Titania colloidal stability

The role of colloidal interactions is further supported by the observation that as the background concentration of HC1 is raised to 10-3 M, the rate of loss of soluble titania is not altered, particle charge increases, and final particle size decreases (13, 14). Again, these observations demonstrate the importance of colloidal stability during the precipitation of uniform particles. 

It seems contradictory that dense particles can grow from solution, resulting in a stable sol. Why aren t the polynuclear ions and polymers repelled by the electrostatic double layer that stabilizes the colloidal particles The answer is revealed in Fig. 5 the repulsive barrier increases with the size of the particles, so the nuclei may be unstable against aggregation until they reach a certain size. Kramer et al. [41] directly demonstrated that the rate of growth of titania particles was related to the electrostatic barrier. Particles grew to a diameter of 4nm in 24h at pH 9.7, but reached 50 nm in 4 h if salts were added to compress the double layer, and they reached 6 nm in 1 h if the pH was reduced to 7 (near the lEP of titania). Of course, if the repulsive barrier is too low, the particles do not form a stable sol. According to Her [13], silica sols made by hydrolysis of alkali silicate will precipitate if the salt concentration exceeds 0.3 N. 

If we assume Scheme 2, grafted colloidal titania particles, similar to those obtained from simple hydrolysis of Ti-(0Pr ) with water, should be obtained and therefore their structure should not be very different from that recently proposed by Leaustic et al (ref.11). In fact, the XANES spectrum of our sample Is very similar to the spectrum recorded by these authors for such colloidal particles. However, there are big differences in the EXAFS region that can be explained by the smaller size of the titania particles obtained in our system. Moreover, after heating at 373K, these authors obtain crystalline anatase, as previously did Kozlowski et al.(ref. 12) and Reichmann et al (ref. 3) during the preparation of similar systems, while the crystalline structure of anatase could not be detected by XRD in our samples, even after calcination at 873K thus implying that the layered open structure, remains stabilized on the surface of the silica. 

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