Effect of dilution on compressibility of naproxen in acetonitrile studied by ultrasonic method

Naproxen, ibuprofen, and ketoprofen are non-steroidal anti-inflammatory drugs. All of them belong to chiral 2-arylpropionic acids (2-APAs). Chiral compounds may remain in a patient's body as two antimers, even if administered as a single one, due to transenantiomerization. That is dangerous if therapeutic enantiomer has a toxic antipode. Chromatographic data suggest that solutions of 5-(+)-naproxen in acetonitrile are stiffer than the pure solvent that favours oscillatory transenantiomerisation. Acoustic and volumetric studies of dilute solutions of naproxen in acetonitrile have been undertaken to verify that supposition. The molar adiabatic compressibility and volume depend linearly on the molar percent of naproxen at temperatures from 298.15 K to 313.15 K. Limiting partial compressibility of naproxen is close to zero and decreases slightly with increasing temperature. Thus, the compressibility of dilute solutions is mainly due to compressibility of acetonitrile, while naproxen is virtually incompressible. The hydrogen-bonded dimers of naproxen probably remain intact, even at infinite dilution.