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1.
Layered calcium octyl phosphate (CH3(CH2)7OPO3Ca.1.6H2O: CaOP), which is composed of a multilayer alternating bilayer of octyl phosphates and a dicalcium phosphate dihydrate (DCPD)-like phase, was thermally treated in vacuo and the intercalation of n-alkyltrimethylammonium ions into the materials was examined. The octyl groups in the layer were eliminated by outgassing above 250 degrees C to give the amorphous calcium phosphates. Further, the specific surface area was steeply increased and mesopores with a diameter of ca. 2.0 nm were formed. IR results indicated that the surface P-OH groups were generated by outgassing at 250 degrees C. When the CaOP outgassed at 250 degrees C was treated with n-alkyltrimethylammonium ion solutions (carbon number of alkyl group, n=14-18), three XRD peaks reappeared below 2theta=15 degrees and the d-spacing ratio of these peaks was 1:1/2:1/3. These facts indicate that the n-alkyltrimethylammonium ions were intercalated into the amorphous calcium phosphate phases. 相似文献
2.
A. R. Khayrullin A. V. Severin A. K. Khripunov A. A. Tkachenko V. D. Pautov 《Russian Journal of Applied Chemistry》2013,86(8):1298-1304
Dielectric properties a Gluconacetobacter xylinus bacterial cellulose and its composites with calcium phosphates were studied and used to analyze structural changes in a matrix polymer upon its mechanical treatment and introduction of a mineral fi ller. It was found that, when composites are formed, the ratio between amorphous and crystalline regions is disturbed in the cellulose matrix. 相似文献
3.
Pipich V Balz M Wolf SE Tremel W Schwahn D 《Journal of the American Chemical Society》2008,130(21):6879-6892
Mineralization of calcium carbonate in aqueous solutions starting from its initiation was studied by time-resolved small-angle neutron scattering (SANS). SANS revealed that homogeneous crystallization of CaCO 3 involves an initial formation of thin plate-shaped nuclei which subsequently reassemble to 3-dimensional particles, first of fractal and finally of compact structure. The presence of the egg-white protein ovalbumin leads to a different progression of mineralization through several stages; the first step represents amorphous CaCO 3, whereas the other phases are crystalline. The formation and dissolution of the amorphous phase is accompanied by Ca (2+)-mediated unfolding and cross-linking of about 50 protein monomers showing the characteristic scattering of linear chains with a large statistical segment length. The protein complexes act as nucleation centers for the amorphous phase because of their enrichment by Ca (2+) ions. SANS revealed the sequential formation of CaCO 3 starting from the amorphous phase and the subsequent formation of the crystalline polymorphs vaterite and aragonite. This formation from less dense to more dense polymorphs follows the Ostwald-Volmer rule. 相似文献
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S. Raičević Ž. Vuković S. Lazić M. Mandić 《Journal of Radioanalytical and Nuclear Chemistry》1995,198(2):303-309
The coprecipitation of strontium by a calcium phosphate phase formed at an elevated pH 10.8 was investigated. The first phase obtained under these conditions in the amorphous calcium phosphate (ACP) which is transformed into crystalline hydroxyapatite (HA) after the induction period. Is has been shown that this transformation together with morphological changes of the precipitated solid phase, influences the amount of the sorbed strontium significantly. The possible consequences of this finding on practical application of coprecipitation of strontium by calcium phosphate have been discussed. 相似文献
6.
B. I. Kidyarov 《Journal of Structural Chemistry》2004,45(1):S31-S35
The energy of crystal nucleation from liquid phase was considered, with the following two stages taken into account: (1) the formation of metastable supercooled melt (solution), containing pre-nuclei with intermediate amorphous (quasicrystalline) structure, and (2) the transformation of amorphous clusters into solid crystalline nuclei having different structures. With growth of a nucleus the nucleation energy profile manifests 2–3 maxima corresponding to these stages, and the kinetics of the non-stationary nucleation has five characteristic variations. 相似文献
7.
We present a computer simulation study on crystal nucleation and growth in supersaturated suspensions of mono-disperse hard spheres induced by a triangular lattice substrate. The main result is that compressed substrates are wet by the crystalline phase (the crystalline phase directly appears without any induction time), while for stretched substrates we observe heterogeneous nucleation. The shapes of the nucleated crystallites fluctuate strongly. In the case of homogeneous nucleation amorphous precursors have been observed [T. Schilling et al., Phys. Rev. Lett. 105(2), 025701 (2010)]. For heterogeneous nucleation we do not find such precursors. The fluid is directly transformed into highly ordered crystallites. 相似文献
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V. A. Sinyaev E. S. Shustikova E. N. Sakhipov L. V. Levchenko 《Russian Journal of General Chemistry》2012,82(3):379-383
A group of crystalline and amorphous calcium-copper phosphates was synthesized by the exchange reaction between sodium orthophosphate and calcium and copper(II) nitrates in solutions within a wide range of reagent ratios 0 ≤ Cu×100/(Cu + Ca) ≤ 100. Conclusions on the effect of the Cu×100/(Cu + Ca) ratio on the phase composition of the substances and on the state of chemical bonds therein were made on the basis of the results of X-ray phase analysis and IR Fourier and electronic spectroscopy. 相似文献
10.
Sinyaev V. A. Shustikova E. S. Levchenko L. V. Tokseitova G. A. Griggs D. 《Russian Journal of Applied Chemistry》2003,76(9):1375-1378
A group of crystalline and amorphous calcium-magnesium phosphates were prepared by exchange in solution of sodium orthophosphate and calcium and magnesium chlorides at pH 8-12. The behavior of the phosphates on heating to 1000°C was studied. 相似文献
11.
Mechanism of Crystalline Self‐Assembly in Aqueous Medium: A Combined Cryo‐TEM/Kinetic Study
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Dr. Yaron Tidhar Dr. Haim Weissman Dr. Dmitry Tworowski Prof. Dr. Boris Rybtchinski 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(33):10332-10342
Understanding the crystallization of organic molecules is a long‐standing challenge. Herein, a mechanistic study on the self‐assembly of crystalline arrays in aqueous solution is presented. The crystalline arrays are assembled from perylene diimide (PDI) amphiphiles bearing a chiral N‐acetyltyrosine side group connected to the PDI aromatic core. A kinetic study of the crystallization process was performed using circular dichroism spectroscopy combined with time‐resolved cryogenic transmission electron microscopy (cryo‐TEM) imaging of key points along the reaction coordinate, and molecular dynamics simulation of the initial stages of the assembly. The study reveals a complex self‐assembly process starting from the formation of amorphous aggregates that are transformed into crystalline material through a nucleation–growth process. Activation parameters indicate the key role of desolvation along the assembly pathway. The insights from the kinetic study correlate well with the structural data from cryo‐TEM imaging. Overall, the study reveals four stages of crystalline self‐assembly: 1) collapse into amorphous aggregates; 2) nucleation as partial ordering; 3) crystal growth; and 4) fusion of smaller crystalline aggregates into large crystals. These studies indicate that the assembly process proceeds according to a two‐step crystallization model, whereby initially formed amorphous material is reorganized into an ordered system. This process follows Ostwald’s rule of stages, evolving through a series of intermediate phases prior to forming the final structure, thus providing an insight into the crystalline self‐assembly process in aqueous medium. 相似文献
12.
V. I. Lysov T. L. Tsaregradskaya O. V. Turkov G. V. Saenko 《Russian Journal of Physical Chemistry A, Focus on Chemistry》2014,88(12):2183-2187
The temperature dependence of the volume fraction of the crystalline phase in Fe80B20 amorphous alloy is calculated using equations from the homogeneous nucleation theory of binary systems. It is shown that the crystallization of Fe80B20 alloy is two-stage, as is confirmed by the experimental data obtained by means of highly sensitive dilatometry and X-ray diffractometry. On the basis of results of calculations performed within the theory of the high-temperature stability of amorphous alloys, two areas of its practical application are proposed: (i) enhancing the thermal stability of amorphous alloys by isothermal annealing in the range of temperatures where crystalline nuclei can transition to the amorphous phase; (ii) controlled nanostructuring of the amorphous state with different modes of treatment. Methods are proposed for obtaining the nanostructured state from the initial amorphous state. Alloys in the nanocrystalline state are obtained, as is confirmed by the results from electron microscope investigations. 相似文献
13.
Crystallization through metastable phases, such as polymorphism, plays an important role in chemical manufacture, biomineralization, and protein crystallization. However, the kinetics creating the final stable crystalline phase from metastable phases has so far remained unclear. In this study, crystallization via an amorphous precursor, the so-called multistep crystallization (MSC), is studied quantitatively in a colloidal model system. In MSC, amorphous dense droplets are first nucleated from the mother phase. Subsequently, a few unstable subcrystalline nuclei can be created simultaneously by fluctuation from the tiny dense droplets, which is different from previous theoretical predictions. It is necessary for these crystalline nuclei to reach a critical size N*(crys) to become stable. However, in contrast to subcrystalline nuclei, a stable mature crystalline nucleus is not created by fluctuation but by coalescence of subcrystalline nuclei, which is unexpected. To accommodate a mature crystalline nucleus larger than the critical size N*(crys), the dense droplets have to first acquire a critical size N*. This implies that only a fraction of amorphous dense droplets can serve as a precursor of crystal nucleation. As an outcome, the overall nucleation rate of the crystalline phase is, to a large extent, determined by the nucleation rate of crystals in the dense droplets, which is much lower than the previous theoretical expectation. Furthermore, it is surprising to see that MSC will promote the production of defect-free crystals. The knowledge acquired in this study will also significantly advance our understandings in polymorphism related processes. 相似文献
14.
The thermal interactions of dolomite and calcite with calcium polyphosphates up to 1000°C were studied by thermal (dynamic
heating and isobaric conditions) and X-ray diffraction methods. It was found that in mixtures with Ca-polyphosphates less
than half of the carbonates reacted before decomposition to oxides with the phosphates. The rates of the reactions of phosphates
with MgO were much lower than those with CaO. The scheme of the main crystalline phase transformations was constructed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
15.
Application of polyamorphism in water to spontaneous crystallization of emulsified LiCl-H2O solution
Mishima O 《The Journal of chemical physics》2005,123(15):154506
Aqueous solutions are widely explained by the hydration or the bound waterfree water notion. Amorphous polymorphism (polyamorphism) in pure water, which is presently under vigorous discussion, may provide a different view over the solutions. Here, I changed pressure, P, temperature, T, and concentration, C, of emulsified LiCl-H2O solutions and studied their freezing by detecting its heat evolution. It was experimentally indicated that the homogeneous nucleation of low-density crystalline ice I (phase Ih or Ic), in pure water and in solutions, connects to the polyamorphic transition of high-density amorphous ice (HDA) to low-density amorphous ice (LDA). Thus, the polyamorphism of water relates to the phase behavior of aqueous solution. In accordance with the recent simulation result, the nucleation was thought to occur in two stages: the appearance of the LDA-like state and the crystallization. Usefulness of the polyamorphic point of view about the solutions was seen. 相似文献
16.
V. A. Sinyaev R. Z. Le Geros L. V. Levchenko E. S. Shustikova R. A. Karzhaubaeva 《Russian Journal of General Chemistry》2008,78(5):864-867
Fourier-transform infrared spectra of three samples of deposited amorphous phosphates, calcium monodiphosphate (orthopyrophosphate) with a molar ratio 2P/(2P + 1P) of ~0.3, calcium magnesium monophosphate with an atomic ratio Mg/(Mg + Ca) of ~0.3, and calcium-magnesium monodiphosphate with a molar ratio 2P/(2P + 1P) of ~0.1 and atomic ratio Mg/(Mg + Ca) of ~0.1, were analyzed. On the basis of the observed similarity between the v(OH), δ(OH) and (δ + ω)(OH) lines in the spectra of these phosphates and liquid water analogous states of water molecules therein were assumed. It was presumed that the formation of amorphous phosphates involves deposition of poorly soluble substances on the surface of water nanoclusters. 相似文献
17.
Behaviours of drawn semi-crystalline poly(ethylene terephthalate) films are investigated by DSC, X-ray diffraction and birefringence measurements. The comparison of the different results confirms the coexistence of two structures into the amorphous part of the material: a completely disordered amorphous phase and a mesomorphic amorphous one. Moreover, for the strongest draw ratio, the calorimetric results show that the drawing effect on the strain induced crystalline structure proceeds by a better orientation of this structure rather than by nucleation and growth of new oriented crystallites. 相似文献
18.
Cerruti M Perardi A Cerrato G Morterra C 《Langmuir : the ACS journal of surfaces and colloids》2005,21(20):9327-9333
Morphological and structural variations of particles of Bioglass with two different grain sizes reacted in Tris-buffered solution were analyzed by means of N(2) adsorption/desorption at 77 K and HR-TEM/EDS. A remarkable increase in specific surface area (ssa) was observed after the first hour of dissolution. A plateau value corresponding to an increase of at least 2 orders of magnitude was reached after 2 days of dissolution. The ssa increase was faster for the smaller particle size sample, and the ratio between the ssa of the starting samples was not maintained during dissolution. Both micro- and mesopores were formed at different stages of the reaction for the two samples. Increasing ssa was also connected to the formation of a microcrystalline structure rich in Ca and P, as shown by TEM images. The segregation of both a SiO(2)-rich amorphous phase and a Ca/P-rich crystalline phase was observed after the first hour of dissolution. After 2 days of reaction, Ca/P-rich particles made of fine aciculate crystals were found either in close contact with SiO(2) particles or deposited on a small SiO(2)-rich core. A preliminary analysis of TEM data showed the formation, together with hydroxy carbonate apatite, of different types of calcium phosphates not detectable by powder X-ray diffraction. 相似文献
19.
Saha A Lee J Pancera SM Bräeu MF Kempter A Tripathi A Bose A 《Langmuir : the ACS journal of surfaces and colloids》2012,28(30):11182-11187
We use time-resolved cryogenic transmission electron microscopy (TR-cryo-TEM) on a supersaturated solution of calcium sulfate hemihydrate to examine the early stages of particle formation during the hydration of the hemihydrate. As hydration proceeds, we observe nanoscale amorphous clusters that evolve to amorphous particles and then reorganize to crystalline gypsum within tens of seconds. Our results indicate that a multistep particle formation model, where an amorphous phase forms first, followed by the transformation into a crystalline product, is applicable even at time scales of the order of tens of seconds for this system. The addition of a small amount of citric acid significantly delays the reorganization to gypsum crystals. We hypothesize that available calcium ions form complexes with the acid by binding to the carboxylic groups. Their incorporation into a growing particle produces disorder and extends the time over which the amorphous phase exists. We see evidence of patches of "trapped" amorphous phase within the growing gypsum crystals at time scales of the order of 24 h. This is confirmed by complementary X-ray diffraction experiments. Direct imaging of nanoscale samples by TR-cryo-TEM is a powerful technique for a fundamental understanding of crystallization and many other evolving systems. 相似文献
20.
The cuticules (shells) of the woodlice Porcellio scaber and Armadillidium vulgare were analysed with respect to their content of inorganic material. It was found that the cuticles consist of crystalline magnesium calcite, amorphous calcium carbonate (ACC), and amorphous calcium phosphate (ACP), besides small amounts of water and an organic matrix. It is concluded that the cuticle, which constitutes a mineralized protective organ, is chemically adapted to the biological requirements by this combination of different materials. 相似文献