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

Abstract

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, 1 H 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 longer compared to those in the micelles of their conventional counterpart. Rotational relaxation time increases with increasing tail length of surfactant as a result of increasing microviscosity of micelles with fast relaxation as a major component. With increasing hydrophilicity of the spacer group, the anisotropy decay becomes slower due to the formation of more compact micelles. Rotational relaxation in gemini micelles is also slower compared to that in their conventional counterpart. The anisotropy decay is found to be biexponential with lateral diffusion of the probe along the surface of the micelle as a slow component. Rotational motion of micelle as a whole is a very slow process, and the motion becomes further slower with increasing size of the micelle. The time constants for wobbling motion and lateral diffusion of the probe become longer with increasing microviscosity of micelles.

Effect of Urea on Solvation Dynamics and Rotational Relaxation of Coumarin 480 in Aqueous Micelles of Cationic Gemini Surfactants with Different Spacer Groups

“…For this purpose, we have presented here Figure 11 a for Gemini-A, which has been reported earlier. 74 This figure shows that in the presence of 10 mM Gemini-A the fluorescence peak maximum gets red-shifted from 477 to 480 nm on increasing the percentage of methanol up to 20%. However, Figure 11 b,c shows that there is no such change in fluorescence peak maximum in 10 mM Gemini-B and Gemini-C, respectively, in the presence of the same percentage of methanol.…”

“…We have carried out a separate fluorescence experiment to further support the fact that the hydroxyl group(s) present in the spacer group of gemini surfactants protects the probe molecules from contact with a certain amount of water molecules. 74 It has been reported 61 , 81 that the significant change in micelle size and cmc occurs only when the percentage of organic co-solvent in water–organic solvent mixture is above 20%. In view of this fact, we have recorded fluorescence spectra of C-480 in aqueous micelles and also in micelles in the presence of various percentages of organic co-solvent not exceeding 20%.…”

“…The growth in the decay indicates the solvation of the probe molecule in the exited state. 74 Decays were fitted triexponentially at a shorter wavelength, whereas biexponentially at a longer wavelength. Figure S3a,b shows decays of C-480 in 12-4(OH)-12 (10 mM) at 5 M urea concentration and 12-4(OH) 2 -12 (10 mM) at 5 M urea concentration as representatives.…”

“… 50 Recently, we have reported comparatively slower solvation processes in case of micelles of gemini surfactants containing diethyl ether spacer group causing similar effect. 74 In case of micelles of Gemini-A, i.e., 12-4-12 surfactant, because of the presence of hydrophobic spacer group, the extent of free water molecules is greater than that in the micelles of gemini surfactants possessing hydroxyl groups in the spacer group. Evidence to this hypothesis has been discussed below.…”

“…At a longer wavelength (say 565 nm), fluorescence decay shows a clear growth in the spectrum, followed by the decay. The growth in the decay indicates the solvation of the probe molecule in the exited state . Decays were fitted triexponentially at a shorter wavelength, whereas biexponentially at a longer wavelength.…”