Defining the different phases of premicellar aggregation using the photophysical changes of a surface-probing compound

Fluorescent probe studies of micropolarity, premicellar and micellar aggregation of non-ionic Brij surfactants

“…With reference to our previous study on the micelles of different types of surfactants (CTAB, SDS, and Triton X-100) [13] and characteristics of TICT fluorescence of DMASBT [23], we can state that the probe molecules at higher concentrations of Brijs reside in the aggregates, the polarity of which essentially favors complete TICT fluorescence of DMASBT. Probably two conclusions can be drawn from this observation: (i) the polarity of the microenvironment around DMASBT increases with increasing the size of aggregates and remains constant when micellization is saturated, and (ii) almost all DMASBT molecules reside in the Stern layer (''wet" shell) and/or near to Stern layer of micelles comprising polar head groups of surfactants and a reasonable amount of water [30,31].…”

“…Otherwise, a complete TICT fluorescence band with lowered intensity would have been noted at higher concentrations of surfactant. In correlation with the TICT fluorescence characteristics of DMASBT [23] between the dipole of probe molecule at its TICT state and the polar head groups of surfactants located in the Stern layer which favors the probe molecule to reside at the surface [13].…”

“…With increasing polarity of the microenvironment around DMASBT it is expected that the DMASBT moves toward an excited state with the progressively increased nature of the dipole, as nitrogen atoms of -N(CH 3 ) 2 groups (donor) possess a positive charge, whereas a dispersed negative charge is located on the benzothiazole moiety (acceptor) [23]. Therefore, during the course of micellization through the formation of smaller aggregates, there is a progressive change in the nature of the molecule and its location in the aggregates, which reflects the change in its fluorescence characteristics [13]. Depending on the polarity of aggregates, DMASBT molecules become attached with different parts of aggregates.…”

“…Various methods for the determination of their critical micellar concentrations (CMC) have been described in the literature [7][8][9][10]. It has been observed in many cases of surfactants that the occurrence of the micellar phase is preceded by the formation of different comparatively smaller aggregates known as premicellar aggregates [11][12][13][14][15][16]. Hadgiivanova et al developed a thermodynamic model to study the aggregation mechanism [17,18].…”

“…With decreasing polarity of the medium, the TICT fluorescence quantum yield increases with the blue shift of fluorescence due to the concomitant increase in the energy gap between S TICT 3 and S TICT o states as well as triplet state. In our previous work, the dipolar nature of DMASBT at its excited TICT state has been found to be very useful for the characterization of micelles [13] and cyclodextrins [25][26][27].…”