Porous ‘Ouzo-effect’ silica–ceria composite colloids and their application to aluminium corrosion protection

Abstract

By exploiting spontaneous emulsification to prepare porous SiO 2 particles, we report the formation of porous CeO 2 @SiO 2 hybrid colloids and their incorporation into a silica-zirconia coating to improve the corrosion protection of aluminium.The ''Ouzo effect'' is probably better known as a drinkers' conundrum than as a technique for the formation of colloidal particles. The careful addition of water to e.g. Pastis, Absinthe, etc. causes the spontaneous emulsification of a metastable droplet phase, driven by the aqueous insolubility of the flavour compound (trans-anethole). Several studies have sought to characterise the underlying process 1 and to use it for the production of polymer particles and capsules.2 As a synthesis method, spontaneous emulsification has the advantage of requiring neither externally applied energy, nor stabilizing or templating species. Using a simple system consisting of water, ethanol, ammonia, hydrochloric acid and silicon alkoxides, porous silica particles were formed.3 However, few other reports use this method to form hard, porous colloidal particles and there have been no reports of their application. This is remarkable given that while porous particles have been developed for many applications (e.g. catalysis, drug delivery, chromatography, corrosion inhibition), almost all of the preparation methods suffer the requirement of templating species, hightemperature syntheses or high shear pre-emulsification. [4][5][6][7] A spontaneous emulsification-based route to form porous materials would be cheap, simple, non-hazardous and therefore warrants further consideration. One way to functionalise the resultant particles (e.g. zeolite or silica) is to adsorb active nanoparticles.8 Such hybrid colloids would have the benefit of easy solution uptake and facile incorporation into e.g. coatings.Here, a spontaneous emulsification process was used to prepare porous silica particles, 3 which are capable of nanoparticle and/or small molecule uptake. Ceria was chosen as the active content, given its known use both in oxidation catalysis 4,9 and in corrosion protection. [10][11][12] Several routes exist to form ceria nanocrystals. 9,13 Here, an adapted precipitation method was employed, 14 due to its simplicity and high yield. Fig. 1a shows the characterization of the ceria particles by XRD. Peaks corresponding to the fluorite ceria structure are clearly noted and marked, with substantial broadening indicating the presence of nano-crystalline grains. The minimum grain size was calculated to be 5.5 nm by the Scherrer equation 15 using shape factor = 0.9, in line with an approximate particle size estimated by TEM (d E 5 nm, Fig. S1, ESIw). TEM images of the silica spheres formed by spontaneous emulsification are shown in Fig. 1b. The sample mainly consisted of polydisperse porous particles with d mean E 200 nm (see ESIw). The dependence on the zeta potential with pH ( Fig. 1c) for the silica and ceria particles showed typical isoelectric points (IEP) at pH 3.2 and pH 8.5, respectively. 16 The BET su...

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