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3.4. Results and discussion 3912108t = 0.1 ht = 0.8 ht = 5.8 h64z [mm]20-2-4-6-8-10 -8 -6 -4 -2 0 2 4 6 8 10x [mm]Fig. 3.3: Detected interface (indicated by the symbols) in the two-phase system at three differenttimes. The experiment was performed at 25 ◦ C and the initial volume fraction ofTMOS was 0.40. The solid curves represent the fitted interfaces after optimization.same resolution and MRI parameters as in the two-phase experiments the surface tension(SFT) was determined four times. The average SFT at room temperature was 74 ± 3 mNm −1 . An additional MRI image was obtained with twice the resolution, a higher numberof signal averages and a turbo factor of 1. In this case the SFT was 71 ± 1 mN m −1 , whichagrees well with literature [67]. Furthermore, the IFT of a n-hexadecane/water samplewas measured at room temperature with the same resolution and MRI parameters as theother two-phase systems. The average value derived from four images was 47 ± 7 mNm −1 .Effect of temperature and concentrationIn the left-hand graph of Figure 3.4 the IFT is shown as a function of time for theexperiments with the initial volume fraction of TMOS of 0.40. Despite the scatter, whichis especially observed for the measurement at 45 ◦ C, it is obvious that the IFT in thesystems without initial methanol starts around 10 mN m −1 and increases gradually duringthe first two hours to at least 20 mN m −1 .A clear effect of temperature on the IFT is not found in the temperature range considered.Interestingly, the IFT in the system with the initial methanol present in theaqueous phase starts at 8 mN m −1 and remains constant in the first three hours, afterwhich it increases slowly to about 15 mN m −1 . Although the experiments without theinitial methanol had similar concentration decays of TMOS in n-hexadecane as a functionof time (see Figure 3.2), the concentration profile for the experiment with methanol wasdifferent. The latter shows a lower mass transfer rate. On the whole, the presence ofadditional methanol in the aqueous phase lowers the IFT.
40 3. Interfacial effects during reactive transport403025 °C35 °C45 °C25 °C, MeOH403025 °C35 °C45 °C12 [mN m -1 ]γ2012 [mN m -1 ]2010γ1000 2 4 600 2 4 6time [h]time [h]Fig. 3.4: Interfacial tension, derived from the image analysis and optimization procedure, as afunction of time. (left) φ 0 T = 0.40. (right) φ0 T = 0.20.A similar analysis was done on the images obtained from the experiments where theinitial volume fraction of TMOS was 0.20. The IFT as a function of time is shown in theright-hand graph of Figure 3.4 for three different temperatures. Again the graphs show asignificant amount of scatter. For all temperatures the first image yields an IFT around20 mN m −1 . The values for 25 ◦ C appear to be fluctuating slightly below 20 mN m −1 inthe initial two hours, after which the IFT approaches an average value of 21 mN m −1 .The measurement done at 35 ◦ C yields also fluctuating IFT values, initially below 20 mNm −1 , and for later times above 20 mN m −1 . A clear difference between the results for theprevious temperatures and the results for 45 ◦ C is found for this volume fraction. Afterthe first acquisition, the IFT fluctuates around 15 mN m −1 .3.4.2 IFT from the tensiometerA series of IFT measurements of the two-phase system was performed at room temperature.The initial volume fraction of TMOS in the oleic phase was 0.40. Figure 3.5 showsthe IFT as a function of time. The initial IFT starts at 10.4 mN m −1 after which itremains approximately constant around 10 mN m −1 in the first 1.2 hours. The other datapoints, measured between 3.5 and 6.9 hours after the start, show an increasing IFT from11.4 to 17.4 mN m −1 . The experiment was stopped after 7 hours in order to preventgelation of the aqueous phase during the measurement.The Huh and Mason correction factor is too small for the initial points because whenTMOS is present in the oleic phase the real density difference ∆ρ is smaller than thespecified difference. This means that the real IFT is slightly higher at times when thereis still TMOS present in the oleic phase. The error in the correction factor is 7% for thefirst point but decreases as the TMOS is transferred to the aqueous phase.The initial IFT around 10 mN m −1 is similar to the IFT determined in the MRIexperiments using the same initial concentration of TMOS. Also the ring measurement
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158 Appendix EAs the saturations, d
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160 Bibliography[12] B. Vinot, R. S
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168 Bibliography[127] G. W. Scherer
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170 Bibliography
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172 Summaryand gelation rates. The
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174 Samenvattingwaterfase. De T 1 v
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