Marcio J. E. De M. Cardoso

Activity coefficients in mixed solvent electrolyte solutions

Abstract A brief, rigorous derivation is given for activity coefficients used in phase equilibruim calculations of multicomponents salts and solvents where an extended Debye—Huckel theory for long-range interactions is combined with short-range interaction effects using an excess Gibbs energy model, such as UNIQUAC. Calculations indicate only minor differences from approximate forms previously used for the salt effect on azeotropes.

Theoretical determination of the theta temperature for solvent–polymer systems

Abstract The miscibility behavior of five binary polymer–solvent systems was studied by means of the Flory–Huggins theory. The theta temperature and the entropic and energetic Flory–Huggins parameters were calculated for these systems. The coordination number in the Flory–Huggins model and the interaction energy parameters were determined using a molecular simulation technique based on a Monte Carlo approach, which takes into account the constraints associated with excluded volume. In all cases, the critical temperature (Tc) values were calculated and compared with the experimental data. Thermodynamic parameters were obtained by plotting 1/Tc versus 1/x 2 1 2 +1/2x 2 curve, where x2 is the degree of polymerization. This study shows a further application of the Flory–Huggins theory combined with molecular simulation techniques, for the model parameter determination, which was previously proposed by C.F. Fan, B.D. Olafson, M. Blanco, Macromolecules 25 (1992) 3667 [1] . In addition, the limitations of the methodology proposed by C.F. Fan, B.D. Olafson, M. Blanco, Macromolecules 25 (1992) 3667 [1] , for miscibility calculations of binary polymer systems, are further discussed with special attention given for the model parameter determination.

Calculation of the Osmotic Pressure and Theta Temperature of Polymer Solutions Through Cubic Equations of State and the McMillan–Mayer Solution Theory Framework

The osmotic compressibility factor of several polymer solutions was correlated with the polymer concentration and temperature through different cubic equations of state, with particular focus on the McMillan–Mayer solution theory. The equations of state employed were those of van der Waals, Redlich–Kwong, Peng–Robinson, and Soave—Redlich–Kwong. All of these equations present two parameters that take into account the attractive and repulsive interactions between the solute molecules in a given solvent, with these parameters being dependent on the nature of the system (solute and solvent) and on the temperature. As the attractive and repulsive interactions are well defined in the parameters of the cubic equations of state, the theta temperature for each polymer system studied may be calculated by a simple and efficient procedure. For the purposes of comparison, the virial equation truncated at the third term was also included in this study. It was confirmed that the attractive parameter has a linear dependence on the temperature, while the repulsive parameter varies according to a quadratic profile. Accordingly, the model to be minimized presents five adjustable parameters that depend only on the nature of the polymer system. The agreement between the experimental and calculated values is within the experimental error.

On the derivation of thermodynamic equilibrium criteria

Abstract It is well established that, besides the two thermodynamic formulations based on the fundamental equations (i.e., entropic formulation S = S ( U , V , N ); Energetic Formulation U = U ( S , V , N )), the equilibrium states of a system can be represented by equivalent formulations derived by making Legendre transforms of either of the fundamental equations [H.B. Callen, Thermodynamics and an Introduction to Thermostatics, 2nd edn., Wiley, NY, 1985]. The main purpose of this article is to present in a new and consistent way, the derivation of the equilibrium criteria for both thermodynamic potentials (i.e., Legendre transforms of internal energy) and Massieu functions (i.e., Legendre transforms of entropy) with the aid of concepts such as boundaries, reservoirs, and certain auxiliary devices known as equilibrium or intermittent connections.