Entropy dS Research Articles

In Russian

The results of experimental data obtained using UV spectroscopy, due to a comparative study on the interaction of α- and β-cyclodextrins with alpha aromatic amino acids differing in molecular size and substituent structure at the α-carbon atom in aqueous solutions with various pH (L-phenylalanine, L-tyrosine and L‑tryptophan) have been analyzed. The features of the interaction of selected amino acids with cyclodextrins have been clarified. Molecular cationic and zwitterionic forms of amino acids at the same content of components in the solution and an excess of cyclodextrin form 1: 1 host-guest inclusion complexes, regardless of the pH value of the medium. This is manifested by a long-wavelength shift and an increase in the intensity of the characteristic absorption band in the UV spectra of solutions. It has been proved that inclusion complexes are formed due to the van der Waals interactions by partial displacing water molecules and incomplete (in the case of α-cyclodextrin) immersion of the amino acid aromatic radical into the internal hydrophobic cavity of α-cyclodextrin and of β-cyclodextrin. Stabilization of the complexes is due to hydrogen bonds, and the geometric parameter makes the main contribution to the formation of inclusion complexes. It has been found that the hydrogen bond is formed between the nitrogen atom of a-amino group or the oxygen atom of a-carboxyl group of the amino acid molecule with hydrogen of the primary hydroxyl group located on the surface of the wide edge of the torus, as well as between the nitrogen atom of the imino group of the L‑tryptophan molecule and glycoside hydrogen atom within the cyclodextrin cavity. The values of the stability constants (Ks) of the complexes and the thermodynamic parameters of the formation of inclusion compounds (ΔG, ΔH, and ΔS) have been calculated. The use of a smaller macrocycle (a-CD) leads to a decrease in the stability of the inclusion complexes. Negative values of the change in the free Gibbs energy indicate a spontaneous complexation reaction between a- or b-cyclodextrins and alpha aromatic aminoacids. Negative values of the enthalpy DH and entropy DS indicate that the complexes are mostly stabilized due to the entropy contribution.

Spectrophotometric study of the interaction of methylene blue with poly(styrene-co-sodium styrene sulfonate)

The interaction of the cationic phenothiazine dye, the Methylene Blue (MB) with poly-(sodium styrene sulfonate) f –co-(styrene) 1-f , (PSSNa f ), has been investigated by spectrophotometric method. The polyelectrolyte induced metachromasy resulting in a blue shift of the absorption maxima of the dye, in agreement with the formation of dye H-aggregates. The stability of the PSSNa-MB complexes was studied as a function of polyelectrolyte chain length, polyelectrolyte electrostatic charge density f , polyelectrolyte concentration, NaCl salt addition, tetrahydrofurane (THF) addition and THF treatment. The stoichiometry of PSSNa-MB complex evaluated by the molar ratio method was found 4:1 for the fully charged PSSNa f = 1. Reversal of metachromasy was observed upon salt and THF addition, while THF treatment does not affect the complex and allows recovering the initial complex. Finally, thermodynamic parameters of the interaction between the polyelectrolyte and the dye at different temperatures, namely free energy DG, the enthalpy DH and the entropy DS have been evaluated to determine the binding constant and as a consequence the stability of the complex. The metachromasy effect was found to be more high as the chemical charge f increases and reaches its maximum value f = 1, when operating at optimal conditions. So, the PSSNa f = 1-MB complex is the most stable in comparison to the others based on lower charge density PSSNa f .

Salting-out effect of alkali metal chlorides (lithium, sodium, and potassium) on 2-cyanoguanidine aqueous solution: A solid–liquid equilibrium study

Solubility of 2-cyanoguanidine in three alkali metal chlorides aqueous solution (LiCl, NaCl and KCl) was measured by visual polythermal method at temperature range from 298.15 K to 328.15 K. The experimental values show that solubility of 2-cyanoguanidine decreased with the concentrations of alkali metal chlorides increasing at a given temperature and salt concentration from wt = 0.01 to 0.05. Solubility data of 2-cyanoguanidine in KCl aqueous solution is significantly higher than that in NaCl or LiCl aqueous solutions. Each set of experimental measured values were correlated using the modified Apelblat equation as a function of temperature. It's obvious that modified Apelblat model shows good agreements with experimental solubility data for three systems. Simultaneously, the salting-out yields (S–O) of three alkali metal salts (LiCl, NaCl, and KCl) on the solubility of 2-cyanoguanidine in water were computed, among all the temperatures and three alkali metal chlorides aqueous solutions, which increase from (0.0407, 0.0089, and 0.0087) to (0.5234, 0.4669 and 0.3561), respectively. Also, salting-out behaviors of three alkali metal chlorides were investigated by Setschenow equation, which shows the relative mole fraction solubility of 2-cyanoguanidine decreased in the following order: LiCl > NaCl > KCl. This result keeps high consistency with the variational tendency of the solubility data. The dissolution enthalpy dH° (27.8–38.6 kJ mol−1), dissolution entropy dS° (52.3–87.6 J mol−1 K−1) and molar Gibbs energy dG° (9.5–13.2 kJ mol−1) of 2-cyanoguanidine in three chlorides aqueous solutions calculated on the basis of modified Apelblat equation analysis are all positive, which suggests that dissolution of 2-cyanoguanidine in alkali metal chloride aqueous solutions was endothermic, spontaneous, and entropy-driven process.

Study of temperature dependencies of saturated vapor pressure of ruthenium(III) beta-diketonate derivatives

Complexes of ruthenium(III) with the following beta-diketones: 2,4-pentanedione (Ru(acac)3), 1,1,1-tri- fluoro-2,4-pentanedione (Ru(tfac)3), 2,2,6,6-tetramethyl-3, 5-heptanedione (Ru(thd)3), 2,2,6,6-tetramethyl-4-fluoro- 3,5-heptanedione (Ru(tfhd)3) and 1,1,1-trifluoro-5,5-dime- thyl-2,4-hexanedione (Ru(ptac)3) were synthesized and identified by means of mass spectrometry. By effusion Knudsen method with mass spectrometric registration of gas phase composition the temperature dependencies of satu- rated vapor pressure were measured for ruthenium(III) compounds and the thermodynamic characteristics of vaporization processes enthalpy DHT* and entropy DS oof

Probability, thermodynamics, and dispersion space for a statistical model of equilibria in solution. 3. Aqueous solutions of monocarboxylic acids at different temperatures

The thermal moments of the grand canonical partition function for solutes are related to the coefficients of a Taylor-MacLaurin expansion because the solutes form a statistical ensemble distributed according to the Boltzmann law. When treating equilibria in solution, the solvent is present in large excess and its concentration is in general assumed as constant. The molecules of solvent can be considered to form a canonical subsystem. For this subsystem, the change of temperature d ln T produces a change of entropy dS=C p,w d ln T, where C p,w is the molar heat capacity of water, exactly equivalent to the change of entropy produced by a change of dilution dS=-d ln [W]

A simple, unified approach to the first and second laws of thermodynamics

The article gives a new verbal formulation of the second law of thermodynamics. It is claimed that the physical content of this statement as well as the derivation of the mathematical consequences normally referred to as the first and second parts of the second law are simpler and more easily grasped by beginners than the standard formulations. The argument is so designed as to be closely modelled on one which pertains to the derivation of the mathematical formulation of the first law. 1. MOTIVATION FOR THIS AR11CLE There is little advantage, from the point of view of advancing progress in physics, in reopening the question of the optimal formulation of the second law of thermodynamics. However, a case can be made for returning to this fundamental topic in the interests of those who are engaged in transmitting existing knowledge. Regardless of which primary formulation of the second law is adopted, it is commonly agreed that it must lead, by an easy logical and mathematical derivation, to three statements: (a) There exists a property called entropy, S, which is additive for subsystems and which possess the mathematical properties of a potential. (b) There exists a variable, called thermodynamic temperature, T, which has the mathematical property of being that integrating denominator, among infinitely many, for an element of heat, dQ°, in a reversible process1 which turns the latter into the perfect differential of entropy dS = dQ°/T (Carnot's theorem) (1) The thermodynamic temperature, T, is a unique function of any empirical temperature, t. (c) There exists a quantity called entropy production, 0, which is positive in any irreversible process. In an adiabatic irreversible process between an initial state 1 and a final state 2, we define (2a) and must have 0>0 (2b) All symbols with the superscript ° refer to reversible processes.