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66 5. Cross-linking of silica with a time-dependent monomer source3.53.0no bufferpH = 2.95pH = 9.482.5n O[mmol]2.01.51.00.50.00 2 4 6 8 10 12 14 16 18 20time [h]Fig. 5.4: Number of TMOS molecules in oleic phase as a function of time derived from the T 1measurement of the oleic phase (with φ T (0) = 0.20). The measurements were doneat 25 ◦ C using buffered and normal D 2 O. The solid curves indicate the fit using Eq.5.3 modified with an additional offset.Two-phase bulk experimentsThe number of TMOS molecules n o in the oleic phase versus time for three bulk experiments,done at 25 ◦ C with varying pH of the aqueous phase, are shown in Figure 5.4. Allconcentration profiles were fitted with Eq. 5.3, modified to include an additional offset(to account for noise present in the profiles), yielding an excellent description of the data.From the fits the mass transfer rate coefficients α T were derived, which are listed in Table5.2.The transverse relaxation time (T 2 ) profiles of 2 H in the aqueous phase are shown inFigures 5.5 and 5.6 for T = 25 ◦ C and T = 50 ◦ C. At T = 25 ◦ C, T 2 is initially about350 ms (similar to the bulk value of D 2 O) and decreases gradually to values below 100ms within several hours. However, the profiles show a significant difference. In the pH =9.48 system T 2 decreases relatively quickly to 100 ms within the first 5 hours, after whichit decreases slowly and monotonically to 30 ms during the remainder of the experiment(up to t = 47 hours). In the pH = 2.95 system T 2 decreases quickly from 335 ms to 250ms in the first hour, after which it decreases more slowly to 34 ms in the remainder ofthe experiment (up to t = 47 hours). In the unbuffered system T 2 starts at 360 ms anddecays to 73 ms in the first 10 hours of the experiment. In the systems at T = 50 ◦ C, T 2is initially about 630 ms in the unbuffered system and drops to 58 ms within two hours.In the buffered systems T 2 is initially shorter, around 540 ms, which is due to the alreadysignificant degree of hydrolysis in the initial minutes of the experiment. In the lower pHsystem 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 with respect to the same systems at T = 25 ◦ C,