Effect of Hydrophobicity of Tails and Hydrophilicityof Spacer Group of Cationic Gemini Surfactants on Solvation Dynamicsand Rotational Relaxation of Coumarin 480 in Aqueous Micelles

摘要

Solvation dynamics and rotational relaxation of coumarin 480 in aqueous micelles of cationic gemini surfactants with diethyl ether (EE) spacer group (m–EE–m) and tails with varying tail lengths (m = 12, 14, and 16) have been studied. Studies have been carried out by measuring UV–visible absorption, steady-state fluorescence and fluorescence anisotropy, time-resolved fluorescence and fluorescence anisotropy, ¹H NMR spectroscopy, and dynamic light scattering. Effects of hydrocarbon tail length and hydrophilicity of spacer group on solvation dynamics and rotational relaxation processes at inner side of the Stern layer of micelles have been studied. With increasing hydrophobicity of tails of surfactants, water molecules in the Stern layer become progressively more rigid, resulting in a decrease in the rate of solvation process with slow solvation as a major component. With increasing hydrophilicity of the spacer group of gemini surfactant, the extent of free water molecules is decreased, thereby making the duration of the solvation process longer. Solvation times in the micelles of gemini surfactants with hydrophilic spacer are almost 4 times longercompared to those in the micelles of their conventional counterpart.Rotational relaxation time increases with increasing tail length ofsurfactant as a result of increasing microviscosity of micelles withfast relaxation as a major component. With increasing hydrophilicityof the spacer group, the anisotropy decay becomes slower due to theformation of more compact micelles. Rotational relaxation in geminimicelles is also slower compared to that in their conventional counterpart.The anisotropy decay is found to be biexponential with lateral diffusionof the probe along the surface of the micelle as a slow component.Rotational motion of micelle as a whole is a very slow process, andthe motion becomes further slower with increasing size of the micelle.The time constants for wobbling motion and lateral diffusion of theprobe become longer with increasing microviscosity of micelles.