Morio Okazaki

Porous structure of organic and carbon aerogels synthesized by sol-gel polycondensation of resorcinol with formaldehyde

Abstract Organic aerogels were synthesized via the sol-gel polycondensation of resorcinol with formaldehyde in a slightly basic aqueous solution and followed by supercritical drying with carbon dioxide. As a result of characterization by nitrogen adsorption, the resorcinol-formaldehyde (RF) aerogels were mesoporous materials with high surface areas and had few micropores. The surface area of the RF aerogel was controlled by the mole ratio of resorcinol to basic catalyst ( R C ). On the other hand, the mesopore volume of RF aerogel changed greatly with R C or the ratio of resorcinol to water ( R W ). The RF aerogel having a monodisperse porous structure was prepared under the conditions of low R C or high R W . As R C increased or R W decreased, the porous structure of the aerogel became dispersed. Carbon aerogels were obtained by pyrolyzing the RF aerogels at 1223 K, and the aerogel had larger surface areas than the RF aerogels. It was found that micropores in the carbon aerogels were formed during pyrolysis of the RF aerogels. When the RF aerogel having a monodisperse porous structure was pyrolyzed, the mesopore volume of the carbon aerogel was smaller than that of the original RF aerogel. The mesopore volume increased by the pyrolysis of the RF aerogel having a dispersed porous structure.

Determination of adsorption equilibria in pores by molecular dynamics in a unit cell with imaginary gas phase

We developed a new molecular dynamics (MD) scheme, introducing the concept of the potential buffering field through which an adsorbed phase could interact with an imaginary gas phase. This simulation cell allowed us to conduct a MD simulation that allowed a change in the number of molecules to attain equilibrium with given equilibrium pressure, like a grand canonical Monte Carlo simulation. By taking another choice for the setting of the cell, the number of molecules stayed constant but the equilibrium pressure was able to be obtained easily by a new technique of “particle counting method.” The thus obtained equilibrium vapor-phase pressure agreed with that obtained by Widom’s particle insertion method. Some adsorption simulations within slitlike pores of 2 and 3 nm were carried out. Adsorption phenomena could be observed from monolayeradsorption on a pore wall under a low relative pressure to the capillary condensation under a high relative pressure. Thus the adsorption equilibrium relation could be determined. The critical relative pressure for capillary condensation was smaller than that predicted by the modified Kelvin equation. This MD method shall provide much benefit in studying interfaces, which is important for analyzing condensation in pores.

An effective medium treatment of the transport properties of a Voronoi tesselated network

The present work stresses the significance of the effective medium theory in the computation of the macroscopic transport coefficients from the microgeometry of porous media. The porous ‘‘material’’ is simulated as a two‐dimensional network of interconnected slits of irregular shape and a random distribution using the Voronoi–Delaunay tesselation technique. The calculation procedure for the macroscopic transport coefficients is based on two concepts, the first one being the approximation of the microscopic field by a smooth field (SFA), and the second one being the average of the network random parameters to a mean/effective value by the effective medium theory (EMT). For the latter we apply an improved version of the EM equation derived for regular lattices by Kirkpatrick [Rev. Mod. Phys. 45, 4 (1973)]. This equation takes into account the irregularity in the slit‐length distribution and is applicable on both regular and irregular lattices. The EMT/SFA results of the improved version for ordinary diffusi...

Measurement of diffusion coefficient of water and ethanol in aqueous maltodextrin solution

Abstract Capillary cell and absorption-desorption methods were used to measure the diffusion coefficients of water and ethanol in aqueous maltodextrin solution over a wide range of water content. The diffusion coefficients showed a surprisingly large dependence on water content, particularly at low water content, as reported by other workers. Free volume theory was applied to correlate the diffusion coefficients. It was found that this theory was adequate to explain their large dependence on water content. Furthermore, experimental results on ethanol retention in drying an aqueous maltodextrin droplet could be predicted well, using the present experimental results for the diffusion coefficients of water and ethanol.

STRUCTURE FORMATION OF COATED FILMS WITH DISPERSED PIGMENTS DURING DRYING

ABSTRACT In the drying of coated films with dispersed pigments, such as floppy disks, the structure of the film is formed during the drying process and depends on the drying condition. It is important to understand the structure formation during drying for the design of the dryer and die better quality of the product. We measured die drying characteristics of the film and determined the structure of dried film experimentally. A qualitative model for the structure formation during drying of the coated film is suggested.

Transport properties of porous media from the microgeometry of a three-dimensional voronoi network

In this work a way of calculating effective transport coefficients from the microgeometry of a porous medium is presented. The model material consists of a random packing of uniform spheres, and by applying the Voronoi—Delaunay tessellation technique the void between the spheres is simulated as a network of cylindrical pores. The tessellation yields all the necessary information for the structural characterization, such as the pore diameter, pore angle and pore length distribution functions and the topological interconnection. The effective transport coefficients of ordinary diffusion, Knudsen flow and viscous flow are calculated numerically by mass balancing at each network node and over all nodes of the system. The results obtained agree very well with the experimental ones, especially for ordinary diffusion. For Knudsen and viscous flow, inaccuracies in the estimation of the pore overlapping volume cause a relative error between the numerical and experimental results of the order of 16%–33%.

ROLE OF SUCROSE-LDH HYDROGEN BOND FOR THERMAL STABILIZING EFFECT OF SUCROSE ON FREEZE-DRIED LDH

ABSTRACT Relationship between thermal stabilizing effect of sugar on freeze-dried protein and sugar-protein hydrogen bond is studied. Sucrose and lactate dehydrogenase (LDH) are used as models for sugar and protein. Samples of freeze-dried LDH involved in sucrose of different crystallinity were prepared, and measurement of X-ray diffractometry (XRD)/ and Fourier-Transform Infrared (/FT-IR)/ spectroscopy was done. It is found that when sucrose is amorphous, the degree of hydrogen bond formation is high and LDH is stabilized; when sucrose is crystalline, hydrogen bond is less formed and LDH is inactivated. These results indicate that the stabilizing effect of sugar is closely related to sugar-protein hydrogen bond. It is also found that there is an optimum sucrose content for the thermal stabilizing effect. This is because amorphous structure of sucrose is stabilized and protected from crystallization by LDH. Thus we car deduce that sugars and proteins work together to keep the activities of proteins.

Principle and Applications of Drying Characteristic Functions

ABSTRACT This article focuses on the concept of a ‘drying characteristic function,’ which is an effective way to correlate drying rate curves for convective drying of homogeneous nonporous, hygroscopic porous and non-hygroscopic porous materials. The characteristic function, obtained bv a certain transformation of a drying rate curve, is independent of drying conditions and hence characterizes the transport kinetics in the material. The principle and some applications of the functions are reviewed. The first application is estimation of dryinge rate curves. Because any drying rate curve can be transformed into the characteristic function and vice versa, the drying rate can be estimated for various drying conditions from a single drying experiment. Another aoolication is determination of the moisture diffusivity. Using the ‘flux ratio method’ an analytical expression of the characteristic function can be obtained for any aiven moisture diffusivity. The exprcssion enables one to determine the moisture diffu...

Approximate isothermal drying curves of hygroscopic porous materials with given desorption isotherm

Abstract The drying curves of hygroscopic porous materials can usually only be predicted by solving the mass transfer equation (isothermal case) numerically with the corresponding limit conditions because of the strong dependence of the apparent diffusion coefficient on the moisture content and the influence of the desorption isotherm on the drying rate. A simple approximate method which gives an analytical expression for the drying curve under isothermal conditions for both drying periods (penetration period and regular regime) for any given diffusion coefficient and desorption isotherm, both defined as a function of the moisture content, is presented here. In this method, the observation that the drying rate normalized by an appropriate steady-state flux hardly depends on the form of the diffusion coefficient is used.

Characterization of porous media by means of the Voronoi-Delaunay tessellation

Abstract The Voronoi-Delaunay tessellation technique for subdividing the space between entities is used in this paper in order to characterize a porous medium. The model material consists of a bundle of cylinders with uniform diameter, randomly arranged so that they are closest to each other. The resulting network consists of quadrilaterals of different width and height and infinite depth. The width is defined as the closest distance between two adjacent cylinders (narrowest neck). On the other hand, the height is defined as the length of a Voronoi polygons edge (distance between two nodes). The height vector lies perpendicular to the width vector. The coordination number, denoting the number of edges meeting at a node, was always found to be 3. The porosity of the packing was 0.47 and of the resulting network 0.41. The simulation consists of about 150 realizations of a model network, which, on average, accommodated about 89 cylinders, 180 nodes and 270 slits. Topologically, the slits do not show any preference for a specific orientation, so that in a large-scale network all possible orientations of the slits will be equally represented. The relation between the slit width and the slit height, of great importance for calculating dynamic properties, is very satisfactorily approximated by a linear expression. A way to predict the saturation-dependent transport coefficient of ordinary diffusion, Knudsen diffusion and viscous flow is proposed. The network showed a percolation threshold at a number based saturation of 0.67, the same as for the honeycomb regular lattice. The tortuosity factor was found to be 2.95.