A theoretical study on spontaneous dipole orientation in ice structures.

Spontaneous dipole orientation is studied for a set of simulated porous ASW ice films on a substrate held at temperatures ranging from 10 K to 140 K. It is found that the water dipoles in the films obtained at the lower temperatures are oriented such that a negative electric field with a magnitude of 108-109 V m-1 is obtained. The magnitude of the field increases approximately linearly with height above the substrate, akin to experimental observations, although the magnitude of our field increases faster. A strong temperature dependence of the surface potential resulting from the spontelectric field is found, where the surface potential decreases when the substrate temperature increases. The surface potential finally becomes close to zero for temperatures around and above 110 K.

Neutron Scattering Analysis of Water's Glass Transition and Micropore Collapse in Amorphous Solid Water.

The question of the nature of water's glass transition has continued to be disputed over many years. Here we use slow heating scans (0.4 K min^{-1}) of compact amorphous solid water deposited at 77 K and an analysis of the accompanying changes in the small-angle neutron scattering signal, to study mesoscale changes in the ice network topology. From the data we infer the onset of rotational diffusion at 115 K, a sudden switchover from nondiffusive motion and enthalpy relaxation of the network at <121 K to diffusive motion across sample grains and sudden pore collapse at >121 K, in excellent agreement with the glass transition onset deduced from heat capacity and dielectric measurements. This indicates that water's glass transition is linked with long-range transport of water molecules on the time scale of minutes and, thus, clarifies its nature. Furthermore, the slow heating rates combined with the high crystallization resistance of the amorphous sample allow us to identify the glass transition end point at 136 K, which is well separated from the crystallization onset at 144 K-in contrast to all earlier experiments in the field.

Small-angle neutron scattering study of micropore collapse in amorphous solid water.

Vapor-deposited amorphous solid water (ASW) is the most abundant solid molecular material in space, where it plays a direct role in both the formation of more complex chemical species and the aggregation of icy materials in the earliest stages of planet formation. Nevertheless, some of its low temperature physics such as the collapse of the micropore network upon heating are still far from being understood. Here we characterize the nature of the micropores and their collapse using neutron scattering of gram-quantities of D2O-ASW of internal surface areas up to 230 ± 10 m(2) g(-1) prepared at 77 K. The model-free interpretation of the small-angle scattering data suggests micropores, which remain stable up to 120-140 K and then experience a sudden collapse. The exact onset temperature to pore collapse depends on the type of flow conditions employed in the preparation of ASW and, thus, the specific surface area of the initial deposit, whereas the onset of crystallization to cubic ice is unaffected by the flow conditions. Analysis of the small-angle neutron scattering signal using the Guinier-Porod model suggests that a sudden transition from three-dimensional cylindrical pores with 15 Å radius of gyration to two-dimensional lamellae is the mechanism underlying the pore collapse. The rather high temperature of about 120-140 K of micropore collapse and the 3D-to-2D type of the transition unraveled in this study have implications for our understanding of the processing and evolution of ices in various astrophysical environments.

Comparative studies of O2 and N2 in pure, mixed and layered CO ices.

We present laboratory data on pure, layered and mixed CO and O2 ices relevant for understanding the absence of gaseous O2 in space. Experiments have been performed on interstellar ice analogues under ultra high vacuum conditions by molecular deposition at 14 K on a gold surface. A combination of reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD) is used to derive spectroscopic and thermodynamic properties of the ices. It is found that for pure ices the desorption energy of O2 is larger than that of CO and N2. TPD spectra reveal similar desorption processes for all examined CO-O2 ice morphologies. The different amorphous and crystalline components of pure 13CO RAIR spectra are analyzed. The RAIRS data of the 13CO stretching vibration show a significant difference between layered and mixed CO-O2 ices: layered CO-O2 ices resemble that of pure 13CO whereas the spectra of mixed ices are broadened. The experiments also show that the sticking probabilities of O2 on CO and O2 on O2 are close to unity. These new results are compared with recently analyzed data of CO-N2 ices. The differences in the TPD and RAIRS spectra of the CO-N2 and CO-O2 ice systems are explained by differences in quadrupole intermolecular interactions and by different crystallization processes of these ices.