Surface Aggregate Structure of Nonionic Surfactants on Silica Nanoparticles

TL;DR

This study investigates how two nonionic surfactants, C12E5 and { extss}-C12G2, adsorb and assemble on silica nanoparticles, revealing micelle formation and surface coverage behaviors using scattering techniques.

Contribution

It provides detailed structural insights into surfactant aggregation on silica nanoparticles, highlighting differences between two surfactants and their adsorption mechanisms.

Findings

C12E5 strongly adsorbs as spherical micelles on silica.

{ extss}-C12G2 exhibits weak adsorption and forms ellipsoidal micelles.

SANS data best fit by micelle-decorated silica bead model.

Abstract

The self-assembly of two nonionic surfactants, pentaethylene glycol monododecyl ether (C12E5) and n-dodecyl-{\ss}-maltoside ({\ss}-C12G2), in the presence of a purpose-synthesized silica sol of uniform particle size (diameter 16 nm) has been studied by adsorption measurements, dynamic light scattering and small-angle neutron scattering (SANS) using a H2O/D2O mixture matching the silica, in order to highlight the structure of the surfactant aggregates. For C12E5 strong aggregative adsorption onto the silica beads, with a high plateau value of the adsorption isotherm above the CMC was found. SANS measurements were made at a series of loadings, from zero surfactant up to maximum surface coverage. It is found that the spherical core-shell model nicely reproduces the SANS data up to and including the local maximum at q = 0.42 nm-1 but not in the Porod region of high q, indicating that the surface area of the adsorbed surfactant is underestimated by the model of a uniform adsorbed layer. A satisfactory representation of the entire scattering profiles is obtained with the model of micelle-decorated silica beads, indicating that C12E5 is adsorbed as spherical micellar aggregates. This behaviour is attributed to the high surface curvature of the silica which prevents an effective packing of the hydrophobic chains of the amphiphile in a bilayer configuration. For the maltoside surfactant {\ss}-C12G2 very weak adsorption on the silica beads was found. The SANS profile indicates that this surfactant forms oblate ellipsoidal micelles in the silica dispersion, as in the absence of the silica beads.