  The surface aggregates structure of dimethyldodecylamine-N-oxide (C12DAO) in
three silica dispersions of different particle sizes (16 - 42 nm) was studied
by small-angle neutron scattering (SANS) in a H2O/D2O solvent mixture matching
the silica. At the experimental conditions (pH 9) the surfactant exists in its
nonionic form and the structure of the adsorbed layer is not affected by added
electrolyte. It is found that C12DAO forms spherical surface micelles of 2 nm
diameter on the 16 nm silica particles, but oblate ellipsoidal surface micelles
are formed on the 27 and 42 nm particles. The dimensions of these oblate
surface aggregates (minor and major semi-axes Rn and Rlat) are similar to those
of C12DAO micelles in the aqueous solutions. It is concluded that the
morphological transition from spherical to ellipsoidal surface aggregates is
induced by the surface curvature of the silica particles. A comparison of the
shape and dimensions of the surface aggregates formed by C12DAO and C12E5 on
the 16 nm silica particles demonstrates that the nature of the surfactant head
group does not determine the morphology of the surface aggregates, but has a
strong influence on the number of surface aggregates per particle, due to the
different interactions of the head groups with the silica surface.
