The improved isopiestic method have been used to obtain activities of water for aqueous solutions of poly(ethylene glycol) 400/NaCl at 293.15, 298.15, 303.15, 308.15 and 313.15 K. The measured water activities were used to calculate the vapor pressure of solutions, activity coefficient of water, activity coefficient of PEG, mean molal activity coefficients of ions and excess Gibbs energies of solution. Also, from the measured water activity data, the hydration number of PEG molecules has been calculated. The effect of temperature on the vapor-liquid equilibrium of poly(ethylene glycol) + NaCl + H2O systems has been studied. It was found that, the slope of constant water activity lines increased with increasing the temperature. The results have been discussed on the basis of the effect of temperature on the hydrophobicity of the polymer. Also it was found that the vapor pressure depression for an aqueous PEG + NaCl system is more than the sum of those for the corresponding binary solutions. Furthermore, the segment-based local composition Wilson model has been used for the correlation of the experimental water activity data. The agreement between the correlation and the experimental data is good.
