Surfactant effects on the spectrophotometry of the gadolinium—chrome azurols complex

Increased molar absorptivities and red-shifted absorbance maxima were noted only upon the addition of cetylpyridinium chloride (CPC), a cationic surfactant. A 1:2:4 gadolinium/chrome azurol S/CPC complex was forned but dissociated at CPC]/ Gd³⁺] > 4 apparently because gadolinim(III) was displaced from this complex by additional surfactant monomers. Ternary complexes having different stoichiometries formed in the presence of excess dye. Sodium dodecyl sulfate (SDS), an anionic suffactant, induced dissociation of the binary complex at micellar concentrations, suggesting that dissociation resulted from adsorption of Gd³⁺ cations on the negatively charged micellar surface. Addition of Triton X-100, a nonionic surfactant, had little effect at either micellar or submicellar concentrations. These results confirm that complex stability is an important factor in the use of surfactants as sensitizers in quantitative spectrophotometry.