The complex formation of 'flexible' dimethyl-pyridino-18-crown-6 (M(2)P18C6) or 'rigid' dimethyl-diketo-pyridino-18-crown-6 (M(2)K(2)P18C6) host molecules with potassium ions in methanol were investigated by UV spectroscopy. Results show same stability of flexible or rigid crown ether complexes with potassium ion. However, negative entropy change has been detected when M(2)P18C6 forms complexes with potassium ions while entropy increasing was obtained when M(2)K(2)P18C6 associates with potassium ions. Theoretical calculations suggest that among other possible effects characteristic vibrations of crown rings could be identified as background of this unexpected complexation phenomena. This finding may contribute to the development of design of sensitive and selective chemical sensors. (C) 2012 Elsevier B. V. All rights reserved.