Preparation of stable dispersions

We have demonstrated a new approach towards stabilization of colloidal particles with hydrophobic surfaces in oily media. The core of this approach is to use a short steric stabilizer that is strongly (probably covalently) bonded onto the surface of hydrophobic particles (conventionally, the stabilizing molecules are rather long and merely adsorbed on the substrate). The approach has been demonstrated on the example of lauric acid which is strongly (covalently) attached to a BTSE-coated TiO₂ (the BTSE coating here serves for capturing the TiO₂-generated radicals). The strong (covalent) bonding has been proven using a comparative DTA analysis of different samples that contained carefully chosen compositions of interest. To the best of our knowledge, such an analysis has not been used before.
The advantages of strong bonding of steric stabilizer have been demonstrated on a practical example of oil-based suncreens. In fact, without such a bonding, TiO₂-based oily sunscreens are even not stable enough to be used in practical applications. In our case, the strong bonding of lauric acid onto silica-coated TiO₂ has given a very stable suspension with very small tendency towards particle agglomeration. As a result, already a very small concentration of BTSE-modified TiO₂ (0.5 wt. %) can give the same UV filtering efficiency as conventional sunscreens. This qualitatively perfectly agrees with our theoretical prediction for the case of significant particle absorption ability: the smaller the particles the higher the UV filtering ability.

Further reading:
UKMAR, Tina, GODEC, Aljaž, MAVER, Uroš, PLANINŠEK, Odon, BELE, Marjan, JAMNIK, Janko, GABERŠČEK, Miran. Suspensions of modified TiO₂ nanoparticles with supreme UV filtering ability. J. mater. chem., 2009, vol. 19, no. 43, pp. 8176-8183.

Schematics showing the materials prepared in the present paper. BTSE layer is attached to TiO2 through covalent Ti-O-Si bonding. Lauric acid (LA) is bonded to BTSE through the (Si-O-C(=O)-) group.

SEM micrographs of initial TiO2, TiO2 coated with BTSE layer (TiO2-BTSE) and TiO2-BTSE with covalently attached lauric acid (TiO2-BTSE -LA). b) Distribution of particle sizes in olive oil: TiO2 (red line), TiO2-BTSE (blue) and TiO2-BTSE-LA (green).