This work presents, for the first time, a detailed and comprehensive thermodynamic description of the binary NaOH–H₂O system. Modeling aqueous solutions of sodium hydroxide electrolytes is complicated by the high charge density of hydroxide ions, which leads to pronounced ionic association phenomena. To address these challenges, the current study utilizes the Association electrolyte nonrandom two-liquid (Association-eNRTL) activity coefficient model, effectively correlating and predicting critical thermodynamic properties as well as the solubility behavior of aqueous NaOH solutions. The approach successfully accounts for the unique molecular interactions within this electrolyte system, significantly enhancing the reliability of predictions compared to conventional models.
The binary interaction parameters of the Association-eNRTL model for the pairs H₂O–(Na⁺, OH⁻) and (Na⁺, OH⁻)–H₂O, are determined through regression analysis using experimental data. This fitting includes mean ionic activity coefficients, vapor pressure, and osmotic coefficients of the NaOH–H₂O binary system, spanning temperatures up to 623.15 K and NaOH concentrations up to 29 molal.
