Use of 2,4-dinitrophenylhydrazone of glyoxylic acid for the determination of glyoxylic acid by the chromatographic-spectrophotometric method and by differential pulse polarography

Two different methods (one based on chromatography and spectrophotometry and the other on polarography) have been developed for the determination of glyoxylic acid in the form of a derivative with 2,4-dinitrophenylhydrazine (DNPH. GA). The TLC method allows the separation of two DNPH. GA isomers ("trans" and "cis"). Spectrophotometric measurements of the eluents of the separated compounds (lambda=360 nm) allow the determination of GA within the range from 4 to 30 microg. Using differential pulse polarography, the conditions of DNPH. GA formation were examined. The reduction peak of this derivative (E(P)=-0.430 V), observed by dpp, was used to develop a polarographic determination of GA within the concentration range from 1.10(-4) to 7.10(-4) mol/l.     

Stability of Colloids in Chemically Reacting Network-Forming Solvent

We consider a model for a two-component solution which consists of a chemically reacting solvent and a chemically inert hard-sphere solute. A directional associative interaction between the solvent molecules promotes the formation of a network. A multidensity associative Ornstein-Zernike equation is solved analytically using the Percus-Yevick approximation. On the basis of the analysis of the osmotic compressibility, an instability curve for different parameters of the model dispersion is constructed. It is shown that concentration and size of colloids affect the formation of associative complexes in a solvent and directs in such a way the location and behavior of the spinodal curves. The effective interaction between colloids is calculated and discussed. Copyright 1999 Academic Press.     

A simplified approach to vapor–liquid equilibria calculations with the group-contribution lattice-fluid equation of state

Equations of state that are based on the lattice-statistics approach use Guggenheim's quasi-chemical approximation to describe the non-randomness in the mixture due to the energetic interactions between the molecules. For ternary and higher-component systems the non-randomness expression is complex and requires an iterative calculation procedure. We have shown that the non-randomness parameters play a negligible role in the application of the GCLF-EoS model (based on the Panayiotou–Vera EoS) for predicting vapor–liquid equilibria. Omission of the non-randomness parameters from such calculations can significantly improve the computation efficiency. Binary, ternary, and quaternary vapor–liquid equilibria predictions were made including polystyrene, polyvinyl acetate, polyethylene, and polypropylene in polar and non-polar solvents to test the theory.     

Negative linear compressibility in confined dilatating systems

The role of a matrix response to a fluid insertion is analyzed in terms of a perturbation theory and Monte Carlo simulations applied to a hard sphere fluid in a slit of fluctuating density-dependent width. It is demonstrated that a coupling of the fluid-slit repulsion, spatial confinement, and the matrix dilatation acts as an effective fluid-fluid attraction, inducing a pseudocritical state with divergent linear compressibility and noncritical density fluctuations. An appropriate combination of the dilatation rate, fluid density, and the slit size leads to the fluid states with negative linear compressibility. It is shown that the switching from positive to negative compressibility is accompanied by an abrupt change in the packing mechanism.     

A planar complex phase interface in BaTiO<Subscript>3</Subscript>

A model is proposed for a complex phase interface separating the cubic and tetragonal phases in barium titanate, which consists of an intermediate layer of the induced phase with small monoclinic distortions near the interface. It is demonstrated that the induced phase can have two different structures, which are formed during the direct and reverse phase transitions, respectively. The parameters of these structures, equations of phase boundaries, relative orientations of the phases upon the phase transition, and the number of possible orientation states are determined.     

A theoretical model for evaluation of configurational entropy of mixing with respect to shape and size of particles

A general method of evaluation of configurational entropy of a liquid mixture is presented. It is based on a generalized lattice model with no restrictions due to particle shape being introduced. A general formula for the entropy is derived. Achieved results open a way for a rigorous analysis of particle shape effect on mixing process. As an example, a new formula for the entropy of mixing of hard spheres in continuous space is derived which may respect a physical bound for packing ratio. A systematic approach to improve the model accuracy is proposed. The resultant alternative models are discussed in details. A comparison with literature data and the Mansoori-Carnahan-Starling formula is presented. Very good agreement is shown.     

Langmuir-blodgett films on the basis of an amphiphilic nitronylnitroxyl radical

The 2-(4-hexadecylmethylamino)phenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-oxy-3-oxide radical, which contains a long alkyl chain and a strong donor substituent conjugated with a nitronyl nitroxide group (an acceptor), was synthesized. The behavior of monolayers formed by this radical on a water surface and the conditions of their transfer onto solid substrates by the Langmuir-Blodgett method were examined. UV-VIS and IR spectroscopies, atomic force microscopy, and small-angle X-ray scattering were used to study the optical and structural characteristics of films prepared from the pure radical and its mixtures with behenic acid. It was revealed that radical aggregates are formed both in solutions and Langmuir-Blodgett films.