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6.3. Results and discussion 910.250.2A03. No bufferA05. pH = 2.84A07. pH = 6.20A09. pH = 9.480.15φ T0.10.0500 2 4 6 8 10time [h]Fig. 6.10: Concentration of TMOS in the oleic phase, φ T , as a function of time and averagedover all slices. The profiles were smoothed but not corrected for the effect of hydrogenpresent in the aqueous phase. The experiments were done at 25 ◦ C and the pH ofthe aqueous phase was varied (see Legend).of hydrolysis. At the lowest pH the rate of hydrolysis is the highest, and therefore themass transfer is the fastest. The lowest values for α T are found for the unbuffered and pH= 6.20 systems. It is noted that the pH of the aqueous phase in the unbuffered systemsdecreases to an unknown extent as the hydrolysis of TMOS proceeds. At 50 ◦ C the masstransfer rate is increased roughly by a factor 2 in case of the unbuffered and pH = 6.20systems. For the pH = 9.48 system α T increased by a factor 4 to 2.9 ± 0.3 h −1 . For thepH = 2.84 system (A06) the mass transfer was already completed within the dead time ofthe experiment, so that α T > 20 h −1 . Exact quantitative comparison between the masstransfer results of the core experiments and the bulk experiments are not made due tothe different spatial distribution and ratio of the liquid phases. However, like in the bulk(see Chapter 4) the mass transfer is predominantly driven by the hydrolysis reaction, therate of which is strongly dependent on the pH of the aqueous phase, and on temperature,especially for the alkaline systems. In the acid-catalyzed system, with a pH around 3, thehydrolysis rate and the mass transfer rate are the fastest, like in the bulk experiments.At a pH of about 6, the hydrolysis rate should be minimum [33]. The mass transfer ratesat a pH of 6.20 in the core experiments are similar to those for the unbuffered system,however, in the bulk it was found that the mass transfer was much faster in the unbufferedsystems compared to the buffered systems at a pH of 5.5.