Effect of pH, polymer concentration and molecular weight on the physical state properties of tolfenamic acid

Tolfenamic acid (TA) has been transformed from crystalline to amorphous state through freeze-drying by using varying ratios of polyacrylic acid (PA) at various pH values. The characterization of the films has been carried out using X-ray diffraction, differential scanning calorimetry, Fourier transform infrared (FTIR) spectrometry and scanning electron microscopy. The results showed a gradual change in the solid state properties of TA and a complete transformation into its amorphous form in 1:8, 1:4, 1:2 and 1:1 ratios at pH 3, 4, 5 and 6, respectively. FTIR spectrometry reveals the formation of a yellow polymorphic form of TA. Polymer molecular weight has also been observed to affect the drug transformation and interaction as the low molecular weight PA (Mw ∼ 1800) was found to be most effective followed by its medium (Mv ∼ 450 000) and high molecular weight (Mv ∼ 3 000 000) forms. No signs of recrystallization in the TA-PA films were noted during the 12-week storage period. PA of low molecular weight has also been found more effective in inhibiting the recrystallization of the melt upon cooling thus proving a valuable polymer in producing stable amorphous solid dispersions of TA.