Modification of the silica nanoparticle surface with organosilanes: introducing hydrophobicity for neutral wetting behavior

Abstract

Purification processes performed in the chemical industry require enormous amounts of energy. By utilizing bicontinuous interfacially jammed emulsion gels (bijels) as separative reaction media, the need for these energy costing purification processes can potentially be eliminated. The bicontinuous structure of bijels, consisting of two interwoven continuous channels, allows for the reaction and separation process to be combined in one medium. Continuous fabrication of bijels can be achieved by the solvent transfer induced phase separation (StrIPS) method. In this method CTA+ (cetyltrimethylammonium) modified silica nanoparticles are utilized to stabilize the bicontinuous structure. Using surfactants like CTA+ to create a hydrophobic silica surface can, however, be disadvantageous for potential applications of bijels. A possible surfactant-free approach to introduce hydrophobicity is by functionalizing the silica surface with organosilanes. In this thesis, the ability of three organosilanes, 3-(trimethoxysilyl)propyl methacrylate (TMSPM), n-dodecyltriethoxysilane (DTES), and hexamethyldisilazane (HMDS), to generate a hydrophobic Ludox TM-50 silica nanoparticle surface for potential application in water/DEP/1-propanol based bijels is researched. The different functionalizations were examined by FTIR, zeta potential, DLS, contact angle, and TGA measurements. The Ludox TM-50 particles were successfully functionalized with TMSPM, DTES, and HMDS. Due to post-hydrolyzation the silane-functionalized particles could not be stored in water as was shown by time-dependent studies. In addition, a maximum silane coverage of 40% was found for Ludox TM-50 particles. The trifunctional silanes, TMSPM and DTES, did not only introduce hydrophobicity but also hydrophilicity. All three silanes were unable to hydrophobize the Ludox TM-50 nanoparticle surface to the extent where they exhibit neutral wetting behavior regarding water and diethyl phthalate (DEP). Therefore, the three researched silanes are not suitable for the stabilization of bijels formed using a water/DEP/1-propanol precursor mixture. Additionally, it was found that there is a mismatch between the hydrophobicity needed for stability in the water/DEP/1-propanol precursor mixture and for stabilization of the bijel.