Tomoko Ikeda-Fukazawa

Molecular-dynamics studies of surface of ice Ih

We performed molecular dynamics calculations of surface of ice Ih in order to investigate formation mechanism of melting layer on the surface. The results showed that the vibrational amplitude of the atoms in the surface layer greatly depends on the crystal orientation, whereas that in the ice bulk is isotropic. The anisotropy of the vibration is due to a dangling motion of the free O-H bonds exist at the surface layer. The dangling motion enhances the rotational motion of the water molecules. The vibrational density of state showed a coupling between the rotational vibration and the lattice vibration of the water molecules in the surface layer. The coupling of the vibrations causes a distortion of ice lattice. Through the hydrogen-bonding network, the distortion transmits to the interior of the crystal. We conclude that the dangling motion of the free O-H bonds exist at the surface layer is one of the dominant factors governing the surface melting of ice crystal.

Structural changes of water in a hydrogel during dehydration

Raman spectra of poly-N,N,-dimethylacrylamide hydrogel were measured in order to investigate the mechanism of the structural changes in water and the polymer network during dehydration. The results show that the vibrational energies of the C=O and the O-H stretching modes increase with the extent of dehydration, whereas that of the CH(3) rocking mode decreases. The energy shifts observed in the C=O stretching and CH(3) rocking modes indicate that the polymer network shrinks with the dehydration and undergoes a glass transition at some point. The energy shifts of the O-H stretching modes are attributed to changes in the water structure with the structural change of the polymer network. By applying a structural model of bulk water to the spectra of the O-H stretching region, the local water structures in the gel and the dried glassy polymer were analyzed. The result shows that a tetragonal water structure consisting of four hydrogen bonds increases in the residual water of the dried glassy polymer, suggesting that the residual water forms a two-dimensional hydrogen-bonded network. The local water structure in a polymer might have important implications for the interpretation of properties of localized water, for instance, water in a mineral crack.

A Hybrid Hydrogel Biomaterial by Nanogel Engineering: Bottom-Up Design with Nanogel and Liposome Building Blocks to Develop a Multidrug Delivery System

New hybrid poly(ethylene glycol) (PEG) hydrogels crosslinked with both nanogels and nanogel-coated liposome complexes are obtained by Michael addition of the acryloyl group of a cholesterol-bearing pullulan (CHP) nanogel to the thiol group of pentaerythritol tetra(mercaptoethyl) polyoxyethylene. The nanogel-coated liposome complex is stably retained after gelation and the complexes are well dispersed in the hybrid gel. Microrheological measurements show that the strength and gelation time of the hybrid hydrogel can be controlled by changing the liposome:nanogel ratio. The hydrogel is gradually degraded by hydrolysis under physiological conditions. In this process, the nanogel is released first, followed by the nanogel-coated liposomes. Hybrid hydrogels that can incorporate various molecules into the nanogel and liposomes, and release them in a two-step controllable manner, represent a new functional scaffold capable of delivering multiple drugs, proteins or DNA.

Variation in N2/O2 ratio of occluded air in Dome Fuji antarctic ice

Ancient atmospheric gases are trapped in polar ice sheets. The gas molecules are stored in air bubbles at shallow depth and are incorporated into clathrate hydrates below a depth at which the hydrostatic pressure becomes greater than the formation pressure of the air clathrate hydrate. Significant gas fractionation has been found from measurements of the depth profile of the N2/O2 composition ratios in clathrate hydrates and air bubbles of Vostok antarctic ice. To investigate the effect of the ice condition on the fractionation process, we measured the N2/O2 ratios in clathrate hydrates and air bubbles from Dome Fuji antarctic ice using Raman spectroscopy. The results showed that the N2/O2 ratios in the clathrate hydrates of the Dome Fuji ice are slightly lower than those of the Vostok ice, although the tendency of the variation of the N2/O2 ratio with depth is similar. The difference in the N2/O2 ratio between the Dome Fuji ice and the Vostok ice for the transition zone is attributed to the difference of the ice temperature and the snow accumulation rate. On the other hand, it is concluded that the difference in the bubble-free ice zone was caused by gas loss from the ice core after coring. The N2/O2 ratio of clathrate hydrate increases after coring because of higher diffusion rate and lower dissociation pressure of O2 than of N2. Our data suggest that the effect of gas loss in the Dome Fuji ice is relatively small, and so the gas composition in the Dome Fuji ice can be a precise paleoenvironmental indicator.

Molecular dynamics studies of molecular diffusion in ice Ih

We performed molecular dynamics simulations of the diffusion of interstitial He and H2O in ice Ih and found diffusion hops for these interstitial molecules from a stable site to an adjacent site. By observing the jumps of these diffusing species, we determined the jump frequencies, the crystal orientation dependence of the diffusion coefficients, and the diffusion activation energies. Most jumps are along the c axis, because the energy barrier for diffusion along the c axis is lower than that in the a–b plane. Furthermore, the diffusion mechanism for He significantly differ from that for H2O; interstitial H2O diffused by distorting the ice lattice, whereas He atom migrated by jumping from a stable interstitial site to an adjacent site without distorting the lattice. The transverse optic mode for translational lattice vibrations of the lattice surrounding the interstitial H2O shifts to high energy in comparison with that of the pure ice Ih. This upward shift is attributed to a strong coupling between local...

Mechanism of Molecular Diffusion in Ice Crystals

We used molecular dynamics (MD) simulations to investigate the diffusion of O2, N2, CH4, and CO2 in a crystal of ice Ih. The results show that the diffusion mechanism for the molecules differs significantly from the interstitial mechanism that applies to small atoms such as helium. The air molecules hopped between stable sites by a new mechanism called the breaking-bond mechanism in which hydrogen bonds in the lattice are broken. The diffusion coefficients determined from the MD calculations are several orders of magnitude larger than the estimates under the assumption of the interstitial mechanism. We conclude that the repulsive interactive between the air and water molecules in ice is the dominant factor governing the diffusion mechanism.

Structural changes of water in poly(vinyl alcohol) hydrogel during dehydration

To investigate the mechanism of structural changes of water and polymer networks with drying and swelling, we measured the Raman spectra of a physically cross-linked poly(vinyl alcohol) (PVA) hydrogel synthesized using the freezing-thawing method. The results show that the vibrational frequencies of the O-H and C-H stretching modes decrease with dehydration. The frequency shifts observed are attributed to reduction of free water inside the polymer network. The C-H bonds elongate as the water density decreases, and the average length of the O-H bonds increases with increasing proportion of bound water to the total amount of water. On the basis of the dependence of the frequency shifts on the PVA concentration of the original solution, it was found that the structure of the polymer network in the reswollen hydrogel becomes inhomogeneous due to shrinkage of the polymer network with drying. Furthermore, to investigate the effects of the cross-linking structure on the drying process, these results were compared with those of a chemically cross-linked PVA hydrogel synthesized using glutaraldehyde as a cross-linker. The result shows that the vibrational frequency of the O-H stretching mode for the chemically cross-linked hydrogel increases with dehydration, whereas that of the C-H stretching mode decreases. The opposite trend observed in the O-H stretching mode between the physically and chemically cross-linked hydrogels is due to the difference in the shrinkage rate of the polymer network. Because the rate of shrinking is slow compared with that of dehydration in the chemically cross-linked hydrogel, water density in the polymer network decreases. For the physically cross-linked hydrogel, the polymer network structure can be easily shrunken, and the average strength of hydrogen bonds increases with dehydration. The results show that the structures of the polymer network and water change with the gel preparation process, cross-linking method, and drying and reswelling processes. The structure of the polymer network and the behavior of water accommodated in the network are important factors governing the chemical and physical properties of gel materials.

Temperature Dependence of the Structure of Bound Water in Dried Glassy Poly-N,N,-dimethylacrylamide

Raman spectroscopy was used to investigate the temperature dependence of structural changes of bound water in dried glassy poly-N,N,-dimethylacrylamide in the temperature range 286.1-329.7 K. The results show that the frequency of the O-H stretching mode of the bound water that is present in the dried glassy polymer shifts to the higher side with increasing temperature. The rate changes at around 310 K, while that for the bulk water is constant in the temperature range studied. The rates of change of the frequencies for the C=O stretching mode and CH(3) rocking mode also change at around 310 K. These results indicate a significant change in the interaction between the bound water and polymer chains at 310 K. Temperature dependence of the local structure of the bound water was analyzed by applying a structural model of bulk water to the spectra of the O-H stretching region. The result shows that the density of a tetragonal water structure consisting of four hydrogen bonds increases with increasing temperature below 310 K and begins to decrease at temperatures above 310 K. Further, estimates of the water content indicate that the evaporation rate of the bound water significantly changes at around 310 K. These results suggest that the bound water present in the dried glassy polymer can be classified as being in two states. At temperatures below 310 K, the water that forms a shell layer around the polymer chains evaporates, while at temperatures above 310 K the water that is bound to polar groups of polymer chains begins to evaporate. The structural changes of bound water might have important implications for the interpretation of properties of hydrated polymer systems, including both biological and synthetic polymers.

Structure and dynamics of water in mixed solutions including laponite and PEO

To investigate the structure and dynamics of water in mixed solutions including laponite clay particles and poly(ethylene oxide) (PEO), we measured the Raman spectra of the mixed solutions in the temperature range 283-313 K. The results show that the vibrational energies of the O-H stretching modes in the mixed solutions depend on the water content and temperature. The energy shifts of the O-H stretching modes are attributed to changes in the water structure. By applying a structural model of bulk water to the spectra in the O-H stretching region, the local structures of water in the solutions were analyzed. The result shows that the formation probability of hydrogen bonds in the solutions decreases as the water content decreases. Laponite and PEO have effects to disrupt the network structure of hydrogen bonds between water molecules. Further, it was found that laponite and PEO cause increase in the strength of hydrogen bonds of surrounding water,although the strength of the hydrogen bonds increases with the order water-laponite < water-water < water-PEO. It is concluded that water in laponite-PEO mixed solutions has a less-networked structure with strong hydrogen bonds compared with bulk water.

Structure and dynamics of empty cages in xenon clathrate hydrate

We performed molecular dynamics calculations of xenon clathrate hydrate to investigate the effects of empty cages on the structure and dynamics of the surrounding lattice. The distinct structure and dynamics of the empty cages, and cages including Xe, which coexist in the lattice, were analyzed. The results show that the ellipsoidal tetrakaidecahedral cage shrinks along the minor (100) axis and expands along the major (100) axis due to the absence of Xe from the cage, whereas the dodecahedral cage shrinks isotropically. These distortions of the empty cages cause a reduction in the lattice constant and an enhancement of the thermal vibrations of the surrounding lattice. The vibrational density of states shows that the hydrogen bonds consisting of the tetrakaidecahedral cage are strengthened by the absence of Xe, whereas those of the dodecahedral cage are weakened. These results show differing mechanisms of guest-host interaction for the two types of cages including Xe. Repulsion is the dominant guest-host interaction for the dodecahedral cage, as proposed by previous studies. For the tetrakaidecahedral cage, however, attractive interaction is dominant along the major (100) axis, whereas repulsive interaction is dominant along the minor (100) axis. The present results suggest that a small number of empty cages can affect not only the local structures but also the macroscopic properties of the crystal. It is concluded that the distortions of the empty cages are one of the important factors governing the density and phase equilibrium of clathrate hydrates.