Effect of hydrocarbon structure of the headgroup on the thermodynamic properties of micellization of cationic gemini surfactants: An electrical conductivity study

A series of cationic gemini surfactants butanediyl-1,4-bis(dodecyldialkylammonium bromide), Cs12H25N+ (CmH2m+1)(2)C4H8N+(CmH2m+1)(2)C12H25 center dot 2Br(-), where m = 1, 2, 3, 4, referred to as C12C4C12(Me), C12C4C12(Et), C12C4C12(Pr), and C12C4C12(Bu), respectively, were synthesized, and their thermodynamic properties of micellization were studied by electrical conductivity measurements. There existed a minimum critical micelle concentration (cmc) in the curve of cmc versus temperature, and the temperature of the minimum of cmc (T-min) increased with increasing the headgroup alkyl chain length. The values of log (cmc) depended linearly on carbon number of the alkyl chains, but that was not true for the carbon number of the headgroup substituents. The temperature dependence of cmc and degree of counterion association (beta) were used to calculate the Gibbs free energy (Delta(mic)G degrees), enthalpies (Delta H-mic degrees) and entropies (Delta S-mic degrees) of micelle formation for these gemini surfactants, and well correlated enthalpy-entropy compensation was observed. The analyses showed C12C4C12(Me) and C12C4C12(Et) behaved similarly in terms of thermodynamics of micellization, but they behaved differently from C12C4C12(Pr) and C12C4C12(Bu). which could be ascribed to the hydrophobicity and the location of the headgroup alkyl chains in the aggregates. These initial results indicate the headgroup alkyl chain plays an important role in influencing the thermodynamic properties of gemini surfactants. (C) 2012 Elsevier Inc. All rights reserved.