Properties and Structure of Aqueous Solutions of Hydroxypropyl‐beta‐Cyclodextrin

Molecules of hydroxypropyl‐beta‐cyclodextrin (HPBCD) in aqueous solution interact with each other via hydrogen bonds. Especially for concentrations beyond 50% (w/w), a strong increase in viscosity and a considerable decrease of freezable water can be determined. The addition of substances which are able to break the water structure (urea, KSCN) reduces these effects remarkably. The formation of inclusion compounds with urea could not be detected. Endothermic phase transformations confirm the presence of liquid crystalline structures at low temperatures. These liquid crystalline structures are not detectable at room temperatures, but the freeze fracture technique shows micelle‐like associates of a size of about 20 nm. Studies on the complexation of phenolphthalein as well as surface tension measurements of HPBCD solutions indicate that a minimum concentration of HPBC is necessary for the association phenomena.