Dominique Labarre

Modification of silica by an organic monolayer in aqueous medium using octylphosphonic acid and aluminium species

In the present work we report a “green” method for the grafting of organic monolayers at the surface of silica nanoparticles using water as a solvent. This method is based on the use of water-stable phosphonic acid coupling molecules grafted on an intermediate layer of aluminium species, as the sensitivity of Si–O–P bonds toward hydrolysis precludes a direct anchoring onto silica. Two approaches were explored: anchoring of octylphosphonic acid on aluminated silica and one-pot modification of silica by AlCl3 then by octylphosphonic acid. The modified powders were characterized using elemental analysis, 31P and 27Al MAS NMR spectroscopy, FTIR spectroscopy and N2 physisorption. The one-pot approach appears particularly promising, as it allows the anchoring on silica of phosphonic acid monolayers with controlled densities in a significantly shorter time.

Organo-lined alumina surface from covalent attachment of alkylphosphonate chains in aqueous solution

The reaction of octylphosphonic acid with the surface of alumina nanoparticles has been investigated in order to prepare a close packing of grafted-alkyl chains. This goal was attained through a fitting selection of the experimental conditions in terms of pH, reactant amount, reaction time and temperature. DRX, TEM and 31P MAS NMR spectroscopy are all consistent with an efficient covalent anchorage of the alkylphosphonate chains without degradation of the support, demonstrated by the morphology and texture preservation during the modification. In addition to the textural analysis, nitrogen adsorption isotherms provide additional pieces of information on the interaction energy between the nitrogen molecule and the surface. These data, combined with those supplied by the empirical test of floatability of powders (methanol number test), as well as the adsorption properties of the differently functionalized alumina samples using other probes, such as hexane and water, as a function of the chain loading, provided converging information about surface coverage and the hydrophilic/hydrophobic properties of the various materials. Different conformations are proposed to take into account the different results obtained from vapor adsorption measurements.