Micelle formation by N-alkyl-N-methylpyrrolidinium bromide in aqueous solution

Abstract

The micellization of the ionic liquid N-alkyl-N-methylpyrrolidinium bromide (C_nMPB, n = 12, 14 and 16) in aqueous solutions was investigated by surface tension measurements, electrical conductivity and static luminescence quenching. The effectiveness of the surface tension reduction (Π_cmc), maximum surface excess concentration (Γ_max) and the minimum area (A_min) occupied per surfactant molecule at the air/water interface can be obtained from the surface tension measurements at 25 °C. The critical micelle concentration (cmc) at different temperatures and a series of thermodynamic parameters (ΔG⁰_m, ΔH⁰_m and ΔS⁰_m) of micellization were evaluated from electrical conductivity measurements in the temperature range of 25–45 °C. The thermodynamic parameters show that the micelle formation is entropy-driven at low temperature and enthalpy-driven at high temperature. Furthermore, the micelle aggregation number (N_agg) of C_nMPB was calculated according to the Turro-Yekta method through static luminescence quenching and found that N_agg (49, 55, and 59) increased with the hydrophobic chain length of C_nMPB.