Estimation of diffusion coefficient of Ba++ ions in silica gel from periodic precipitation of BaMoO4

Formation of Liesegang rings on the growth of BaMoO₄ crystals in silica gel by single diffusion is discussed. The rhythmic precipitation is found to be influenced profoundly by (a) the molarity of inner and outerelectrolytes (b) the age of gel and (c) the pH of the gel medium. The time law, spacing law and the law relating diffusion depth and width are verified. A method to estimate the diffusion coefficient of the outer electrolyte in the gel medium is developed and calculated for Ba⁺⁺ions.     

Three dimension Liesegang rings of calcium hydrophosphate in gelatin

Three dimensional Liesegang spherical layers of CaHPO₄ in gelatin ball were performed by employing CaCl₂ and Na₂HPO₄ as the inner and outer electrolyte, respectively. Effects of concentrations of inner and outer electrolyte as well as pH on the morphologies of Liesegang rings (LRs) were investigated. As a result, it was observed that the time law, spacing law and width law found in 1D/2D gel systems were obeyed in this 3D gelatin system. The interaction of Ca²⁺ and HPO₄ ^2? with gelatin matrix played a key role to the formation of LRs due to the existence of carboxylic groups on the gelatin chains. Using Ca²⁺ as the inner electrolyte, LRs were prepared. However, employing HPO₄ ^2? as inner electrolyte, LRs were not obtained. Moreover, pH of gelatin solution greatly impacted on the formation of LRs. The number of LRs increased with the decrease of pH, whereas the width inversely decreased. pH 4.40 was a turn point, from which the spacing coefficient abruptly increased as pH increased. All these results indicated that the network was created by the interaction of Ca²⁺ and –COO^? of gelatin chains, which dominated the formation of CaHPO₄ LRs in gelatin.     

Liesegang patterns: Estimation of diffusion coefficient and a plausible justification for colloid explanation

The Liesegang phenomenon is interpreted as a moving boundary problem. The existing time law, spacing law, and width law are revisited and reformulated on the basis of the new scenario. A better understanding of the basic facts associated with pattern formation is made possible with the new concept. The phenomenon is explained on the basis of colloidal kinetics. Diffusion coefficients of the outer electrolyte in the gel for various experiments in the literature are calculated. The diffusion coefficients so calculated range from 1.792-5.996᎒^-10 m² s^-1. It is suggested that such values confirm the fast colloidal dynamics during the process of periodic precipitation pattern formation.     

Hierarchical Tubular Structures Grown from the Gel/Liquid Interface

Three dimensional hierarchical materials are widespread in nature but are difficult to synthesize by using self‐assembly/organization. Here, we employ a gel–liquid interface to obtain centimeter‐long ～100 μm diameter tubes with complex mineral wall structures that grow from the interface into solution. The gel, made from gelatin, is loaded with metal chloride salt, whereas the solution is a high pH anion source. Tubes were obtained with a range of cations (Ca²⁺, Sr²⁺, Ba²⁺, Cu²⁺, and Zn²⁺) and anions (CO₃^2? and PO₄^3?). The crystalline phases found in the tube walls corresponded to expectations from solution chemistries and phase solubilities. The growth mechanism is found to be akin to that of chemical gardens. The divalent cations modify the strength of the gelatin gel in a manner that involves not only simple electrostatic screening, but also ion‐specific effects. Thus, tubes were not obtained for those ions and/or concentrations that significantly changed the gel’s mechanical structure. At high Cu²⁺ loading, for example, vertical convection bands, not Liesegang bands, were observed in the gels.     

Crystal growth in syndiotactic poly(vinyl alcohol) hydrogels

Aqueous solutions of syndiotacticity-rich poly(vinyl alcohol) (s-PVA) form gels easily. The optimum condition of growth of the calcium tartrate crystal formed by diffusing calcium chloride into hydrogels containing tartaric acid was studied with use ofs- PVA of a syndiotacticity of 56 % and a degree of polymerization of 1460. The crystal grew in the gel of the concentrations of 2 % s-PVA and of 0.5 N tartaric acid at pH=4. The relation between the formation of Liesegang rings and shear modulus of a gel was studied by diffusing silver nitrate into gels containing potassium chromate. The distance between rings decreased with increasing shear modulus of a gel in the range from 670 to 7500 dyne/cm². The Liesegang rings were not formed for the shear modulus gel for 280 and 16200 dyne/cm₂.     

Justification for the colloid explanation of Liesegang ring formation

 The persuasive evidence for the role of colloid in the formation of Liesegang rings is nullified by the low diffusion constants (less than 2 × 10⁻¹¹ m² s⁻¹) of sol particles; however, those values were obtained for sols suspended in otherwise homogeneous solutions. The essential randomness of Brownian motion in such situations is absent in Liesegang experiments, where the large excess of outer electrolyte diffusing into the gel creates a bias in molecular bombardment resulting in sol particles moving a given distance in fewer steps, hence in a shorter time. From Einstein's equation (D=x ²/2t) values for D of 2–4 × 10⁻¹⁰ m² s⁻¹ have been calculated for Liesegang experiments in the literature. It is suggested that such values could well pertain to sol particles diffusing in the heterogeneous conditions existing in those experiments. Received: 13 April 1999 Accepted: 16 November 1999     

Chemical pattern formation in gels

Precipitate forming chemical reactions have been studied in chemically crosslinked poly(vinyl alcohol) gel medium. One of the reactive components was incorporated into the gel, the other was allowed to diffuse into the swollen network. This reaction-diffusion process often results in pattern formation which can occur intermittently in terms of time and space or both. Experiments were carried out in order to investigate the effect of various factors (crosslinking density, swelling degree as well as the concentrations of the outer and inner electrolyte). The following precipitates were intensively studied: Mg(OH)₂, Cr(OH)₃, PbCl₂. Three basically different morphologies (Liesegang band formation, moving bands and tree-like structure) have been observed and investigated. One- and twodimensional computer simulations were carried out in order to establish the influence of various factors on the precipitation.     

TLC Separation of some closely Related Synthetic dyes Impregnated Silica Gel Layers

Abstract

Ammonium molybdate and Copper sulphate were found as good impregnants for improving the separation of some closely related thirty synthetic dyes on silica gal ‘G’ layers using the solvent system BuOH-AcOH-H₂O (25:5:10). Tables are presented to illustrate the comparison in resolution of dyes on plain and impregnated silica gel layers.     

Water vapor molecule adsorption onto 'Ajwa' dates: Analytical investigation via infinite multilayer statistical physics model

A gravimetric technique was used to examine the water activity onto the ‘Ajwa’ dates. The equilibrium adsorption isotherms of water molecules were carried out at three temperatures (between 303 K and 323 K). A theoretical method was developed using statistical physics treatment to describe the experimental data at the ionic scale. The date’s isotherms were analyzed via the infinite multilayer adsorption model (formation of a high number of adsorbed layers) which is established based on the ideal gas law (there are no lateral interactions influences on the adsorption mechanism). The chosen model gave significant interpretation of the adsorption of water on the Ajwa dates based on the physicochemical model’s parameters (the density of binding sites (D_m), the number of water molecules per site (n) and the energetic parameters (a₁) and (a₂)). The physicochemical interpretation of the appropriate model indicated that the adsorption of water on the Ajwa dates occurred via a multi-anchorage process since the n values are lower than 1 for the three tested temperatures. The Ajwa dates adsorption was found typical to an exothermic process by the intermediate of the steric parameter D_m (D_m (303 K) = 0.58 kg/kg? D_m (323 K) = 0.33 kg/kg). Moreover, the energies values |?ε₁| and |?ε₂|, which varied from 27.8 KJ/mol to 51.2 KJ/mol, confirmed that the ‘Ajwa’ dates adsorption was a chemical process presenting covalent bonds between the water molecules and the dates’ sites.     

Influence of polydispersity and amine–epoxide ratio on molecular mobility in epoxy networks

Using nuclear magnetic resonance (NMR) T₂ relaxation and pulsed-gradient spin-echo diffusion experiments at 175.5°C, molecular motions of the sole and gel of several epoxies of the type diglycidyl ether of bisphenol-A (DGEBA; Shell Epon 1007F and 1009F) cured with 4,4′-diaminodiphenyl sulfone (DDS) have been studies as a function of curative content. It was found that the fraction of protons associated with the shorter T₂ component cannot be identified as the gel fraction until the substantial bimodal polymer polydispersity is accounted for in the spin relaxation model. The gel fraction and both relaxation rates have maxima near curing stoichiometry, and fall off more rapidly on the curative-poor side. The diffusion spectrum of the sol fraction was consistent with a light species (Epon 828 remnants) plus a polydisperse (M?_w/M?_n ?2) heavier species, in agreement with resin and sol gel permeation chromatography (GPC) results. Numerical simulations also show that polymer polydispersity is likely to affect the interpretation of T₂ relaxation found in the literature.     

Determination of Activation Energy for the Diffusion of Fe<Superscript>3+</Superscript> Ions in Agar Gel Medium Containing Various Transition Metal Sulfates

The study of the diffusion of ferric ions in agar gel containing transition metal sulfates was carried out. The effect of gel concentration, electrolyte concentration and temperature on the diffusion of Fe³⁺ ions in various transition metal sulfates was studied with a view to verify Wang’s model of diffusion and the applicability of transition state theory to diffusion in a gel medium. The diffusion coefficients were measured using the zone diffusion technique. For a given concentration of electrolyte the activation energy (E) is found to decrease with an increase in the charge density of the cation of the supporting electrolyte and for a given system it is found to decrease with increasing electrolyte concentration in agreement with Wang’s model. This observation is explained on the basis of the distortion in the water structure caused by ions and agar molecules. At a given electrolyte concentration the magnitude of the Arrhenius parameters, E and D _o, is found to decrease with increasing gel concentration in agreement with the transition state theory of diffusion.     

Phenolic Derivatives from Fruits of Dipteryx lacunifera Ducke and Evaluation of Their Antiradical Activities

The fruits of Dipteryx lacunifera, known as ‘fava de morcego’ and ‘garampara’, comprise pleasant tasting kernels that contain high amounts of fatty acids (mainly oleic acid) and are commonly consumed by inhabitants of the northeast of Brazil. In the present study, the crude EtOH extract of the fruit kernels was separated into hexane‐, Et₂O‐, AcOEt‐, and H₂O‐soluble fractions. The Et₂O fraction was found to exhibit the highest 1,1‐diphenyl‐2‐picrylhydrazyl (DPPH) radical‐scavenging activity in vitro, and was subjected to further fractionation. Column chromatography over silica gel and Sephadex LH‐20, followed by preparative HPLC‐C₁₈, afforded (?)‐eriodictyol ( 1 ), (?)‐butin ( 2 ), luteolin ( 3 ), 3′,4′,7‐trihydroxyflavone ( 4 ), butein ( 5 ), and sulfuretin ( 6 ). The antiradical activities of compounds 1, 2, 4 , and 6 , together with the positive controls rutin, butylated hydroxy toluene (BHT), and tert‐butylhydroquinone (TBHQ), were evaluated with the DPPH assay and were found to decrease in the order rutin> 4 > 1 > 6 > 2 >TBHQ>BHT.     

Determination of effective diffusion coefficients in calcium alginate gel plates with varying yeast cell content

Effective diffusion coefficients (D_e) have been determined for lactose, glucose, galactose, and ethanol in calcium alginate gel with varying yeast cell concentration. The measurements have been performed in a diffusion cell, and the results evaluated with the quasisteady-state method. An ultrasonic meter was used for gel thickness determination with an accuracy of 1.5% and a new method for the reproducible preparation of gel plates was developed. It was found thatD _e in pure alginate gel decreased to about 90% of the diffusivity in water and did not vary with alginate concentration.D _e decreased considerably with increasing yeast cell concentration. For the solutes studied, the effective diffusion coefficient can be estimated according to the equationD _e =D _eo (1 - ?)/[1+(?/2)], whereD _eo is the effective diffusivity in pure gel and ? is the volume fraction of yeast cells.     

Synthesis and characterization of novel organonickel–organosilicon alternating copolymers

The synthesis and characterization of five novel organonickel-organosilicon alternating copolymers having the repeating unit ? C₆F₄? Ni‘PBu₃’₂? C₆F₄? SiR₂? [where SiR₂ = ? SiMe₂? , ? SiMe‘Hex’? , ? SiPh₂? , ? SiMe₂? O? SiMe₂? , and ? SiMe₂? ‘CH₂’₆? SiMe₂? ] are reported. The model compounds Ni‘PR₃’₂‘1,4-C₆F₄SiMe₃’₂‘PR₃ = PMePh₂ or PBu₃’ were prepared via reactions of Ni‘PR₃’₂‘1,4-C₆F₄Li’₂ with 2 equiv of SiMe₃Cl, and were characterized by conventional analytical and spectroscopic measurements. The Polymers were prepared from the reactions of Ni‘PBu₃’₂‘1,4-C₆F₄Li’₂ with 1 equivalent of SiMe₂Cl₂ ‘polymer 1 , M _w = 15,800’, SiMe ‘Hex’ Cl₂ ‘polymer 2 , M _w = 7300’, SiPh₂Cl₂ ‘polymer 3 , M _w = 8600’, O‘SiMe₂Cl’₂ ‘polymer 4 , M _w = 13,900’ and ‘CH₂’₆‘SiMe₂Cl’₂ ‘polymer 5 , M _w = 19,700’. The molecular weights for each polymer were fully determined by both GPC and VPO. The multinuclear ‘¹H-, ¹⁹F-, and ³¹P [¹H]’-NMR, FTIR, and UV-Visible spectroscopic data for each polymer unambiguously establishes its repeating unit structure. The observations indicate that introduction of the silyl or siloxane units into the organonickel backbones has remarkably decreased the chain rigidity of the organonickel-organosilicon polymers compared to their rigid rod organonickel analogues ‘i.e., ? [? C₆F₄? Ni‘PR₃’₂? ]_n? ’. © 1994 John Wiley & Sons, Inc.     

Self‐diffusion in a hyaluronic acid–albumin–water system as studied by NMR

Hyaluronic acid (HA) is an anionic biopolymer that is present in many tissues and can be involved in cancerous neoformations. HA can form complexes with proteins (particularly, serum albumin) in the body. However, HA structures and processes involving HA have not been extensively studied by NMR because the molecule's rigid structure makes these studies problematic. In the current work, self‐diffusion of HA and bovine serum albumin (BSA), and water in solutions was measured by ¹H pulsed field gradient NMR (PFG NMR) with a focus on the HA‐BSA‐D₂O systems at various concentrations of BSA and HA. It was shown that in the presence of even a small amount of HA, the self‐diffusion coefficient (SDC) of BSA decreases. To explain this fact, three hypotheses were proposed and analyzed. The first one was based on the effect of slowing down of water mobility in the presence of HA. The second hypothesis suggested an effect of mechanical collisions of BSA with HA molecules. The third hypothesized that BSA and HA molecules form a complex where BSA molecules reduced in mobility. It was shown that the third mechanism is the most likely. The state of the BSA molecules in the BSA‐HA‐D₂O system corresponds to a ‘fast exchange’ condition from the NMR point of view: BSA molecules reside in the ‘free’ and ‘bound’ (with HA) states for much shorter time than the diffusion time of the PFG NMR experiment, 7 ms. The fractions of ‘bound’ BSA molecules in the BSA‐HA complex were estimated. Copyright © 2012 John Wiley & Sons, Ltd.     

Influence of processing parameters on regeneration kinetics and morphology of porous cellulose from cellulose–NaOH–water solutions

The kinetics of cellulose regeneration in acetic acid bath from cellulose–8% NaOH–water solutions and gels is studied as a function of gelation conditions, acid concentration and bath temperature. The diffusion coefficient of NaOH from cellulose solution or gel into regenerating bath was calculated. It does not depend either on gelation mode or on acid concentration. On the contrary, cellulose regeneration from non-gelled solutions is slower than from a gel. The increase in bath temperature induces diffusion coefficient increase obeying Arrhenius law. Scanning electron microscopy images of regenerated swollen-in-water freeze-dried cellulose and of the same samples dried in supercritical CO₂ show highly porous morphology. CEMEF is a Member of the European Polysaccharide Network of Excellence (EPNOE), .     

Cosynthesis, coexistence, and self-organization of alpha- and beta-cobalt hydroxide based on diffusion and reaction in organic gels

We report the cosynthesis of highly stable laminated single crystal alpha- and beta-Co(OH) 2 using the reaction and diffusion of a hydroxide solution into a gel containing Co(II). The obtained alpha-Co(OH) 2, which is known to be thermodynamically unstable and transforms in a short period of time to the beta form, has been stabilized in the gel medium for weeks. The system also exhibits Liesegang banding where complicated spatial dynamics during the formation of the two polymorphs are shown to take place.     

CO2/N2-switchable viscoelastic fluids based on a pseudogemini surfactant system

We prepared a CO₂/N₂-switchable pseudogemini surfactant system composed of sodium oleate (NaOA) and N, N, N’, N’-tetramethyl-1, 6-hexanediamine (TMHDA) at a mole ratio of 2:1. The two tertiary amine groups of the TMHDA can be protonated into quaternary ammonium salt when the system was bubbled with CO₂, which can ‘‘bridge’’ two NaOA molecules via electrostatic attraction to form a pseudogemini surfactant. The formed pseudogemini surfactant can further self-assemble to wormlike micelles, causing a sharp increase in viscosity. The viscoelastic property and structure transitions of the pseudogemini surfactant system were investigated before and after bubbling of CO₂. The pseudogemini surfactant system transformed from water-like to gel-like fluid with the bubbling of CO₂, followed by white precipitate. The cryo-transmission electron microscope (cryo-TEM) characterization and rheological measurements exhibited that the sol–gel transition was attributed to a spherical-wormlike micelle transition. Moreover, this transition was switchable at least in three cycles. Finally, a reasonable mechanism of aggregate behavior transition was proposed from the viewpoint of the molecular states, micelle structures, and intermolecular interactions.     

Kinetics of the formation of Liesegang rings

The kinetics of the formation of Liesegang rings is considered and they are classified. The relationship between the position of the propagating diffusion front and the moments of the formation of Liesegang rings is shown. In order to describe the formation processes of the Liesegang rings a generalized model based on the diffusion equation hierarchy is proposed. The kinetics of changes in the dispersed phase sizes at the initial stage of the formation of Liesegang rings is studied.