Effect of hydrocarbon structure of the headgroup on the thermodynamic properties of micellization of cationic gemini surfactants: An electrical conductivity study
2012
Zhang, Qi | Gao, Zhinong | Xu, Feng | Tai, Shuxin
A series of cationic gemini surfactants butanediyl-1,4-bis(dodecyldialkylammonium bromide), C₁₂H₂₅N⁺(CₘH₂ ₘ ₊₁)₂C₄H₈N⁺(CₘH₂ ₘ ₊₁)₂C₁₂H₂₅·2Br⁻, 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 (Tₘᵢₙ) 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 (β) were used to calculate the Gibbs free energy (ΔₘᵢcG°), enthalpies (ΔₘᵢcH°) and entropies (ΔₘᵢcS°) 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.
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