An extension of the Pitzer equation for the excess Gibbs energy of aqueous electrolyte systems to aqueous polyelectrolyte solutions |
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Authors: | Pedro de Alcntara Pessa Filho Gerd Maurer |
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Institution: | aDepartment of Chemical Engineering, Escola Politécnica, Universidade de São Paulo, Caixa Postal 61548, CEP 05424-970, São Paulo, SP, Brazil;bChair of Applied Thermodynamics, Department of Mechanical and Process Engineering, University of Kaiserslautern, Erwin Schrödinger Strasse, D-67653 Kaiserslautern, Germany |
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Abstract: | Pitzer's equation for the excess Gibbs energy of aqueous solutions of low-molecular electrolytes is extended to aqueous solutions of polyelectrolytes. The model retains the original form of Pitzer's model (combining a long-range term, based on the Debye–Hückel equation, with a short-range term similar to the virial equation where the second osmotic virial coefficient depends on the ionic strength). The extension consists of two parts: at first, it is assumed that a constant fraction of the monomer units of the polyelectrolyte is dissociated, i.e., that fraction does not depend on the concentration of the polyelectrolyte, and at second, a modified expression for the ionic strength (wherein each charged monomer group is taken into account individually) is introduced. This modification is to account for the presence of charged polyelectrolyte chains, which cannot be regarded as punctual charges. The resulting equation was used to correlate osmotic coefficient data of aqueous solutions of a single polyelectrolyte as well as of binary mixtures of a single polyelectrolyte and a salt with low-molecular weight. It was additionally applied to correlate liquid–liquid equilibrium data of some aqueous two-phase systems that might form when a polyelectrolyte and another hydrophilic but neutral polymer are simultaneously dissolved in water. A good agreement between the experimental data and the correlation result is observed for all investigated systems. |
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Keywords: | Aqueous solution Polyelectrolyte Excess Gibbs energy Pitzer's model |
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