Martin Wendland

Aerogels from Unaltered Bacterial Cellulose: Application of scCO2 Drying for the Preparation of Shaped, Ultra-Lightweight Cellulosic Aerogels

Bacterial cellulose produced by the gram-negative bacterium Gluconacetobacter xylinum was found to be an excellent native starting material for preparing shaped ultra-lightweight cellulose aerogels. The procedure comprises thorough washing and sterilization of the aquogel, quantitative solvent exchange and subsequent drying with supercritical carbon dioxide at 40 degrees C and 100 bar. The average density of the obtained dry cellulose aerogels is only about 8 mg x cm(-3) which is comparable to the most lightweight silica aerogels and distinctly lower than all values for cellulosic aerogels obtained from plant cellulose so far. SEM, ESEM and nitrogen adsorption experiments at 77 K reveal an open-porous network structure that consists of a comparatively high percentage of large mesopores and smaller macropores.

High-pressure multiphase behaviour of ternary systems carbon dioxide–water–polar solvent: review and modeling with the Peng–Robinson equation of state

Abstract Mixtures of water and a hydrophilic organic solvent, e.g. an alcohol, a ketone, or a carboxylic acid, reveal a complex phase behaviour when pressurized with near critical carbon dioxide at temperatures near the critical temperature of carbon dioxide. Although water and the hydrocarbonaceous solvent are completely miscible, by pressurization with carbon dioxide a liquid–liquid phase split is observed, resulting in a ternary liquid–liquid–vapor equilibrium. In some cases even a four-phase equilibrium is observed. Such equilibria are discussed for systems with the organic solvents methanol, ethanol, 1-propanol, 2-propanol, acetone and propionic acid. That phase behaviour is modelled using the Peng–Robinson equation of state with several mixing rules. A short description of the mathematical tools for such phase equilibrium calculations is given. From pure component and binary data alone, the calculations usually result in a qualitative agreement with the experimental data, i.e. typical effects like for example the existence of ternary critical endpoint lines and tricritical points can be predicted. However, in most cases the quantitative agreement with experimental results is poor. It can be considerably improved by fitting some interaction parameters to ternary high-pressure three-phase equilibrium data.

Cellulose aerogels: Highly porous, ultra-lightweight materials

Abstract Cellulosic aerogels are intriguing new materials produced by supercritical drying of regenerated cellulose obtained by solvent exchange of solid Lyocell moldings. From N-methylmorpholine-N-oxide (NMMO) solutions with cellulose contents between 1 and 12%, dimensionally stable cellulose bodies are produced, in which the solution structure of the cellulose is largely preserved and transferred into the solid state, the material having densities down to 0.05 g cm-3 and surface areas of up to 280 m2 g-1. In this study, several aspects of cellulosic aerogel production are communicated: the stabilization of the cellulose solutions against degradation reactions by agents suitable for later extraction and drying, a reliable extraction and drying procedure by supercritical carbon dioxide, the advantages of DMSO/NMMO in this procedure as a solvent/non-solvent pair, and some data on the physical properties of the materials.

Cellulosic aerogels as ultra-lightweight materials. Part 2: Synthesis and properties

Abstract Ultra-lightweight cellulose aerogels can be obtained in three steps: (1) preparation of a cellulose solution in molten N-methylmorpholine-N-oxide monohydrate (NMMO·H2O) at 110–120°C and casting of the viscous mass into moulds; (2) extraction of the solidified castings with ethanol to initiate cellulose aggregation and to remove NMMO·H2O so that the fragile, fine-porous texture of cellulose II is largely retained; and (3) drying of the lyogel using supercritical carbon dioxide (scCO2). According to this approach, cellulosic aerogels were prepared from eight commercial cellulosic materials and pulps and analysed for selected chemical, physicochemical and mechanical parameters. The results reveal that all aerogels obtained from 3% cellulose containing NMMO·H2O melts had a largely uniform mesoporous structure with an average pore size of ∼9–12 nm, surface area of 190–310 m2 g-1, and specific density of 0.046–0.069 g cm-3, but rather low mechanical stability expressed as compressive yield strain of 2.9–5.5%. All samples showed viscoelastic behaviour, with Youngs modulus ranging from ∼5 to 10 N mm-2. Doubling the cellulose content in the NMMO·H2O melt from 3% to 6% increased Youngs modulus by one order of magnitude. Shrinkage of the fragile cellulose bodies during scCO2 drying was still considerable and is subject to further investigations. Influencing parameters such as scCO2 pressure, cellulose content, regenerating solvent and the number of regenerating baths were optimised.

Simulation of Forces between Humid Amorphous Silica Surfaces: A Comparison of Empirical Atomistic Force Fields.

Atmospheric humidity strongly influences the interactions between dry granular particles in process containers. To reduce the energy loss in industrial production processes caused by particle agglomeration, a basic understanding of the dependence of particle interactions on humidity is necessary. Hence, in this study, molecular dynamic simulations were carried out to calculate the adhesion between silica surfaces in the presence of adsorbed water. For a realistic description, the choice of force field is crucial. Because of their frequent use and transferability to biochemical systems, the Clay and CWCA force fields were investigated with respect to their ability to describe the water–silica interface in comparison to the more advanced Reax force field, ab initio calculations, and experiments.

Multiphase high-pressure equilibria of carbon dioxide-water-isopropanol

Abstract Homogeneous aqueous solutions of alcohols can be split in two liquid phases by pressurization with gases like carbon dioxide or ethene. At conditions near the critical point of the gas, the two liquid phases coexist with a gaseous phase. It has been proposed to use this effect for the separation of water and alco hols, but it might also be promising to use it for extracting products from aqueous solutions. For the design of such processes, data on the phase behavior of the gas-water-alcohol system is needed. In order to take such data, an analytical equilibrium apparatus was developed. To test this apparatus, three- and four-phase equilibria in the carbon dioxide-water-isopropanol system at temperatures between 303 and 333 K and pressures up to 13 MPa were investigated. The new apparatus is described and the results are critically compared to literature data.

Description of alternative refrigerants with BACKONE equations

Abstract The BACKONE equations are a family of physically based equations of state, in which the Helmholtz energy (F) is written as a sum of contributions from characteristic intermolecular interactions. For dipolar fluids F is given by the DIBACKONE equation as F=FH+FA+FD, where FH is the hard-body contribution, FA the attractive dispersion force contribution, and FD the dipolar contribution. For quadrupolar fluids F is given by the QUABACKONE equation as F=FH+FA+FQ, where FQ is the quadrupolar contribution. FD and FQ have been determined on the basis of extensive molecular simulations [B. Saager, J. Fischer, Fluid Phase Equilibria, 72 (1992) 67–88]. Both the DIBACKONE and the QUABACKONE equation need only four substance specific parameters: a characteristic temperature T0, a characteristic density ρ0, an anisotropy parameter α and either a reduced squared dipole moment μ*2 or a reduced squared quadrupole moment Q*2. In the present work these parameters were determined for the alternative refrigerants R123, R124, R125, R134a, R143a, R152a, R218, and R236ea by fitting them to saturated liquid densities and vapour pressures at four temperatures. It turned out that all these substances can be quite well described by the QUABACKONE equation with the exception of R152a which is better described by DIBACKONE. Moreover, a description in the form of F=FH+FA+FQ+FD with five parameters called D+QBACKONE is explored. Comparisons of the BACKONE results for the saturated liquid densities, the saturated vapour densities and the vapour pressures with experimental data or with data from reference Helmholtz function (RHF) equations show satisfying agreement. For R123, R125, R134a, and R152a enthalpies and entropies for coexisting liquid and vapour states are also compared with RHF equation results and show maximum relative deviations in the enthalpy of less than 1.5% and in the entropy of less than 1.3%. Moreover, coefficients of performance for an idealized refrigeration and a heat pump cycle are compared with RHF equation results and show for R134a deviations less than 0.25% and for R152a deviations less than 1.00%. Finally, a thermodynamic table is given for R236ea on the basis of BACKONE.

On the hard sphere plus attractive mean field approximation for inhomogeneous fluids

As many theories for inhomogeneous fluids use a mean field approximation for the attractive intermolecular forces the thermodynamic consequences of this approximation are investigated for the homogeneous phases and the saturation curve of the Lennard-Jones fluid. The pressure at a given density is reasonably accurate for the gas but considerably too high for the liquid; the consequences for the density at a given chemical potential are discussed. The bubble densities are generally too low by more than 10 per cent, the dew-line is too steep, and the critical temperature is kT/ϵ = 1·412. Moreover, a coarse graining prescription introduced earlier into the Born-Green-Yvon equation is tested for hard spheres near a hard wall. The density profiles are qualitatively correct but the layering is not pronounced enough. We also prove that for hard spheres near a hard wall the sum rule n(0) = p/kT is true for a wide class of approximations to the Born-Green-Yvon equation.

Born-Green-Yvon results for adsorption of a simple fluid on plane walls. Structure, adsorption isotherms, and surface area determination

The Born-Green-Yvon approach using a coarse grained density is applied to study the adsorption of a subcritical Lennard-Jones fluid. In case of a strongly adsorbing wall the adsorbed gas forms several layers with increasing pressure. Close to the vapour pressure the layers are followed by a film of nearly saturated liquid density the thickness of which increases rapidly. At low temperature the layer formation occurs rather suddenly causing type VI (multistepped) adsorption isotherms whilst at high temperature type II (concave one-step BET) adsorption isotherms are obtained. The adsorbed liquid shows the same layered structure as the gas. In case of a weakly adsorbing wall the adsorption mechanism close to saturation at high temperature is similar to that at a strongly adsorbing wall; comparison with the results from the density functional theory of Meister and Kroll is made. At low temperature, however, the gas forms only one layer in approaching the vapour pressure. For all temperatures the adsorption is...

Influence of capillary bridge formation onto the silica nanoparticle interaction studied by grand canonical Monte Carlo simulations.

Adhesion forces between nanoparticles strongly depend on the amount of adsorbed condensed water from ambient atmosphere. Liquid water forms bridges in the cavities separating the particles, giving rise to the so-called capillary forces which in most cases dominate the van der Waals and long-range electrostatic interactions. Capillary forces promote the undesirable agglomeration of particles to large clusters, thereby hindering the flowability of dry powders in process containers. In process engineering macroscopic theories based on the Laplace pressures are used to estimate the strength of the capillary forces. However, especially for low relative humidity and when the wetting of rough or small nanoparticles is studied, those theories can fail. Molecular dynamic simulations can help to give better insight into the water–particle interface. The simulated force versus distance curve as well as adhesion forces and the adsorption isotherm for silica nanoparticles at varying relative humidity will be discussed in comparison to experiments, theories, and simulations.