In this paper, the solid–liquid equilibrium of N-methylpyrrolidone (NMP) + MCl (M = Na, K, Rb, and Cs) + H2O ternary systems was investigated at three temperatures (T = 288.2, 298.2, and 308.2 K). The isothermal solubility, density, and refractive index of the saturated ternary systems were obtained. At the same temperature, as the NMP content in the mixed solvent increases, the solubility of the salt decreases significantly, and the density change trend is consistent with the solubility. However, as the NMP content in the mixed solvent increases, the refractive index increases. As the experimental temperature increases, the solubility and density increase, while the refractive index decreases. Moreover, the semi-empirical four-parameter equation was used to fit the experimental data.

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