An Aqueous Orange II/γ-Cyclodextrin Complex: Calculation of the Guest/Host Binding Constant and the Superstructure...

An anisotropic phase can form in water when orange II (O-II) and γ-cyclodextrin (γ-CD) are each present at 0.025 M or higher. Anisotropy is not readily observed in water with only one component present. Both components are needed to observe anisotropy due to a complexation that occurs where 2 orange II molecules are included within the γ-cyclodextrin cavity that subsequently builds into a larger rod-like superstructure. The naphthol moiety has been considered to be the most likely portion of O-II to reside within the cavity, which was further confirmed by birefringence observations with various derivatives of O-II in the presence of aqueous γ-CD. Further understanding of the equilibrium binding constant of the O-II₂:γ-CD complex was obtained through conductivity titrations in comparison to those in the literature with UV-vis studies. Using binding constants from both approaches, calculations of the O-II₂:γ-CD concentration were made at the isotropic/anisotropic transition for a variety of solution compositions. Using the O-II₂:γ-CD concentrations, subsequent calculations were made to predict superstructure rod lengths based on various levels of participation of the O-II₂:γ-CD complex in rod formation. In comparison to dynamic light scattering measurements, rod lengths on the order of 300 nm may occur where 50-75% of O-II₂:γ-CD could be involved in rod formation.