Spherical Gel Research Articles

Preparation and Chiral Separation of a Novel Immobilized Cellulose-Based Chiral Stationary Phase in High-Performance Liquid Chromatography

The chiral selector 6-azido-2, 3-di(p-chlorophenylcarbamoylated) cellulose was synthesized and further chemically immobilized onto 5-μm amino functionalized spherical porous silica gel. It was used as chiral stationary phase in high-performance liquid chromatography. Thirty racemates were successfully separated into enantiomers in either normal phase mode or reversed-phase mode. Good reproducibility and stability of the chiral stationary phase have been demonstrated.

Preparation of pyridino-crown ether-based new chiral stationary phases and preliminary studies on their enantiomer separating ability for chiral protonated primary aralkylamines

This paper reports the preparation and testing of three new pyridino-18-crown-6 ether-based chiral stationary phases (S,S)-CSP-12, (S,S)-CSP-17 and (S,S)-CSP-20. Secondary amine (S,S)-7 was first transformed to triethoxysilyl derivative (S,S)-11, which contains a urea unit by treating the former with 3-(triethoxysilyl)propyl isocyanate. Next, (S,S)-11 was heated with spherical HPLC quality silica gel in toluene to obtain (S,S)-CSP-12. In order to acetylate the 3-aminopropylsilyl groups bonded to the silica gel during immobilization of the triethoxysilyl derivative (S,S)-11, we pumped acetic anhydride and triethylamine in DMF through the column to give the modified chiral stationary phase (S,S)-CSP-20.Triflate (S,S)-13 was first transformed to a pyridino-18-crown-6 ether derivative (S,S)-15, which contains a 4-(methoxycarbonyl)phenyl substituent at the 4-position of the pyridine ring by a Suzuki carbon–carbon coupling reaction. The hydrolysis of ester (S,S)-15 gave carboxylic acid (S,S)-21. Carboxylic acid (S,S)-21 was reacted with an excess of thionyl chloride to form the appropriate acyl chloride, which was treated with 3-(triethoxysilyl)propylamine in the presence of triethylamine in THF to furnish triethoxysilyl derivative (S,S)-16 containing an amide unit. Triethoxysilyl derivative (S,S)-16 was heated with spherical HPLC quality silica gel in toluene to give the chiral stationary phase (S,S)-CSP-17.The enantiomer separating ability of chiral stationary phases (S,S)-CSP-12, (S,S)-CSP-17 and (S,S)-CSP-20 were tested by using mixtures of enantiomers of 1-(1-naphthyl)ethylamine hydrogen perchlorate (1-NEA), 1-(2-naphthyl)ethylamine (2-NEA), 1-(4-bromophenyl)ethylamine (Br-PEA) and 1-(4-nitrophenyl)ethylamine hydrogen chloride (NO2-PEA). Chiral stationary phase (S,S)-CSP-17 showed the best enantiomer separating ability for the mixtures of enantiomers of amine compounds amongst the pyridino-crown ether-based CSPs ever synthesized. The high enantioselectivity is probably due to the strong π–π interaction of the extended π system of the aryl–substituted pyridine unit.

A microwave-based method for the synthesis of carbon xerogel spheres

Carbon xerogel spheres with millimeter-scale diameters were synthesized by a simple process using microwave radiation as the heating source. Using this type of heating it is possible to establish the gelation point of different resorcinol–formaldehyde solutions and stop the gelation step of the material at the exact time of gelation. Organic gel spheres can then be directly obtained by stirring in a silicone bath at 80°C. Finally, carbonization is performed to obtain carbon xerogels with a spherical shape. The size and porous texture of the spheres can be controlled by adjusting the synthesis conditions.

Drying dissipative structures of thermo-sensitive gel spheres of poly(N-isopropylacrylamide). Influence of degree of cross-linking of the gels

Drying dissipative patterns of de-ionized suspensions (colloidal crystal state above the critical concentrations of crystallization) of the thermo-sensitive gels of poly(N-isopropylacrylamide) with degrees of cross-linking of 10% and 2% (pNIPAm(200–10) and pNIPAm(200–2)) were observed at 20 °C and 45 °C on a cover glass, a watch glass, and a Petri glass dish. The broad rings were observed, and their size decreased as micro-gel concentration decreased. Formation of the monodispersed agglomerated particles and their ordered arrays were observed. Microscopic drying structures of (a) flickering ordered spoke-lines, (b) ordered rings, (c) net structure, and (d) lattice-like ordered structures of the agglomerated particles are observed. The net and lattice structures formed more favorably at high degrees of cross-linking, at high concentrations of the gels, and/or high temperatures. By the addition of sodium chloride, very large dendrite-like and net structures of the large agglomerated particles formed at 20 °C and 45 °C, respectively. Importance of the cooperated convectional flow of the agglomerated particles during the drying processes is supported for the ordered array formation. The role of the electrical double layers around the agglomerated particles and the interaction of the particles with the substrate surfaces during dryness are also important for the ordering. The microscopic drying patterns of gel spheres were different from those of linear type polymers and also from typical colloidal spheres, though the macroscopic patterns of gel system such as broad ring formation at the edges of the dried film were similar to other two systems.

ハイドロゲル微粒子の結晶化に見られる高分子性とコロイド性のバランス

ハイドロゲル微粒子の結晶化に見られる高分子性とコロイド性のバランス

Core–shell separation of a hydrogel in a large solution of proteins

Upon absorption of large oppositely charged electrolytes such as proteins, polyionic hydrogels are frequently observed to separate into dense shell–swollen core states. We have developed a theory that in a detailed way takes into account the inhomogeneous swelling and distribution of the protein within such a core–shell separated gel. With this we investigated whether the core–shell separation can be an equilibrium state or if it must be understood as a dynamical phenomenon. Restricting ourselves to spherical gels with an unlimited supply of protein, we found that as an intermediate between a swollen and a collapsed gel the core–shell state can indeed be the one of lowest free energy but this state is not stable. In such cases where formation of a shell could occur spontaneously there was no further thermodynamic barrier to complete collapse of the gel (but possibly dynamical ones). The core–shell separation was favourable in systems of high charge and low ionic strength and was explained, within our theory, by the fact that the energy gain in packing proteins and polyions closely together outweighs the entropy loss of the uneven distribution.

Surface pattern formation in a swelling gel

We study the time evolution of surface pattern formation in a swelling spherical gel using proton nuclear magnetic resonance microimaging of water molecules migrating into the gel. Relaxation-weighted images permit mapping of the local evolution of the swelling ratio which, within the framework of a nonlinear poroelastic theory, can be identified with the determinant of the deformation gradient tensor. The wavelength of the surface pattern is examined and some analogies with the extensively studied problem of the wavelength at threshold for swelling rings are pointed out. By controlling the degree of cross-linking within a surface layer of the gel it is possible to monitor the transition from an undulating folding pattern to the formation of singular creases.

Drying dissipative structures of thermo-sensitive gel spheres of poly(N-isopropylacrylamide) with low degree of cross-linking

Drying dissipative patterns of deionized suspensions (colloidal crystal state at high concentrations) of the thermo-sensitive gels of poly(N-isopropylacrylamide) with low degree of cross-linking of 0.5% (318 nm and 116 nm in the hydrodynamic diameter at 25 °C and 45 °C) were observed at 20 °C and 45 °C on a cover glass, a watch glass and a Petri glass dish. The broad rings were observed and their size decreased as micro-gel concentration decreased. Formation of the monodispersed agglomerated particles and their ordered arrays were observed. This work clarified the formation of the drying microscopic structures of (a) flickering ordered spoke-lines, (b) ordered rings, (c) net structure, and finally (d) lattice-like ordered structures of the agglomerated particles. The net and lattice structures formed more favorably at higher temperatures and/or higher degree of cross-linking of the gels. Importance of the convectional flow of the agglomerated particles during the drying processes is supported for the ordered array formation. The role of the electrical double layers around the agglomerated particles and the interaction of the particles with the substrate surfaces during dryness are also important for the ordering. The microscopic drying patterns of gel spheres were quite different from those of linear-type polymers and also from typical colloidal spheres, though the macroscopic patterns such as broad ring formation at the edges of the dried film were similar to each other.

Colloidal crystallization of thermo-sensitive gel spheres of poly (N-isopropyl acrylamide). Influence of gel size

Influence of the gel size on the morphology, phase diagram, and reflection spectroscopy of the colloidal crystals of thermo-sensitive gel spheres, poly (N-isopropylacrylamide) (pNIPAm), was discussed by adding the data of two gel samples of pNIPAm(400–5) and pNIPAm(600–5) of 412 nm (at 25 °C) and 220 nm (at 45 °C) and of 517 nm (at 20 °C) and 294 nm (at 45 °C), respectively. Colloidal single crystals formed, but not so large compared with the giant crystals of small pNIPAm gels reported previously. The suspensions even with ion-exchange resins were turbid and hard to observe the single crystals clearly with the naked eyes as gel size increased. The critical concentration of melting decreased sharply as the suspensions were deionized with coexistence of the mixtures of cation- and anion-exchange resins. The critical concentration increased as the gel size increased and/or dispersion temperature increased. Density of the gel spheres increased as their size increased. These results demonstrated that the colloidal crystallization takes place by the extended electrical double layers formed around the gel spheres in addition of the excluded volume effect of the gels. Contribution of the electrical double layers on the crystallization increased sharply as temperature increased and gel concentration decreased, respectively. The contribution also increased slightly as sphere size increased, when comparison was made at the same gel concentration in wt.%. The present work clarified that the colloidal interfaces, which are inevitable for the formation of the electrical double layers, are formed between the water phase and gel spheres, though the gel spheres contain a lot of water molecules at the inner sphere region.

Colloidal crystallization of thermo-sensitive gel spheres of poly (N-isopropyl acrylamide). Influence of degree of cross-linking of the gels

Morphology, phase diagram, and reflection spectroscopy of the colloidal crystals of thermo-sensitive gel spheres, poly (N-isopropylacrylamide) having degrees of cross-linking 10 and 2 mol.% (pNIPAm(200–10) and pNIPAm(200–2)) were studied. Giant colloidal single crystals formed at very low gel concentrations. Critical concentrations of melting increased as the degree of cross-linking decreased in the range from 10 to 0.5 mol.% and/or suspension temperature increased from 20 to 45 °C. The critical concentration decreased sharply as the suspensions were deionized with coexistence of the mixtures of cation- and anion-exchange resins. Density of a gel sphere (gel concentration in weight percent divided by that in volume percent) increased sharply as the degree of cross-linking and/or temperature increased. These results demonstrated that the colloidal crystallization takes place by the extended electrical double layers formed around the gel spheres in addition of the excluded-volume effect of the gels. Most of the researchers including the authors have believed that the crystallization of the gel spheres takes place by the excluded-volume effect. However, the present work clarified that the colloidal interfaces, which are inevitable for the formation of the electrical double layers, are formed firmly between the water phase and gel spheres, though the gel spheres contain a lot of water molecules in the sphere region.

Second-order elastic solutions for spherical gels subjected to spherically symmetric dilatation

The problem of a spherically symmetric dilatation in a spherical gel, important for understanding the mechanical behavior of hydrogels and biogels (e.g., drug-delivery vehicles, mineralized organic gels), is solved analytically using second-order elasticity theory. The solutions are developed via a perturbation method up to the cubic terms of the strains. Using elastic constants for a gel derived from an earlier paper ( Wu and Kirchner, 2010), the stresses, displacements and energy due to a dilatation in an unconstrained sphere and in a spherical shell bonded to a rigid core are calculated. The results have the advantage of being in analytical closed-form, and are consistent with previous numerical simulations. The nonlinear part of the stress, displacement and energy is shown to be significantly larger than the linear part, and they may have different signs.

Drug Delivery Using Stimuli-Responsive Polymer Gel Spheres

Polymeric gels that undergo deformation upon appropriate changes in pH or temperature have considerable promise as drug delivery vehicles. Uptake of drug macromolecules into swelling and nonswelling gel spheres and release of drug macromolecules from deswelling and nondeforming gel spheres into a target fluid are investigated here. A mathematical model for gel–solution composite, a composite of a distributed parameter system (gel spheres) and a lumped parameter system (surrounding solution), is developed. The polymer network displacement in swelling/deswelling gels is described by a stress diffusion coupling model. The analytical solution for network displacement is used to predict solvent intake by swelling gels, solvent efflux from deswelling gels, and changes in pressure, porosity, and effective drug diffusivity resulting from network displacement. These in turn influence drug uptake during and after gel swelling and drug release from gel during and after gel deswelling. Numerical results illustrate benefits of gel swelling for drug loading and merits of different modes of drug release. Comparisons are made, as concerns drug uptake and drug release, with gels not subject to deformation.

Preparation and characterization of spherical mesoporous ZrO2-Al2O3 composites with high thermal stability

Spherical mesoporous ZrO2-Al2O3 composites containing different zirconia content have been synthesized by an oil-column sol-gel method. A mixed alumina-zirconia hydrosol and hexamethylenetetramine solution were mixed together and added dropwise into a hot oil column. Due to the surface tension, spherical gel particles were formed in the oil column. The spherical gel particles were then aged and washed by deionized water and dried at 120 °C for 12 h and then calcined at 600 °C for 8 h, 960 °C for 8 h or 1200 °C for 12 h. X-ray diffraction and nitrogen adsorption-desorption measurements indicated that the presence of zirconia prevents the sintering of alumina and the obtained ZrO2-Al2O3 composites have much larger surface areas than pure alumina. Temperature-programmed desorption of ammonia results illustrated that the addition of zirconia leads to an increase in the number of strong acid sites and the total number of acid sites compared with pure alumina. Thus, the spherical mesoporous ZrO2-Al2O3 composites prepared in this way were shown to be suitable for high temperature catalytic processes as a catalyst support.

Colloidal crystallization of thermosensitive gel spheres of poly(N-isopropylacrylamide) with low degree of cross-linking

Morphology, phase diagram, and reflection spectroscopy of the colloidal crystals of thermosensitive gel spheres, poly(N-isopropylacrylamide) ((200–0.5), 318 and 116 nm in the hydrodynamic diameter at 25°C and 45°C, and 0.5% in the degree of cross-linking) were studied. Giant colloidal single crystals formed at very low gel concentrations. Densities of the gel spheres were 0.030 and 0.61 at 25°C and 45°C, respectively. Critical concentration of melting of gel spheres (0.8 wt.% without ion-exchange resins) decreased sharply to 0.015 wt.% at 25°C as the gel suspension was deionized exhaustively with coexistence of the mixtures of cation and anion exchange resins. These results demonstrate that the colloidal crystallization takes place by the extended electrical double layers formed around the gel spheres in addition of the excluded volume effect of the gels. Extent of the contribution of the electrical double layers on the crystallization increased sharply when the degree of cross-linking increased, the gel spheres shrank, and/or the density of the gel spheres increased.

Low-cost production of mesoporous carbon/carbon composite cryogels

A carbon/carbon (C/C) composite cryogel was prepared by dispersing cotton fibers (CF) in a resorcinol and formaldehyde (RF) sol, followed by solvent exchange with t-butanol, freeze drying and pyrolysis under inert atmosphere. The mass ratio of CF to RF was increased from 0% up to 25% to observe the mesopore structure change of C/C composites under a selected RF synthesis condition. The porous structures were characterized by nitrogen adsorption and mercury porosimetry, and their structure was observed by a scanning electron microscope (SEM). The mesopore diameter of the C/C composites increases slightly with the increase of CF content. The preparation can increase the mesopore volume and also reduce the production cost compared with carbon cryogels prepared without CF. The SEM images show that nanoporous gel spheres appeared after the freeze drying step. These spheres are converted to be nanoporous carbon spheres with surrounding nanocages, and CF are converted into carbon fibers at the same time by pyrolysis.

The incorporation and release of imipramine from alumina gel spheres [proceedings

The incorporation and release of imipramine from alumina gel spheres [proceedings

Drying dissipative structures of thermo-sensitive gel spheres of poly(N-isopropylacrylamide)

Drying dissipative patterns were observed at 25 °C, 33 °C, and 45 °C on a cover glass, a watch glass, and a Petri glass dish during the course of dryness of colloidal crystals of the thermo-sensitive gels of poly(N-isopropylacrylamide) (PNIPA). Two kinds of broad rings, i.e., transparent ring at the outside edge and the ring in the inner area from the edge, were observed. Sizes of the former were the same as those of the initial liquids irrespective of gel concentration, whereas sizes of the latter decreased as gel concentration decreased. These broad rings were composed mainly of the monomeric and the agglomerated gel particles, respectively. Formation of the monodispersed agglomerated particles and their ordered arrays in the inner area of the dried film were observed especially on a Petri glass dish and a watch glass. The important role of the electrical double layers formed around the agglomerated particles is supported for the ordering of the agglomerated particles. The essential differences in the drying patterns between PNIPA gel spheres and the typical colloidal particles did not appear.

Intraparticle Mass Transfer in Adsorption Heat Pumps: Limitations of the Linear Driving Force Approximation

Adsorption heat pumps and chillers (ADHPCs) can utilize solar or waste heat to provide space conditioning, process heating or cooling, or energy storage. In these devices, intraparticle diffusion is shown to present a significant mass transfer resistance compared with interparticle permeation. Therefore, accurate modeling of intraparticle adsorbate mass transfer is essential for the accurate prediction of overall ADHPC performance. The linear driving force (LDF) approximation is often used to model intraparticle mass transfer in place of more detailed equations because of its computational simplicity. This paper directly compares the adsorbate contents predicted using the LDF and Fickian diffusion (FD) equations for cylindrical and spherical geometries. These geometries are typical of adsorbents commonly used in adsorption refrigeration such as cylindrical activated carbon fibers (ACFs) and spherical silica gel particles. In addition to the conventional LDF approximation, an empirical LDF approximation proposed by El-Sharkawy et al. (2006, “A Study on the Kinetics of Ethanol-Activated Carbon Fiber: Theory and Experiments,” Int. J. Heat Mass Transfer, 49(17–18), pp. 3104–3110) for ACF-ethanol (cylindrical geometry) is compared with the FD solution. By analyzing the relative error of the LDF approximation compared with the FD solution for an isothermal step-change boundary condition, the conditions under which the LDF approximation agrees with the FD equation are evaluated. It is shown that for a given working pair, agreement between the LDF and FD equations is affected by diffusivity, particle radius, half-cycle time, initial adsorbate content, and equilibrium adsorbate content. A step change in surface adsorbate content for an isothermal particle is shown to be the boundary condition that yields the maximum LDF error, and therefore provides a conservative bound for the LDF error under nonisothermal conditions. The trends exhibited by the ACF-ethanol and silica gel-water working pairs are generalized through dimensionless time and dimensionless driving adsorbate content, and LDF error is mapped using these two variables. This map may be used to determine ranges of applicability of the LDF approximation in an ADHPC model.

Colloidal crystallization of thermo-sensitive gel spheres of poly (N-isopropyl acrylamide)

Morphology, phase diagram, and reflection spectroscopy of the colloidal crystals of thermo-sensitive gel spheres, poly (N-isopropylacrylamide) (224 nm in the hydrodynamic diameter at 25 °C) were studied. Giant colloidal single crystals formed at very low gel concentrations. Critical concentration of melting of gel spheres (0.8 wt.% without ion-exchange resins) decreased sharply to 0.01 wt.% as the gel suspension was deionized exhaustively with coexistence of the mixtures of cation- and anion-exchange resins and increased substantially as concentration of sodium chloride increased. These studies demonstrated that the colloidal crystallization takes place by the extended electrical double layers formed around the gel spheres in addition of the excluded-volume effect of the gels. Most of the researchers including the authors have believed that the crystallization of the gel spheres takes place by the excluded-volume effect, in other words, by the hard-sphere model, exclusively. However, the present work clarified that the colloidal interfaces, which are inevitable for the formation of the electrical double layers, are formed firmly between the water phase and gel spheres, though the gel spheres contain a lot of water molecules in the inner the sphere region.

XPCS study of dynamic correlation in polyurethane gel-carbonyl iron composite under magnetic field

An X-ray photon correlation spectroscopy (XPCS) study of composite-type material consisting of polyurethane gel and carbonyl iron micrometric spheres was performed under magnetic fields of 0, 300 and 600 mT. The onion-like spheres structure was destroyed during the composite processing. The following conclusions were obtained from the study: -The polyurethane matrix is preferred as a source for the observed dynamic effects. -Below 300mT the material dynamics in direction of the outer magnetic field are very clear. -For 600 mT the dependence of the dynamics on magnetic field direction disappears, but the correlation rate is much higher. These findings may be caused by a disturbance of the polymer mesostructure by larger strain leading to its cross-linking.