Inorganic Nano-Ions as Non-amphiphilic surfactants

Abstract

International audienceNanometric ions (or nano-ions) with low charge density have the propensity to adsorb onto neutral hydrated surfaces or chemical functions by an entropic phenomenon, which was recently established as the superchaotropic effect.[1, 2] In this context, the cloud point (CP) determination of the nonionic polyethoxylated surfactant C8EO4 (around 1% in volume) has served as a powerful tool to compare the superchaotropy of various polyoxometalates (POMs) as a function of their concentration.[2] We expand this approach by studying the cloud point of different nonionic surfactants in the presence of various nano-ions such as POMs, dodecaborates, metallabisdicarbollides and tetraphenylborate in comparison with mixed micelles formed by adding the ionic surfactant SDS.[3] Moreover, the respective micellar morphologies and interaction were monitored with DLS, SANS and SAXS. A strong cloud point increase, in the micromolar range, was observed for the non-amphiphilic nano-ions (Fig.1 left). This CP-increase, being comparable to the one with SDS, is due to a charging of the non-ionic micelles inducing intermicellar electrostatic repulsion. In the case of the nano-ions, their adsorption onto the micellar surface is driven by their superchaotropic property, while SDS is incorporated into the micelle due to the hydrophobic effect. Additionally, upon changing the nonionic surfactant, a different behavior was observed, i.e. a closed two-phasic region appeared in the presence of nano-ions but not in presence of SDS (Fig.1 right). The distinct behavior of related surfactant systems might be due to a preferential interaction of the nano-ions with longer ethoxy chains as observed by Buchecker et al.[4] This observed diversity shows the richness of phenomena induced by nano-ions in intriguingly simple systems emphasizing their interesting and complex behavior.References[1] K.I. Assaf, W.M. Nau, Angew. Chem. Int. Ed. 57 2018, 13968-13981.[2] T. Buchecker, et al., Chem. Comm. 54 2018, 1833-1836.[3] K. Panchal, A. Desai, T. Nagar, J. Dispersion Sci. Technol. 27 2006, 33-38.[4] T. Buchecker, et al., Chem.: Eur. J. 23 2017, 8434-8442