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4.5. Conclusion 55However, the final value of T 2 is much higher at T = 50 ◦ C, which indicates that therelaxation behavior is similar to that of a normal liquid. Although the calibration gelswere aged for 20 days, the final values of T 2 in the bulk experiments agree well with thoseof the calibration gels, and show a similar dependence of T 2 on pH and temperature.It was found in a previous work [88] that a minimum (plateau) in T 2 , for 1 H in H 2 Oin that case, corresponded with macroscopic gelation of the aqueous phase as determinedby tilting test tube experiments [88]. Similar experiments using systems of equivalentdimensions were done here at 35 ◦ C to confirm the relation between the gel transition andthe minimum (plateau) in T 2 for 2 H. Again, a good agreement was found and thereforethe point at which the minimum or plateau in T 2 is found is used to derive the gel time.The results are listed in Table 4.4. The gel time varies between 0.5 and 30 hours. Theshortest gel times are found for the pH = 5.50 system followed by the unbuffered systemand the pH = 9.48 system. The longest gel times are found for the pH = 2.95 system.Slow condensation rates are expected when the pH is close to 2.5, which corresponds tothe isoelectric point of silica [24, 33]. The gel time decreases roughly by a factor two whenφ T (0) = 0.35 compared to the case when φ T (0) = 0.20. Increasing the temperature from25 to 50 ◦ C results in a reduction of the gel time of at least 50%. The effect of pH onthe reaction rates was only determined for a limited set of buffers (chemicals). Preparingthe buffers with other chemicals or concentrations is likely to quantitatively influence theresults [33].4.5 ConclusionThe mechanisms of coupled mass transfer and gel reaction of TMOS in the two phasesystems were revealed using a combined 1 H and 2 H NMR technique. The pH of theaqueous phase is the main parameter that affects the mass transfer and the gelationrates. The mass transfer of TMOS is driven by the rate of hydrolysis. Around pH = 6the rate is minimum, whereas the hydrolysis rates, and thus the mass transfer rates, areincreased in the acid- and base-catalyzed systems (due to enhanced protonation of TMOSunder acid conditions, respectively, enhanced attack by hydroxyl anions under alkalineconditions). At a temperature of 50 ◦ C the rates are increased by a factor between 1.6and 13 with respect to a temperature of 25 ◦ C, depending on the pH used. The T 2 of thecalibration gels decreases with increasing initial TMOS content in the gelling solutions.The differences in T 2 for a given concentration among the various series, prepared with thedifferent buffers, can be explained by the pH, which influences the microscopic structure ofthe silica gel. The gel transition times, derived from the T 2 versus time profiles revealedthat the gelation rates were the highest for the pH = 5.50 systems (gel time on theorder of 1 hour), and the lowest for the pH = 2.95 systems (gel time up to 30 hours).Slow condensation rates are expected when the pH is close to 2.5, which corresponds tothe isoelectric point of silica. Increasing the temperature from 25 to 50 ◦ C results in areduction of the gel time of at least 50%.