Raman spectroscopy investigations of hydrated hydroxypropyl cellulose mixtures with low-soluble salicylic acid: Molecular interactions and the water-binding structure

Abstract

Highly hydrophilic polymer matrices are limited their in use to the delivery of hydrophilic drugs, and are ineffective for hydrophobic compounds. Therefore, studying hydrogels that can effectively deliver hydrophobic drugs is still an important challenge. The molecular interactions occurring in model hydrated binary mixtures of hydroxypropyl cellulose (HPC) and low-soluble and hydrophobic salicylic acid (SA) (1:1 w/w) were investigated by means of Raman spectroscopy. During the study, the OH stretching vibrations region (in the 3600–3100 cm−1 range) was examined and bands for the three assessed fractions of water were assigned at the wavenumbers 3400 cm−1 (non-freezing water), 3320 cm−1 (freezing bound water) and 3220 cm−1 (free water). The contribution of the OH vibrations of SA (3205 cm−1) was also considered. A significant area for the analysis of changes in the HPC polymer is the glycosidic linkage vibration region ranging from 1250 cm−1 to 1000 cm−1 (at ∼1151 cm−1 and at ∼1093 cm−1) and the 3000–2850 cm−1 region due to the CH stretching of the CH2 and CH3 groups. SA is able to form hydrogen bonds to polymer chains and to water, as well as to another SA molecule to form a dimer. Therefore, bands derived from SA were observed to monitor these interactions (in the 1600–1500 cm−1 range).The obtained results were discussed and partially compared to the corresponding previously published spectroscopic and theoretical outcomes. All observed interactions between the components of the mixture and water molecules were also analyzed as a function of time, which is a novel approach providing a large amount of valuable information about the system. The applied methodology allowed for establishing that the stabilization of the hydrated HPC/SA system took place after one week. The greatest dynamics of changes were observed for the lowest studied water concentration. For the highest water concentration, several low-intensity bands in the CO vibration range seemed to indicate dimer-type interactions.