Novel Pyridinium Surfactants with Unsaturated Alkyl Chains: Aggregation Behavior and Interactions with Methyl Orange in Aqueous Solution

Abstract

This paper presents the synthesis and a study of the aggregation behavior of 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 surfactant on the critical micelle concentration (cmc), degree of counterion binding (β), Krafft temperature, and thermodynamic parameters of micelle formation has been investigated. Also, a pyridinium surfactant with a triple bond in the alkyl chain has been studied. The cmc values of the surfactants increase after the introduction of a double bond or a triple bond in the tail as compared to the saturated equivalents. The largest effect was observed for surfactants with a cis-oriented double bond in the alkyl chain. The enthalpies of micelle formation (ΔH°) were shown to be exothermic. 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) complexed to the surfactant was studied by UV−vis spectroscopy. Low concentrations of 4-alkyl-1-alkylpyridinium amphiphiles induce a new, ca. 80-nm, blue-shifted band. However, the surfactants 4-(10-undecenyl)-1-methylpyridinium iodide, 4-(5-cis-undecenyl)-1-methylpyridinium iodide, and 4-(5-trans-undecenyl)-1-methylpyridinium iodide do not induce this absorption band at short wavelengths. In contrast, 4-(1-undecenyl)-1-methylpyridinium iodide and 4-(1-undecynyl)-1-methylpyridinium iodide showed this short wavelength absorption band positioned at 383 nm.