A calorimetric investigation into the interaction between paracetamol and polyethlene glycol 4000 in physical mixes and solid dispersions.

The solubility, heat of solution and dissolution rate of paracetamol and polyethyelene glycol 4000 (PEG 4000) systems have been studied in order to clarify the nature of the interaction between the two components during dissolution of solid dispersions. The logarithmic solubility of paracetamol demonstrated a non-linear increase with concentration of PEG 4000, while linear relationships between heat of solution in water and concentration were seen for both individual components. However, the heat of solution of paracetamol was found to decrease with increasing concentrations of PEG 4000. Similarly, the heats of solution in water of physical mixes and solid dispersions prepared using two manufacturing protocols were found to be lower than the theoretical values calculated from those corresponding to the individual components. Drug release studies showed a marked increase in paracetamol dissolution rate when prepared as a solid dispersion, with behaviour consistent with carrier controlled dissolution observed at low drug contents which was ascribed to enhanced dissolution of the drug into the diffusion layer of the PEG 4000. The implications of the understanding of this mechanism for the choice of carrier and manufacturing protocol for solid dispersion products is discussed.