Effects of colloidal SiO₂ and cyclodextrins on the aggregation of dyes in aqueous solutions

Aggregation of two positively charged dyes, Methylene Blue, MB⁺ (1), and pinacyanol chloride, PC⁺, (2) is induced in aqueous SiO ₂ colloids. In colloidal SiO₂, MB⁺ aggregates into the dimeric form, and PC⁺ is aggregated into H-assemblies. The aggregation processes are a result of high local dye concentrations in the vicinity of the colloid owing to binding of the positively charged dyes to the negatively charged colloidal interface. De-aggregation of thionin, (3), is accomplished in water by the addition of cyclohexaamylose, α-CD, and cyclohepta-amylose, β-CD. Monomeric thionin binds to α-CD and β-CD (K_ass. = 6.67 x 1O² M^-1 and 1.78 x 10³ M^-1, respectively). Conversion of thionin dimer into the monomeric form, upon addition of α-CD or β-CD, is attributed to association of the monomeric form to the hydrophobic cavity of CD. This conversion is accompanied by changes in the absorption and fluorescence spectra of the dye.