0.40 eV.

Freezing Properties of Disaccharide Solutions: Inhibition of Hexagonal Ice Crystal Growth and Formation of Cubic Ice

Numerous studies have been undertaken to preserve living bodies and cells from freezing by addition of cryoprotective agents including natural substances (e.g., sugars and proteins) and synthetic chemicals (e.g., glycerol and dimethyl sulfoxide). Trehalose and sucrose, which consist of fructose and glucose rings connected by a glycosidic bond, are naturally occurring disaccharide compounds found in cryoprotectants. Trehalose is primarily found in animals capable of enduring cold temperatures, whereas sucrose is typically found in plants (Crowe et al., 1988). Since these molecules are too large to permeate the biomembrane, the cryoprotective mechanism is considered to be different from those of cellpermeable substances, such as glycerol and dimethyl sulfoxide. One of the considerable mechanisms of disaccharide molecules operating as a cryoprotective agent of living cells is that they protect the lipid bilayer by making the hydrogen bonding during extracellular ice formation. Based on the interactions between disaccharide molecules and the lipid bilayer, it has been suggested that the molecular mechanism underlying this cryoprotective effect is the hydrogen bonding of trehalose molecules to the bilayer head group (Sum et al., 2003). A simulation of the interaction of the lipid bilayer with trehalose has revealed that only marginal changes occur in the lipid bilayer. Another considerable mechanism of disaccharide molecules of the cryoprotective effect for living cells is that they inhibit the growth of ice crystals in extracellular space. To reveal the inhibition mechanism of disaccharides on ice crystal growth, the interaction between the disaccharide and water molecules has been investigated both by macroscopic observations (for example, Sei et al., 2001; 2002; Sei & Gonda, 2004), where the melting point of disaccharide solutions has been determined, and by microscopic observations (for example, Wang & Tominaga, 1994; Kanno & Yamazaki, 2001; Akao et al., 2001; Branca et al., 1999a) where the interaction has been investigated via Raman or infra-red spectroscopy. Sei et al. (2001; 2002) found that the melting points of trehalose and sucrose solutions were lower than those expected by the molar freezing point depression. They explained this phenomenon by considering the ratio of the hydrated water around disaccharide molecules