Study on the solid-liquid equilibrium of l-malic acid in binary solvents

In this study, the solubility of l -malic acid in three binary solvents (ethyl acetate +  tetrahydrofuran , acetonitrile  + tetrahydrofuran, butyl acetate  + acetonitrile) at atmospheric pressure from T = (278.15 to 323.15) K was measured via a gravimetric method. The results show that in the temperature range studied, the solubility of l -malic acid in the selected solvent gradually increases with the increase of temperature. The solubility of l -malic acid increases with the increase of the mass fraction of tetrahydrofuran at the binary system (ethyl acetate + tetrahydrofuran, acetonitrile + tetrahydrofuran), the solubility of l -malic acid increases with The mass fraction of butyl acetate increases and decreases at the butyl acetate + acetonitrile system. In addition, the dissolution behavior of l -malic acid was explained using Hansen solubility parameter (HSP). Furthermore, the solubility data were correlated with using the modified Apelblat equation, the NRTL model and the newly proposed Apelblat-Sun model, the results have shown that the modified Apelblat equation has the best fitting effect. After research found, the hydrogen bonding force plays a key role in the solubility of l -malic acid. So solubility of l - resulted malic acid from the integrated effect of solute–solvent and solvent–solvent interactions. In addition, the van't Hoff equation was used to calculate and discuss all thermodynamic parameters of dissolution, Including standard Gibbs dissolution (Δ sol G o ), standard dissolution enthalpy (Δ sol H o ) and standard dissolution entropy (Δ sol S o ), Where Δ sol H o ，Δ sol S o ，Δ sol G o were all greater than 0, it have been shown that the dissolution process is driven by heat absorption and entropy. This study can provide basic thermodynamic data for the crystallization and industrial separation of l -malic acid.