Volume 74, Issue 10

An analysis is given of several biologically important processes, connected with the crystallization and kinetics of liophilization in aqueous physiological solutions. These are glass formation and vitrification of life, precipitation of slightly soluble salts and kinetics of crystallization in human urine, and the solution of CaOX concretions under the influence of biocompatible solubilizators, which are normal constituents of human urine. The problems investigated experimentally are treated in the framework of the capillary theory of nucleation and the Debye–Hückel model of ionic solutions.

The solubility isotherms of the systems Na 2 SO 4 ·10H 2 O–Na 2 SeO 4 ·10H 2 O–H 2 O and Na 2 SO 4 ·7H 2 O–Na 2 SeO 4 ·7H 2 O–H 2 O have been investigated at 15 °C. It is established that discontinuous series of mixed crystals are formed in both systems. Two methods of calculation of the free Gibbs energy of phase transition at the interruption point of the solubility diagrams are used: (i) on the basis of the composition of the two mixed crystal types, which are in equilibrium with the saturated solution in the eutonic point of the system, and (ii) from the distribution coefficients between each of the mixed crystal phases and their saturated solution, using both experimentally obtained values and calculated distribution coefficients for ideal isomorphic mixing. The data found by the two methods exhibit very good agreement. It is established that the free Gibbs energy of the phase transition in the case of decahydrates is lower than in the case of heptahydrates. This fact is considered as a confirmation of the buffering action of water molecules with respect to the distortion of the crystal structures of the pure salts provoked by the formation of mixed crystals.

The dissolution of gases, liquids,or solids in liquids is a heterogeneous reaction, and in many cases, the reaction rate is limited by a diffusive transfer of substance in the boundary layer at the face interphase. In modern industry, intensive interphase mass transfer processes are accomplished by large concentration gradients. This can result in a set of nonlinear effects and the difference between experimental data and predictions of the linear mass transfer theory. One of the main nonlinear effects is a result of the induction of a secondary flow at the interface boundary. This can lead to essential changes of the dissolution rate and the hydrodynamic stability of the system.

The main goal of this presentation is to tentatively forecast or improve an extractive process. The sea-water system is described using a graphical representation of the quinary ion-pair system Na + , K + , Mg 2+ /Cl − , SO 4 2- //H 2 O. The operations required by extraction or purification of a sea-water component are simulated by a path through the solubility diagram.The procedure is applied to isothermal evaporation of Black Sea water at 25 °C, and the balance of the crystallization sequence is determined. In the same way, the cooling of residual brine, after precipitation of pure sodium chloride, is studied.

The bioconcentration factor (BCF) is a parameter that describes the ability of chemicals to concentrate in aquatic organisms. Traditionally, it is modeled by the log–log quantitative structure -activity relationship (QSAR) between the BCF and the octanol- water partition coefficient ( K ow). A significant scatter in the parabolic log(BCF)/log( K ow ) curve has been observed for narcotics with log( K ow ) greater than 5.5. This study shows that the scatter in the log(BCF)/log( K ow ) relationship for highly hydrophobic chemicals can be explained by the molecular size. The significance of the maximal cross-sectional diameter on bioconcentration was compared with the traditionally accepted effective diameter. A threshold value of about 1.5 nm for this parameter has been found to discriminate chemicals with log(BCF) > 3.3 from those with log(BCF) < 3.3. This critical value for the maximum diameter is comparable with the architecture of the cell membrane. This threshold is half thickness of leaflet constituting the lipid bilayer. The existence of a size threshold governing bioconcentration is an indication of a possible switch in the uptake mechanism from passive diffusion to facilitated diffusion or active transport. The value of the transition point can be used as an additional parameter to hydrophobicity for predicting BCF variation. The effect of molecular size on bioconcentration has been studied by accounting for conformational flexibility of molecules.

Models that accurately predict solid–solute phase equilibria in aqueous electrolyte solutions are of mounting importance for numerous industrial processes, especially those operating at high temperatures, pressures, and concentrations. The incorporation of such electrolyte models into process simulators is in great demand. This communication will focus on thermodynamically consistent models that can simultaneously predict densities, heat capacities, and apparent molar enthalpies of multicomponent electrolyte mixtures together with activities of their constituents. Data optimization issues to be discussed include the CALPHAD (CALculation of PHAse Diagrams) method applied to electrolyte systems and the generation of robust models that extrapolate well outside the parameterization space. Recent development of software suitable for the simulation of industrial processes involving concentrated electrolyte solutions will be outlined.

Arsenic is a toxic element for animals and the majority of plants, in spite of evidence that it is also an essential element. The long-term intake of small doses of arsenic has a carcinogenic effect. There are well-identified regions where arsenic ground water concen- trations can reach values higher than 2 mg/L. Water purification and waste treatment techniques based on (1) precipitation of calcium, magnesium, and iron(III) arsenates, and/or (2) adsorption or coprecipitation of arsenic oxyanions are unlikely to produce aqueous solutions with arsenic concentrations below the guideline values proposed for arsenic dissolved in potable water and treated sewage effluents. As(III) species are more toxic than As(V) species. Arsenate species are predominant at moderate and high redox potentials, while arsenite species occur under more reducing conditions. Metal arsenites are much more soluble than the corresponding metal arsenates, and arsenites are adsorbed less by solid phases. Remediation techniques must consider the available information on solubility and adsorptive properties of As(III) and As(V). The less-soluble lead and barium arsenates are not suitable for arsenic decontamination. New remediation methods must consider solubility data for arsenic-containing materials and minerals.

A requirement for the determination of the solubility of minerals is to ensure that equilibrium has been reached. Recent constant composition dissolution studies of sparingly soluble calcium phosphates have revealed an interesting and unusual behavior in that the rates decreased, eventually resulting in effective reaction suppression, even though the solutions remained undersaturated. Traditional theories of dissolution assume a volume diffusion-controlled mechanism with reaction continuing until true equilibrium has been reached. The new results for sparingly soluble salts point to the importance not only of particle size on the dissolution rate but also the participation of critical phenomena. Although the crystal size decreases during dissolution, when the reaction is controlled by poly-pitting (the formation and growth of pits), the edge free energy increases at the very first stage of reaction owing to the creation of pits and dissolution steps. The constant composition experimental results demonstrate the development of surface roughness as the dissolution steps are formed, implying an increase of the total edge length during the reaction. This is an exactly analogous mechanism to that of crystal growth, in which the formation of embryos of critical size plays a key role in the overall mechanism. In contrast to crystal growth, dissolution is a process of size reduction, and, when the particle size is sufficiently reduced, critical phenomena become important so that the influence of size must be taken into consideration. It is interesting to recognize that these critical phenomena are readily apparent for sparingly soluble minerals for which the critical conditions are attained much more readily. The results point to the importance of understanding the detailed mechanism of dissolution when attempts are made to measure, experimentally, the solubilities of sparingly soluble minerals.

In the context of a long study of the dynamical properties of water and inorganic aqueous solutions using infrared (IR) and Raman spectroscopy,we present here recent results of the influence of salt concentration on the dynamics of water molecules. The main systems studied were salts of X n Y m type (where X = Li + , Na + , K + , Rb + , Cs + , Mg 2+ , Zn 2+ and Y = Cl – ,ClO 4 – , NO 3 – , SO 4 –2 ). The whole spectrum reflects the molecular and ionic interactions between solvent molecules and solvent–solute and solute–solute molecules. Nevertheless, each spectral region supplies different information about these intermolecular interactions. The strong modifications in intensity and band profile undergone by the v(OH) band depends mainly on the nature of anions while cations have a weak influence. A picture of these different situations in a particular salt solution needs the support of a model of the dynamic structure of the water solvent. A discussion of the current state of the dynamical models of liquid water and new ideas on water dynamics are presented on the basis of the results obtained from the study of internal OH/OD vibrations of water in diluted mixtures of D 2 O/H 2 O. Finally, a comparative analysis of the influence on the dynamics of water molecules under the concentration of perchlorate and chloride salts of monovalent cations in water and isotopic D 2 O/H 2 O mixtures was undertaken using Raman spectroscopy.

Four main types of binary fluid-phase diagrams and available experimental data on binary systems are used as a starting point for derivation of the systematic classification of binary complete phase diagrams by the method of continuous topological transformations. This method and the classification of binary phase diagrams, containing the boundary versions of phase diagrams with ternary nonvariant points, are applied to derive the main types of fluid and complete phase diagrams for ternary systems with one volatile component and immiscibility phenomena in two constituent binary subsystems. The results gained from this analysis of derived fluid and complete phase diagrams of ternary systems are represented.

Results on the local cation ordering in layered lithium-nickel/cobalt oxides and metal-substituted lithium-manganese spinels are presented. It is shown that electron spin resonance of Ni 3+ and Mn 4+ and magnetic susceptibility measurements are powerful tools to monitor the short-range cation ordering in these compounds, which is not accessible by diffraction techniques. Thus, owing to the different strength of the 90° and 180° Ni 3+ –O–Ni 3+/2+ exchange interactions, the distribution of Ni 3+ /Ni 2+ between the lithium and nickel layers in Li 1– x Ni 1+ x O 2 with 0 < x < 0.4 can be determined. For layered LiNi 1– y Co y O 2 and spinel LiMn 2– x Co x O 4 solid solutions, analysis of the temperature-independent EPR line width in terms of dipole–dipole and exchange interactions has been used to examine the local Ni 3+ /Co 3+ and Mn 4+ /Co 3+ ordering. The results obtained are correlated with the electrochemical intercalation of lithium in these compounds.

The present paper gives an overview of the solubility behavior of actinide oxides/hydroxides, taking into account hydrolysis reactions up to colloid formation. The analogies, systematic trends, and differences in the hydrolysis and solubility constants of actinides in the oxidation states An(III) to An(VI) correlate with the charge and size of the actinide ions. The formation of amorphous and crystalline solids and the discrepancies between the corresponding experimental solubility data may be explained as an effect of particle size. However, using thermodynamic data for the stable crystalline solids, the predicted solubilities are often significantly lower than experimental data (even in long-time experiments), indicating that the solubility is controlled by the surface properties. Typical examples are the known U(VI) solids schoepite and sodium diuranate. The most striking example is provided by the tetravalent actinides. Above the threshold of hydrolysis, the dissolution of microcrystalline or crystalline An(IV) oxides is found to be irreversible. The measured concentrations approach those of the amorphous hydroxides or hydrous oxides.

Enthalpy of melting can be used to store heat in a simple way for time periods of hours and days. Knowledge of the solid –liquid equilibria represents the most important presumption for systematic evaluations of the suitability of hydrated salt mixtures. In this paper, two approaches for predicting solid–liquid equilibria in ternary or higher component systems are discussed using the limited amount of thermodynamic data available for such systems. One method is based on the modified Brunauer–Emmett–Teller (BET) model as formulated by Ally and Braunstein. In cases of a strong tendency toward complex formation of salt components, the BET model is no longer applicable. Reaction chain models have been used to treat such systems. Thereby, the reaction chain represents a method to correlate step-wise hydration or complexation enthalpies and entropies and, thus, reduce the number of adjustable parameters. Results are discussed for systems containing MgCl 2 , CaCl 2 , ZnCl 2 , and alkali metal chlorides.

A structure-based nomenclature for regular single-strand organic polymers is described. In concept, a generic name for the polymer (ABC) n is poly(ABC), in which (ABC) is a constitutional repeating unit (CRU) representing the chemical structure of the polymer chain, and A, B, and C are the subunits that comprise the CRU. To provide a unique and unambiguous name, rules are given to identify the preferred CRU and to name it using the names of A, B, and C based on current organic nomenclature. Provisions are made for naming end-groups of the polymers and the polymer substituents. In addition, the document contains a glossary of concepts and definitions, a list of common subunit names, and a variety of examples of structure-based polymer names. The document is a revision of the 1975 Rules.

There is a widespread belief on the part of the general public that natural substances are inherently superior to synthetic substances with regard to efficacy and safety in matters related to human health. This question is examined by reviewing the therapeutic use of drugs and herbal medicine preparations, the role of vitamins and nutrients, and the effects of toxic substances. A comparison of the characteristics of natural and synthetic substances within these categories shows a similar range of favorable and unfavorable effects. It is apparent that molecular structure and dose determine the effect of substances on human health, not whether they are of natural or synthetic origin.

Documented variations in the isotopic compositions of some chemical elements are responsible for expanded uncertainties in the standard atomic weights published by the Commission on Atomic Weights and Isotopic Abundances of the International Union of Pure and Applied Chemistry. This report summarizes reported variations in the isotopic compositions of 20 elements that are due to physical and chemical fractionation processes (not due to radioactive decay) and their effects on the standard atomic-weight uncertainties. For 11 of those elements (hydrogen, lithium, boron, carbon, nitrogen, oxygen, silicon, sulfur, chlorine, copper, and selenium), standard atomic-weight uncertainties have been assigned values that are substantially larger than analytical uncertainties because of common isotope-abundance variations in materials of natural terrestrial origin. For 2 elements (chromium and thallium), recently reported isotope-abundance variations potentially are large enough to result in future expansion of their atomic-weight uncertainties. For 7 elements (magnesium, calcium, iron, zinc, molybdenum, palladium, and tellurium), documented isotope variations in materials of natural ter- restrial origin are too small to have a significant effect on their standard atomic-weight uncertainties. This compilation indicates the extent to which the atomic weight of an element in a given material may differ from the standard atomic weight of the element. For most elements given above, data are graphically illustrated by a diagram in which the materials are specified in the ordinate and the compositional ranges are plotted along the abscissa in scales of (1) atomic weight, (2) mole fraction of a selected isotope, and (3) delta value of a selected isotope ratio.