Surface modification of alumina-coated silica nanoparticles in aqueous sols with phosphonic acids and impact on nanoparticle interactionsElectronic supplementary information (ESI) available: Details regarding the synthesis of PAs with short alkyl groups, the NP size, and complementary data related to IR, TGA, ICP-OES, and small-angle scattering. See DOI: 10.1039/c5cp01925g

It is often necessary to tailor nanoparticle (NP) interactions and their compatibility with a polymer matrix by grafting organic groups, but the commonly used silanization route offers little versatility, particularly in water. Herein, alumina-coated silica NPs in aqueous sols have been modified for the first time with low molecular-weight phosphonic acids (PAs) bearing organic groups of various hydrophobicities and charges: propyl, pentyl and octyl PAs, and two PAs bearing hydrophilic groups, either a neutral diethylene glycol (DEPA) or a potentially charged carboxylic acid (CAPA) group. The interactions and aggregation in the sols have been investigated using zeta potential measurements, dynamic light scattering, transmission electron microscopy, and small-angle scattering methods. The surface modification has been studied using FTIR and 31 P MAS NMR spectroscopies. Both high grafting density ρ and high hydrophobicity of the groups on the PAs induced aggregation, whereas suspensions of NPs grafted by DEPA remained stable up to the highest ρ . Unexpectedly, CAPA-modified NPs showed aggregation even at low ρ , suggesting that the carboxylic end group was also grafted to the surface. Surface modification of aqueous sols with PAs allows thus for the grafting of a higher density and a wider variety of organic groups than organosilanes, offering an increased control of the interactions between NPs, which is of interest for designing waterborne nanocomposites. We report on aggregation of alumina-coated silica nanoparticles in suspensions modified with phosphonic acids of various hydrophobicities and charges.