Comparison of single phases and emulsion rheology under 3Φ conditions

Although the systems coalesce faster under 3Φ conditions, an experiment to compare the viscosity of the single phases and the emulsion viscosity was performed. Figure 64 shows the viscosity of a water/1-dodecene/Marlophen NP6 system. The phases (aq, mi, org) were separated at T = 61.6^◦C under 3Φ conditions and their single phase viscosity was determined. Furthermore, the complete emulsion rheology was measured after stirring the com-plete system with all three phases. The emulsion consists of an organic continuous phase, while the aqueous and microemulsion droplets are dispersed in form of single and multiple emulsion drops. The data for the emulsion show a declining dynamic viscosity for increasing shear rates but also declining values with shear rate reduction caused by the fact that the emulsion coalesces under these conditions. The fluctuations at the lowest shear rates are caused by the measurement device and could be avoided if a longer time per measurement point is used, but would increase the influence of evaporation and coalescence on the experimental results.

0.0001 0.001 0.01

1 10 100

Dynamic viscosityƞ and ƞeff[Pas]

Shear rate γ[1/s]

aq mi org Emulsion T = 61.6°C

.

emulsion rheology (Pal)

pure water pure 1-dodecene

Fig. 64: Viscosity curves of water, 1-dodecene, Marlophen NP6 systems: single phases and complete emulsion at T = 61.6^◦C (α= 0.5,γ= 0.05, 3Φ)

Table 19: Dispersed phase fractionsφand dynamic viscosityη

Phase fractions phase

φc[-] 0.44 org

φd1[-] 0.22 mi

φd2[-] 0.34 aq

φ[-] 0.56 mi + aq

dynamic viscosity phase

ηc[Pa s] 7.463E-04 org ηd1[Pa s] 3.860E-03 mi η_d2[Pa s] 6.054E-04 aq

ηd[Pa s] 9.417E-04 mi + aq (weighted)

The emulsion viscosity at high shear rates is in the range of the pure microemulsion phase viscosity, which might be used as a simplification to estimate the emulsion rheology in 3Φ systems. The model by Pal [103] was applied to calculate the emulsion viscosity using the overall dispersed phase fraction of the systems φ =φmi

+φ_aq = 0.56 and an average dispersed phase viscosity ofη_d = 9.417E-04 Pa s. This value was achieved with the viscosity of the aqueous and the microemulsion phase weighted by their respective volume fractions, see Table 19. Although the model overpredicts the measured emulsion viscosity, the results are in the right order of magnitude especially considering the instability of the system towards coalescence.

A more detailed analysis under 3Φ conditions is necessary in future investigations in order to identify the influence of system composition, a change of the continuous phase or of multiple emulsion droplets on the emulsion rheology.

5.5.4 Emulsion rheology during phase separation

During the phase separation process, both drop sedimentation and coalescence occur, which changes the drop size distribution and the local drop density. This section briefly depicts how this may influence the emulsion rheology. The rheological behavior of 2

̄Φ systems with Marlophen NP6 and Marlipal 24/70 was determined in the complete emulsions before phase separation and in an emulsion that separated for approximately three months at T = 20^◦C. After this period of time, the system had separated into a clear aqueous phase at the bottom of the systems and a turbid upper phase which did not coalesce completely. Since an o/w emulsion occurs under 2

̄Φ conditions, the turbidity is caused by a dense packed zone of organic droplets, as illustrated on the right side of Figure 65. The height of this dense-packed zone indicated, that barely any part of the organic phase had coalesced and the droplets mainly sedimented which led to an accumulation of the droplets in the upper part of the system. The viscosity of the aqueous phase, the dense packed zone and the complete emulsion before phase separation are depicted in Figure 65 for Marlophen NP6 and Marlipal 24/70. In both cases, the aqueous phase shows Newtonian rheological behavior at a dynamic viscosity value slightly higher than water, which is induced by the presence of surfactant. The dense-packed zone has a dynamic viscosity which is several

0.0001 0.001 0.01 0.1 1 10

1 10 100 1000

Dyn. viscosityηand ηeff[Pas]

Shear rate γ[1/s]

aq dpz emulsion Marlophen NP6

emulsion rheology (Pal) dpz rheology (Pal)

clear aqueous phase dense-packed zone (dpz)

0.0001 0.001 0.01 0.1 1 10

1 10 100 1000

Dyn. viscosityηand ηeff[Pas]

Shear rate γ[1/s]

aq dpz emulsion

Marlipal 24/70

emulsion rheology (Pal)

.

.

dpz rheology (Pal)

Fig. 65: Viscosity of water, 1-dodecene, surfactant systems of the initial emulsion and after incomplete phase sepa-ration under 2

̄Φ conditions into a clear aqueous and an organic phase with remaining droplets with Marlophen NP6 (α= 0.5,γ = 0.05) and Marlipal 24/70 (α= 0.5,γ= 0.1)

magnitudes higher than the aqueous phase. This deviation is beyond the pure effect of surfactant in 1-dodecene, even if high concentrations are assumed.

The dense-packed zone of the Marlipal 24/70 system furthermore shows shear thickening behavior at shear rates between ̇γ = 20 - 150s⁻¹, which is attributed to the high droplet density [102]. For both surfactants, the dynamic viscosities of the complete emulsion before separation lie at an intermediate values in the range of 0.01 - 0.1 Pa s. The model by Pal [103] was used to estimate the emulsion and the dense packed zone viscosity.

To achieve this for the complete emulsion, the dispersed phase fraction calculated from the initial water and 1-dodecene volumes φ= 0.568, the viscosity of the aqueous phase at ̇γ= 1000 s⁻¹ and of pure 1-dodecene at T = 20^◦C were used in combination with the maximum packing factor φmax = 0.64, similar to the previous sections. For the dense-packed zone, the following additional assumptions were made: The dispersed phase fraction increases to φ= 0.724, because the organic droplets are only located in approximately half of the aqueous phase volume and the maximum packing factor increases to φmax = 0.74 which is valid for deformed droplets in dense emulsions [49, 141] (see also Fig. 20, section 3.4). Although the model again does not include the effect of shear rate, these approaches are an easily accessible possibility to estimate the emulsion viscosity of the systems at least for high shear rates with reasonable accuracy.

To describe the processes in stirred tanks, the models need to be adjusted to account for the local shear rates in the systems, which can be done for example using a combination of Computational Fluid Dynamics [12] and more detailed experimental investigations. For the phase separation, especially the lower range of shear rates will be of importance.

5.6 Phase separation in two phase systems

Phase separation experiments and simulations were performed in different water, 1-dodecene, surfactant systems while varying temperature, agitation speed, composition and tank geometry. Surfactant-free systems were deliberately not analyzed because the separation occurs within several seconds under all conditions. This chapter provides the validation of the use of the separation model by Henschke [48, 49] and the applicability of the model to 2

̄Φ microemulsion systems withC4E2.