The influence of the poly(ethylene glycol) on the mean activity coefficients of NaCl aqueous solutions. The application of the MSA and HNC method

A primitive model electrolyte in mixture with uncharged hard spheres was used to model the concentration dependence of the mean activity coefficient of sodium chloride in aqueous solutions of poly(ethylene glycol) (PEG) of different degree of polymerization and concentration. Ornstein-Zernike integral equation was numerically solved using the hypernetted chain (HNC) closure and the results were compared with the results of an analytical solution valid within the mean spherical approximation (MSA) closure. Good agreement between the HNC and MSA activity coefficients was obtained. However, at higher NaCl concentrations the convergence of the numerical solution was not satisfied for large model PEG molecules. The presence of the neutral component in the solution increases the mean activity coefficients in the whole concentration range due to the confinement. For a given NaCl and PEG concentration, decreasing the dielectric constant of the solution diminishes the activity coefficient due to stringer interaction between the ions. Mean activity coefficients of NaCl in two aqueous solutions differing in the concentrations of PEG (molecular weight of 12,000 g mol(-1)) were determined also experimentally and compared with MSA results. The agreement was semi-qualitative, mainly due to difficulties in estimating good model parameters for the PEG molecules and the potential decrease of the dielectric constant of the solvent. (C) 2017 Elsevier B.V. All rights reserved.