Freezing and melting transitions of liquids in mesopores with ink-bottle geometry

Freezing and melting behavior of nitrobenzene in mesoporous silicon with different pore size and with different porous structure have been studied using H-1 NMR cryoporometry. With the bulk phase surrounding the porous monoliths, in materials with uniform channel-like pores distinct pore-size-dependent freezing and melting transitions have been measured. These data were further used for the analysis of the fluid behavior in samples with modulated porous structure, namely linear pores with alternating cross-section. We have, in particular, considered two materials consisting of channel sections, which were separated by almost identical channel `necks' but notably differed in the respective channel diameters. In the smaller channel segments, the observed shift in the freezing temperature provides direct evidence of the relevance of a pore-blocking mechanism, i.e. of the retardation in the propagation of a solid front by the channel necks. In the channel segments with larger diameter, on the other hand, freezing is found to be initiated by homogeneous nucleation.DF

Controlled crystallisation of mesoscopic particles

EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Special Issue \u201cMagnetic Resonance in Porous Media\u201d: A selection of contributions presented as posters at the 10th International Bologna Conference Magnetic Resonance in Porous Media (MRPM 10), September 2010, Leipzig, Germany

Diffusion Fundamentals is an interdisciplinary open-access Online Journal publishing original research articles in the field of diffusion and transport, energized by a biannual Conference Series. It addresses a broad community of scientists in all research disciplines and provides a Forum of Communication

Special Issue \u201cMagnetic Resonance in Porous Media\u201d

Editorial del Volume contenente una selezione di contributi presentati al "10th International Bologna Conference Magnetic Resonance in Porous Media" (MRPM 10) September 2010, Leipzig, Germany, www.diffusion-fundamentals.or

Influence of the Methane-Zeolite A Interaction Potential on the Concentration Dependence of Self-diffusivity

Studies on the diffusion of methane in a zeolite structure type LTA (as per IZA nomenclature) have indicated that different types of methane zeolite potentials exist in the literature in which methane is treated within the united-atom model. One set of potentials, referred to as model A, has a methane oxygen diameter of 3.14 angstrom, while another set of potential parameters, model B, employs a larger value of 3.46 angstrom. Fritzsche and co-workers (1993) have shown that these two potentials lead to two distinctly different energetic barriers for the passage of methane through the eight-ring window in the cation-free form of zeolite A. Here, we compute the variation of the self-diffusivity (D) with loading (c) for these two types of potentials and show that this slight variation in the diameter changes the concentration dependence qualitatively: thus, D decreases monotonically with c for model A, while D increases and goes through a maximum before finally decreasing for model B. This effect and the surprising congruence of the diffusion coefficients for both models at high loadings is examined in detail at the molecular level. Simulations for different temperatures reveal the Arrhenius behaviour of the self-diffusion coefficient. The apparent activation energy is found to vary with the loading. We conclude that beside the cage-to-cage jumps, which are essential for the migration of the guest molecules, at high concentrations migration within the cage and guest guest interactions with other molecules become increasingly dominant influences on the diffusion coefficient and make the guest zeolite interaction less important for both model A and model B

Assessing Molecular Transport Properties of Nanoporous Materials by Interference Microscopy:Remarkable Effects of Composition and Microstructure on Diffusion in the Silicoaluminophosphate Zeotype STA-7

The influence of the chemical composition and of the storage and activation protocol on the diffusion of methanol into strongly chemically zoned crystals of the silicoaluminophosphate zeotype STA-7 has been investigated by interference microscopy. Analysis of the evolution of transient intracrystalline concentration profiles reveals that just-calcined SAPO STA-7 crystals with lower Si content (Si/(Si + P) = 0.18) exhibit higher surface permeability and bulk diffusivity than those with higher Si content (S/(Si + P) = 0.37). Remarkably, crystals with the higher Si content which were stored in the calcined form crack during activation along planes of weakness already present in the as-prepared crystals, creating fresh surfaces through regions of lower Si that are much more easily penetrated by the adsorbing methanol than are the original surfaces.</p

FunResDB-A web resource for genotypic susceptibility testing of Aspergillus fumigatus

Therapy of invasive aspergillosis is becoming more difficult due to the emergence of azole resistance in Aspergillus fumigatus. A majority of resistant strains carries mutations in the CYP51A gene. Due to a lack of sensitivity of culture-based methods, molecular detection of A. fumigatus has become an important diagnostic tool. We set up the database FunResDB (www.nrz-myk.de/funresdb) to gather all available information about CYP51A-dependent azole resistance from published literature. In summary, the screening resulted in 79 CYP51A variants, which are linked to 59 nonsynonymous mutations. A tailor-made online sequence analysis tool allows for genotypic susceptibility testing of A. fumigatus

How Hydrogen Bonds Influence the Mobility of Imidazolium-Based Ionic Liquids. A Combined Theoretical and Experimental Study of 1-n-Butyl-3-methylimidazolium Bromide

The virtual laboratory allows for computer experiments that are not accessible via real experiments. In this work, three previously obtained charge sets were employed to study the influence of hydrogen bonding on imidazolium-based ionic liquids in molecular dynamics simulations. One set provides diffusion coefficients in agreement with the experiment and is therefore a good model for real-world systems. Comparison with the other sets indicates hydrogen bonding to influence structure and dynamics differently. Furthermore, in one case the total charge was increased and in another decreased by 0.1 e. Both the most acidic proton as well as the corresponding carbon atom were artificially set to zero, sequentially and simultaneously. In the final setup a negative charge was placed on the proton in order to introduce a barrier for the anion to contact the cation via this most acidic hydrogen atom. The following observations were made: changing the hydrogen bonding ability strongly influences the structure while the dynamic properties, such as diffusion and viscosity, are only weakly changed. However, the introduction of larger alterations (stronger hydrogen bonding and antihydrogen bonding) also strongly influences the diffusion coefficients. The dynamics of the hydrogen bond, ion pairing, and the ion cage are all affected by the level of hydrogen bonding. A change in total charges predominantly influences transport properties rather than structure. For ion cage dynamics with respect to transport porperties, we find a good correlation and a weak or no correlation for the ion pair or the hydrogen bond dynamics, respectively. Nevertheless, the hydrogen bond does influence ion cage dynamics. Therefore, we confirm that ionic liquids rather consist of loosely interacting counterions than of discrete ion pairs. Hydrogen bonding affects the properties only in a secondary or indirect manner