Aggregation properties of supralong-chain surfactants with double or triple quaternary ammonium head groups

Quantification of energy of activation to supramolecular nanofibre formation reveals enthalpic and entropic effects and morphological consequence

We show a self-assembly process leading to fibres from a system that starts far from equilibrium because of fast solvent - anti-solvent mixing and analyse the activation energies associated with the aggregation. It is in some ways reminiscent of diverse natural fibrous materials that have kinetic behaviour dominated by a rate limiting induction period followed by rapid growth. A full thermodynamic rationale for these systems and related synthetic ones is required for a full understanding of the driving force of their non-equilibrium self-assembly. Here we determine quantitatively the enthalpy and entropy of activation for the processes leading to the growth of fibres of this type, that contrasts with analysis of other systems where final energetic states are analysed. A dramatic effect is revealed whereby comparatively small changes in temperature or solvent composition (the ratio of water to ethanol) lead to alterations in the relative importance of enthalpy and entropy of activation and massive changes in the speed of fibre formation. The characteristics of the kinetic model adopted show a correlation with the fibre morphology of the self-assembled materials, which are isostructural according to diffraction experiments: the control of growth can lead to fibres only two bilayers thick. The crossover in behaviour is characteristic of the solvent mixture and the thermodynamic analysis points to the origins of this effect where different assembly routes are viable under only marginally different conditions.

Synthesis and properties of multiheaded and multitailed surfactants based on tripentaerythritol

The surface and self-assembly properties of a family of multiheaded, multitailed surfactants based on a tripentaerythritol backbone are described. Critical aggregation concentrations of these unusual surfactant systems have been determined from surface tension measurements; aggregate sizes in the presence and absence of a small amount of added electrolyte have been obtained via dynamic light scattering, and the morphologies of the aggregates were examined from electron microscopy measurements. In general, when compared to conventional ionic and two-headed surfactants (and other recently synthesized pentaerythritol derived surfactants from this group), these multiheaded surfactants exhibited some unusual trends in their aggregation behaviour and interesting aggregate structures in aqueous solution, as a function of alkyl chain length.