ALBERT

Description: Recent studies of saccharides’ peculiar anti-freezing and anti-dehydration properties point to a close association with their strong hydration capability and destructuring effect on the hydrogen bond (HB) network of bulk water. The underlying mechanisms are, however, not well understood. In this respect, examination of the complex dielectric constants of saccharide aqueous solutions, especially over a broadband frequency region, should provide interesting insights into these properties, since the dielectric responses reflect corresponding dynamics over the time scales measured. In order to do this, the complex dielectric constants of glucose solutions between 0.5 GHz and 12 THz (from the microwave to the far-infrared region) were measured. We then performed analysis procedures on this broadband spectrum by decomposing it into four Debye and two Lorentz functions, with particular attention being paid to the β relaxation (glucose tumbling), δ relaxation (rotational polarization of the hydrated water), slow relaxation (reorientation of the HB network water), fast relaxation (rotation of the non-HB water), and intermolecular stretching vibration (hindered translation of water). On the basis of this analysis, we revealed that the hydrated water surrounding the glucose molecules exhibits a mono-modal relaxational dispersion with 2–3 times slower relaxation times than unperturbed bulk water and with a hydration number of around 20. Furthermore, other species of water with distorted tetrahedral HB water structures, as well as increases in the relative proportion of non-HB water molecules which have a faster relaxation time and are not a part of the surrounding bulk water HB network, was found in the vicinity of the glucose molecules. These clearly point to the HB destructuring effect of saccharide solutes in aqueous solution. The results, as a whole, provide a detailed picture of glucose–water and water–water interactions in the vicinity of the glucose molecules at various time scales from sub-picosecond to hundreds of picoseconds.

Description: Water conformation around hydrophobic side chains of four amino acids (glycine, L-alanine, L-aminobutyric acid, and L-norvaline) was investigated via changes in complex dielectric constant in the terahertz (THz) region. Each of these amino acids has the same hydrophilic backbone, with successive additions of hydrophobic straight methylene groups (–CH 2 –) to the side chain. Changes in the degree of hydration (number of dynamically retarded water molecules relative to bulk water) and the structural conformation of the water hydrogen bond (HB) network related to the number of methylene groups were quantitatively measured. Since dielectric responses in the THz region represent water relaxations and water HB vibrations at a sub-picosecond and picosecond timescale, these measurements characterized the water relaxations and HB vibrations perturbed by the methylene apolar groups. We found each successive straight –CH 2 – group on the side chain restrained approximately two hydrophobic hydration water molecules. Additionally, the number of non-hydrogen-bonded (NHB) water molecules increased slightly around these hydrophobic side chains. The latter result seems to contradict the iceberg model proposed by Frank and Evans, where water molecules are said to be more ordered around apolar surfaces. Furthermore, we compared the water–hydrophilic interactions of the hydrophilic amino acid backbone with those with the water–hydrophobic interactions around the side chains. As the hydrophobicity of the side chain increased, the ordering of the surrounding water HB network was altered from that surrounding the hydrophilic amino acid backbone, thereby diminishing the fraction of NHB water and ordering the surrounding tetrahedral water HB network.

Description: Rapid thermal oxidation, in which samples are intensely heated to a preset temperature, is used to grow silicon oxide on Si substrates while avoiding significant diffusion of impurities into the substrate. In previously proposed reaction models for rapid thermal oxidation, the oxidation rate is only determined by the temperature and O 2 pressure. Therefore, it is believed that the rate of oxidation at a preset temperature is independent of the initial substrate temperature. In this study, the interfacial oxidation reactions that follow Si(001) surface oxidation were observed using real-time Auger electron spectroscopy. Interfacial oxidation was enhanced when the substrate temperature was increased from temperature T 1 to temperature T 2 at the end of Si(001) surface oxidation. As a result, strong T 1 and T 2 dependences of the interfacial oxidation rate were observed. The interfacial oxidation rate at T 1 = room temperature was more than 10 times higher than that at T 1 = 561 °C, even for the same T 2 (682 °C). Additionally, the activation energy of interfacial oxidation was 0.27 eV, and independent of T 1 . This activation energy corresponds to the “no elementary step” proposed as the rate-limiting reaction in previous studies. These results can be explained using the unified Si oxidation reaction model mediated by point defect generation: high magnitude thermal strain is generated when the difference between T 2 and T 1 is large, and this strain generates point defects that become reaction sites for interfacial oxidation.