The interactions of water with cellulose- and starch-derived pharmaceutical excipients

Water associated with polymeric pharmaceutical excipients derived from cellulose and starch can have a profound effect on the properties of the excipient and on the other ingredients making up a solid dosage form. Important questions which need to be addressed include How much water will be sorbed or desorbed at various relative humidities and temperatures? and What is the thermodynamic state of water associated with the solid as a function of moisture content? A critical review of the literature is presented to demonstrate the most likely answers to these questions. It appears that water exists in at least three thermodynamic states in starch, cellulose, and their derivatives: (1) water directly and tightly bound, with a stoichiometry of one water molecule per anhydroglucose unit; (2) water in a relatively unrestricted form, approaching the properties of bulk or pure liquid water; and (3) water in an intermediate state or states, with properties reflecting a much higher level of structure than bulk water but less than that of tightly bound water. Some implications of such behavior for pharmaceutical systems are discussed.