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18 2. Coupled mass transfer and sol-gel reaction in a two-phase bulk systemCPMG measurements and the constant read-out duration result in a varying resolutionof the 1D profiles. Nevertheless, the liquid phases are easily reconstructed and identifiedfrom the profiles. The magnetic susceptibility of water and n-hexadecane is −9.0×10 −6 ,respectively, −8.0×10 −6 [67]. At 4.7 Tesla the resulting frequency mismatch is about 200Hz. With respect to the applied read-out gradient of at least 2.6 kHz mm −1 , susceptibilityartifacts at the interface between n-hexadecane and water are insignificant. The acquisitiontime for each sequence is between 2 and 3 minutes, so that the loop time is about15 minutes (given one Saturation Recovery, one 2D TSE and three CPMG measurementsper loop).In order to measure the gel time of the aqueous phase similar experiments were performedinside glass vials with equal dimensions as the Teflon vials. The vials were placedin a water bath at specific temperatures. By gently tilting the vials at times near the predictedgel time with intervals of 5 minutes, the gel time could be determined by checkingwhether the oil-water interface is still able to follow the tilting motion. The reproducibilitywas checked by using multiple glass vials, and the accuracy of the gel time is about30 minutes.2.4 Results and discussion2.4.1 Calibration resultsViscosity measurementsThe viscosities of TMOS, n-hexadecane and the mixtures were measured in order to checkwhether the relaxation times can be correlated with viscosity and temperature. Figure2.3 shows ln η mix at 25 ◦ C as a function of the mole fraction x (where x = 0 representspure oil, and x = 1 represents pure TMOS). The viscosity for x = 0 is 3.01 ± 0.01 mPas, and it decreases monotonically with x to 0.63 ± 0.01 mPa s when x = 1. The figureshows that ln η mix follows a straight line with increasing mole fraction. The values werefitted with the mixture law (Eq. 2.18) yielding an excellent fit with G = 0.T 1 relaxation in TMOS/n-hexadecane mixturesThe calibrations are used to determine the concentration, φ, of TMOS in n-hexadecanein the two-phase bulk experiments. The Saturation Recovery data of the TMOS/nhexadecanemixtures were evaluated by fitting mono-exponential and bi-exponential decaycurves as given by Eqs. 2.3 and 2.16, respectively. For each concentration and temperaturethe decay appeared to be virtually mono-exponential and a robust bi-exponential fit couldnot be obtained. The individual T 1 -s for TMOS and n-hexadecane in the mixtures areobviously relatively close. Noise present in the spin-echo data results in significant errorsin the fit parameters when applying a multi-exponential fit, even at high signal-to-noiseratios.The Saturation Recovery data of the pure liquids were fitted straightforwardly with asingle exponent. As for the mixtures, a slight deviation from mono-exponential behavior