Effect of Polymethylene Spacer of Cationic Gemini Surfactants on Solvation Dynamics and Rotational Relaxation of Coumarin 153 in Aqueous Micelles

Abstract

The present work demonstrates the solvation dynamics and rotational relaxation of Coumarin 153 (C-153) in the micelles of a series of cationic gemini surfactants, 12-s-12, 2Br(-) containing a hydrophobic polymethylene spacer with s = 3, 4, 6, 8, 12. Steady-state and time-correlated single-photon counting (TCSPC) fluorescence spectroscopic techniques have been used to carry out this study. Steady-state and TCSPC fluorescence data suggest that C-153 molecules are located at the Stern layer of micelles. While probe molecules feel more or less the same micropolarity in the micellar phase, the microviscosity of micelles decreases with spacer chain length. Solvation dynamics at the Stern layer is bimodal in nature with fast solvation as a major component. Counter ions and water molecules bonded with the polar headgroups of surfactant molecules are responsible for the slow component. Average solvation time increases with spacer chain length because of the increased degree of counter ion dissociation. Some water molecules are involved in the solvation of counter ions themselves, resulting in the decrease in "free" water molecules to be available for the solvation of C-153. The hydrophobic spacer chain also has an effect on increasing the solvation time with increasing chain length. The average rotational relaxation time for C-153 decreases with spacer chain length with a rapid decrease at s > 4. The anisotropy decay of C-153 in micelles is biexponential in nature. The slow rotational relaxation is due to the lateral diffusion of C-153 in micelles. Lateral diffusion is much faster than the rotational motion of a micelle as a whole. The rotational motion of the micelle as a whole becomes faster with the decreasing size 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

“…It depicts that C-480 is present neither at the core of the micelles nor in the bulk water, but somewhere in between these two. According to literature reports, the micropolarity of the Stern layer is similar to methanol, 50 , 51 , 53 , 69 , 70 on the basis of which we state that C-480 molecules are mostly residing in the Stern layer. It is pertinent to note that C-480 being present in the Stern layer of micelles, microenvironment remains more or less same with the addition of urea in micellar systems.…”

Gemini Surfactants with Polymethylene Spacer: Supramolecular Structures at Solid Surface and Aggregation in Aqueous Solution

“…Larger values around 4 nm are reported for 12-s-12 aggregates which may have resulted from the selection of a SANS data fitting model different to the usual core-shell prolate spheroidal model [37]. The 12-s-12 micellar diameters resulting from fluorescence anisotropy measurements give substantially smaller values from 0.86 to 1.23 nm [26].…”

“…In the field of nonionic systems, they interact with cyclodextrins [22] and nonionic triblock copolymers [23,24] and form mixed micelles with single-chain nonionic surfactants [25]. The bisammonium gemini molecules with the polymethylene spacer have been found to be an efficient solvation tool for electronically excitable molecules [26]. The investigations of the aggregation properties at the air-water interface showed that the area per gemini surfactant molecule gives the maximum value at the spacer length of 12 CH 2 groups which is interpreted in terms of diminishing spacer rigidity and increased flexibility with the increasing spacer length [27].…”

Binding Interaction of Gemini Surfactants with Nanotubes of β‐Cyclodextrin and Controlled Release of Guest Molecules: Effect of Spacer Chain Length and Concentration of Surfactants

“…also have reported that gemini surfactants with small spacer chain form bigger micelles . We have reported earlier the diameter of the micelle of each of pure 12–3‐12, 12–6‐12 and 12–12‐12 as 1.24 nm, 1.01 nm and 0.86 nm, respectively . At higher concentration of surfactants (data given in Table ), sizes of small particles observed are in these ranges only.…”

“…Zana et al have reported, and we have also seen in one of our earlier studies, that the microviscosity of gemini micelles increases with decreasing polymethylene spacer chain length. Thus the above mentioned trend in fluorescence intensity could be due to increasing microviscosity of micelles with decreasing spacer chain length.…”