Front propagation in patterned precipitation. 3. Composition variations in two-precipitate stratum dynamics

This paper is a study of the composition dynamics of Liesegang band strata of Co(OH)2 and Ni(OH)2 from NH4OH, with redissolution by complex formation with ammonia. At a fixed time, the cobalt hydroxide composition was found to exhibit a random variation with band number, yet within a general overall decrease. The decrease with band number becomes more pronounced as the initial concentrations of Co2+ and Ni2+ get closer to each other. At equal concentrations, periodic oscillations in Co(OH)2 composition appear over consecutive bands. The time evolution of the total Co(OH)2 mass percent (over the entire pattern of strata) passes through a maximum. The dynamics of this complex system has been simulated by theoretical calculations using the model of Müller and Polezhaev, modified by Al-Ghoul and Sultan in a series of two papers in J. Phys. Chem. A; the present paper is the third in the series. The simulations capture the essential features of the experimentally observed dynamics.     

Band propagation, scaling laws and phase transition in a precipitate system. I: Experimental study

In the first part of this work, we present an experimental study of the precipitation/redissolution reaction-diffusion system of initially separated components in two distinct organic gels: agar and gelatin. The system is prepared by diffusing a concentrated ammonia solution into a gel matrix that contains nickel sulfate. In agar, the system exhibits a pulse propagation due to the concomitant precipitation reaction between Ni(II) and hydroxide ions and redissolution due to ammonia. At a later stage of propagation, a transition to Liesegang banding is shown to take place. The dynamics of the distance traveled by the precipitation pulse, its width, and mass are shown to exhibit power laws. Moreover, the mass of the bands is shown to oscillate in time, indicating the emergence of a complex mass enrichment mechanism of the formed Liesegang bands. At the microscopic level, we show evidence that the system undergoes a continuous polymorphic transition concomitant with a morphological change whereby the solid in the pulse, which consists of nanospheres of α-nickel hydroxide transforms to form the bands, which consists of larger platelets of β-nickel hydroxide. This clearly indicates the existence of a dynamic Ostwald ripening mechanism that underlies the dynamics on both scales. On the other hand, in gelatin, although we can still obtain similar power laws as in the case of agar, no transition to bands was observed. It is shown that in this case, the propagating pulse is made of nanoparticles of α-nickel hydroxide with an average diameter ~50 nm.     

Simulation of a crossover from the precipitation wave to moving Liesegang pattern formation

Model simulations to investigate the precipitation wave phenomenon and a crossover from the precipitation wave to moving Liesegang patterns were performed. The chemical scheme contains four chemical species via the interaction of precipitation and redissolution (complex formation), in which the precipitation reaction term was based on Ostwald's supersaturation theory. In this article, for the first time, all the features and behaviors of the heterogeneous traveling waves are reproduced, which were observed experimentally in the work of Zrínyi et al. (Zrínyi, et al. J. Phys. Chem. 1991, 95, 1618.). The detailed investigation of the pattern formation showed three possible states of the system, which depend on the initial concentration of the inner and outer electrolytes, respectively. These are the precipitation wave (single moving precipitation band), the moving Liesegang pattern (moving precipitation bands), and the state where these two patterns coexist.     

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.     

Patterns with high rhythmicity levels in multicomponent Liesegang systems.

Liesegang banding is the display of rhythmic strata of precipitate as co-precipitate ions interdiffuse in a gel medium. Complex periodic patterns as well as aperiodic structures could emerge, notably in systems where more than one salt is precipitated. The use of three cations (Co2+) Ni2+, and Mg2+) in the banded precipitation of their hydroxides resulted in an unusual pattern with a consistently increasing rhythmicity. A periodic structure marked by the succession of band multiplets with increasing number of bands (from singlets to doublets to triplets to quadruplets, consistently) was observed. Such rhythmic patterns are obtained as the initial Mg2+ concentration ([Mg2+]0), chosen as a control parameter, increases through a critical value. At high [Mg2+]0, the trend breaks after a long time elapses. Two types of bifurcation are therefore experienced by such a system: concentration bifurcation and diffusive (time/space) bifurcation. The dynamics is elucidated on the basis of an analysis of the bands in certain groups, and gel regions between these groups, as well as between group blocs (here, a bloc denotes a succession of multiplet groups, with repetitively the same number of bands). Finally, similarities between our system and naturally occurring rhythmic patterns are emphasized and discussed.     

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.     

Near-infrared spectroscopic study of [AlO4Al12(OH)23(H2O)12]7+-O-Si(OH)3 nitrate crystals formed by forced hydrolysis of Al3+ in the presence of TEOS

The polymer [AlO4Al12(OH)23(H2O)12]7+-O-Si(OH)3 was prepared by forced hydrolysis of Al3+ up to an OH/Al molar ratio of 2.0 in the presence of monomeric orthosilicic acid. Crystalline material was obtained by slow evaporation. Although the near-infrared spectra of the Al13-sulfate and Al13-O-Si(OH)3 are very similar, there are differences related to the bonding of the -O-Si(OH)3 group to the Al13-unit. The strong complex of bands around 7000 cm(-1) associated with the overtones and combination bands of the OH-stretching modes for Al13-sulfate is much weaker for Al13-O-Si(OH)3 and the opposite is true for the complex of bands around 5000 cm(-1) associated with the water overtone and combination modes, suggesting that the outer OH-groups of the Al13-unit are involved in the formation of the new Al13-O-Si(OH)3 units. A weak band around 7370-7631 cm(-1) is interpreted as the overtone of the Si-OH stretching vibration around 3740 cm(-1). A low intensity band, absent for Al13-sulfate and -nitrate is observed around 5550-5570 cm(-1) and is interpreted as the overtone of the OH-stretching mode of the OH-groups in the vicinity of the central AlO4 in the Al13-unit around 2890-2935 cm(-1). The interaction between the -O-Si(OH)3 group and the Al13-unit has a small influence on other bands like the combination modes of water in the 4400-4800 cm(-1) region, which show a small shift towards higher wavenumbers. The internal OH-groups in the Al13-complex are relatively shielded by the water molecules and therefore do not reflect the influence of the -O-Si(OH)3 in their band positions.     

The intensity change of IR OH bands by H-bonds

The integrated intensity change by H-bonds are measured for CH₃OH solved in different solvents of fundamental, 1. and 2. overtone OH stretching bands. A function A=f(ν) for the strong intensity change by H-bonds of the fundamental band is given, it shows a kink between pure van der Waals solvents and H-bond acceptors. - The contrary behavior of fundamental and 1. overtone bands for the T-dependence of pure CH₃OH and its LiClO₄-solutions could be canceled if the fundamental spectra are intensity corrected by A=f(ν). It is shown that the discussions between species and continuum models of water could become unique taking into account the function f(ν) and its kink, different for fundamental and overtone bands.     

Contrast effect of hydrogen bonding on the acceptor and donor OH groups of intramolecularly hydrogen-bonded OH pairs in diols

We studied the influence of hydrogen bonding on the fundamental and overtone bands of the OH-stretching vibration of each OH group in the intramolecularly hydrogen-bonded OH(I)::OH(II) pair in 1,2-, 1,3- and 1,4-diols. The hydrogen bonding between the two OH groups significantly increases in strength from the five-membered ring of a 1,2-diol to the seven-membered ring of a 1,4-diol. Although the hydrogen bonding does not affect the vibrational property of the OH(II) (or acceptor), it significantly influences the OH(I) (or donor). As the hydrogen bonding becomes stronger from a 1,2- to a 1,4-diol, the fundamental band of the OH-stretching shifts downwards by from about 50 to 140 cm(-1), and the overtone band markedly decreases in intensity, although the effect on the intensity and bandwidth of the fundamental band varies among 1,2-, 1,3- and 1,4-diols. The quantum-mechanically calculated normal frequencies of the acceptor and donor OH groups in the hydrogen-bonded ring are in good agreement with the observed frequencies. The calculated interatomic distance between the O of an acceptor OH and the H of a donor OH is the shortest for a 1,4-diol, which is consistent with the largest frequency shift caused by the hydrogen bonding.     

Removal of OH Absorption Bands Due to Pyrohydrolysis Reactions in Fluoride-Containing Borosilicate Glasses

The purpose of this study is to decrease and to remove OH ions and H₂O in borosilicate glasses. Fluoride-containing borosilicate glasses followed by dry-air-bubbling showed the significant decrease of OH absorption bands around 3500 cm⁻¹. The decrease of OH absorption bands was elucidated by the use of pyrohydrolysis reactions in these glasses where fluoride ions react with OH ions or H₂O during melting. The rates of the decrease of OH absorption bands substantially depend on high valence cations of fluorides. Particularly, the decrease rates of OH absorption coefficients were in the order of ZrF₄-containing glass>AlF₃-containing glass>ZnF₂-containing glass. ZrF₄-containing glass treated by dry-air-bubbling showed a good capability to remove OH absorption band. Fluoride-containing glasses showed the low flow point in comparison with fluoride-free glasses.     

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。     

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.     

Nitrogen as a Probe Molecule for the IR Studies of the Heterogeneity of OH Groups in Zeolites

One of the methods of IR studies of the heterogeneity of Si–OH–Al groups in zeolites is the investigation of the frequency shift of the band of free OH bands restored upon the adsorption of ammonia and subsequent desorption at increasing temperatures. We extended this method by following the shift of the band of the OH group interacting by hydrogen bonding with nitrogen. The advantage of nitrogen, compared with CO, which has been commonly used as a probe molecule in studies on hydrogen bonding, is that for nitrogen the frequency shift is smaller than for CO and therefore there is no overlapping of shifted OH band with the bands of ammonium ions. For zeolites NaHY, HMFI, and HBEA, the frequency shift of IR bands of both free and hydrogen-bonded Si–OH–Al with the increase of ammonia desorption temperature evidences the heterogeneity of these hydroxyls. On the other hand, in zeolite HFAU of Si/Al = 31, Si–OH–Al were found to be homogeneous. Heterogeneity of OH groups may be explained both by the presence of Si–OH–Al of various number of Al near the bridge and of Si–OH–Al of various geometry.     

Identification of the rosasite group minerals--an application of near infrared spectroscopy

The ability of near infrared reflectance spectroscopy to classify the rosasite group minerals from spectral characteristics is demonstrated. NIR spectroscopy can be regarded as an alternative tool for structure analysis. The spectra show that rosasite group minerals with different cations can be distinguished. Ni2+ in nullaginite [Ni2(CO3)(OH)2] is conspicuous through a single broad band absorption feature at 8525 cm-1, extended from 11,000 to 7000 cm-1. The effect of Ni on Cu is seen in the spectrum of glaukosphaerite [(Cu, Ni)2(CO3)(OH)2] both by a red shift of the spectrum and reduction in intensity of bands with variable positions of band maxima for Cu2+ at 6995 cm-1 and Ni2+ at 7865 cm-1. The spectrum of rosasite [(Cu, Zn)2(CO)3(OH)2] is characterised by Cu2+ band at 7535 cm-1. Kolwezite [(Cu, Co)2(CO)3(OH)2] is a spectral mixture of Cu and Co but optically separated by Co2+ and Cu2+ peaks at 8385 and 7520 cm-1. Vibrational spectra of carbonates show a number of bands in the 7000-4000 cm-1 region attributable to overtones, combination of OH stretching and deformation modes. They appear to be uniform in nature since the structure of rosasite group minerals is identical. The complexity of these features varies between samples because of the variation in composition and hence is useful for discriminating different hydrous carbonates.     

温度对掺杂球形Ni(OH)_2质子扩散的影响

应用微电极恒电位阶跃法研究了在Ni(OH) 2 电极的阳极及阴极过程反应中 ,温度对球形Ni(OH) 2 的质子扩散系数和表观扩散活化能的影响 .研究表明 ,于Ni(OH) 2 中掺杂Co和Co +Zn后可降低其阳极和阴极过程的表观扩散活化能 ,增大质子扩散系数 ,掺Co的效果更加明显 ,而掺Zn则增大表观扩散活化能 ,降低了扩散系数 .这说明前者的掺入其作用是降低了质子扩散阻力使电极的反应活性增加、而后者的掺入则是增大了质子的扩散阻力而使电极反应活性降低 .     

叶状Cu（OH）2的合成及其向带有纳米孔的CuO的转化

通过乳液界面反应法,用以Span80(sorbitan monooleate)作为稳定剂的乳液体系控制合成了叶状Cu(OH)2单晶.通过热处理,可以得到表面有纳米孔的CuO,且保持了原有的叶状形貌.通过X射线衍射(XRD)、Fourier红外光谱(FTIR)、扫描电镜(SEM)和透射电镜(TEM)观测了其形貌和结构特征.实验结果表明,叶状Cu(OH)2为单晶,且沿[111]晶面定向生长.孔的形成是由于相转变过程中Cu(OH)2失去H2O分子所致.通过观测不同反应时间产物的形貌,深入探讨了叶状Cu(OH)2纳米结构的组装机理.整个组装过程是由能量高的颗粒状纳米粒子通过端部取向连接定向生长而得到能量相对较低的叶状结构.并且得到的CuO的紫外光谱相对于其块体材料发生了蓝移,显示出比较大的禁带宽度.     

Proton dynamics in the strong chelate hydrogen bond of crystalline picolinic acid N-oxide. A new computational approach and infrared, raman and INS study

Infrared, Raman and INS spectra of picolinic acid N-oxide (PANO) were recorded and examined for the location of the hydronic modes, particularly O-H stretching and COH bending. PANO is representative of strong chelate hydrogen bonds (H-bonds) with its short O...O distance (2.425 A). H-bonding is possibly well-characterized by diffraction, NMR and NQR data and calculated potential energy functions. The analysis of the spectra is assisted by DFT frequency calculations both in the gas phase and in the solid state. The Car-Parrinello quantum mechanical solid-state method is also used for the proton dynamics simulation; it shows the hydron to be located about 99% of time in the energy minimum near the carboxylic oxygen; jumps to the N-O acceptor are rare. The infrared spectrum excels by an extended absorption (Zundel's continuum) interrupted by numerous Evans transmissions. The model proton potential functions on which the theories of continuum formation are based do not correspond to the experimental and computed characteristics of the hydrogen bond in PANO, therefore a novel approach has been developed; it is based on crystal dynamics driven hydronium potential fluctuation. The envelope of one hundred 0 --> 1 OH stretching transitions generated by molecular dynamics simulation exhibits a maximum at 1400 cm-1 and a minor hump at approximately 1600 cm-1. These positions square well with ones predicted for the COH bending and OH stretching frequencies derived from various one- and two-dimensional model potentials. The coincidences with experimental features have to be considered with caution because the CPMD transition envelope is based solely on the OH stretching coordinate while the observed infrared bands correspond to heavily mixed modes as was previously shown by the normal coordinate analysis of the IR spectrum of argon matrix isolated PANO, the present CPMD frequency calculation and the empirical analysis of spectra. The experimental infrared spectra show some unusual characteristics such as large temperature effects on the intensity of some bands, thus presenting a challenge for theoretical band shape treatments. Our calculations clearly show that the present system is characterized by an asymmetric single well potential with no large amplitudes in the hydronium motion, which extends the existence of Zundel-type spectra beyond the established set of hydrogen bonds with large hydronic vibrational amplitudes.     

纳米Co(OH)2/HY复合物的制备及其电化学电容性能

当材料以纳米尺度存在时，某些物理及化学性质将发生根本性变化。因而，纳米技术的概念绝不仅仅是尺寸的缩小，更应体现在物理概念、系统设计、材料合成及制造等方面所发生的根本性革命。随着21世纪的来临，人们正努力地将越来越多的纳米材料功能化，Co(OH)2通常用作Ni(OH）2电池活性材料的添加剂。关于Ni(OH）2的许多电化学性质已有大量报道，而对于Co(OH）2的研究却很少涉及。本文报道了一种新的纳米级Co(OH）2／HY复合物的制备方法，并将制得的复合物制成超级电容器电极，研究了其超电容特性。此外，还初步提出了Co(OH）2各向异性的形貌形成机理。     

Isotope effects in liquid water by infrared spectroscopy. V. A sea of OH4 of C2v symmetry

The two water gas OH stretch vibrations that absorb in the infrared (IR) near 3700 cm(-1) are redshifted to near 3300 cm(-1) upon liquefaction. The bathochromic shift is due to the formation of four H-bonds: two are from the labile hydrogen atoms to neighbors and two are received from neighbors by the oxygen free electron pairs. Therefore, the water oxygen atom is surrounded by four hydrogen atoms, two of these make covalent bonds that make H-bonds and two are oxygen H-bonded. However, these permute at rate in the ps range. When the water molecules are isolated in acetonitrile (MeCN) or acetone (Me(2)CO), only the labile hydrogen atoms make H-bonds with the solvent. The bathochromic shift of the OH stretch bands is then almost 130 cm(-1) with, however, the asymmetric (ν(3)) and symmetric (ν(1)) stretch bands maintained. When more water is added to the solutions, the oxygen lone doublets make H-bonds with the available labile hydrogen atoms from neighboring water molecules. With one bond accepted, the bathochromic shift is further displaced by almost 170 cm(-1). When the second oxygen doublet is filled, another bathochromic shift by almost 100 cm(-1) is observed. The total bathochromic shift is near 400 cm(-1) with a full width at half height of near 400 cm(1). This is the case of pure liquid water. Notwithstanding the shift and the band broadness, the ν(3) and ν(1) band individualities are maintained with, however, added satellite companions that come from the far IR (FIR) absorption. These added to the fundamental bands are responsible for the band broadness and almost featureless shape of the massive OH stretch absorption of liquid water. Comparison of light and heavy water mixture spectra indicates that the OH and OD stretch regions show five different configurations: OH(4); OH(3)D; OH(2)D(2); OHD(3); and OD(4) [J. Chem. Phys. 116, 4626 (2002)]. The comparison of the OH bands of OH(4) with that of OHD(3) indicates that the main component in OHD(3) is ν(OH), whereas in OH(4) two main components are present: ν(3) and ν(1). Similar results are obtained for the OD bands of OD(4) and ODH(3). These results indicate that the C(2) (v) symmetry of H(2)O and D(2)O is preserved in the liquid and aqueous solutions whereas C(s) is that of HDO.