(87a) Dispersion and Coalescence in Liquid/Liquid Systems Stabilized with Nanoparticles

Nanoparticles can be applied as emulsifying agents to stabilize
liquid/liquid systems towards coalescence (Pickering emulsions). Due to the
adsorption of nanoparticles at the liquid/liquid interface, coalescence is
hindered or arrested and smaller drop size distributions and a higher
interfacial area can be achieved. Since this promotes mass transfer,
nanoparticles can be used as innovative additives in liquid/liquid reaction
systems. In this work, physical properties such as rheology and interfacial phenomena
are investigated experimentally. Furthermore, the impact of different silica
nanoparticles on drop size distributions in liquid/liquid systems is determined
using microscopy or an in-situ endoscope technique. Thereby, emulsions
generated with two different devices are compared: 1) High-energy devices such
as ultrasonication, leading to emulsions that are nearly completely stabilized
towards coalescence. 2) Low energy input induced by stirring, leading to only
partly stabilized emulsions. This allows quantifying drop breakage and
coalescence phenomena in steady state and dynamic conditions. The experiments
can provide essential information for model development, e.g., breakage and
coalescence rates used in population balance equations.

Figure 1:
Apparent emulsion rheology, frequency sweep results and drop size distributions
in w/o emulsions stabilized by two silica nanoparticles (HDK H18 and H20) that
differ in wettability. Emulsions were prepared with ultrasonication and are
stable towards coalescence. (Weight percent of water w_H20 = 25%, oil
phase = 1-dodecene, particle weight percent wp = 0.75%) [1]

The presence of nanoparticles in suspensions and emulsions leads to
complex rheological behaviour. The systems become shear thinning and
viscoelastic with gel-like character especially at high particle concentrations
and small droplet sizes. The nanoparticles are initially suspended in one
liquid phase and move to the liquid/liquid interface as the systems are
dispersed and droplets are created. The free nanoparticles, the nanoparticles
at the interface and the droplet sizes influence the rheology of the system.
Comparing silica nanoparticles with different wettability (HDK H18 and H20) in
completely stabilized w/o emulsions leads to significant changes in emulsion
rheology (Figure 1 left, middle). The nanoparticle type affects the
particle-particle and particle-fluid interactions, so that the rheology also
depends on particle morphology, size and concentration. Furthermore, the drop
size distribution of the system also is shifted towards smaller diameters for
the particles with intermediate wettability HDK H20 in comparison to the more
hydrophobic HDK H18 [1].

Figure 2:
Dynamic changes in Sauter mean diameter (left) and other characteristic
diameters (right) after abrupt changes in energy input in a w/o emulsion
dispersed in a stirred tank. (Weight percent of water w_H20 = 15%,
oil phase = n-heptane, particle weight percent wp = 0.05 – 1.00%)  [2].

The aforementioned parameters also influence the drop sizes of the
emulsions in systems with low energy input that are only partly stabilized
towards coalescence. Figure 1 (left) shows transient Sauter mean diameters for
different particle concentrations after agitation start and after an abrupt
change of agitation speed from 900 to 700 rpm. The effect of coalescence
inhibition for different particle concentrations is clearly visible. With
rising particle content, the Sauter mean diameters barely increase after energy
input reduction. Figure 1 (right) depicts other characteristic diameters for
one particle concentration and indicates that the lowest drop fraction is
stabilized against coalescence, whereas the larger drops still coalesce after
reduction in energy input. This shows that the shape of the drop size
distribution is affected [2].

 [1] Hohl, L.; Röhl, S.; Stehl, D.; von Klitzing, R.; Kraume M.:
Influence of nanoparticles and drop size distributions on the rheology of w/o
Pickering emulsions. Chem. Ing. Tech. 88, No. 11., 1818-1826

[2] Röhl, S.; Hohl, L.; Kempin, M.; Enders, F.; Jurtz, N.; Kraume,
M.: Influence of different silica nanoparticles on drop size distributions in
agitated liquid/liquid system. submitted to Chem. Ing. Tech. in 2019