The DSC approach to study non-freezing water contents of hydrated hydroxypropylcellulose (HPC)

Abstract

The research consisted of investigation of non-freezable bound water (NFW) contents of raw hydroxypropylcellulose (HPC) Klucel® Pharm HF, MF, and LF by differential scanning calorimetry. Polymers (HF, MF and LF) used in the research differed significantly in their molecular mass and viscosity. Mixtures of HPC and low soluble salicylic acid or highly soluble sodium salicylate were used to examine the effect of counter-ions presence and drug solubility on NFW contents. The dependencies of determined enthalpies of melting (ΔH) and respective contents of water Wc (water fraction in the dry mass of HPC or HPC mixture) ΔH = f(Wc) were linear. The sought values of non-freezing water were calculated (by extrapolation) as the concentration at which ΔH = 0. It was found that the viscosity and molecular mass of the raw HPCs did not affect the contents of non-freezing water, which ranged between 0.54 and 0.51 g g−1 for all types LF, MF, HF. Poorly soluble salicylic acid (SA) reduced the non-freezing water content to 0.19–0.17 g g−1. On the other hand, matrices composed of highly soluble sodium salicylate (NaSA) showed variable decrease of NFW content—0.44 g g−1 for HF and 0.54–0.51 g g−1 for LF and MF. It was also found that the maximum temperature Tmax of melting was strongly influenced by the solubility of the drug. Highly soluble NaSA shifted Tmax toward the lower negative values, while SA toward the higher positive values. Assessing NFW contents in hydrated HPC matrices may contribute to a better understanding of the drug release and dissolution mechanisms of tablet formulations.