2, Institut Européen des Membranes, Montpellier, , France
In the nuclear field, functionalized porous silica are envisaged for the decontamination of radioactive effluents . Indeed, molecules having specific head functions can enhance the adsorption capacity and the selectivity of the sorbent material. More particularly, phosphonate group is of great interest because of their selectivity regarding actinides. However, classical grafting of alkoxysilanes requires the use of organic solvents, which presents many drawbacks. Supercritical carbon dioxide (SC CO2) is a green option and has already given good results on the grafting of Self-Assembly Monolayers (SAM) of different alkoxysilanes. Moreover, SC CO2 is a green solvent with physical properties facilitating the access of soluble molecules to low dimension gap such as nanopores . Here for the first time, we report an experimental method using supercritical CO2 grafting process to functionalize planar and concave silica surfaces (Si/SiO2 wafer, SiO2 nanochannels consisting in two parallel and planar SiO2 surfaces spaced of few nm and mesoporous silica) with a phosphonate head group. The method consists on the SC CO2 grafting of 3-(iodopropyl)trimethoxysilane (IPTMS), followed by a post-functionnalization with a triethylphosphite. This method avoids the possible reactions of the phosphonate head group with surface silanols and allows a homogeneous grafting all over the surface .
First, in order to find the optimal process parameters allowing the obtention of a monolayer, the IPTMS SC CO2 grafting is performed on silica planar surfaces, and characterized by X-Ray Reflectivity, X-ray photoelectron spectroscopy, contact angle and Atom Force Microscopy. Then, to determine the well adapted pore size to the grafting process, silica nanochannels having gap comprise between 3 and 5 nm were also functionalized using the optimal process and characterized by Hard X-Ray Reflectivity at 27 keV (BM32 – ESRF) . A confinement size of 5 nm being the well adapted size to our functionalization method, ordered mesoporous silica (SBA-15 type having a 6 nm pore size) was prepared  and characterized using are Small Angle X-Ray Scattering (SAXS), N2 adsorption-desorption, FTIR-ATR and NMR to validate the presence of phosphonate groups at the pore surface.
Finally, the stability of the grafted silica regarding aqueous solutions at various pH was determined using in-situ SAXS measurements and infrared spectroscopy .
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