Novel pyridinium surfactants with unsaturated alkyl chains: Aggregation behavior and interactions with methyl orange in aqueous solution

This paper presents the synthesis and a study of the aggregation behavior o f 4-undecyl-1-methyl- and 4-undecenyl-1-methylpyridinium iodide surfactants . The effect of the position of the double bond in the alkyl chain of the s urfactant on the critical micelle concentration (cmc), degree of counterion binding (beta), Krafft temperature, and thermodynamic parameters of micell e formation has been investigated. Also, a pyridinium surfactant with a tri ple bond in the alkyl chain has been studied. The cmc values of the surfact ants increase after the introduction of a double bond or a triple bond in t he tail as compared to the saturated equivalents. The largest effect was ob served for surfactants with a cis-oriented double bond in the alkyl chain. The enthalpies of micelle formation (DeltaH degrees) were shown to be exoth ermic. Introduction of an unsaturation in the tail leads to an increase of the enthalpy of micelle formation. The largest effect is observed for 4-(1- undecynyl)-1-methylpyridinium iodide. The influence of a double bond in the tail of the surfactants on the absorption spectrum of methyl orange (MO) c omplexed to the surfactant was studied by UV-vis spectroscopy. Low concentr ations of 4-alkyl-1-alkylpyridinium amphiphiles induce a new, ca. 80-nm, bl ue-shifted band. However, the surfactants 4-(10-undecenyl)-1-methylpyridini um iodide, 4-(5-cis-undecenyl)-1-methylpyridinium iodide, and 4-(5-trans-un decenyl)-1-methylpyridinium iodide do not induce this absorption band at sh ort wavelengths. In contrast, 4-(1-undecenyl)-1-methylpyridinium iodide and 4-(1-undecynyl)-1-methylpyridinium iodide showed this short wavelength abs orption band positioned at 383 nm.