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

Abstract 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.