For the description of dispersion and separation processes, it is crucial to know which phase is continuous and how the dispersed phases phases interact with each other in case of 3Φ conditions. A methodology for the identification of the continuous and dispersed phases that includes the previously discussed phase volume fractions and conductivity values in stirred systems was developed based on a preliminary study by Hamerla et al. [42] and mainly reported in [I, II, IV]. A summary of the results is presented in the following subsections.
5.2.1 Interfacial tension and free energy
Due to the changing solubility of the surfactant in water and 1-dodecene, and in accordance with the Bancroft rule [13], the systems tend to form o/w emulsions under 2
̄Φ conditions and w/o emulsions under 2̄Φ conditions, as was verified by using the electrical conductivity measurements. Under 3Φ conditions, the conductivity measurements can also be used as an indicator for the continuous phase, as already described in the previous section [I, II]. Additionally, the different refractive indices lead to a corresponding change of droplet appearance [I]. The assessment of these droplets to the respective dispersed phase is critical if one intends to determine two distinct droplet size distributions. As already discussed by Kahlweit et al. [60], there are some restrictions concerning the occurring interfacial tensions in microemulsion 3Φ systems (Fig. 40a). Due to these trajectories of the interfacial tensions, some cases of droplet interactions can be ruled out since they result in an increasing free energy of the systems which makes these conditions inefficient and unstable [IV]. The schematic diagrams in Figure 40 b - d depict the three possible continuous phases (aq, mi and org) and the respective droplet interactions between the two dispersed phases from the energetic point of view.
If one of the excess phases is continuous, the formation of dimers and multiple emulsions with a surrounding microemulsion droplet can occur. The formation of these complex droplets can reduce the free energy by minimizing interfacial area with high interfacial tension such as the interfacial tension between the excess phases σaq/org [III, IV]. This is the case if a droplet of the dispersed excess phase is surrounded by a microemulsion droplet [II]. The opposite case with an inner microemulsion and an outer excess phase droplet is unlikely, since
InterfacialtensionσmN/m]
Temperature T [°C]
σaq/org
σaq/mi σmi/org
Tmin Tma
σaq/org/σaq/mi [-]
1
1 σmi/org/σaq/mi [-]
complete engulfing σaq/org > σmi/org + σaq/mi σmi/org > σaq/org + σaq/mi
non-engulfing σaq/mi > σmi/org + σaq/org
d) a)
1
1 σmi/aq/σmi/org [-]
complete engulfing σaq/org> σaq/mi + σmi/org σaq/mi> σaq/org + σmi/org
non-engulfing σmi/org> σaq/org + σaq/mi
b)
1
1 σmi/org/σaq/org [-]
σmi/aq/σaq/org[-]
complete engulfing σmi/org > σaq/mi + σaq/org σaq/mi > σmi/org + σaq/org
non-engulfing σaq/org> σmi/org + σaq/mi
c)
σaq/org/σmi/org [-]
Fig. 40: a) Schematic interfacial tensions in microemulsion 3Φ systems [60] and stability diagrams in respect to the interfacial tensions for systems with different continuous phases: b) aqueous, c) microemulsion and d) organic (acc.
to [41] for simple three phase systems. Red colours indicate which formations are not expected due to the occuring interfacial tensions
the interfacial tension between the excess phases is the highest one in the systems. For the same reason, the formation of dimers and multiple emulsions does not occur if the microemulsion phase is the continuous one.
Hence, these considerations from the energetic point of view can definitely rule out some dispersion conditions.
However, they are only based on the free energy and interfacial tension aspect and completely neglect other influencing factors such as external forces and fluid dynamics.
5.2.2 Fluid dynamics in the stirred tank
To directly identify the phases in the agitated systems, a combination of different measurement techniques was applied [I, II, IV]: Conductivity measurements in stirred systems over temperature were performed as described in the previous Section 5.1 to determine which phase is the continuous one. The phase volume fractions indicate which phase more likely becomes the continuous phase in agitated systems. Furthermore, an analysis of the droplet interactions such as multiple emulsions, the droplet appearance and the droplet sizes was performed via endoscope images. The optical analysis of the turbidity of each phase after complete phase separation can be compared to the droplet appearance. Furthermore, the position within the 3Φ temperature and the corresponding expected interfacial tensions can be set in relation to the observed droplet sizes. One additional possibility would be to use dye to specifically change the appearance of one phase based on the dye solubility.
However, additives can affect the whole system characteristics and should be avoided if possible. Furthermore, the dye is barely visible on the images made with the endoscope technique. As an example, the observation of multiple emulsions provides the following information: a) One of the excess phases and not the microemulsion phase is the continuous phase and b) the outer droplet consists of microemulsion phase.
In this case, conductivity measurements are a simple way to distinguish if the aqueous or the organic phase is continuous and the respective other excess phase has to form the inner droplet. The flow conditions in agitated tanks or during phase separation need to be taken into consideration for a thorough description of possible droplet formations. The fact that the energetic consideration favors a specific dispersion condition does not necessarily mean that all the droplets in the system will follow this rule. For all microemulsion systems investigated in this work, the cases depicted in Figure 41 occurred in agitated 3Φ systems. If one of the excess phases (aq, org) is continuous, the droplets can form single droplets, dimers (Janus droplets) and multiple emulsions. The formation of single droplets is not favored by the stability diagram because the value of σaq/minever exceeds the sum of σmi/organdσaq/org(respectivelyσmi/orgnever exceeds the sum of σaq/mi and σaq/org). This effect is induced by the fluid dynamics in the stirred tank, as well as the probability of droplet collisions and the need for a successful penetration and/or adherence process in order to form a dimer or a multiple emulsion [IV]. For agitated systems, the dimers are a transitional state between the single droplets and
2φ 3φ 3φ
2φ 3φ 3φ
Continuous phase
org
mi
aq
Single droplets
Dimers/
Janus droplets
Multiple emulsions
Fig. 41: Left: Observed complex droplets in 3Φ systems [I, II, III, IV]. Right: Examples for image quality and droplet appearance in 2
̄Φ, 2̄Φ and 3Φ systems [I, II].
multiple emulsions. Hence, the dimers mainly occur at low agitation speeds or during phase separation where the conditions during collision of the two droplets do not suffice for a penetration. In turbulent systems mainly single droplets or multiple emulsions were observed. Although the formation of dimers or multiple emulsions would be energy efficient in some cases, only single droplets are formed if the other system properties such as the size differences between the droplets prevent a high probability of a penetration process. Beside the question if multiple emulsions are formed or not, one important factor is the exact nature of the multiple emulsions. Two relevant characteristics are the number of droplets-per-droplet, which can reach from one to dozens and the ratio of single droplets to multiple emulsion droplets. Several factors influencing the formation of the multiple emulsion droplets and dimers are described in [IV]. In conclusion, fluid dynamics in the stirred tank as well as the interfacial tensions and other physical properties need to be considered to understand and describe the dispersion conditions in 3Φ systems. Changes in the multiple emulsion characteristics also affect the dispersion and the separation process [III, IV], as will be discussed in following sections.