Physical properties of the coexisting phases

The density difference between the solvent and feed, as well as the viscosity of the phases, are essential parameters for the design of an efficient liquid-liquid extraction. The higher the density difference, the faster is the phase separation (see chapter 2.3). Additionally, if the viscosity of the disperse phase is too high, mass transfer limitation could occur [14]. Therefore, it was of importance to determine those properties for the new studied aqueous two-phase systems.

The measurement described in chapter 5.11.6 was applied for the determination of the density difference in the new surfactant-based biphasic systems. The density of the coexisting phases in the cloud point system of Silwet L-7230 and ROKAnol NL5 are presented in Figure 6.10.

Figure 6.10: Density of the corresponding micellar and aqueous phases of the Silwet L-7230/water system (T=39 °C) and for the ROKAnol NL5/water system (T=45 °C). Error bars indicate the standard deviation, N = 3.

When comparing the densities of the coexisting phases, the surfactant-rich phase in the system Silwet L-7230/water was the denser one (see Figure 6.10). Hence, the extract was the dense phase and was collected at the bottom. The phase separation at 45 °C of the ROKAnol NL5/water system led to the formation of a lighter micellar phase. These results were in agreement with the pure substance densities of both surfactants. However, the density difference between the coexisting phases in the ROKAnol NL5 system (density difference = 0.002 g·cm^-3) was much lower in comparison to the Silwet L-7230 system (density difference = 0.011 g·cm^-3). The values were also lower than the general minimum density difference of 0.05 g·cm^-3, which was acceptable for a continuous extraction in a stirred column (see chapter 2.3). Nevertheless, Ritter et al. reported, that phase separation took place even at extremely low density differences, (e.g., density difference equal 0.002 g·cm^-3 in the ternary two-phase system Triton X-114/glucose/water) [146]. Therefore, the Silwet L-7230/water and the ROKAnol NL5/water system was considered to be applicable for the cloud point extraction.

The viscosity is a further influencing parameter in the design of the extraction process. For instance, a disperse phase with high viscosity leads to broader droplet distribution in a stirred extraction column and thus to a lower interfacial surface area. In the best case, the viscosity of the disperse phase must be similar to the one of water (see chapter 2.3). Hence, the viscosity of the coexisting phases was measured as described in chapter 5.11.7. The shear rate within the column at an

0.97

agitation speed of 25 rpm was calculated according to Equation 5-7 and found to be 5.0 s^-1. Therefore, results are shown for this shear rate in Figure 6.11.

Figure 6.11: Viscosity of the corresponding micellar and aqueous phases of the Silwet L-7230/water system (T=39 °C) and for the ROKAnol NL5/water system (T=45 °C) at a shear rate of 5.0 s^-1. Error bars indicate the standard deviation, N=3.

The results of the corresponding viscosity measurements are presented in Figure 6.11. As expected, the viscosities of the surfactant-lean aqueous phases were similar to the values for water at the tested temperatures. The micellar phase of ROKAnol NL5 had a viscosity comparable to the surfactant-rich phase in the previously studied Trion X-114 system [43]. Therefore, ROKAnol NL5 was suitable for the extraction concerning its viscosity.

Further, the Silwet L-7230 – rich phase had a higher viscosity. Moreover, the measured values were strongly fluctuating. Samples from the extract phase showed a gel-like appearance at room temperature – although being of a lower Silwet L-7230 concentration than the solvent feed stream, which was a homogeneous liquid solution.

Due to this phenomenon, two different samples were investigated for their viscosity as a function of temperature: (1) an aqueous sample without previous clouding, with a surfactant concentration of 20 wt% Silwet L-7230; (2) an extract (20 wt% Silwet L-7230) obtained from the cinnamic acid extraction at 39°C. The results are presented in Figure 6.12.

0.00 0.01 0.02 0.03

Silwet L-7230 ROKAnol NL5

viscosity [Pa∙s]

micellar phase aqueous phase

Figure 6.12: Viscosity of extract samples (retrieved at 39°C) and untreated aqueous Silwet L-7230 samples (20 wt%) without previous clouding as a function of temperatures between 2°C and 30°C at a shear rate of 5.0 s^-1.

Overall, the viscosity increased with decreasing temperatures, as presented in

Figure 6.12. However, the extract samples’ viscosity increased by up to three orders of magnitude if cooled below 15 °C. In comparison, the non-clouded aqueous Silwet L-7230 samples of equal concentrations showed no such behavior. That difference between the behavior of a micellar extract and an untreated surfactant solution has not been described yet. He et al. found trimethylsilane surfactants to be able to form crystalline structures at deficient concentrations [87].

Furthermore, Lavergne et al. stated that liquid crystalline phases in nonionic surfactant solutions can exhibit very high viscosities [34]. Hence, it can be considered that the sudden change in viscosity and rheological behavior denoted the formation of a liquid crystalline phase within the extract samples at the observed transition temperatures. Besides, it was further reported that micelles form entangled networks, whereby the extent of cross-linking increased above the cloud point [175]. Therefore, it is reasonable to assume that such entanglement could be responsible for the increased tendency of pre-clouded micellar phase to form liquid crystalline structures, compared to untreated surfactant solutions of equal concentration.

It is important to note that the micellar phase contained cinnamic acid. It was important to clarify the influence of cinnamic acid on the formation of a liquid crystalline structure. Surfactant-rich phases from clouding experiments without CA were compared to aqueous Silwet L-7230 solutions to and found to exhibit the

same behavior. Thus, the presence of cinnamic acid as a cause of the observed phenomenon was excluded.

Ultimately, the appearance of liquid crystalline structures of high viscosity further emphasizes that the phase behavior of Silwet L-7230/water was challenging for process control and a further processing of the extract. However, the sufficient density difference and high surfactant concentration in the extract made the system attractive for the design of a cloud point extraction.

The phase behavior, physical properties and extracting ability of the studied food- and cosmetic-grade surfactant systems indicated their applicability as solvents in the extraction of biological substances. Therefore, the CPE of cinnamic acid with Silwet L-7230 and ROKAnol NL5 was conducted in both batch and continuous mode. The performance of the single-stage process is discussed in the next chapter.