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48 4. The effect of pH on mass transfer and gelation4.3.2 NMR measurementsRelaxation times for 1 H and 2 H in the samples were measured with a 0.95 Tesla NMRscanner (home-built) operating at a frequency of 40.5 MHz for hydrogen and 6.21 MHzfor deuterium. The setup consists of an electro-magnet with a vertical, narrow-boreinsert with an inner diameter of 31 mm. The insert is capable of producing a staticmagnetic gradient up to 0.2 T/m in the vertical direction. A single rf coil is used with twodifferent front-end rf circuits for 1 H, respectively 2 H. Switching between the two circuitstakes less than a second, allowing for a fast sequential NMR measurement of hydrogenand deuterium. A polyvinylchloride (PVC) sample holder was constructed in which afluorocarbon fluid (Galden TM HT135, manufactured by Solvay Solexis) was circulated inorder to control the temperature of the sample. The fluid is invisible to the NMR set-upand the temperature can be controlled between 10 and 65 ◦ C with an accuracy of about1 degree.Details about the sequences can be found in a previous work [88]. To summarize,the T 1 of the hydrogen content in the oleic phase is measured to determine the TMOSconcentration in oil, and the T 2 of the aqueous phase is measured to monitor the progressof gelation. In this case T 2 is measured for the deuterium content. Here a 1D staticmagnetic gradient is used. Since in previous measurements [88] the phases showed nosignificant spatial profiles in T 1 or T 2 it was decided to measure a single slice in the oleicphase and a single slice in the aqueous phase, in order to limit the total acquisition time.The measurements were repeated for each sample over an interval of 20 to 40 hours.4.4 Results and discussion4.4.1 Calibration experimentsMethanol-D 2 O calibrationsThe results of the relaxation time measurements are shown in Figure 4.3. At 25 ◦ C the T 2for D 2 O is 0.389 s and decreases with increasing molar ratio of methanol, x M , to 0.186 swhen x M = 0.511. At 50 ◦ C the T 2 for D 2 O is 0.647 s and decreases with increasing molarratio of methanol, x M , to 0.319 s when x M = 0.642. The effect of mixing methanol withD 2 O on the observed T 2 of the water molecules appears to be less pronounced than incase of H 2 O [88], but agrees well with other studies on methanol-water mixtures [97, 98].Calibration gel samplesThe transverse relaxation time T 2 of deuterium was measured in the gels at 25 ◦ C and50 ◦ C, respectively. The results are listed in Table 4.2. It is noted that the T 2 was notmeasured for the buffers without TMOS (i.e. φ g T = 0) at 25 ◦ C, but only for pure D 2 O,since at 50 ◦ C the T 2 appeared to be insensitive to the presence of the buffer chemicals.In all cases the relaxation time T 2 decreases with increasing initial TMOS concentrationφ g T. The reciprocal relaxation times are shown for the pH = 2.95 gels in Figure 4.4.The relaxation times for deuterium in the gels were fitted with Eq. 4.6 using Eq. 4.7 to