I. V. Melikhov

Simulation of batch crystallization

Abstract The closed mathematical model of a well-mixed batch crystallizer has been presented. This model takes into consideration crystals growth rate fluctuations producing an increased spread of crystal sizes. The numerical methods for solving partial integro-differential system of this model equations are proposed for the cases of when growth rate is expressible as product of supersaturation and size functions. Simultaneous population and mass balance equations have been calculated together with power law kinetics of nucleation and growth to determine crystal-size distribution functions (CSD), variation coefficients (CV), produced crystal sizes and numbers. The case of size-dependent crystal growth determined by sequential processes of diffusion and surface reaction is also investigated. The computer results are presented in the dimensionless form as functions on both diffusion and fluctuation Peclet numbers and dimensionless cooling rate. In particular, it has been established that the CV of weight CSD increases approximately twice due to fluctuations and becomes close to 40%; the CV also increases owing to the decrease of cooling rate and approaches 28%. Some formulae are suggested for predicting the kinetics of crystallization (or precipitation) and determining the kinetic parameters of crystals growth and nucleation.

Electron microscopic study of nucleation and growth of highly dispersed solid phase

Abstract Crystallization of barium carbonate and barium sulfate at fast mixing of reagents solutions was investigated. The method of high resolution electron microscopy showed that during crystallization ultramicrocrystals of 1- to 5-nm size are formed which have an oriented aggregation and are further transformed into polyhedrons not different in their outer appearance from the single crystals. The change of ultramicrocrystals size distribution is described by the Fokker-Planck equation. Ultramicrocrystal nucleation and growth at large supersaturation interval obey the kinetic reaction equations of the third and the second order, respectively. It is possible to consider nucleation as ion triplet associate formation with further rearrangement requiring activation energy of approximately 70 kJ mole −1 .

The effect of dissolved impurity on calcium phosphate nucleation in supersaturated medium

Abstract Using the example of amorphous calcium phosphate (ACP) precipitation, it was shown that molecules of dissolved impurities can cause nucleation in supersaturated aqueous solutions. Electron microscopy was used as the basic method of analysis. ACP was synthesized under the conditions of rapid reagent mixing at a pH of 10.80 in a wide range of initial reagent concentrations. Transmission electron microscopy indicates that ACP particles are spheroids with an average size of 27 nm, partially joined into larger aggregates. The particle size distribution function can be described by the Fokker-Planck equation, the solution of which is in good agreement with experimental data provided proportionality exists between the number of nucleation centers and the concentration of impurities introduced with the reagents. It can be concluded that the impurity molecules contained in the reagents present a basis for ACP particle formation.

The influence of fullerene additives on the structure of polystyrene films

It was shown by electron microscopy and X-ray diffraction that a polystyrene solid with a three-level hierarchical structure was formed when the solvent was evaporated from a solution of polystyrene in o-xylene (polystyrene molecules stuck together united to form a framework). The kinetics of the transformation of formerly dissolved molecules into aggregates could be described by a Fokker-Planck-type equation. Fullerenes introduced into a solution of polystyrene in amounts less than 0.1 wt % transformed aggregated polystyrene molecules into nanocrystals and accelerated the formation of aggregates. The influence of fullerenes on the kinetics of the processes could be correctly described in the continuum approximation taking into account aggregation rate fluctuations.

Use of solid-phase inhomogeneities to increase the efficiency of ultrasonic therapy of oncological diseases

Authors’ concepts on the use of solid-phase sonosensitizer nanoinclusions in biological structures as ultrasonic energy concentrators in the therapy of oncological diseases are developed. The possibility of directed synthesis of nanoparticles and their aggregates in tumor tissue depending on its growth features is discussed. It was found that acoustic effects in polymer structures containing solid-phase inclusions depend on the nature of these inclusions and their bond with the polymer matrix. Using model gel systems, it was shown that solid-phase sonosensitizers enhance local thermal effects and amplitude-dependent scattering of ultrasound during its propagation in gel. Experimental studies on animals showed that the ultrasound exposure of malignant tumors containing nanoparticles of gold and some complex compounds results in a significant therapeutic effect.

Crystallization behavior of nanodisperse phases

Extensive experimental work demonstrates that the crystallization leading to the formation of a nanocrystalline phase is characterized by the following specific features: particles of the solid phase nucleate in a small region of the system where a high supersaturation is produced (local character of the process); crystallization involves a few, kinetically self-similar stages (multistage character); several structural and morphological forms may develop concurrently during crystallization (multiple paths). The information accumulated to date provides the basis for quantitatively describing all the stages of crystallization and optimizing the synthesis of nanomaterials

Strategy and tactics in the search for new materials technology

We discuss ways of overcoming the crisis in which an exponential accumulation of information about any specific material is not accompanied by the development of theories allowing the use of the accumulated information to create new materials. To systematize the existing information and identify process regularity leading to the transformation of a substance into material, the Fokker-Planck equation is proposed, with frequency functions subject to experimental determination. It is shown that the use of this equation will supplement currently adopted methods for the creation of functional materials so that materials are the most consistent with consumer requirements and that they contribute to development of the theory. This upgrade is an important tactical problem that will prepare the development of a material theory that ensures a priori selection its optimal technology.

Electron microscopy of biomaterials based on hydroxyapatite

Three types of biomaterials based on hydroxyapatite are synthesized and investigated. Hydroxyapatite nanocrystals or microcrystals precipitated from low-temperature aqueous solutions serve as the initial material used for preparing spherical porous granules approximately 300–500 μm in diameter. Sintering of hydroxyapatite crystals at a temperature of 870°C for 2 h or at 1000°C (for 3 h) + 1200°C (for 2 h) brings about the formation of solid ceramics with different internal structures. According to the electron microscopic data, the ceramic material prepared at 870°C is formed by agglomerated hydroxyapatite nanocrystals, whereas the ceramics sintered at 1200°C (with a bending strength of the order of 100 MPa) are composed of crystal blocks as large as 2 μm. It is established that all the biomaterials have a single-phase composition and consist of the hydroxyapatite with a structure retained up to a temperature of 1200°C.

Localization of acoustic energy in gel systems on solid-phase inhomogeneities

The thermal effects of ultrasound on an agarose gel containing nanoparticles of iron(III) hydroxide and barium sulfate are comparatively studied. The agarose matrix is shown to interact differently with iron(III) hydroxide and barium sulfate. The relative change in ultrasound absorption due to modifier particles located in the gel is estimated. The highest thermal effect is observed for systems in which modifiers are located on separate elements of the matrix bulk. Production of “containers” with ultrasound-controlled drug release on the basis of thermosensitive gels containing solid-phase inclusions is discussed as an example of possible application of the effects described.

Secondary nucleation in the formation of methane crystal hydrate

The kinetic data on crystallization and a morphological analysis of a layer of CH4 · 6H2O hydrate crystals formed on the surface of water as a result of methane absorption showed that secondary nucleation occurred during hydrate crystallization. The mutual arrangement of crystals in the layer revealed photographically in situ was evidence that part of nuclei produced on the surface of previously formed crystals went away from the surface into solution and grew there independently of “mother” crystals, although the probability of such transfer into an immobile solution remained low. In view of this, a model of crystal growth generating secondary crystals was developed.