Abstract
Aqueous methyl p-hydroxybenzoate (MPHB) solutions containing various amounts of α-cyclodextrin (α-CD) were freeze-dried. The crystalline states of MPHB and the freeze-dried products have been investigated. From a comparison of the powder X-ray diffraction patterns, it was found that rapid freezing provided amorphous products, while slow freezing provided crystalline inclusion complexes. To estimate the amorphous MPHB fraction, crystalline MPHB was removed by means of sublimation treatment, and residual MPHB was determined by ultraviolet spectrophotometry. It was found that the amount of amorphous MPHB in a rapidly frozen sample was greater than that in a slowly frozen sample.Amorphous MPHB molecules were considered to exist in two different states, namely the included state in the α-CD cavity and the dispersed state in the intermolecular hydrogen-bonding network of α-CDs. From studies with linear oligosaccharides, it was suggested that the freezing condition influenced the amount of MPHB molecules dispersed in the intermolecular hydrogen-bonding network.
