A simple experiment demonstrating the temperature effect in supramolecular chemistry

The aim of the experiment described in this article is to demonstrate the temperature influence on creation of an inclusion complex in which beta-cyclodextrin is the host molecule and phenolophtalein is the guest. The experiment in the simplest form requires minimal laboratory equipment, however various modifications of the experiment are possible.

Thermochromic solution solution was prepared by mixing 5 mL of 0.1 M sodium hydroxide, 0.5 mL of 10 mM phenolophtalein and 20 mL of 10 mM beta-cyclodextrin solutions and adding water up to 100 mL. The reference solution was prepared in the same manner but without beta-cyclodextrin. In iced water (0 degrees C) the thermochromic solution due to highg stability constant of the inclusion complex is colorless, whereas the reference solution is purple colored. When the temperature was raised to 40 degrees C the thermochromic solution became pale purple. In a 70 degree C water bath the color intensity of both solutions is identical. After moving the colored thermochromic solution to iced water, the color will disappear. The complexation processes ocurring in solution are reversible and hysteresis phenomena is not observed. The formation of inclusion complexes between small organic molecules and cyclodextrins has proven to be an excellent model system for studying the nature of noncovalent binding forces in solution. They have provide valuable insights concerning electrostatic interactions and are good models for understanding the specificity of enzyme-substrate interactions. The described experiment is simple, requires minimal laboratory equipment and provides the most useful information needed in separation science (e.g. chromatography, electrophoresis) and also in the pharmaceutical, food, and chemical industries.