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.     

Revert and direct type Liesegang phenomenon of silver iodide

The experimental conditions for Liesegang phenomenon of silver iodide in agar gel columns, and particularly the transition from revert to direct type of periodic precipitations, have been investigated. The theoretically derived Mathur's spacing law, which suits both revert and direct type of Liesegang's rings, has been experimentally verified. An attempt has been made to explain the mechanism of revert and direct type and of the transition of revert to direct type of periodic precipitation on the basis of adsorption on the precipitated silver iodide and of flocculation. The dependence of the transition point on the concentrations of the outer and inner electrolytes has been studied. The time law of Morse and Pierce has been verified. The dependence of the velocity constant ( ) on the concentrations of the outer and inner electrolytes has been studied.     

A model of the formation of liesegang rings under stimulated precipitation conditions

A mathematical apparatus for describing the kinetics of chemical reactions accompanied by phase transitions is suggested. The phenomenon of stimulated precipitation is singled out. Together with spontaneous precipitation-solution, it allows periodic sedimentation (Liesegang rings) to be correctly described and the concept of solubility product to be corrected and generalized.     

Secondary structure of periodic precipitation patterns in gelatin

The primary and secondary Liesegang rings in the silver dichromate-gelatin system were studied microscopically and microdensitometrically. The close mutual connection of both structures is evident from these measurements. Examination of the temporal sequence of ring development showed that the formation of fine secondary rings precedes the growth of primary precipitation patterns. The experimental results confirm the view that the formation of both primary and secondary rings has a common basis and pattern.     

Liesegang图案

介绍了Liesegang图案的生长规律、实验研究方法、形成影响因素、理论进展、应用及存在的问题,同时对当前两个活跃的研究方向即复杂Liesegang环和Liesegang图案的微观形成机制进行了讨论.该领域研究的新突破有助于理解自然界中一些复杂图案的形成和控制材料制备中的自组装结构及动物代谢中的结石生长.     

Design of equidistant and revert type precipitation patterns in reaction-diffusion systems

In the past years considerable attention has been devoted to designing and controlling patterns at the microscale using bottom-up self-assembling techniques. The precipitation process proved itself to be a good candidate for building complex structures. Therefore, the techniques and ideas to control the precipitation processes in space and in time play an important role. We present here a simple and technologically applicable technique to produce arbitrarily shaped precipitation (Liesegang) patterns. The precipitation process is modelled using a sol coagulation model, in which the precipitation occurs if the concentration of the intermediate species (sol) produced from the initially separated reactants (inner and outer electrolytes) reaches the coagulation threshold. Spatial and/or temporal variation of this threshold can result in equidistant and revert (inverse) type patterns in contrast to regular precipitation patterns, where during the pattern formation a constant coagulation threshold is supposed and applied in the simulations. In real systems, this threshold value may be controlled by parameters which directly affect it (e.g. temperature, light intensity or ionic strength).     

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.     

Elastic light scattering from nanoparticles by monochromatic vacuum-ultraviolet radiation

Elastic light scattering is reported using monochromatic vacuum-ultraviolet radiation to study free, spherical silica nanoparticles prepared by approaches from colloidal chemistry, with diameters between 100 and 240 nm. The colloidal nanoparticles of defined size are transferred from an aqueous solution into the gas phase using a particle beam experiment. After focusing of the particle beam by an aerodynamic lens, the scattered light from monochromatic synchrotron radiation is measured. Angle-resolved elastically scattered light is detected, showing a strong forward-scattering component. Additional evidence for the detection of elastically scattered light comes from plotting the scattered light intensity as a function of the dimensionless parameter qR, where q is the magnitude of the scattering wave vector and R is the particle radius. This yields different power-law regimes that are assigned to scattering from the surface and the bulk of the nanoparticles. Furthermore, there is evidence for modulations in the scattered light intensity as a function of scattering angle, which is clearly distinguished from the forward-scattering component. The experimental results are compared to Mie scattering simulations for isolated particles, yielding general agreement with the experimental results. Deviations from Mie simulations are observed for samples consisting of significant amounts of aggregates. The present results indicate that the optical properties of free nanoparticles are sensitively probed by vacuum-ultraviolet radiation.     

Periodic precipitation of lanthanum molybdate crystals in silica gel

Formation of Liesegang rings on the growth of La₂(MoO₄)₃·XH₂O in silica gel by single diffusion technique is discussed. The nature of the periodic precipitation is found to depend on the concentration of inner and outer electrolytes, gel medium and its pH. The ‘time law’, ‘spacing law’ and ‘the law relating diffusion depth and width’ are found obeyed in this system. The one-dimensional Liesegang phenomena is thus investigated quantitatively making use of the simple laws of diffusion.     

Probing the micellization kinetics of pyrene end-labeled diblock copolymer via a combination of stopped-flow light-scattering and fluorescence techniques

A pyrene end-labeled double hydrophilic diblock copolymer, poly(2-(diethylamino)ethyl methacrylate)-b-poly(2-(dimethylamino)ethyl methacrylate) (Py-PDEA-b-PDMA), was synthesized by sequential monomer addition via oxyanionic polymerization using a 1-pyrenemethanol-based initiator. This diblock copolymer exhibits reversible pH-responsive micellization behavior in aqueous solution, forming PDEA-core micelles stabilized by the soluble PDMA block at neutral or alkaline pH. Taking advantage of the pyrene probe covalently attached to the end of the PDEA block, the pH-induced micellization kinetics of Py-PDEA-b-PDMA was monitored by stopped-flow light scattering using a fluorescence detector. Upon a pH jump from 4.0 to 9.0, both the scattered light intensity and excimer/monomer fluorescence intensity ratios (IE/IM) increase abruptly initially, followed by a more gradual increase to reach plateau values. Interestingly, the IE/IM ratio increases abruptly within the first 10 ms: a triple exponential function is needed to fit the corresponding dynamic trace, leading to three characteristic relaxation time constants (tau(1,fluo) < tau(2,fluo) < tau(3,fluo)). On the other hand, dynamic traces for the scattered light intensity can be well-fitted by double exponential functions: the resulting time constants tau(1,scat) and tau(2,scat) can be ascribed to formation of the quasi-equilibrium micelles and relaxation into their final equilibrium state, respectively. Most importantly, tau(1,scat) obtained from stopped-flow light scattering is in general agreement with tau(2,fluo) obtained from stopped-flow fluorescence. The fastest process (tau(1,fluo) approximately 4 ms) detected by stopped-flow fluorescence is ascribed to the burst formation of small transient micelles comprising only a few chains, which are too small to be detected by conventional light scattering. These nascent micelles undergo rapid fusion and grow into quasi-equilibrium micelles and then slowly approach their final equilibrium state. The latter two processes can be detected by both techniques.     

Aided self-assembly of porphyrin nanoaggregates into ring-shaped architectures

The formation of micrometer-sized, highly ordered porphyrin rings on surfaces has been investigated. The porphyrin-based nanoarchitectures are formed by deposition from evaporating solutions through a surface dewetting process which can be tuned by variations in the substitution pattern of the molecules used, the coating of the surface and the conditions under which the evaporation takes place. Control over the combined self-assembly and surface dewetting results in nanorings possessing a defined internal architecture. The ordering of the molecules within the rings has been studied by a variety of microscopy techniques (TEM, AFM, fluorescence microscopy) and the exact ordering of the porphyrins within the rings has been quantified.     

存在外加浓度梯度时的沉淀斑图形成

本文主要讨论了内电解质非均匀分布情况下的沉淀图案形成。通过施加反应物的不同浓度梯度,可以观察到几类新的沉淀斑图结构,如分叉状,苞芽状斑图,以及有断环间隔的Liesegang环。分析表明,尽管外加了浓度梯度,但第n个环形成的位置与形成的时间还是满足普通Liesegang环的简单定律。所不同的是,反应体系中一旦有断环形成,相邻沉淀条纹间距比值X_n+1/X_n就会偏离原来的线性关系。     

Rings of nanoparticle-decorated honeycomb-structured polymeric film: the combination of pickering emulsions and capillary flow in the breath figures method

The self-assembly of nanoparticles at the fluid/fluid interface (Pickering emulsions) in the breath figures (BF) method have been explored to direct nanoparticles onto BF microarrays and adjust the BF assembly in microsize. Circular rings of nanoparticle-decorated honeycomb-structured polymeric film can be obtained by a one-step process. The combination of Pickering emulsions and capillary flow in the BF method may be responsible for the formation of this intriguing structure.     

Opto‐electrochemical In Situ Monitoring of the Cathodic Formation of Single Cobalt Nanoparticles

Single‐particle electrochemistry at a nanoelectrode is explored by dark‐field optical microscopy. The analysis of the scattered light allows in situ dynamic monitoring of the electrodeposition of single cobalt nanoparticles down to a radius of 65 nm. Larger sub‐micrometer particles are directly sized optically by super‐localization of the edges and the scattered light contains complementary information concerning the particle redox chemistry. This opto‐electrochemical approach is used to derive mechanistic insights about electrocatalysis that are not accessible from single‐particle electrochemistry.     

Study of the structure of solutions of the H-complexes of diesters of tetracarboxylic acids with diamines by the light scattering method 1. H-complex of the dimethyl ester of benzophenonetetracarboxylic acid with hexamethylenediamine

Conclusions about structure formation in solutions of the H-complexes of benzo-phenonetetracarboxylic acid with hexamethylenediamine have been drawn from the angular dependence of the intensity of scattered polarized light. The calculated statistical structural parameters show that the H-complexes form dipentameric associations. At high concentrations of the complexes in aqueous-methanolic solution the supermolecular structure is disrupted, while in the pure alcohol the reverse process is observed.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1338–1343, June, 1991.     

Polymeric Nanoparticles Stabilized by Surfactants Investigated by Light Scattering,Small-Angle Neutron Scattering,and Cryo-TEM Methods

We have investigated self-organization of polymers with surfactants through solvent shifting process resulting in formation of stable and uniform nanoparticles. We studied polymeric nanoparticles made of poly(methylmethacrylate) and of polystyrene dispersed in water. The dispersion was prepared by a fast mixing of a solution of the polymers with a solution of several ionic and nonionic surfactants in pure water. We observed the formation of well defined nanoparticles by light scattering, small-angle neutron scattering (SANS), and cryogenic transmission electron microscopy (Cryo-TEM) methods. The study shows how nanoparticle properties are changed by the chemical composition of surfactants, molar mass of polymers, concentrations of both components and finally, by variations in method of nanoparticles preparation. Dynamic light scattering (DLS) and static light scattering (SLS) provide the hydrodynamic radii and radii of gyration for selected types of nanoparticles. Cryo-TEM experiments prove that the nanoparticles have good spherical shape. Analysis of SANS data and Cryo-TEM micrographs suggest that the prepared particles are composed of polymer and surfactant that are evenly distributed.     

Dynamic and static light scattering by weakly ionized gels and solutions

Dynamic light scattering (DLS) and static light scattering (SLS) experiments have been performed on partially neutralized poly(acrylic acid) and poly(methacrylic acid) solutions and gels. The gels exhibit a non-ergodic behavior, much less marked however than that observed in neutral systems. By combining DLS and SLS, the fluctuating part of the light scattered from PAA gel was separated from the total scattered intensity and found to be almost equal to the intensity scattered by the solution. Also the diffusion coefficient associated with the dynamic fluctuation was found to be the same in the PAA gel and the PAA solution.     

Kinetics of nucleation and growth of metal nanoparticles in the presence of surfactants

Numerical simulation has been performed for the process of nucleation and growth of nanoparticles in the presence of surfactants. Surfactant molecules are adsorbed on the surface of nanoparticles and decelerate their growth in supersaturated solutions. It has been assumed that nanoparticles are completely blocked after a certain degree of surface coverage is achieved, and they cease to grow. It has been demonstrated that, at low concentrations, surfactants influence the average size and the width of the size distribution of nanoparticles; i.e., the average particle size decreases and the distribution becomes narrower with the growth of surfactant concentration. At high concentrations, the effect of surfactants is more dramatic; namely, the particle size distribution becomes bimodal. At high surfactant concentrations, the periodic supply of a precursor, which serves as a source of monomers (metal atoms), may result in the formation of polymodal ensembles of the nanoparticles.     

Formation of calcium phosphate having a hierarchically laminated architecture through periodic precipitation in organic gel

Hierarchically laminated calcium phosphate was produced through Liesegang periodic precipitation in a gel matrix of poly(acrylic acid) containing phosphate anions by diffusion of calcium cations.     

The influence of electrolyte concentration on the adsorption of octadecanol on Au(111)

The influence of electrolyte concentration on the potential dependent adsorption and desorption of octadecanol to/from a Au(111) electrode was investigated utilizing electrochemical and elastically scattered light techniques. The electrolyte concentration was found to influence the potential driven changes of the adsorbed layer (adsorption and desorption). The capacitive changes in the adsorbed layer were found to occur at more negative potentials with lower electrolyte concentration. The changes in the optical measurement, used to measure the characteristics of the desorbed species, or aggregates, were also found to be affected similarly. The magnitude of the overall change in the scattered light intensity was slightly dependent on electrolyte concentration. The re-adsorption of the aggregates was influenced by electrolyte concentration. The scattered light signal for an intermediate adsorbed state (adsorbed aggregate) was more prevalent for higher electrolyte concentration, suggesting that these intermediates were somewhat different compared to lower electrolyte concentrations. The lower electrolyte concentration displayed a larger potential region where this intermediate was stable, but the intensity of the scattered light was much lower. The electrolyte concentration most strongly influenced the potentials of adsorption and desorption, as well as the potential region of stability for the adsorbed intermediates. The sweep rate also has an influence on the scattering characteristics of the desorbed species, suggesting a possible method for measuring the kinetics of the adsorption–desorption process or for controlling the character of the desorbed species. These changes were explained in terms of a mechanism for the wetting or de-wetting of a surface. The influence of electrolyte concentration provides another opportunity for investigating the dynamics of this adsorption–desorption process.