Ice Nucleation in Periodic Arrays of Spherical Nanocages

A silicious material containing massive array of spherical nanocages connected to each other by small micropores was used to study ice nucleation in confined water under conditions of well-defined pore geometry. By purposefully selecting small size of the interconnecting pores below 2 nm, ice nucleation and growth were limited to occur only within the nanocages. By exploitation of nuclear magnetic resonance, ice nucleation rates at different temperatures were accurately measured. These rates were obtained to be substantially higher than those typically observed for micrometer-sized water droplets in air. In addition, the occurrence of correlations between ice nucleation in one nanocage with the phase state in the adjacent cages were observed. These results have important implication for a deeper understanding of ice nucleation, especially in confined geometries

Water Transport in Periodic Mesoporous Organosilica Materials

Water transport in periodic mesoporous organosilicas (PMOs) was studied using pulsed field gradient NMR. A series of isogeometric PMO materials with different chemical compositions of the pore walls were investigated and compared to a purely siliceous MCM-41 material with an identical pore size. The long-range water diffusivities measured were found to be largely controlled by the macroscopic textural properties of the materials, namely, by the particle geometry and a degree of the particle agglomeration, and by thermodynamic conditions under which the experiments were performed. It is shown that their combined effect caused water molecules either to propagate predominantly along the capillary-condensed water domains or to frequently alternate their trajectories between these domains and the water phase in the interparticle space. Because the transport rates in these two regimes differ substantially, it is suggested that by a purposeful choice of the PMO composition, both the long-range transport rate and the chemical functionality can deliberately be tuned

Modeling the Influence of Side Stream and Ink Bottle Structures on Adsorption/Desorption Dynamics of Fluids in Long Pores

We apply dynamic mean field theory to study relaxation dynamics for lattice models of fluids confined in linear pores with side streams and with ink bottle structures. Our results show several mechanisms for how the pore structure affects the dynamics, and these are amplified in longer pores. An important conclusion of this work is that features such as side streams and ink bottle segments can substantially slow the equilibration of fluids confined in long pore systems where the pore lengths can be more than 100 micrometers, such as in porous silicon. This may make it difficult to properly equilibrate these systems for states close to those where the pores should be completely filled with liquids. The presence of trapped bubbles in the system may change the desorption characteristics of the system and the shape of the hysteresis loops

Cooperative Dynamics of Highly Entangled Linear Polymers within the Entanglement Tube

We present a quantitative comparison of the dynamic structure factors from unentangled and strongly entangled poly(butylene oxide) (PBO) melts. As expected, the low molecular weight PBO displays Rouse dynamics, however, with very significant subdiffusive center-of-mass diffusion. The spectra from high molecular weight entangled PBO can be very well described by the dynamic structure factor based on the concept of local reptation, including the Rouse dynamics within the tube and allowing for non-Gaussian corrections. Comparing quantitatively the spectra from both polymers leads to the surprising result that their spectra differ only by the contribution of classical Rouse diffusion for the low molecular weight melt. The subdiffusive component is common for both the low and high molecular weight PBO melts, indicating that in both melts the same interchain potential is active, thereby supporting the validity of the Generalized Langevin Equation approach

Monitoring Molecular Mass Transfer in Cation-Free Nanoporous Host Crystals of Type AlPO-LTA

Micro-imaging is employed to monitor the evolution of intra-crystalline guest profiles during molecular adsorption and desorption in cation-free zeolites AlPO-LTA. The measurements are shown to provide direct evidence on the rate of intra-crystalline diffusion and surface permeation and their inter-relation. Complemented by PFG NMR and integral IR measurements, a comprehensive overview of the diffusivities of light hydrocarbons in this important type of host materials is provided