Abstract: To investigate the inclusion of Trimethoprim (TMP) with Hydroxypropyl-β-cyclodextrin (HP-β-CD) in aqueous soliutions,change of thermodynamic parameters during the inclusion process and inclusion molar ratio of the host and guest molecules. The measurements of apparent inclusion constant of TMP with HP-β-CD, the change of entropy,enthalpy and free energy of the reaction, inclusion molar ratio were carried out by the following methods separately were phase solubility method,equimolar series method and molar ratio method,respectively. That all the phase solubility diagrams showed a typical A_L-type in various temperature aqueous solutions, suggested the formation of a inclusion complex of 1∶1 molar ratio.The change in the thermodynamic parameters suggested that the inclusion could proceed spontaneously along with the release of heat and the increase of entropy. An 1∶1 molar ratio inclusion complex of TMP with HP-β-CD could be formed spontaneously and the solubility of TMP in aqueous solution increased. Appropriate temperature probably favor the progress of inclusion procedure.

Key words: Trimethoprim; Hydroxypropyl-β-cyclodextrin; inclusion constant; inclusion molar ratio; thermodynamic parameters