Solvent effects on chemical processes. 10. Solubility of alpha-cyclodextrin in binary aqueous-organic solvents:...

Area of Science:

• Physical Chemistry
• Supramolecular Chemistry
• Materials Science

Background:

• Alpha-cyclodextrin (α-CD) is a cyclic oligosaccharide with significant applications in drug delivery and separation science.
• Understanding the solubility of α-CD in various solvent systems is crucial for optimizing its use in different formulations and processes.
• Previous studies have explored α-CD solubility, but detailed investigations into the influence of specific binary aqueous-organic mixtures and solid-phase behavior are ongoing.

Purpose of the Study:

• To investigate the equilibrium solubility of alpha-cyclodextrin (α-CD) in binary aqueous-organic solvent mixtures at 25°C.
• To identify the influence of different organic cosolvents (methanol, 2-propanol, ethylene glycol, acetone) on α-CD solubility profiles.
• To elucidate the relationship between observed solubility maxima and the nature of the stable solid phases in equilibrium with the solutions.

Main Methods:

• Equilibrium solubility measurements of α-CD were performed across a range of compositions in binary aqueous-organic solvent mixtures.
• Karl Fischer titration was employed to analyze the hydration state of the solid phases in equilibrium with selected solvent systems.
• Solvent compositions were systematically varied from pure water to pure organic cosolvent to observe solubility trends.

Main Results:

• Solubility maxima for α-CD were observed in all tested binary solvent systems, with varying degrees of prominence.
• The solubility maximum was pronounced in 2-propanol and acetone systems, less so in methanol, and two maxima were observed in ethylene glycol.
• Solid-phase analysis revealed a transition from α-CD hexahydrate in pure water to a less hydrated form (three water molecules) in the presence of 2-propanol.

Conclusions:

• The presence of solubility maxima in aqueous-organic mixtures is attributed to the existence of multiple stable solid phases of α-CD.
• The specific organic cosolvent influences the hydration state of the stable solid phase, thereby affecting the solubility behavior.
• These findings provide critical insights into the phase behavior of α-CD in mixed solvents, essential for formulation development and process optimization.