Equilibrium solubility of 6-methylcoumarin in some (ethanol + water) mixtures: determination, correlation, thermodynamics and preferential solvation

ABSTRACT

The solubility of 6-methylcoumarin in water, ethanol and nine aqueous ethanol mixtures was determined from 293.15 to 323.15 K. 6-Methylcoumarin solubility expressed in mole fraction was correlated with some well-known correlation/prediction models. Gibbs energy, enthalpy and entropy, for dissolution and mixing processes, were computed using van’t Hoff and Gibbs equations. The plot of enthalpy vs. Gibbs energy of dissolution exhibited negative slopes from water to w₁ = 0.20 and from w₁ = 0.90 to ethanol but positive slopes in the interval 0.20 < w₁ < 0.90. 6-Methylcoumarin transfer from water to w₁ = 0.20 is entropy-driven that could be attributed to water molecules releasing originally bounded as ‘icebergs’ around the drug-non-polar groups. In mixtures of 0.20 < w₁ < 0.90, enthalpy-driving mechanism for drug transfer from more-polar to less-polar systems is observed, probably owing to better solvation by ethanol. Inverse Kirkwood–Buff integrals show that 6-methylcoumarin is preferentially solvated by water in water-rich mixtures but preferentially solvated by ethanol in 0.24 < x₁ < 1.00.

KEYWORDS:

• 6-methylcoumarin
• (ethanol + water) mixtures
• solubility
• solution thermodynamics
• Jouyban–Acree model
• preferential solvation

Acknowledgements

We would like to thank Aksaray University Scientific Research Projects Coordination for its financial support (Project Number: 2021-007) for this study.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the Aksaray Üniversitesi [Project Number: 2021-007].