Surface aggregate structure of nonionic surfactants on silica nanoparticles

Abstract

The self-assembly of two nonionic surfactants, pentaethylene glycol monododecyl ether (C₁₂E₅) and n-dodecyl-β-maltoside (β-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 β-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.