Aggregation behavior and intermicellar interactions of cationic Gemini surfactants: Effects of alkyl chain,spacer lengths and temperature

The aggregation behavior of the cationic Gemini surfactants C_mH_2m+1N(CH₃)₂(CH₂)_S (CH₃)₂ N C_mH_2m+1,2Br^? with m = 12, 14 and s = 2, 4 were studied by performing surface tension, electrical conductivity, pulsed field gradient nuclear magnetic resonance (PFG-NMR), and cyclic voltammetry (CV) measurements over the temperature range 298 K to 323 K. The critical micelle concentration (CMC), surface excess (Г_max), mean molecular surface area (A_min), degree of counter ion dissociation (α), and the thermodynamic parameters of micellization were determined from the surface tension and conductance data. An enthalpy–entropy compensation effect was observed and all the plots of enthalpy–entropy compensation exhibit excellent linearity. The micellar self-diffusion coefficients (D_m) and intermicellar interaction parameters (k_d) were obtained from the PFG-NMR and CV measurements. These results are discussed in terms of the intermicellar interactions, the effects of the chain and spacer lengths on the micellar surface charge density, and the phase transition between spherical and rod geometries. The intermicellar interaction parameters were found to decrease slightly with increasing temperature for 14–4–14, which suggests that the micellar surface charge density decreases with increasing temperature. The mean values of the hydrodynamic radius, R_h, and the aggregation number, N_agg, of the Gemini surfactants’ m–4–m micelles were calculated from the micellar self-diffusion coefficient. Moreover, the N_agg values were calculated theoretically. The experimental values of N_agg increase with increases in the chain length and are in good agreement with both previous results and our theoretical results.