Precipitation of different calcite crystal morphologies in the presence of sodium stearate

The influence of sodium stearate (NaSt) on the precipitation of calcium carbonate during the semicontinuous process of slaked lime carbonation was studied in the systems in which process parameters, like concentration of total dissolved calcium, temperature, CO(2) flow rate and initial addition rate of slaked lime, were controlled. It was found that calcite was the only calcium carbonate polymorph that appeared under the investigated experimental conditions, while FT-IR spectroscopy and thermogravimetric analysis of samples confirmed the presence of stearate on the surface of precipitated calcium carbonate (PCC). Specific surface area of PCC increased with increasing stearate content: the highest value, s = 52.8 m(2) g(-1), was obtained at t = 20 degrees C, c(tot) = 17.0 mmol dm(-3) and the stearate content of m(NaSt)/m(CaO) = 0.03. It was also found that hydrophobic calcite crystals in the form of rhombohedral and scalenohedral morphology can be produced at m(NaSt)/m(CaO) > 0.01. The exception is the case of nanosized PCC production, when much higher concentration of NaSt is needed, m(NaSt)/m(CaO) = 0.22. Minimal amount of stearate necessary to build up the monolayer and corresponding cross sectional area of one stearate molecule were estimated for the obtained calcite morphologies.     

The structure and rheology of mixed micellar solutions of sodium dodecyl sulfate and dodecyldimethylamine oxide

The paper presents the results of viscosity measurements by capillary and rotational viscometry for aqueous solutions of sodium dodecyl sulfate and dodecyldimethylamine oxide and data on the hydrodynamic radii of aggregates obtained from dynamic light scattering measurements. A wide range of solution compositions predominantly containing dodecyldimethylamine oxide was studied at 308.15 K; the total content of surfactants was from 1 to 15 wt %, pH ～ 8. Solution viscosities were found to change by several orders of magnitude when the composition of the system was varied; the dependence of viscosity on the ratio of surfactants passes through maximum and inflections. The dependences of viscosity and hydrodynamic radii on the ratio between two surfactants were similar in character. The conclusion was drawn that various nanostructural rearrangements occurred in the solutions studied and synergistic effects were observed.     

The epitaxial growth of cholesterol crystals from bile solutions on calcite substrates

Epitaxial relationships between the surfaces of inorganic and bioorganic crystals can be an important factor in crystal nucleation and growth processes in a variety of biological environments. Crystalline cholesterol monohydrate (ChM), a constituent of both gallstone and atherosclerotic plaques, is often found in association with assorted mineral phases. Using in situ atomic force microscopy (AFM) and well-characterized model bile solutions, the nucleation and epitaxial growth of ChM on calcite (104) surfaces in real-time is demonstrated. The growth rates of individual cholesterol islands formed on calcite substrates were determined at physiological temperatures. Evidence of Ostwald's ripening was also observed under these experimental conditions. The energetics of various (104) calcite/(001) ChM interfaces were calculated to determine the most stable interfacial structure. These simulations suggest that the interface is fully hydrated and that cholesterol hydroxyl groups are preferentially positioned above carbonate ions in the calcite surface. This combination of experimental and theoretical work provides a clearer picture of how preexisting mineral seeds might provide a viable growth template that can reduce the energetic barrier to cholesterol nucleation under some physiological conditions.     

Interaction between bilirubin and sodium deoxycholate and their adsorption from mixed solutions on the surface of silica sorbents

The interaction between the bile pigment bilirubin and sodium deoxycholate is studied in aqueous buffer solutions at pH 7.4. It is established that bilirubin forms strong complexes with deoxycholate trimers. The complexation constant is determined by spectrophotometry. The adsorption of bilirubin and sodium deoxycholate from their mixed solutions on the surface of a hydrophilic and two hydrophobic silica sorbents is investigated. It is shown that bilirubin is adsorbed on the surface of all these sorbents only in the free state. Sodium deoxycholate is adsorbed in the forms of monomers and trimers. The affinity of all adsorbed particles is higher for hydrophobic silica sorbents. The binding constants of bilirubin, as well as monomers and trimers of deoxycholate, with the surfaces of all examined sorbents are determined.     

Unusual mixed solvent supramolecular crystal framework formed of a new tecton-like tetracarboxylic building block

A new tecton-like tetracarboxylic compound 1 featuring two isophthalic acid groups attached to both ends of a rigid 1,3-butadiyne spacer unit is synthesised using acetylene blocking/deblocking and coupling techniques. Crystallisation of 1 from DMSO-chloroform solution gives rise to the formation of an unusual mixed solvent crystalline framework structure 1a containing DMSO and chloroform as secondary and tertiary components in 1:2:1 stoichiometric ratio. The X-ray crystal structure of 1a shows interesting stacking mode and hydrogen-bonded, 3D network architecture with the two solvent species being involved in different behaviour pattern.     

Hydrothermal synthesis and crystal structure of two luminescent three-dimensional supramolecular complexes generated from mixed ligands

Two 3D metal-organic supramolecular complexes [Zn₂(btec)(2, 2′-bpy)₂(H₂O)₂] (1), [Cd₂(dpa)₂(phen)₂(H₂O)₂] (2) have been prepared by hydrothermal reaction and characterized by IR and single-crystal X-ray diffraction analysis. The 3D architectures of 1 and 2 both possess rectangular cavities. Furthermore, compounds 1 and 2 both show intense photoluminescent properties at room temperature.     

Two-dimensional crystal growth from undersaturated solutions

The solubility of a substance is commonly understood as the minimum concentration necessary for the condensation of a solid phase from solution. Here we report the nucleation and growth of ionic compounds from aqueous concentrations on the order of 0.1 times the solubility. The condensation is catalyzed by a foreign substrate, and the new phase grows as a crystalline monolayer. Undersaturated growth is observed only in cases where the dissolved compound is isomorphic with the substrate and the interaction strength between a dissolved-ion/substrate-ion pair exceeds that between the two dissolved ions. These results are consistent with a simple model in which favorable ion-surface interactions lead to ion enrichment and supersaturation in the two-dimensional interfacial zone.     

Smectic layer structure of ferroelectric liquid crystal formed between fine polymer fibres

This paper describes the alignment of ferroelectric liquid crystal (FLC) structures formed between aligned polymer fibres, where the FLC smectic layers are determined by polarising microscopy and X-ray diffraction. The FLC/polymer composite films were formed from a nematic phase FLC/monomer solution using a photopolymerisation-induced phase separation method. It was found that bending of the FLC smectic layers was induced in both the film plane and the cross-sectional plane at the phase transition from smectic A to chiral smectic C of the FLC material. The light transmittance properties of the composite film between crossed polarizers was analysed by light propagation simulation in several optical anisotropic media, based on the evaluated smectic layer model.     

Smectic layer structure of ferroelectric liquid crystal formed between fine polymer fibres

This paper describes the alignment of ferroelectric liquid crystal (FLC) structures formed between aligned polymer fibres, where the FLC smectic layers are determined by polarising microscopy and X-ray diffraction. The FLC/polymer composite films were formed from a nematic phase FLC/monomer solution using a photopolymerisation-induced phase separation method. It was found that bending of the FLC smectic layers was induced in both the film plane and the cross-sectional plane at the phase transition from smectic A to chiral smectic C of the FLC material. The light transmittance properties of the composite film between crossed polarizers was analysed by light propagation simulation in several optical anisotropic media, based on the evaluated smectic layer model.     

Determination of phosphate based on inhibition of crystal growth of calcite

The growth of calcium carbonate seed crystals (calcite) is strongly inhibited by the presence of phosphate ions. The kinetics of crystal growth were followed potentiometrically using a calcium ion-selective electrode or through the use of a pH electrode. The study of different variables on such a process was carried out with the aim of developing kinetic methods to determine phosphate ions (50–400 ng ml^?1). The selectivity and sensitivity of these processes allow the application of crystallization reactions to the determination of phosphate in human urine.     

A hierarchical self-similar structure of oriented calcite with association of an agar gel matrix: inheritance of crystal habit from nanoscale

Calcite crystals grown with association of an organic gel matrix led to the generation of a hierarchical, self-similar architecture possessing rhombohedral habit under ambient conditions.     

Investigation on the basic hydrolysis of crystal violet in mixed reverse micelles formed with AOT and nonionic surfactants

 The kinetics and thermodynamics of the basic hydrolysis of crystal violet (CV) in mixed reverse micelles formed with anionic surfactant AOT and nonionic surfactants have been investigated. It was found that the mixed reverse micelles had inhibitory effects on CV hydrolysis compared with the normal aqueous solution, and the equilibrium constant K of the reaction in mixed reverse micellar systems is smaller than that in pure water. The influence of water content and surfactant composition in reverse micelles on the second-order rate constant k ₁ of the positive reaction, on the first-order rate constant k _-1 of the reverse reaction, as well as on the equilibrium constant K of the reaction has been studied, and the results obtained were interpreted in terms of the nature of surfactants and the properties of microenvironment where the reaction took place. Received: 24 October 1997 Accepted: 18 March 1998     

Study of mixed micellar aqueous solutions of sodium dodecyl sulfate and amino acids

Thermodynamic properties of sodium dodecyl sulfate (SDS) in micellar aqueous solutions of L-serine and L-threonine were determined by fluorescence spectroscopy and dynamic light scattering techniques. The values of Gibbs free energy, enthalpy and entropy of the process of micelle formation were calculated using the critical micelle concentration and degree of dissociation. Changes in critical micelle concentration of SDS with the addition of amino acids were examined by both conductivity and pyrene I ₁/I ₃ ratio methods at different temperatures. The pyrene fluorescence spectra were used to study the change of micropolarity produced by the interaction of SDS with amino acids. The aggregation behavior of SDS was explained in terms of structural changes in mixed solutions. The data on dynamic light scattering suggest that size of SDS micelles was influenced by the presence of amino acids.     

Effects of supercharging reagents on noncovalent complex structure in electrospray ionization from aqueous solutions

The effects of two supercharging reagents, m-nitrobenzyl alcohol (m-NBA) and sulfolane, on the charge-state distributions and conformations of myoglobin ions formed by electrospray ionization were investigated. Addition of 0.4% m-NBA to aqueous ammonium acetate solutions of myoglobin results in an increase in the maximum charge state from 9+ to 19+, and an increase in the average charge state from 7.9+ to 11.7+, compared with solutions without m-NBA. The extent of supercharging with sulfolane on a per mole basis is lower than that with m-NBA, but comparable charging was obtained at higher concentration. Arrival time distributions obtained from traveling wave ion mobility spectrometry show that the higher charge state ions that are formed with these supercharging reagents are significantly more unfolded than lower charge state ions. Results from circular dichroism spectroscopy show that sulfolane can act as chemical denaturant, destabilizing myoglobin by ∼1.5 kcal/mol/M at 25 °C. Because these supercharging reagents have low vapor pressures, aqueous droplets are preferentially enriched in these reagents as evaporation occurs. Less evaporative cooling will occur after the droplets are substantially enriched in the low volatility supercharging reagent, and the droplet temperature should be higher compared with when these reagents are not present. Protein unfolding induced by chemical and/or thermal denaturation in the electrospray droplet appears to be the primary origin of the enhanced charging observed for noncovalent protein complexes formed from aqueous solutions that contain these supercharging reagents, although other factors almost certainly influence the extent of charging as well.     

Fluctuation structure of solutions of polyamic acid salts in mixed solvents

Structural organization is studied for dilute solutions of the comblike N,N-dimethylhexadecylammonium salt of polyamic acid based on 3,3′,4,4′-diphenyltetracarboxylic dianhydride and o-tolidine by the method of polarized light scattering in mixed dimethylacetamide-benzene and N-methylpyrrolidone-benzene solvents. The statistical structural parameters of the solutions are calculated according to the Debye-Stein theory. The dimensions of ordered scattering fluctuation-type formations are shown to be much greater than the molecular dimensions and to depend on the chemical nature and composition of the mixed solvent. At polymer concentrations below 0.2 g/dl, the corresponding concentration dependences of the statistical structural parameters are characterized by extreme patterns for all systems under study.     

Glycine phases formed from frozen aqueous solutions: Revisited

Glycine phases formed when aqueous solutions were frozen and subsequently heated under different conditions were studied by Raman scattering, x-ray diffraction, and differential scanning calorimetry (DSC) techniques. Crystallization of ice I(h) was observed in all the cases. On cooling at the rates of 0.5 K∕min and 5 K∕min, glassy glycine was formed as an intermediate phase which lived about 1 min or less only, and then transformed into β-polymorph of glycine. Quench cooling of glycine solutions (15% w∕w) in liquid nitrogen resulted in the formation of a mixture of crystalline water ice I(h) and a glassy glycine, which could be preserved at cryogenic temperatures (80 K) for an indefinitely long time. This mixture remained also quite stable for some time after heating above the cryogenic temperature. Subsequent heating under various conditions resulted in the transformation of the glycine glass into an unknown crystalline phase (glycine "X-phase") at 209-216 K, which at 218-226 K transformed into β-polymorph of glycine. The "X-phase" was characterized by Raman spectroscopy; it could be obtained in noticeable amounts using a special preparation technique and tentatively characterized by x-ray powder diffraction (P2, a = 6.648 A?, b = 25.867 A?, c = 5.610 A?, β = 113.12[ordinal indicator, masculine]); the formation of "X-phase" from the glycine glassy phase and its transformation into β-polymorph were followed by DSC. Raman scattering technique with its power for unambiguous identification of the crystalline and glassy polymorphs without limitation on the crystallite size helped us to follow the phase transformations during quenching, heating, and annealing. The experimental findings are considered in relation to the problem of control of glycine polymorphism on crystallization.