Salt effects on caffeine solubility, distribution, and self-association.

Area of Science:

• Physical Chemistry
• Chemical Thermodynamics
• Solution Chemistry

Background:

• The Setschenow equation is commonly used to describe salt effects on solubility but is inadequate for self-associating compounds.
• Separating salt effects from self-association is crucial for understanding compound behavior in solution.
• Caffeine, theophylline, and theobromine are relevant compounds for studying these phenomena.

Purpose of the Study:

• To differentiate the impact of salts on monomeric solubility and distribution from their effects on caffeine self-association.
• To investigate the influence of various salts (Na(2)SO(4), NaCl, NaClO(4), NaSCN) on caffeine, theophylline, and theobromine solubility and distribution.
• To apply a modified approach using distribution coefficients at infinite dilution to accurately determine salting parameters for caffeine.

Main Methods:

• Determined solubilities of caffeine, theophylline, and theobromine in water and various salt solutions at 25°C.
• Measured caffeine distribution coefficients (D(W/O)) in the presence of different salts.
• Calculated salting parameters (k(s)) using distribution data at infinite dilution and analyzed self-association using dimerization and isodesmic models.

Main Results:

• Caffeine, theophylline, and theobromine solubilities exhibited salting-out with Na(2)SO(4) and NaCl, and salting-in with NaClO(4) and NaSCN.
• Caffeine distribution coefficients mirrored solubility trends, decreasing with salting-out salts and increasing with salting-in salts.
• Calculated salting parameters (k(s)) differed from Setschenow constants (K), and salt type modulated caffeine self-association constants (k(d) or k(iso)).

Conclusions:

• The Setschenow equation is insufficient for self-associating compounds like caffeine due to its inability to separate salt contributions.
• Distribution coefficients at infinite dilution provide a more accurate method for determining salting parameters.
• Specific salts significantly influence caffeine's self-association behavior, highlighting the importance of considering salt type in solution studies.