A SEARCH FOR A RELATION BETWEEN AGGREGATE MORPHOLOGY AND THE STRUCTURE OF 1,4-DIALKYLPYRIDINIUM HALIDE SURFACTANTS

A detailed study of the relation between the aggregate morphology and the molecular shape of 27 1-alkyl-4-(or 2-)alkyl(or n-alkoxycarbonyl)pyridinium halide surfactants is described. This shape can be expressed in a molecular packing parameter (P) as suggested by Israelachvili. For unbranched 1-methyl-4-n-alkylpyridinium iodides, the packing parameter does not depend on the length of the alkyl chain (n(c)). Spherical micelles formed from these surfactants grow into rodlike micelles. The critical rod concentration (crc) is, however, dependent on n(c). This originates from the dependence of the aggregation number of spherical micelles on n(c), in agreement with predictions based on the ladder model. Alkyl chain branching in 1-methyl-4-(C12-alkyl)pyridinium iodides affects the shape of the surfactant. Branching near the headgroup decreases P and dramatically lessens the propensity of the spherical micelle to grow. Branching near the chain end increases P and decreases the crc. Highly branched surfactant monomers associate into bilayers, which can be transformed into vesicles. The overall results indicate that the morphology of the aggregate is mainly dependent on the shape of the surfactant. The possibility for backfolding of the 1-alkyl chain of 1-alkyl-4-n-dodecylpyridinium iodide surfactants directly affects the preferred morphology of the aggregate. Bilayers are formed if backfolding occurs; otherwise spherical micelles are formed, which, on increasing surfactant concentration, grow into rodlike micelles. Interestingly, 1-methyl-4-(n-alkoxycarbonyl)pyridinium iodides associate into bilayers, independent of the length of the alkyl chain (n(c) = 10-16). This aggregation behavior probably stems from a special kind of interdigitation, which changes the geometrical constraints for packing into a bilayer.