Effect of temperature and surfactant structure on the microviscosity at the micelle - Water interface: Isomeric alkylbenzenesulfonates used as their own probe

We investigated the effect of temperature and surfactant structure on the microviscosity in aqueous micellar solutions formed by isomeric hexadecylbenzenesulfonates (xφC16, where x = 4-6 and indicates the position of the benzene ring [φ] along the alkyl chain) by fluorescence polarization and excimer emission spectroscopy. For a given isomer, the degree of polarization (p) was found to decrease with increasing temperature, with no evidence for changes in micellar structure, η _int/τ ratios, where η _int is the microviscosity of the benzene environment in micelles and τ its natural lifetime, were derived from fluorescence polarization measurements and showed a similar temperature behavior to that observed with the degree of polarization, suggesting that a thermal effect is the determinant factor in the variation of η _int. Interestingly, the microviscosity around the benzene ring was found to depend on the isomer structure in the entire range of temperatures investigated (8-60°C) and is mainly determined by the orientation of the surfactant at the micelle-water interface in which the short alkyl chain is preferentially located at the interface and the long alkyl chain in the micellar core. This micelle conformation was found to prevail in the entire range of temperatures. In contrast to the dependence of p with temperature, excimer to monomer maximum emission ratios (I _E/I _M) were found to increase with increasing temperature, showing that when I _E/I _M is high (strong excimer emission), the degree of polarization is low (low microviscosity) and vice versa. Thus, the two independent measurements (I _E/I _M and p) yield the same information, namely, that the benzene moiety in all xφC16 aqueous micelles resists both translational and rotational diffusion in a similar manner in the entire range of temperatures investigated (∼ 8-60°C).