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.
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.