  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.
