Microcalorimetric studies of cationic gemini surfactant with a hydrophilic spacer group

The aim of this work is to investigate the micellization thermodynamics for gemini surfactants with a hydrophilic spacer group, i.e. Br−n-C12H25N+Me2CH2(CH2OCH2)xCH2N+Me2n-C12H25 Br−, referred to as 12-EOx-12, where x = 1, 3, 6, 7. These oligooxaalkanediyl-α,ω-bis(dimethyldodecylammonium bromide) dimeric surfactants were synthesized by a two step reaction. Their physicochemical properties in aqueous solutions have been studied by microcalorimetry at 298 K, electrical conductivity and surface tension measurements. The critical micellar concentration (cmc) and the head-group area (a0) per molecule at the air-water interface increase with increasing number of oxide ethylene moieties in the spacer and, consequently, with the enhanced hydrophilic character of the molecule. The calorimetric studies of the self-assembly process confirm this trend, since the exothermic values of the differential enthalpy of micellization (ΔmicH°) decrease as the ethylene oxide moieties are lengthened. Moreover, the values of the free energy and entropy of micellization indicate an entropy driven phenomenon for all surfactants. These results are compared with those obtained for gemini cationic surfactants with a hydrophobic spacer and the differences observed are discussed in terms of the hydrophilic head-group conformation in the micelle.