Determination of Association Constants for Cyclodextrin–Surfactant Inclusion Complexes: A Numerical Method Based on Surface Tension Measurements

Abstract

Inclusion complexes of β-cyclodextrin (β-CD) with sodium octyl sulfonate (C8As), sodium dodecyl sulfonate (C12As), and sodium hexadecyl sulfonate (C16As) in aqueous solutions are studied by surface tension measurement at the air/water interface at different temperatures. At fixed concentrations of the surfactants, the surface tension increases with an increase in β-CD concentration to a maximum value, at which it holds. The surface tension curves of the surfactants in the presence of β-CD are higher than those in the absence of β-CD. The curves rise higher with the increase in β-CD concentration for each surfactant. The apparent critical micelle concentrations (CMC*) of the surfactants vary linearly with β-CD concentration. The CMC* and surface tension values (including those after the CMC*) for the same system decrease with increase in temperature. A numerical method based on surface tension measurements is developed to determine the association constants for 1:1 inclusion complexes. This method is very reliable and easy to perform. The results demonstrate that the longer the hydrophobic tail of the surfactant, the greater the association constant with β-CD, and that for the same surfactant the association constant is higher at lower temperatures.