Urea solution as a biopolymer denaturing agent and source of nitrogen fertilizer has a key role in some industries, and it seems necessary to study the phase behavior of this system. This work aims to investigate the vapor−liquid equilibria (VLE) of aqueous urea solutions over a wide range of concentrations and temperatures. For this purpose, water activity measurements were performed for the aqueous urea solution using the dynamic vapor sorption (DVS) technique at 283.15−343.15 K in the various urea concentrations. The results showed good agreement between obtained data using DVS and the isopiestic technique at 298.15 K. The conductivity value of the urea solution at 298.15 K revealed that this system is a very weak electrolyte solution. Thereby, a combination of Pitzer’s extension of the Debye−Huckel function (PDH) to cover the ion interaction and some local composition models including Wilson, nonrandom two-liquid (NRTL), nonrandom factor (NRF), modified Wilson-NRF, and NRTL-NRF to point to physical interaction was considered to correlate the experimental activity data. It was revealed that increasing the temperature led to increasing the accuracy of NRTL-NRF and NRF models and other models can correlate the experimental data at any temperature in this range well.
