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Thermo-Reversible Sol−Gel Transition of Surface Modified Titanium Poly Oxo Building Blocks

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posted on 2010-01-28, 00:00 authored by Renata C. K. Kaminski, Sandra H. Pulcinelli, Patrick Judeinstein, Florian Meneau, Valérie Briois, Celso V. Santilli
In this paper, the thermo-reversible sol−gel transition of titanium poly oxo building blocks dispersed in a solution of p-toluene sulfonic acid (PTSH) in isopropanol is reported. The sol formed by the thermo-hydrolysis at 60 °C of titanium tetraisopropoxide (Ti(OiPr)4) reversibly changes into a gel upon cooling. A large domain of formulations (β-domain) presenting this thermo-reversible behavior has been obtained for samples containing different nominal acidity (A = [PTSH]/[Ti]) and hydrolysis (H = [H2O]/[Ti]) ratios. The EXAFS study of the β-domain evidences the formation of different titanium poly oxo building blocks, depending on the formulation, for which the local-order structure does not change upon sol−gel transition. This result points out that the thermo-reversible behavior is linked to interfacial properties of these building blocks. SAXS results indicate that the thermo-reversible gelation is associated to a reversible aggregation of a disperse set of building blocks. The presence of both immobilized and free PTSH species was revealed by Raman and 1H NMR spectroscopies. Furthermore, NMR clearly evidences the significant decrease of free PTSH amount during the sol−gel transition. In the light of the different structural investigations, we propose that the thermo-reversible gelation is induced by the formation of a supramolecular network, in which the protonated surface of building blocks and/or aggregates are interconnected through cooperative bonds between the PTSH molecules.

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