Micellization behavior of the ionic liquid lauryl isoquinolinium bromide in aqueous solution

Abstract

The micellization behavior of the ionic liquid lauryl isoquinolinium bromide ([C12iQuin]Br) in aqueous solution has been assessed using surface tension, electrical conductivity, and 1H nuclear magnetic resonance (NMR) measurements. The results reveal that the critical micelle concentration (CMC) and constant surfactant tension (γcac) are lower than that of butyl isoquinolinium bromide ([C4iQuin]Br), octyl isoquinolinium bromide ([C8iQuin]Br, and lauryl pyridinium bromide ([C12Pyr]Br). 1H NMR spectra show the evidence of paralleled π-stacking of adjacent isoquinoline rings. To elucidate the effect of the π–π interactions on the aggregation process, thermodynamic parameters such as the standard free energy, enthalpy, and entropy of aggregation have been discussed. These parameters are evaluated from the CMC with temperature by fitting these values to expressions derived from a micellization thermodynamic model. The enthalpy–entropy compensation phenomenon has been observed in the micellization process of [C12iQuin]Br in water, and the presence of isoquinoline cations is responsible for the decrease in the ΔHmic∗, compared with [C12Pyr]Br which has the same alkyl chain and counter-ion.