Nanoparticles 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 Importance of Particle Size and Particle Size Distribution
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.
Particle Size Distribution Data
Figure 1 shows superimposed high-resolution PSD results on two fairly monodispersed polystyrene latex systems and a silica system. The particle sizing instrument, CHDF-2000 from Matec Applied Sciences based on a patented capillary hydronamic fractionation (CHDF) technique, performs high-resolution particle sizing from 3 microns down to 10 nanometers.
Figure 1. Particle size distributions from two monodisperse latex and one broad silica system.
How Measurements are Calculated Using Capillary Hydrodynamic Fractionation
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 and density.
Producing Reliable High Resolution Results Quickly
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.
Limitations of More Traditional Particle Sizing Techniques
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 Particle Size Distribution Data
The importance of relying on true PSD data is illustrated as follows. Figure 1 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.
Limitations of Light Diffraction Techniques
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 determine role.
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.