Characterizing Particles in Nano-Powder Regimes
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By J. Gabriel DosRamos, Ph.D., Vice President, Matec Applied Sciences,
Nano-particles of increasingly smaller particle size and various
material compositions are being developed for the pharmaceutical,
biomedical, electronics, optoelectronics, energy, catalyst and ceramics
industries. These particles are formed, or alternatively dispersed
after formation, in a liquid medium, typically water. Various non-aqueous
continuous media such as alcohols are also used.
The particle size and particle size distribution (PSD) of these
materials are of great importance to the end user because they affect
key colloid properties such as rheology, film gloss, surface area
and packing density. Additionally, to prevent the aggregation of
fine particles into much larger, undesirable units, steps must be
taken to prevent particles from sticking together (aggregating)
due to inter-particle collisions in the liquid medium. This can
be accomplished by creating an interparticle electrical and/or steric
energy barrier. For very fine particles, a combination of both electrical
and steric barriers may be necessary to prevent aggregation.
The strength of the particle electrical barrier is measured in
terms of an electrical potential termed the “zeta potential.”
Zeta potential should be measured at process conditions, i.e., without
prior sample dilution, as ions can adsorb or desorb from the particle
surface upon dilution and change the zeta potential.
Until recently, those wishing to obtain zeta potential results
had to resort to tedious and time-consuming “microelectrophoresis”
methods that require extreme dilutions and consequently stringent
sample handling precautions. As particle sizes decrease to the nanometer
size range, the light scattering methods employed in microelectrophoresis
become increasingly difficult to use due to extreme Doppler broadening
of the scattered light from the fine particles. Thus, detectors
must be placed very close to the incident laser beam direction,
requiring superior mechanical and environmental stability.
However, a new instrument* is changing this process. Developed
during the last 15 years, the instrument employs electroacoustic
principles that provide accurate measurements while preventing the
zeta potential probe from being susceptible to environmental noise
Zeta Potential Measurement in Action
The electroacoustic instrument measures the electrokinetic properties
of a particle by an electroacoustic method. A high frequency electric
field is applied to the dispersion, and the particles move electrophoretically
in the applied field. If there is a density difference between the
particle and the liquid, this motion will generate an alternating
acoustic wave. The application of the electric field and the resulting
acoustic wave provides a truly non-intrusive measurement and does
not alter the properties of the sample.
were carried out on various materials to demonstrate the capabilities
of the new instrument. Figure 1 shows the measured zeta potential
of an 8.3% wt solids fine (~270 nanometer particle diameter) titanium
dioxide system as a function of pH. The variation of electrical
conductivity as a function of pH is also measured simultaneously
during this automated “potentiometric titration.” This
system has a zero of zeta potential (isoelectric point, IEP) at
pH = 6.0. The location of the isoelectric point, in pH, is a characteristic
of the particle surface and depends on the type of the metal oxide
bond, the particle crystal structure, (rutile vs. anatase for titania)
and on the type and level of impurities or other soluble species
bonded to the titania particle surface.
Perhaps the single most important piece of information obtained
from Figure 1 is that this titania system is unstable and will aggregate
in the region of pH = 5 to pH = 7.3. This is because for particles
in this size range (~270 nanometers), aggregation will occur in
a concentrated system at a rapid rate for zeta potentials less than
20 millivolts in magnitude. For finer particles still, the zeta
potential should be of even larger magnitude to prevent aggregation.
In the extreme limit of very small nano-particles, sufficiently
high zeta potentials cannot be achieved, and some level of steric
stabilization is necessary.
The results shown in Figure 1 can be obtained in concentrated systems
(in the range of 0.1 to 50 volume %) by carrying out automatic pH
titrations in conjunction with a computer-controlled burette on
a vigorously stirred and/or flowing sample. Measurements can be
made every thirty seconds while accumulating about 60 data points
over the whole pH range.
2 shows a potentiometric titration result on a 60-nanometer 10%
wt silica particle system. For a pure silica surface, it is believed
that the zeta potential approaches asymptotically an isoelectric
point in the neighborhood of pH = 2.0. The Si – OH2+ configuration,
implying a positive silica surface, is believed not to exist for
the silica surface. This idea seems to be supported by the results
of Figure 2. For this very fine particle system, it is estimated
that the region of stability against aggregation would start from
about pH = 7.0 and go higher in pH. Increasing pH much above 10
will again result in a decrease in the magnitude of the zeta potential
due to compression of the electric double layer. This occurs as
the ionic strength increases with pH. Thus, the stability region
is restricted to pH higher than 7 for a very fine silica particle
The previous two examples display the utility of the zeta potential
concept applied to particle stability considerations. Zeta potential
measurements can also be used to determine the fractions of two
different metals in mixed metal oxide catalyst particles, to determine
the fraction of one crystal structure of a mixed structure metal
oxide and many other applications too numerous to list here.
Particle Size Distribution Data
3 shows superimposed high-resolution PSD results on two fairly monodispersed
polystyrene latex systems and the silica system shown in Figure
2. The particle sizing instrument,** based on a patented capillary
hydronamic fractionation (CHDF) technique, performs high-resolution
particle sizing from 3 microns down to 10 nanometers.
Sample particles are fractionated according to size as they flow
in a capillary tube. The particles are detected at the capillary
outlet by an on-line detector, typically an ultraviolet (UV) detector.
Particle size is given by the elution or transit time of the particles
in the capillary. This elution time depends only on the particle
hydrodynamic size and is independent of particle chemical composition
True PSD data are produced in less than 10 minutes thanks to the
high-resolution particle size fractionation capability of the CHDF
technique. One significant CHDF advantage is that one can reliably
measure PSD width and multimodality without the need for assumptions
regarding the PSD shape.
The more traditional sizing methods employ laser light scattering
by using either diffraction or photon correlation spectroscopy (PCS)
techniques. Both are ensemble methods with inherently low resolution.
As with any ensemble measurement, it is difficult to obtain reliable
and consistent results for many particle systems, especially below
about 100 nanometers in size, using these techniques. Ensemble measurements
produce basically a mean particle size that can be fit by an infinite
number of PSDs. This forces the software or instrument operator
to guess a given PSD shape.
The importance of relying on true PSD data is illustrated as follows.
Figure 3 samples Silica (blue) and Polysty1 (red) have the same
mean particle size of 209 nm. This is unexpected given how dissimilar
both samples’ PSDs are. Despite having identical weight-average
particle size, these two samples will exhibit different properties,
such as packing density, polishing capability, rheology, film gloss
and surface area.
Another light diffraction disadvantage is that one needs to know
the complex index of refraction (real and imaginary components of
the index of refraction) for the particle material, and even then
the particle surface morphology may play a surprising and hard to
New Particle Characterization Methods
As smaller-sized nano-particles are developed using a wider range
of materials, particle characterization methods that require extreme
dilutions and stringent sample handling precautions are quickly
becoming outdated. Today, reliable and consistent results are critical
to achieving a stable dispersion and a high-quality end product.
New, rapid, easy to use, and high-resolution particle characterization
methods have become available that meet these needs, allowing accurate
characterization of particles of ever decreasing size.
*The ESA-9800 from Matec Applied Sciences, Northborough, Mass.
**The CHDF-2000 from Matec Applied Sciences