Ion Interaction: The Energetics and Mechanism of The Competitive Behavior BEtween Two Similarly Charged Molecules. 2. Temperature Effect and Energetics

Abstract

The competition between two molecules of similar polarity for adsorption sites on the stationary phase is discussed in light of the effects of temperature, acetonitrile and surfactant (cyclohexylaminopropane sulfonic acid, CAPS) concentration on the retention of the thyroid hormones (3,5-diiodo-thyronine, T2; 3,3′,5-trillodo-thyronine, T3 and thyroxine, T4). The data are analyzed using a second-order polynomial from which the enthalpy, entropy and heat capacity can be evaluated. The molecular motion of the analyte is reduced with an increase in surfactant concentration as determined from entropy and heat capacity calculations. This effect does not result from micelle formation but rather from molecular interaction between the analyte and a few surfactant molecules. A reduction in enthalpy from competitive and interactive behaviour is proposed. The compensation temperature is half of what is normally observed, which is related to the heat capacity effect and the data treatment.