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Department of Chemistry, University of Patras, GR-26500 Patras, GreeceSamples of conducting nanocomposites of polyaniline (PANI) and polypyrrole (PPy) with zeoliteconcentrations from 0 to 50% w/w and ZnO from 0 to 40% w/w were synthesized through oxida-tive polymerization. Their d.c. conductivity was measured under He atmosphere from 80 to 350 Kand their thermal stability was studied by heating them at 70 C for times from 0 to 600 h underenvironmental conditions. The experimental results are consistent with a granular metal structurein which conductive grains are embedded in an insulating matrix and thermal degradation is dueto the corrosion–like shrinking of them with thermal treatment. In the PANI/zeolite composites theconductivity and the thermal stability remain practically unchanged with increasing zeolite content.On the contrary, in PPy/zeolite composites these properties improve with increasing zeolite concen-tration. Moreover, SEM images and XRD patterns of PANI/ZnO composites reveal that a number ofthe ZnO particles are expelled from the matrix, although in PPy/ZnO composites ZnO is completelyencapsulated into the polymer. These differences can be attributed to the stronger bond betweenPANI chains which justify the formation of crystalline conductive grains in it, in contrast to the muchweaker packing of polymer chains in the amorphous PPy.

The kinetics of calcite particles formation in the presence of soluble polyvinyl-alcohol matrix was investigated by the constant composition method. The presence of the above-mentioned polymer, induces the precipitation of this calcium carbonate polymorph at room temperature possibly through the –OH moieties. The apparent order for the crystallization process was found to be 2.25 ± 0.16 indicative of a surface nucleation mechanism. The number of ions forming the 3D critical nucleus was found to be p = 9. The surface energy of the growing phase was 28 mJ m− 2 relatively small because of imperfections, in the form of vacant sites.

The crystal growth of hydroxyapatite (HAP), the most stable calcium phosphate phase at 37 °C, pH 7.40, and ionic strength of 0.15 M NaCl, in the presence of the cysteine-rich Mdm2 peptide (containing 48 aminoacids in ring finger configuration) has been investigated by the constant composition technique. Crystallization took place exclusively on well-characterized HAP crystals in supersaturated solutions only with respect to this calcium phosphate salt. The kinetic results indicated a surface diffusion spiral growth mechanism. The presence of the Mdm2 peptide inhibited the crystal growth of HAP by 59−92% through adsorption onto the active growth sites of the HAP crystal surface. The kinetic results favored a Langmuir-type adsorption model, and the value of the calculated affinity constant was kaff = (258 ± 21) × 104 dm3 mol-1.

The kinetics of calcite (CaCO(3)) crystallization on calcite seed crystals in the presence of taurine was investigated by the constant composition method. The presence of taurine (4 x 10(- 5)- 4 x 10(-4)M) in the supersaturated solutions lead to calcite crystals with a characteristic discontinuous planes of growth and poor habit, as compared to the hombohedral morphology of the seed crystals. The acceleration effect of taurine on the crystal growth rate was 17-96%. The apparent order of the crystal growth was found to be 2.0+/- 0.2 typical for a surface diffusion-controlled spiral growth processes.

Oxadiazole homopolymer was found to be a substrate favoring the deposition of hydroxyapatite (HAP) crystals from stable supersaturated solutions at pH 7.40, 37 °C, and 0.15 M NaCl. The second-order dependence of the precipitation of HAP on oxadiazole polymer on the solution supersaturation suggested a surface diffusion controlled mechanism. The surface energy of the HAP nuclei growing on oxadiazole homopolymer was calculated to be 158 mJ m-2 from the dependence of crystal growth rate on the solution supersaturation. The overgrowth of HAP on the polymer was done selectively possibly through active site formation on −N−N− groups on the macromolecules as confirmed by computational chemistry calculations at 310 K.

The effect of taurine, used in a number of food supplements for athletes, on the crystal growth of hydroxyapatite is investigated at conditions of sustained supersaturation employing the constant composition technique. It was found to increase the crystal growth rate possibly through blocking of the less active growth sites by taurine molecules. The apparent order for the crystallization reaction was determined to be ca. 2, thus suggesting a surface diffusion controlled spiral growth mechanism.

The effect of sodium alginate on the crystal growth of hydroxyapatite (HAP) was investigated at sustained supersaturation by the constant composition technique. Sodium alginate was found to inhibit HAP crystal growth at low concentrations and reduced the crystal growth rates by 42–86% for inhibitor concentrations of 2.1 × 10− 7–12.6 × 10− 7 mol/l. The inhibition effect on the crystal growth rate may be explained possibly through adsorption onto the active growth sites. A detailed kinetics analysis suggested a Langmuir-type adsorption of the alginate on HAP surface and a value of 1.63 × 107 l/mol was obtained for the affinity constant of sodium alginate for the surface of HAP. The apparent order for the crystallization reaction was determined to be approximately 2, thus suggesting a surface diffusion controlled spiral growth mechanism.

The nucleation and further crystallization of calcite on oxadiazole-terpyridine copolymer were investigated by the constant composition method. The apparent order of the crystallization process was found to be 2.3±0.2 indicative of a surface diffusion-controlled spiral growth mechanism. The number of ions forming the critical nucleus was found experimentally to be ( n * = ) 3 in accordance with the simulation by the PM3 method included in the MOPAC program package. The surface energy of the growing phase was found to be 29.5 mJ m −2 .

The dc electrical conductivity (σ) of HCl-protonated polyaniline, polypyrrole, and their blends was measured from 80 to 300 K for thermal aging times between approximately 0 and 600 h. The thermal aging took place at 70°C under room atmosphere. The change of σ with the temperature (T) and the decrease of σ with the thermal aging time (t) are consistent with a granular metal type structure, in which conductive grains are randomly distributed into an insulating matrix. Aging makes the grains shrink in a corrosion-like process. From σ = σ(T) measurements the ratio s/d, where s is the average separation between the grains and d their diameter, as well as the rate d(s/d)/dt of their decrease with t were calculated. These revealed that the conductive grains consist of a shell, in which aging proceeds at a decreasing rate, and a central core, which is consumed at a much slower rate. Our measurements not only permitted the estimation of the shell thickness, which lies between 0 and 5 A, but also gave quantitative information about the quality of the shells and the cores from their aging rates. The shells are consumed with an average rate of d(s/d)/dt = 6.6 × 10−4 (h−1), which is about 5 times greater than the more durable cores. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 117–122, 2005

The dielectric properties of polypyrrole-zeolite composites up to 50%w∕w zeolite are studied in the frequency range from 10−2to2×106Hz from room temperature to liquid nitrogen temperature. The complex permittivity formalism reveals a temperature dependent relaxation in all samples except for the 25%w∕w zeolite composite. The frequency fmax where a maximum of a loss peak is located varies with temperature by the Williams-Lander-Ferry law. The values of the activation energy of the relaxation process (which are of the order of polaronic dc conductivity) have the tendency to reach a minimum in the 25%w∕w composition, which is a loss-free composite. The 50%w∕w zeolite behaves as a dielectric where ionic relaxation dominates. The temperature variation of the strength of the dielectric mechanism follows a Curie law, apart from 50%w∕w zeolite where the dielectric strength is practically constant. The frequencies, where loss peaks are maximum, as well as dc conductivity follow qualitatively the same temperature l...

The kinetics of vaterite (CaCO3) crystallization in the presence of leucine was investigated by the constant composition technique. The presence of leucine in the supersaturated solution stabilizes this carbonate polymorph by a modulation procedure. The number of ions forming the critical nucleus was found to be two and a surface energy of 23 mJ m−2 was estimated. The apparent order was found to be 1, typical for a surface diffusion controlled spiral growth mechanism.

The effect of Prothymosin α (Protα) fragments (used as drugs for various applications) in supersaturated solutions of calcium phosphate was investigated under plethostatic conditions. The rates of crystal growth measured in the presence of Protα were drastically reduced on the size of the fragment. This inhibitory effect may be explained by adsorption and subsequent blocking of the active growth sites. Kinetic analysis suggested Langmuir-type adsorption of Protα on the hydroxyapatite (HAP) surface with a relatively high affinity for the substrate in the concentration range investigated. In all cases, the measured crystal growth rates, R, were proportional to the relative supersaturation, S, with respect to HAP. Also, the rate of crystal growth was reduced upon increasing the size of the Protα fragment reaching a plateau, a fact that may be ascribed to further blocking of the active growth sites.

The ability of zeolite to accommodate and neutralize a great number of cations with all three mechanisms, adsorption, intercalation, and cation exchange reaction, combined with the high conductivity and stability of HCl—protonated polyaniline, which interposes into its laminar structure and the good contact between zeolite surface and polyaniline, make polyaniline/zeolite very suitable for a cathode electrode in dry electrical primary coin cells. Polyvinylalcohol (PVA) gel was used as the electrolyte and a Mg foil as the anode. The e.m.f. values of the cells ranged from 1.62 to 1.94 V, their specific energies from 4.34 to 8.88 Wh/kg and their energy densities from 3.10 to 6.34 mWh/cm 3 for cathodes containing 10–50% in zeolite. The parameter τ, which characterizes the aging rate of the polyaniline/zeolite cathode ranges between 70 and 320 h. The higher τ is, the slower is the aging of the sample. The best combination of high conductivity, slow aging of the cathode and high specific energy of the cells was obtained between 25 and 35% of zeolite content. Moreover, surface examination of the polyaniline/zeolite samples, was performed by scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques. © 2003 Elsevier B.V. All rights reserved.

The low frequency (10 −2 to 10 6 Hz) a.c. conductivity of fresh and thermally aged polypyrrole–polyaniline blends of various compositions was studied at room temperature. The cross-over frequency that separates the d.c. regime from the dispersive region is a function of the composition and is influenced by the thermal aging for the polyaniline-rich blends. The frequency-dispersive conductivity is determined by a sublinear law. The fractional exponent is close to unity for the fresh samples. The thermal aging results in values of the fractional exponent, which depend on the composition. This picture reflects what has been observed previously in d.c. vs. temperature studies: thermal aging reduces the size of conducting grains in polyaniline, while in polypyrrole, results in a modification of the backbone matrix. It seems that a.c. measurements at room temperature may well serve as a parallel way to the time consuming d.c. conductivity vs. temperature technique, to detect thermal degradation of the transport properties in conducting polymers.

The nucleation and further crystallization of vaterite on gallium (III)-oxide was investigated by the constant composition technique. The apparent order of the crystallization process was found to be 1.17±0.15 indicative of a surface diffusion controlled mechanism. The number of ions forming the critical nucleus was found to be n*=3. We explore the mechanism of vaterite nucleation on gallia by the PM5 method included in the MOPAC program package. The surface energy of the growing phase was found to be 26.4±2.7 mJ m−2.

The kinetics of vaterite crystallization on styrene (30% w/w)–butadiene copolymer (branched) containing –C(O)CH 3 functional groups was investigated by the constant composition method. The polymer along with the functional groups stabilizes this calcium carbonate polymorph which transforms slowly to calcite. The apparent order for the crystallization process was found to be 1.3±0.1 indicative of a surface diffusion control mechanism. The number of ions forming the critical nucleus was found to be n *=3. The surface energy of the growing phase was 45 mJ m −2 . The formation of vaterite initiated through the interaction of Ca 2+ ions of the supersaturated solution with the negative end of the –CO bond.

The kinetics of hydroxyapatite (HAP) crystallization on polymers containing –C≡N functional group was investigated by the constant composition technique. The apparent order for the crystallization process was found to be 2 indicative of a surface diffusion control mechanism. Critical nucleus formation was confirmed by computational chemistry calculations using the PM3 method in the version 6.0 of MOPAC program package. The surface energy of the growing phase was 109 mJ m −2 . The HAP formation initiated through the interaction of Ca 2+ ions with the negative end of the –C≡N bond.

A styrene (30% w/w)–butadiene copolymer (3-arm configuration), epoxidized and further functionalized with the –S(O) 2 OH groups was found to be a substrate favoring the deposition of aragonite crystals from stable supersaturated solutions at pH 8.50 and 25°C. The crystallization was studied by the constant composition technique, thus making it possible for a relatively large amount of the overgrowths to be formed and to be identified exclusively as aragonite crystals. The apparent order found from kinetic analysis was n =7.2±0.8, thus suggesting a polynuclear mechanism. A surface energy of 44±5 mJ m −2 was calculated for the growing aragonite phase and a four-ion cluster forming the critical nucleus, according to the classical nucleation theory.

The kinetics of calcite crystallization on polymers containing –C ¼ O, functional group was investigated by the constant composition method. The apparent order for the crystallization process was found to be 2.270.3 indicative of a surface diffusion controlled mechanism. The number of ions forming the critical nucleus was found to be 3–6. These values were confirmed by quantum mechanical calculations using the PM3 method of the MOPAC program package. The surface energy of the growing phase was found to be 30–32 mJ m � 2 approximately. The formation of calcite was initiated through the interaction of Ca 2+ ions with the negative end of the –C ¼ O bond as concluded from the quantum mechanical calculations. r 2003 Elsevier Science B.V. All rights reserved.

A dielectric relaxation mechanism, which is attributed to the localized motion of trapped polarons, has been recorded in conducting polyaniline by employing the thermally stimulated depolarization current technique. The signal was analyzed within the frame of the normal distribution in the activation energy value. The experimental dielectric relaxation results were manipulated in order to evaluate the length of the jump distance that the trapped polarons transfer along and the concentration of trap centers. The concentration of trapped carriers is calculated from two different viewpoints: the pair approximation that assumes phonon-assisted tunneling through the barrier separating two adjacent sites and the pinning model that considers the trapped polaron oscillating around its pinning point. Both models provide compatible results.

Organometallic compounds have been used in various fields of chemistry, medicine and materials science. Central metal, stereochemical configuration and functional groups of the substitutes give to the organometallic compounds very special and selective properties. These properties have been used successfully in selective-antitumor-targeting, as well as anti-arthritic drugs. In the present investigation we study the influence of two organometallic compounds on the inhibition of crystallization of hydroxyapatite. These compounds are complexes of Ti(IV) with the general formula [Cp2Ti(aa)2]2+2Cl−, where Cp = η 5-C5H5 cyclopentadienyl and aa the amino acid glycine or alanine. The experiments were conducted according to the constant composition technique in supersaturated solutions containing calcium and phosphate ions. The kinetic results indicate a surface diffusion controlled mechanism of the hydroxyapatite (HAP) crystals. The experiments prove that the presence of [Cp2Ti(Ala)2]2+2Cl− and [Cp2Ti(Gly)2]2+2Cl− complexes affects drastically the profile formation rate of the HAP crystals under biological conditions. The complex with the amino acid alanine provides a stronger inhibition of the formation rate comparing to the complex with glycine. The experimental observations are supported by computer calculations.

The charge trapping centers in fresh and thermally aged conductive polyaniline–polypyrrole copolymers (starting from pure polyaniline and ending in pure polypyrrole by steps of 10 wt%) were studied by thermally stimulated depolarization current (TSDC) spectroscopy and dc conductivity measurements. One low temperature (115–135 K) relaxation mechanism appears in polyaniline-rich copolymers and disappears after thermal anneal. It is attributed to polaron trapping in the vicinity of chlorine anion dopants. Another relaxation mechanism operates close to room temperature and is related to conformational relaxation, which corresponds to the influence of the annihilation of the conformons on the conduction mechanism. The relaxation is sensitive to thermal aging as a result of the thermal annealing on the conductivity. The temperature derivative of the dc conductivity supports the latter assertion. The activation energy values were obtained as a function of composition for both fresh and heated samples. The TSDC spectroscopy is a sensitive tool which is an alternative to the standard conductivity studies for the characterization of polypyrrole–polyaniline conductive copolymers and the study of the thermal degradation.

The kinetics of calcium carbonate hexahydrate crystallization on polymer containing CN, functional group was investigated by he constant composition method. The presence of the above-mentioned functional group stabilize this calcium carbonate polymorph at room temperature which transforms slowly to vaterite. The apparent order for the crystallization process was found to be 4±0.3 indicative of a surface nucleation mechanism. The number of ions forming the critical nucleus was found to be p =5. This value was confirmed by quantum mechanical calculations. The surface energy of the growing phase was 33 mJ m −2 . The formation of hexahydrate initiated through the interaction of Ca 2+ ions with the negative end of the CN bond.

The kinetics of calcite (CaCO 3 ) crystallization on calcite in the presence of titanocene, zirconocene, vanadocene, hafnocene dichlorides and ferricenium salts was investigated by the constant composition technique. The rates of crystal growth measured in the presence of metallocene complexes at concentrations 2.60–4.02×10 −5 mol dm −3 were drastically reduced by 27–66%, possibly through adsorption onto the active sites of crystal growth. Kinetics analysis suggested a surface diffusion controlled spiral growth mechanism.

The kinetics of vaterite (CaCO3) crystallization on calcite in the presence of the aminoacids alanine, glycine, lysine as well as on lysine, polyglycine, polytyrosine and polymethionine was investigated by the constant composition method. The presence of the above mentioned aminoacids stabilizes this calcium carbonate polymorph. The number of ions forming the critical nucleus, the surface energy as well as the apparent growth order for the forming phase was estimated.

The effect of sodium alginate in supersaturated solutions of calcium carbonate was investigated under plethostatic conditions. The rates of crystal growth measured in the presence of sodium alginate at concentrations as low as 0.83 × 10-7mol dm-3 were drastically reduced. Kinetic analysis according to a Langmuir-type adsorption isotherm led to the calculation of an affinity constant Kaff = 999.8 × 10-4mol dm-3. The apparent order found from kinetic data was 3.0±0.2 suggesting a surface nucleation mechanism.

The thermal aging of conducting polypyrrole/polyaniline (PPy/PA) blends heated at 70°C for up to about 600 hours was studied by d.c. conductivity measurements and by thermally stimulated depolarization current (TSDC) spectroscopy in the temperature range from 80 to 300 K. The composition of the samples varied from pure polypyrrole to pure polyaniline with the PA content increasing in steps of 10%. Although the σ = σ(T) curves seem smooth, the corresponding Δσ/ΔT = f(T) curves exhibit systematically ascattering of points in the temperature ranges from 100 to 140 K and from 225 to 320 K approximately for all the samples before and after the heat treatment. TSDC measurements with the MISIM (metal-insulator-sample-insulator-metal) configuration, show a low temperature peak at 100–120 K and a high temperature peak at 280–300 K. The first disappears when the polypyrrole content exceeds 70% and for all the samples after heat treatment, although the high-temperature peak persists. The explanation given to this correspondence between d.c. conductivity and TSDC signals is based on the destruction of conformons and the mobility change of the polymer chains with rising temperature.

Recent studies in the bibliography showed that calcium carbonate was the major constituent (77.8%) in gall stones, and the polymorph calcite was at 62.5% of the cases examined. The kinetics of crystallization of calcite on sodium cholate has been studied using the constant composition technique. Analysis of the initial rates as a function of the solution supersaturation, according to the classical nucleation theory, yielded a value of 33 mJ m-2 for the surface energy of the growing phase and a five-ion cluster, forming the critical nucleus. The apparent order for the calcite crystallization was found to be 4.5±0.7 indicative of a surface nucleation mechanism. The formation of calcite may be initiated through the interaction of Ca2+ ions with the negative end of the C=0 bond of the sodium cholate molecule.

The effect of three homo-aza-steroids, 3-aza-A-homo-4α-androstene-4,17-dione, 4-oxo-3-aza-A-homo-4α-androsteno[16,17-c]pyrazole, and 4-oxo-3-aza-A-homo-4α-androsteno[16,17-c]phenyl pyrazole, used as drugs with anticancer properties, on the crystal growth of hydroxyapatite is investigated at conditions of sustained supersaturation employing the constant composition technique. All three aza-steroids were found to inhibit the crystal growth of hydroxyapatite possibly through adsorption on the active sites available for crystal growth. A detailed kinetic analysis suggested a Langmuir-type adsorption of the polycyclic compound on the surface of hydroxyapatite. The apparent order for the crystallization reaction was determined to be ca. 2, thus suggesting a surface diffusion controlled spiral growth mechanism.

The use of biprosthetic valves remains limited, due to poor long-term durability, primarily as a result of tissue calcification. Porcine and human cardiac valve leaflets were found to be a substrate favoring the deposition of vaterite crystals from stable supersaturated solution at pH 8.5 at 25°C. The apparent order for vaterite crystallization reaction was found to be n=1, suggesting a surface diffusion controlled mechanism. The crystallization was studied by the constant composition technique, thus making it possible for relatively large amounts of the overgrowth phase to be formed and identified exclusively as vaterite. Analysis of the initial rates of the reaction as a function of the solution supersaturation, according to the classical nucleation theory, yielded a value of 23.4 mJ m−2 for the surface energy of the growing phase and a three-ion cluster forming the critical nucleus. A novel “in vitro” anticalcification process based on the treatment of porcine and human heart valves with sodium alginate has been developed. This anticalcification procedure reduced mineralization of porcine and human valves up to 53% and 45%, respectively. Kinetic analysis according to a Langmuir type adsorption isotherm lead to the calculation of an affinity constant for the adsorption of sodium alginate to the substrate.

The crystal growth of hydroxyapatite (HAP), the most stable calcium phosphate phase at 37 °C, pH 7.40, and ionic strength 0.15 M, in the presence of four amino acids with uncharged polar side groups (glycine, cysteine, cystine, and glutamine) has been investigated by the constant composition technique. Crystallization took place exclusively on well-characterized HAP crystals in supersaturated solutions only with respect to this calcium phosphate salt. The kinetic results indicated a surface diffusion mechanism. The presence of the amino acids inhibited the crystal growth of HAP possibly through adsorption onto the active growth sites of the HAP crystal surface. Cysteine in particular caused a reduction in the crystal growth of HAP up to 81%. The kinetic results favored a Langmuir type adsorption model, and the value of the affinity constant for each amino acid was calculated. Each value was related to the nature of the amino acid side group.

A retardation effect (62–76%) was observed on the crystallization of calcium carbonate in the presence of an ultrasonic applied field (0.56 w/cm 2 ). The apparent order of crystal growth of calcite was found to be n =2±0.2 for the seeded growth experiments and n =6.7±0.2, n =5.3±0.1 for the spontaneously precipitated vaterite in the absence and presence of the ultrasonic field, respectively. The mechanism, the nature, the morphology and the size of the crystals formed was not influenced by the applied ultrasonic field, which may possibly affect the dehydration step of the growth units or the migration step of the dehydrated growth units to the active growth sites.

The kinetics of vaterite (CaCO3) crystallization on calcite in the presence of aspartic acid was investigated by the constant composition technique. The presence of aspartic acid in the supersaturated solution stabilizes this calcium carbonated polymorph possibly through the development of active growth sites, which should show chemical and structural affinity to this mineral phase. The number of ions forming the critical nucleus was n* = 4, and a surface energy of 31 mJ m-2 for the growing phase was estimated. The apparent order was found to be 1.75 ± 0.25, indicative for a surface diffusion controlled mechanism.

Benzotriazoles have been used satisfactorily for the last 25 years as a corrosion inhibitor for copper pipelines. The effect of six benzotriazoles on the crystal growth of calcite was investigated at sustained supersaturation. The rates of crystal growth measured in the presence of benzotriazoles at concentrations of 7.5×10 −5 M were drastically increased. The apparent order of the reaction found from kinetic data was n =2±0.2, thus suggesting a surface diffusion controlled mechanism.

Fibrin, a protein endoproduct of blood coagulation was found to be a substrate favoring the deposition of vaterite crystals from stable supersaturated solutions at pH 8.5 and at 25°C. The apparent order for the vaterite crystallization reaction was found to be 1.0±0.1, suggesting a surface diffusion-controlled mechanism. The crystallization was studied by the constant solution composition technique thus making it possible for relatively large amounts of the overgrowth phase to be formed and identified exclusively as vaterite. Fibrin stabilizes this calcium carbonate polymorph, preventing the transformation to the thermodynamically more stable calcite. Analysis of the initial rates of the reaction as a function of the solution supersaturation, according to the classical nucleation theory, yielded a value of 21±1 mJ m−2 for the surface energy of the growing phase and a three-ion cluster forming the critical nucleus.

The crystal growth of hydroxyapatite (HAP), the thermodynamically most stable calcium phosphate phase at 37°C, pH 7.40 and ionic strength 0.15 M in the presence of ferrocene complexes has been investigated by the constant composition technique. The kinetic results showed a surface diffusion controlled mechanism. The presence of ferricenium salts inhibited the crystal growth of HAP possibly through adsorption onto the active growth sites for crystal growth. The kinetic results favored a Langmuir-type isotherm, suggesting a high affinity of ferricenium salts for HAP.

The thermal aging in various conducting polyaniline and polypyrrole composites protonated with hydrochloric acid, containing polymers with sulphonic or phosphoryl groups is investigated. The DC conductivity was measured in the temperature range 300–80 K for aging times from 0 to 300 h at 70°C in room atmosphere. The conductivity decreases with aging time according to the law σ = σ 0 exp[−( t / τ ) 1/2 ], indicating an inhomogeneous structure of the granular metal type. Time τ , which specifies the aging rate, varies with the porosity and the composition of the composites from 40 to 3000 h. The porosity and the presence of sulphonic or phosphoryl groups retards the aging process. The conductivity of the composites with temperature T follows the σ = σ 0 exp[− T 1 /( T + T 0 )] law predicted by Sheng's model of fluctuation-induced tunneling (FIT), which assumes conductive grains separated by insulating barriers in accordance with the thermal aging law mentioned above. Thermal degradation is attributed to the release of HCl from the samples, which reduces the protonated-conducting phase.

Crystallization kinetics of hydroxyapatite, HAP, in the presence of acidic amino acids (aspartic and glutamic acid) were investigated at conditions of sustained supersaturation. In the presence of both aspartic and glutamic acid, the crystal growth rates of HAP decreased markedly. Thier action is due to adsorption and subsequent blocking of the active growth sites onto the surface of the HAP crystals. The kinetic results revealed that a Langmuir-type adsorption isotherm is followed and affinity constants of both amino acids for HAP crystal surface were calculated. The apparent order of the crystal growth reaction was found to be equal to 2, suggesting a surface diffusion controlled mechanism.

Four natural amino acids with hydrophobic nonpolar side group (alanine, phenylalanine, proline, and methionine) were examined and their activity on hydroxyapatite, HAP, crystallization was assessed. The method used was the constant composition technique, at low supersaturation levels. Crystallization took place exclusively on well-characterized HAP seed crystals introduced in supersaturated solutions only with respect to the calcium phosphate salt. The fit of the experimental data to the Langmuir adsorption isotherm indicates an inhibition mechanism based upon molecular adsorption of the amino acids on the HAP crystal surface at active growth sites. The inhibitory effect was related to the adsorption affinity constant of each amino acid to the crystal surface, which depends on the nature of the amino acids' side group.

In the present work we employed fibrin in order to assess its capability to induce biological mineralization. Fibrin is a very important factor in the blood-clotting system. Structurally, fibrin is an ordered organic matrix which has a periodic structure that repeats every 230 A. Hydroxyapatite, HAP and octacalcium phosphate (OCP) are the most interesting calcium phosphate salts. Hydroxyapatite is thermodynamically the most stable calcium phosphate which is mostly used as a model compound for the study of biological-calcification processes. On the other hand, octacalcium phosphate has been proposed as a precursor of hydroxyapatite whose formation is favoured kinetically in solutions supersaturated to both salts. The kinetics of crystallization of HAP and OCP on fibrin were studied using the constant composition technique. The onset of HAP crystallization started immediately after introducing the substrate in the supersaturated solution. Unlike HAP crystallization induction periods were observed before the appearance of OCP precipitate in a solution supersaturated with respect to both HAP and OCP. Using nucleation rate equations derived from the classical homogeneous nucleation theory, interfacial energies and the size of the critical nucleus for both HAP and OCP were calculated. Phosphate was taken up extensively by the biological molecule studied. The dependence of adsorption upon ionic strength and pH of the medium suggests an appreciable contribution of electrostatic forces. Controversially calcium ions did not exhibit any detectable adsorption from solutions containing calcium dichloride at concentrations ranging from 1×10 −4 to 5×10 −3 M in 0.15 M NaCl supporting electrolyte, 37°C, pH=7.4. From the results above it follows that formation of HAP on fibrin may be initiated via adsorption of inorganic phosphate on the biological substrate.

The precipitation and dissolution processes of the iron phosphate system may be of particular interest for the mechanism of phosphorus binding in the soils. The system of Fe(NO3)3–H3PO4–H2O was investigated at pH 2.00 and 25°C. At high supersaturations a crystalline phase of the formula Fe(H2PO4)(HPO4) · xH2O forms spontaneously at rates strongly dependent on the solution supersaturation. The solubility product of the spontaneously precipitating phase was found. Seeding of stable supersaturated solutions with the spontaneously precipitating solid resulted in ferric phosphate dihydrate overgrowth, with a second-order dependence of the rate on the relative solution supersaturation, suggesting a surface diffusion – controlled mechanism.

Among semiconductors, metal sulfides are very interesting materials that may be used for solar energy conversion arrangements. The authors prepared a series of Cu{sub x}Ni{sub 1{minus}x}S powders, where x = 0.05, 0.20, 0.50, and 0.80, by spontaneous precipitation. Various stoichiometries of Cu(NO{sub 3}){sub 2}, Ni(NO{sub 3}){sub 2}, and (NH{sub 4}){sub 2}S solutions at 25 C and pH 2.50, at conditions in which the aqueous solutions were supersaturated with respect to CuS and NiS, yielded precipitates corresponding to intermediate mixed sulfides. The appearance of new peaks in the powder X-ray diffraction spectra suggested the formation of new phases. Measurements of the resistance of the bulk solids as a function of temperature for the preparations with x = 0.05, 0.20, and 0.50 showed a decrease with increasing temperature. A hysteresis was found upon thermal cycling of the sulfide preparations, possibly due to changes in the solid state that affect the energy bands of the semiconductors. The Cu and Ni sulfide preparations with x = 0.80 showed metallic behavior. All preparations were highly charged with suspended in aqueous media, and the electrical charge was found to depend strongly on the activity of copper ion in solutions equilibrated with the solid phase. Finally, currentmore » potential curves of electrodes prepared from the precipitated sulfides under constant illumination gave values for the open circuit potential as high as 0.868 V, with 0.119 mA for the photocurrent and 0.58 for the field factor. These values are encouraging for the potential application of these materials in cells for the conversion of solar into electric energy.« less

Poly(vinyl chloride-co-vinyl acetate-co-maleic acid) (vinyl chloride 83%, vinyl acetate 13%, carboxylated 1%) was found to be a substrate favoring the deposition of vaterite crystals from stable supersaturated solutions at pH 8.50 and 25 °C. Induction times preceding calcium carbonate precipitation were inversely proportional to the solution supersaturation, and a surface energy of 23 mJm-2 was calculated according to classical nucleation theory. The relatively low value may be attributed to the heterogeneous character of vaterite nucleation. The linear dependence of the rates of vaterite formation on the solution supersaturation, in which the crystallization took place, in combination with the independence of the measured rates on the fluid dynamics, suggested that vaterite overgrowth was controlled by surface diffusion. This finding was in agreement with the results obtained for the crystallization of vaterite on cholesterol. Our results suggest that the kinetics of overgrowth may be very important for ...

A series of Cu−Cd sulfides were prepared by coprecipitation in aqueous supersaturated solutions at 25 °C, pH 2.50. The solids obtained were characterized by powder X-ray diffraction and their elemental composition was determined by chemical analysis. The results showed that in all cases mixed CuS−CdS precipitates were prepared, the ratio of each phase depending on the composition of the supersaturated solutions. A series of precipitates, of (CuS)x (CdS)1-x, were prepared with x = 0.0, 0.20, 0.25, 0.30, 0.35, 0.50, and 1.0. All polycrystalline preparations with x = 0.30 showed metallic behavior typical for CuS, while the mixed sulfides with x = 0.20 and 0.25 showed semiconducting behavior, which however could not be described by any of the models reported in the literature. The band gap energies of 2.40 and 2.39 eV calculated for the two preparations showed that at this composition the material was dominated by the behavior of CdS. The activation energies estimated from the dependence of the solids conduct...

Elastin, a highly cross-linked protein polymer found in the arterial walls was found to be a substrate favoring the deposition of calcite crystals from stable supersaturated solutions at pH 8.50 and at 25°C. The apparent order for the calcite crystallization reaction was found to be 2.0±0.1, suggesting a surface diffusion controlled mechanism. The crystallization was studied by the constant solution composition technique thus making it possible for relatively large amounts of the overgrowth phase to be formed and be identified exclusively as calcite. The induction periods varied markedly with supersaturation. Analysis of the initial rates of the reaction as a function of the solution supersaturation, according to the classical nucleation theory, yielded a value of 25 mJ m−2 for the surface energy of the growing phase and a three-ion cluster forming the critical nucleus.

The spontaneous precipitation of calcium sulfate in supersaturated solutions over the temperature range between 25.0 and 80.0 °C was investigated by monitoring the solution specific conductivity during desupersaturation. From measurements of the induction times preceding the onset of precipitation the surface energy of the forming solid, identifield as gypsum, was found between ca. 12 and 25 mJ m-2 for the temperature range between 80.0 and 25.0 °C, respectively. Kinetics analysis showed that over 50 °C it is possible that anhydrous calcium sulfate is forming as a transient phase converting into the more stable calcium sulfate dihydrate. The linear dependence of the rates of precipitation on the relative solution supersaturation suggested a mechanism according to which the growth units are integrated into the active sites of the supercritical nuclei by surface diffusion. According to the morphological examination of the crystals it is possible that crystal growth occurs by the advancement of steps.

Transition metal sulfides, because of their electronic and spectroscopic properties, are a very interesting class of materials that may be used efficiently for the construction of photoelectric chemical solar cells. In the present work we have prepared a series of mixed Cu and Zn sulfides with the general stoichiometry Cu x Zn 1-x S by coprecipitation in aqueous solutions. Precipitates with x = 0.10, 0.15, 0.20, 0.22, 0.25, and 0.50 were synthesized by mixing metal nitrates with ammonium sulfide at pH 2.50. The precipitates behaved as n-type semiconductors. The dependence of the conductivity of the materials on temperature and calculations of the thermoelectric power coefficient showed the presence of metallic conducting islands separated by nonconducting parts, for copper content corresponding to x = 0.15, 0.20, and 0.22. For x = 0.10, the precipitates behaved as insulators, whereas forx = 0.50, the precipitates showed metallic character. The morphology and the mean particle size of the precipitates varied with increasing Zn content, yielding for pure CuS, prismatic crystals with a mean size of ∼5 μm. Upon increasing Zn content, polycrystalline materials were obtained, forming aggregates of submicron, spherullitic particles. The measurements of the electrical properties of the materials showed that the conditions of precipitation play an important role for the design of their properties.

The decrease of the electrical conductivity of polyaniline and polypyrrole due to thermal aging is reported. The d.c. conductivity is measured in the temperature range 300-80 K for aging times from 0 to 11 h at 120°C in room atmosphere. The conductivity of polyaniline decreases with aging time according to the law σ = σ 0 exp [ − ( t τ ) 1 2 ] . Polypyrrole diverges either from the above or σ 0 − σ ∝ t 1 2 . Moreover, polyaniline follows a σ = σ 0 exp [ − ( T 0 T ) 1 2 ] law with T0 increasing with aging time. This can be explained by a conduction mechanism consisting of electron tunnelling between conducting grains embedded into an insulating matrix. Polypyrrole follows a σ = σ0exp[ − T1/(T + T0)] law until about 5 h of aging time. For longer heating its behaviour diverges from the predictions of known models of conduction in polymers. The above can be attributed to differences of the aging process in the two compounds. In polyaniline, aging is accompanied simply by a decrease of the grain size, the separation of which increases from 27 to 54 A after 10 h of heating at 120°C. In polypyrrole, for short aging times, the grain size remains constant, their separation being about 60 A. Longer aging leads to a thermally activated conductivity whose mechanism is obscure.

The effect of vanadocenes, used as drugs for various therapeutic applications, on the crystal growth of hydroxyapatite was investigated at sustained supersaturation using the constant composition technique. All vanadocene dichlorides were found to inhibit crystal growth of hydroxyapatite possibly through adsorption onto the active growth sites for crystal growth. The apparent order for the crystallization was n = 2 thus, suggesting a surface-diffusion-controlled mechanism. The kinetic results favoured a Langmuir-type isotherm, suggesting a high affinity of vanadocene dichloride for hydroxyapatite.