(325h) ZnO Nanoparticle Morphology in the Stability and Properties of O/W Emulsions

ZnO
Nanoparticle Morphology in the Stability and Properties of O/W Emulsions

Tomás-E.
Chávez-Miyauchi*, Adriana Benitez-Rico*, Martín Romero-Martínez†

* School of Chemistry,
Universidad La Salle México; Benjamin Franklin 47, Col. Condesa, Del. Cuauhtemoc,
Cd. De México 06140 Mexico

† Facultad de Ciencias,
Universidad Nacional Autónoma de México; Circuito Ext s/n Coyoacán, Ciudad
Universitaria Cd. De México 04510 Mexico

Nanoparticles
have been used for stabilizing emulsions in different oil – water systems
[1-3]. Depending on the nature of the particle, emulsion properties can be
tuned: their type (O/W or W/O), the droplet size and their stability.
Nanoparticles lay at the oil-water interface and interact with other
surface-active components such as ions or surfactants.

The properties of nanoparticles are mainly governed by their
surface energies, and therefore, adsorption of nanoparticles on a solid surface
can significantly change the surface energy and wettability of the system. The
interaction between nanoparticle and surfactants can also make considerable
change in the surface activity of surfactant molecules.

Nanoparticle
morphology seems to have a direct effect in the emulsion stability: authors
have observed that the roughness of the particle surface decreases the
stability of the emulsion [4], in the same line, reports show that the shape of
the nanoparticle promotes its specific arrangement at the interface promoting
interactions with other molecules in the system and leading to different
effects such as phase inversion (O/W to W/O) [2].

In
this work, we present a study of emulsion stabilization with ZnO nanoparticles
synthesized by three different methodologies, giving place to three different
samples with specific microstructures and their interactions with surfactants
(anionic, non-ionic and cationic) and the ionic strength of the water phase.

Synthesis of ZnO
nanoparticles is presented from the same precursor salt zinc acetate dihydrate,
Zn(CH3COO)2∙2H2O. Three synthesis methods were performed with different
experimental conditions: sol-gel, precipitation and hydrolysis in a polyol. The
resulting ZnO presents different morphologies and grain sizes depending on the
synthesis method. Crystallite size calculations and phase identification were
obtained from X- ray diffraction spectra. The microstructure, morphology and
grain size was carried out by SEM.

Emulsions
were formed by mixing water and heptol (75% n-heptane and 25% toluene) in same
volume amounts. The concentration of the nanoparticles is 100 ppm. Emulsion
nature and stability are analyzed as a function of NaCl concentration in the
water phase and in the presence of 0.1 M of an anionic (sodium dodecyl sulfate
SDS), non-ionic (Tween 80) and cationic (bromide of octyl terbutyl ammonium
OTBA) surfactants. Nanoparticles were stabilized by sonication. Mixtures of
surfactant, salt and nanoparticle were made in the water phase then put in
contact with the oil phase and mixed by sonication for 30 minutes.

All samples of ZnO were
crystalline and occurs in the hexagonal phase with ICCDD-PDF # 89-0511.
Figure 1, presents the morphology and distribution of the grain sizes obtained
by SEM: a) Sol-gel synthesis shows grains of oval-shaped material with a grain
size distribution of 100 nm. b) Precipitation synthesis presents polyhedral
structures that have a grain size around 100 nm, on a larger
scale this sample presents a crisscrossed wire form c) The hydrolysis synthesis of polyol presents nanoparticles with
spherical shape and a narrow distribution of grain size around 15 nm.

            Figure
1. ZnO micrographs taken at 100 000x from: a) sol-gel method

              
b) precipitation, c) hydrolysis of polyol.

Emulsions
are observed to form either in the light phase or in the heavy phase depending
on the nature of the surfactant, a simple droplet test shows that both kinds of
emulsions are O/W. When analyzing to the optical microscope, dense emulsions
show larger droplet size than emulsions located in the light phase as shown in
Figure 2.

Figure
2. Optical microscope images of emulsions formed at the light and heavy phase.

Emulsions
droplet size vary depending on the morphology of the nanoparticle. Droplets are
bigger when nanoparticles are more spherical and decrease size as the
nanoparticles become turning to wires. Stability of the emulsion increases as
the nanoparticle becomes thinner.

Zeta
potential of the nanoparticles in the solutions was determined to analyze the
electrostatics surrounding the particle. Zeta potential is negative in all
cases and becomes less negative as salt concentration increases. As a function
of surfactants, we observe that anionic surfactants lead to larger negative
zeta potential, followed by the non-ionic and at last the cationic which is
very close to 0. In each case, Zeta potential is more negative as the particles
elongate to nanowires which is in line to the observations in terms of emulsion
stability.

We
continue the work in our laboratory testing different nanoparticle synthesis
methods and other types of nanoparticles and testing the samples in environments
close to real systems for further applications.

[1]
Binks, B.P.; Whitby, C.P. Coll. and Surf. A: Physicochemical and Engineering
Aspects 2005, 253(1-3), 105-115 “Nanoparticle
silica-stabilized oil-in-water emulsions: improving emulsion stability”

[2]
Zhang, J.; Li, L.; Wang, J.; Sun, H.; Xu, J.; Sun, D. Langmuir, 2012,
28, 6769-6775, “Double Inversion of emulsions induced by salt
concentration”

[3]
Simovic, S.; Prestidge, C.A. Langmuir, 2004, 20(19), 8357-8365
“Nanoparticles of varying hydrophobicity at the emulsion droplet-water
interface: adsorption and coalescence stability”

[4]
Vignati, E.; Piazza, R. Langmuir, 2003, 19(17), 6650-6656
“Pickering emulsions: Interfacial tension, colloidal layer morphology and
trapped-particle motion”