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4.4. Results and discussion 53Table 4.4: Fitted hydrolysis rate k and interaction parameter ε 0 for the various bulk experiments.The gel time is derived from the point at which T 2 of 2 H reaches a minimumor plateau.pH T [ ◦ C] φ T (0) k [10 −3 s −1 ] ε 0 [kJ mol −1 ] gel time [h]no buffer 25 0.20 0.246± 0.008 2.8 ± 0.1 8 ± 10.35 0.323 ± 0.009 4.6 ± 0.1 6 ± 150 0.20 0.43 ± 0.02 4.6 ± 0.2 2.0 ± 0.50.35 0.51 ± 0.03 4.9 ± 0.02 1.0 ± 0.52.95 25 0.20 0.32 ± 0.01 0.9 ± 0.1 30 ± 30.35 1.39 ± 0.06 5.1 ± 0.2 14 ± 250 0.20 1.05 ± 0.05 2.9 ± 0.2 5 ± 10.35 2.4 ± 0.3 6.1 ± 0.6 2.5 ± 0.55.50 25 0.20 0.049 ± 0.001 1.95 ± 0.04 2.0 ± 0.50.35 0.032 ± 0.001 3.79 ± 0.02 1.5 ± 0.550 0.20 0.127 ± 0.003 5.45 ± 0.08 1.0 ± 0.50.35 0.094 ± 0.002 5.29 ± 0.08 < 0.59.48 25 0.20 0.090 ± 0.002 1.91 ± 0.08 no percolation0.35 0.088 ± 0.001 2.63 ± 0.04 6 ± 150 0.20 0.54 ± 0.03 3.6 ± 0.2 no percolation0.35 1.18 ± 0.07 5.5 ± 0.2 1.0 ± 0.5increased value for k, which is expected. The magnitude of the increase, however, variesamong the systems, i.e. the factor is between 1.6 and 13. The strongest dependence of k ontemperature is found in the high pH system. The lowest values for k are found when the pHis equal to 5.50, which is close to neutral conditions. Both the lower and higher pH systemshow a higher hydrolysis rate, which agrees well with literature [24, 33] (see also Figure4.1). The pH in the unbuffered system may, due to the formation of silicic acid, slightlydecrease towards 4, as was indicated by test measurements. Hydrolysis is promoted underacid-catalyzed conditions as the alkoxide groups are more easily protonated and attackedby water. Under base-catalyzed conditions the abundant hydroxyl anions attack thesilicon atoms [24].Relaxation in the aqueous phaseFor the same systems the transverse relaxation time T 2 of the 2 H content in the aqueousphase was measured. The results are shown in Figure 4.7 for T = 25 ◦ C and φ T (0) =0.35. The echo trains from which the T 2 was derived all showed a mono-exponential decayat all times. In addition, the subsequent T 2 versus time profiles are smooth and exhibitalmost no scatter. In the systems at T = 25 ◦ C, T 2 is initially about 350 ms (similar tothe bulk value of D 2 O) and decreases gradually to values below 200 ms within severalhours. However, the profiles show a significant difference. In the pH = 9.48 system theT 2 decreases relatively quickly to 40 ms within the first 1.5 hours, after which it decreasesslowly and monotonically to 9 ms during the remainder of the experiment (up to t =