On the effects of head-group volume on the adsorption and aggregation of 1-(n-hexadecyl)-3-Cm-imidazolium bromide and chloride surfactants in aqueous solutions

The effects of the length of alkyl side chain (C-m) of ionic liquid-based surfactants (ILBSs) on their adsorption at the water/air interface, and aggregation in aqueous solutions were investigated for the series 1-(n-hexadecyl)-3-C-m-imidazolium bromides and chlorides, where C-m = C-1-C-4 for the bromides, and C-1-C-5 for the chlorides. These physicochemical properties were calculated from surface tension, conductivity, and fluorescence data. It was found that increasing the length of C-m (i.e., volume of the head group) leads to enhancement of surface activity, increase in the area per surfactant molecule at the water/air interface (A(min)) and the degree of counter-ion dissociation (alpha(mic)). Our data also indicated that increasing the volume of the head group results in a decrease of the critical micelle concentration (cmc), Gibb's free energy of adsorption and micellization, and microscopic polarity of interfacial water. In order to delineate the effects of the presence of unsaturation in the HG, we included members that carry C-m = vinyl and allyl in the bromide series. The effect of these groups was found to be similar to removing a methylene group from C-m. The dependence of the solubilization of a lipophilic dye (Sudan IV) and a drug (nitrendipine) on the length of C-m was also studied. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The study investigates the effects of alkyl side chain length of ionic liquid-based surfactants on their adsorption at the water/air interface and aggregation in aqueous solutions. Results show that increasing the side chain length enhances surface activity and affects various physicochemical properties, such as decreasing critical micelle concentration and increasing the area per surfactant molecule at the water/air interface.