Disc centrifuges fractionate particles according to both size and density, making them ideal for high-resolution particle sizing. X-Ray based detection further simplifies the process of calculating a particle size distribution from a centrifugation-based instrument, eliminating optical corrections and approximations. This has been shown to be especially relevant for metal particles and metal oxide, which have a wide array of industrial uses, and yet are poorly suited for characterization with traditional light scattering.
Disc centrifuges exploit the tendency of particles to sediment according to both size and density, which is a form of fractionation, resulting in high-resolution particle sizing. The Disc Centrifuge Photosedimentometer (BI-DCP) and X-ray Disc Centrifuge (BI-XDC) are two unique technologies that offer high resolution particle sizing capability based on this principle. In both cases the difference in hydrodynamics between particles of different sizes drives separation, while detection based on the Lambert Beer Law allows for quantification of amount (by volume, mass, number for example). For the BI-DCP, this quantification is based on optical turbidity, whereas for the BI-XDC, detection is based on attenuation of X-rays by the particles as they pass in front of the detector. Particles in suspension pass the detector, where the rate of sedimentation is based on a known set of parameters including particle density, the density and viscosity of the fluid which the particles are moving through (the spin fluid), centrifugal forces exerted on the particles and the distance the particles must travel through the spin fluid to reach the detector.
Compared to other sizing technologies based on light scattering, centrifugation has no real bias such as angular dependence and can be performed at much higher concentrations. While a light source is used, these techniques are based on the measurement of the time it takes to spin the material through the spin fluid. It is a rugged technique for which the theory is well developed. A detailed explanation of theory and principles is contained in the appendices.
In this discussion we present three metal oxide sample data using X-ray disc centrifuge technique.
- Zinc Oxide ZnO
- Ferric Oxide Fe(III)2O3
- Titanium Dioxide TiO2
In order to determine the most useful measurement technique for each material class we compare three industrially useful metal oxides. These are: Zinc Oxide, Ferric Oxide, and Titanium Dioxide. Each are sold under a wide variety of different tradenames and can be prepared by numerous methods. In each case submicron particles are formed. In addition to centrifuges date, results using DLS, a common method for submicron particle sizing, are also presented.
However, two characteristics of these materials make this (DLS) technique less suitable. One is high density which results in rapid gravitational sedimentation. Two is high turbidity and for some materials border on chalky or nearly opaque appearances except at very low concentrations, which makes this property less suitable for DLS. Metal oxides are common components of paints and other pigment containing formulations and this turbidity is often unavoidable. Titanium- and Zinc- oxides are commonly used as colorants or opacifiers in a number of common formulations types: personal care formulations such as sunscreens lotions; toothpastes; and creams; industrial formulations such as commercial paints and pigments. In this case their ability to scatter or otherwise attenuate visible light is a desired property used to modify the appearance of a product. Both metal oxides also attenuate ultraviolet light to a degree, which can be exploited to resist UV degradation of paints, or more directly to confer UV resistance to lotions and creams, as with sunscreens. Recent evidence suggests that TiO2 micro- and nanoparticles may have greater toxicity than ZnO, requiring substitution of zinc oxide for titanium oxide, in many products such as sunscreens.
In contrast ferric oxides are mainly of interest as base materials for production of other materials, although they can also be used directly for pigments. While the source iron particles are crystalline in nature, they can adopt a wide variety of properties when incorporated into steel and iron. In most Industrial applications, characterization of the size of primary particles will be necessary to ensure desired material properties in the final alloy.
The three metal oxides that will be compared are TiO2, ZnO, Fe2O3. TiO2 and ZnO were prepared at concentrations of 2% (w/v) in 0.1% (w/v) tetrasodium pyrophosphate (TSPP), whereas Fe2O3 was dispersed in 0.1% Triton X-100 (TX100), a commercial nonionic surfactant.
In each case DLS overestimates the mean particle size relative to XDC. In order to make a more direct comparison, DLS results were transformed from Intensity Weighted- to Volume Weighted- distributions. Light scattering is intrinsically intensity biased, and thus tends to overrepresent larger particles or aggregates. The DLS particle size distribution for Fe2O3 is heavily influenced by the presence of what appear to be several very large, optically resolvable, particles. As a result, the distribution is heavily skewed towards larger sizes, whereas it appears that the BI-XDC is capable of effectively resolving the two populations. This is seen to a much smaller extent with ZnO, in which a second population causes an otherwise broad distribution to appear bimodal.
Titanium and zinc oxides both appear oversized in DLS. Iron oxide appears dramatically oversized by DLS, suggesting that aggregated particles might comprise the majority of the signal obtained from light scattering. This bias is overcome easily with the BI-XDC. Unlike methods based on measuring scattered light, X-Ray detection has no bias towards larger sizes. Because centrifugation exploits physical separation of differently sized particles, it is also capable of much greater resolution.
Read our appendix on the Principles of Operation of Disc Centrifigues for more information.