Hydrophobicity effect on interactions between organic molecules in nanocages of surfactant micelle

Interactions between the cationic polymethine dyes 3,3'-diethyloxacarbocyanaine iodide (DiOC(2)), 3,3'-dioctadecyloxacarbocyanine iodide (DiOC(18)), and 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) in the nano-scale volume of anionic sodium dodecylsulfate (SDS) micelles have been studied using optical spectroscopy techniques. The solubilization of pairs of dye molecules (DiOC(18)/DiI or DiOC(2)/DiI) within SDS micelles was monitored by Forster resonance energy transfer (FRET) between the dyes in each pair. The influence of the hydrophobicity of the dyes DiOC(2) and DiOC(18) on the efficiency of their binding to SDS micelles and, consequently, on the efficiency of FRET between DiOC(18) and DiI or DiOC(2) and DiI fluorophors has been analyzed. It has been shown that hydrophobic interactions in addition to electrostatic interactions are of key importance for the binding between the positively charged dyes and negatively charged surfactant micelles.