Ion Interaction: The Energetics and Mechanism of The Competitive Behavior Between Two Similarly Charged Molecules. 1. The Effect of Ionic Strength, Acetonitrile and Surfactant Concentration

摘要

The competition between two molecules of similar polarity for adsorption sites on the stationary phase is discussed in light of the effects of acetonitrile, surfactant (cyclohexylaminopropane sulfonic acid, CAPS) and salt concentrations on the retention of the thyroid hormones (3,5-diiodo-thyronine, T2; 3,3#x2032;,5-diiodo-thyronine, T3 and thyroxine, T4). The Kinetics of adsorption and desorption for the surfactant are fast which allows for actual competition, otherwise an electrostatic #x201C;wall#x201D; would be seen by the analyte leadiny to exclusion. A three-parameter equation relates the surfactant concentration and ionic strength to the retention of the hormones, which suggests that the retention and selectivity arise from the structure and #x201C;density#x201D; of the outer layer in a Stern-Gouy-Chapman model and confirms that only the outer electrical layer significantly contributes. The data are analyzed with both Langmuir and Freundlich equations which describe an intermediary state where the marginal heat of adsorption begins to be negligible with respect to the surface concentration of the surfactant.