Dependence of aggregate morphology on structure of dimeric surfactants

Abstract

SURFACTANT molecules in water form organized assemblies of various shapes, such as micelles and bilayer lamellae, which are of interest as analogues of biological structures, as model systems for studying complex phase behaviour and because of their technological importance, for example to the food and paint industries. The polar head groups are usually arranged randomly at the surface of these assemblies. We have studied the effect on the microstructure of these assemblies of imposing constraints on the head-group spacing. We investigate the structures formed by 'double-headed' surfactants in which two quaternary ammonium species (C_mH_2m+1N⁺(CH₃)₂) are linked at the level of the head groups by a hydrocarbon spacer (C_sH_2s). Here we report the microstructures formed by these dimeric surfactants with m = 12 and s = 2,3 or 4 in aqueous solution, by rapidly cooling the micellar solutions and investigating the vitrified structures with transmission electron microscopy. The surfactants with a short spacer (s = 2,3) form long, thread-like and entangled micelles even at low concentrations, whereas the corresponding monomeric ammonium surfactants can form only spherical micelles. The dimeric surfactants with s = 4 form spheroidal micelles. Thus short spacers (which impose reduced head-group separation) appear to promote lower spontaneous curvature in the assemblies. This approach may afford a new way to control amphiphile self-aggregation.