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54 4. The effect of pH on mass transfer and gelation0.40.3no bufferpH = 2.95pH = 5.50pH = 9.48T 2[s]0.20.10.00 5 10 15time [h]Fig. 4.7: T 2 of the 2 H content in the aqueous phase as a function of time. The measurementswere done at 25 ◦ C using four different D 2 O solutions. The initial concentration φ T (0)was 0.35.47 hours). A stabilization of T 2 or plateau is observed after 6 hours. In the pH = 2.95system the T 2 decreases quickly from 345 ms to 180 ms in the first hour, after which itdecreases more slowly to 21 ms in the remainder of the experiment (up to t = 47 hours).The T 2 in the unbuffered system starts at 365 ms and decays to 50 ms in the first 6 hoursof the experiment. In the pH = 5.50 system the T 2 decreases most rapidly and reachesa plateau at T 2 = 24 ms after 1.5 hours. The decay of T 2 is due to the cross-linking ofthe silica, which indicates that the condensation rate is low for the pH = 2.95 systemcompared to the other systems. The condensation rate appears to be the highest for thepH = 5.50 system, which corresponds to the fast gel formation observed in the transparentvials. Test measurements with the transparent vials also revealed that the gel formed inthe pH = 9.48 system with φ T (0) = 0.20 did not percolate in the entire volume of theaqueous phase, and remained liquid-like. Despite complete transfer of TMOS and the fastcondensation in the higher pH system the lack of percolation suggests that the silicic acidinitially condenses into larger aggregates compared to the lower pH systems, in agreementwith the literature [24]. As a result the formed clusters consist of colloidal particles ratherthan a highly branched network.In the systems at T = 50 ◦ C, T 2 is initially about 630 ms in the unbuffered systemand drops to 58 ms within two hours. The T 2 in the buffered systems is initially shorter,around 540 ms, which is due to the already significant degree of hydrolysis in the initialminutes of the experiment. In the lower pH system T 2 drops to 20 ms within 5 hours.Both in the unbuffered system as in the pH = 2.95 system the decay of T 2 is faster withrespect to the same systems at T = 25 ◦ C, which implies that the condensation rate ishigher at T = 50 ◦ C. In the higher pH system with φ T (0) = 0.20, where the gel did notpercolate, the decay of T 2 is equally fast in the initial two hours for both temperatures.