Thermodynamics and Structural Properties of the Model Polyelectrolyte-electrolyte Mixture

Jesus Piñero,¹ Lutful B. Bhuiyan,¹ Jurij Reščič² and Vojko Vlachy^2,*

¹Laboratory of Theoretical Physics Department of Physics, University of Puerto Rico San Juan, Puerto Rico 00931-3343
²Faculty of Chemistry and Chemical Technology, University of Ljubljana, A{ker~eva 5, 1000 Ljubljana, Slovenia
* Corresponding author: E-mail: vojko.vlachy@fkkt.uni-lj.si

Abstract
Structural and thermodynamic properties of linear polyelectrolyte solutions in presence of a low-molecular electrolyte are studied using Monte Carlo simulation techniques in conjunction with the cylindrical cell model where a uniformly charged hard cylinder mimics the linear polyion situated in its own cylindrical cell. The simulation data are complemented with the results of the Poisson-Boltzmann theory for the same model, while Manning’s theory for mixing enthalpies is compared with the corresponding simulation results. The reported results suggest a strong influence of added low-molecular electrolyte on the various distribution functions. The influence of electrolyte addition on thermodynamic properties such as the enthalpy of dilution, enthalpy of mixing, and the osmotic coefficient is examined. These latter results are seen to be consistent with an empirical finding, the so called ”additivity rule” for osmotic pressure. The calculated enthalpies of dilution and of mixing are in qualitative agreement with experimental data. The Poisson-Boltzmann results are in semi-quantitative agreement with Monte Carlo simulation results. The same holds true for the enthalpies of dilution and of mixing as calculated via the Manning limiting-law formula.

Keywords: Polyelectrolytes, electrolytes, enthalpy of dilution, osmotic coefficient