Effect of Pressure on Molecular and Ionic Motions in Ultraviscous Acetaminophen-Aspirin Mixture

Effect of pressure and temperature on molecular motions and dc conductivity in ultraviscous, 50 wt% acetaminophen-aspirin melt has been studied by dielectric relaxation spectroscopy. The spectra obtained over the pressure range, 5-300 MPa and temperature range, 295-320 K show a distribution of relaxation times greater than found in pure acetaminophen. The equilibrium dielectric permittivity and relaxation time, τ, of the melt increase with increase in pressure and decrease in temperature and the dc conductivity, σdc, decreases. The pressure and temperature variation of the limiting high frequency permittivity shows significant contribution from infrared polarization. The volumes of activation for σdc and τ vary with both the pressure and temperature, indicating that there is also a structural effect that determine σdc. This effect would add a non-linear term to the Debye-Stokes-Einstein equation for variation of σdc with τ. The ultraviscous liquid remains stable with time, and with change in temperature and pressure, suggesting that a more stable glassy state of a pharmaceutical may be obtained by mixing a second component.