Anna Fureby, YKI Institute for Surface Chemistry, Stockholm, Sweden
Powders are becoming more common as products and ingredients, and the need to characterize these materials is ever increasing. The more advanced functionalities that are sought, the more characterization is needed to confirm the performance and understand the relationship between powder characteristics, formulation, processing and performance. The interest increases in other techniques beyond the more conventional particle sizing, bulk and tap density determinations, flow properties and functional wetting tests. It is known that many of the functional powder properties are determined by the powder particle surface properties. These properties are not that readily studied and detailed studies require advanced instrumentation. However, there is much to be gained for the powder producer and user if the link between formulation, processing and function is made clearer than it is today.
This communication aims to describe some of the more advanced techniques available, in particular focusing on the
information that can be gained from the techniques and the limitations of the techniques.
The shape of powder particles requires the use of microscopy, and sometimes light microscopy may be sufficient. However, for detailed analysis of the powder shape and surface structure, higher resolution is required, and this can be achieved by using Scanning Electron Microscopy (SEM). In this technique, the powder is first coated with a thin conducting layer (typically gold or platinum), and can then be imaged under vacuum, using an electron beam rather than a light source. The scattered electrons are analyzed and an image is generated, with very good focal depth (in contrast to light microscopy). Figure 1 illustrates light microscopy and SEM of a spray-dried powder. The different surface features can be seen in high resolution in SEM, while the light microscopy also shows some of the internal structure in the particles. The best SEM instruments of today provide high-quality images with a resolution down to tens of nanometers, so that very detailed structural investigations can be performed.
Figure 1. Light microscopy of a powder dispersed in oil, and b) SEM image of spray-dried powder.
Some SEM instruments may also be operated in environmental mode, when the sample is left unsputtered and the backscatter form the water molecules in the atmosphere around the sample (still under vacuum) is viewed. Resolution is sacrificed to some extent in this technique, but on the other hand the native surface is imaged and experiments can be conducted, e.g. observing the effect of increased relative humidity on the powder, see Figure 2 that shows how amorphous spray-dried lactose adsorbs water from the atmosphere and starts to flow.
Figure 2. Spray dried lactose at a) RH=35%, b) RH=50%, 5 min, and c) RH=50%, 10 min. Successive moisture adsorption causes the lactose to flow, a pre-stage to recrystallization and severe caking..
Properties such as particle-particle interactions, wetting, dispersability, etc are strongly influenced by the surface chemistry in the outermost surface of the particle, i.e. the first nanometer or nanometers. The composition of the powder surface can differ substantially from the bulk composition of a powder. Research at YKI Institute for Surface Chemistry has shown that the more surface active and fast adsorbing materials in a formulation will be present at the surface of a spray-dried powder particle. The same effect is observed also in freeze-drying, but is less pronounced.
This can be analyzed using X-ray photoelectron spectroscopy, XPS (Electron spectroscopy for chemical analysis, ESCA). In this technique the solid powder sample is placed under high vacuum and irradiate with monochromatic X-rays from a soft X-ray source. The X-ray irradiation excites electrons in the material, which are emitted as photoel ectrons. Due to collisions and energy dissipation only the electrons originating from atoms at or close to the surface can escape form the sample and be detected. This effect causes the surface sensitivity of the technique so that the atomic surface composition in the outermost 2-10 nm of the particle surface (depth depending on material). The atomic surface composition can be converted to molecular surface composition, using reference data for the pure components in the powder and a calculation model. An example of the analysis is shown in figure 3, where the surface composition of a powder with casein micelles and lactose is correlated with transmission electron micrographs of the same powder, showing the presence of casein micelles at the powder surface.
Figure 3. The correlation between measured surface composition and inset TEM images, showing the casein micelles as dark globules at the powder surface.
The XPS technique can also be combined with another very surface sensitive technique, ToF-SIMS (time-of-flight secondary ion mass spectroscopy). This is also a high-vacuum technique, but in this case a pulsed primary ion beam is used to desorb and ionize species from the surface. The emitted ions are detected in a mass spectrometer, and the ion fragments are sued to identify the molecular species at the particle surface. The technique is less quantitative than XPs, but more surface sensitive (1-2 nm).Both XPS and ToF-SIMS can be used to image surfaces, so that compositional maps can be generated, as shown for XPS in Figure 4.
Figure 4. An SEM image of an opened particles and XPS image of the same particle.
Today, more advanced formulation of powders is carried out in order to generate particular nano-structured powders, and in this context it is interesting to investigate the structure in the solid state. Again, microscopy techniques of higher resolution and also providing chemical information are of interest. Confocal microscopy, where the instrument is set-up so that only the light form a thin section (typically 0.5 µm) is collected. The lateral resolution is limited by the light diffraction limit to about 200 nm (depending on light source). Two different types of confocal microscopy are generally used, confocal scanning laser microscopy (CSLM) relying on fluorescent dyes, and confocal Raman microscopy (CRM), relying on the Raman scattering of the components in the sample. Both techniques have merits and draw-backs, but both about the same resolution. The techniques of generating the images are also quite different. In CSLM fluorescent labeling of the sample is necessary, since it is the fluorescence that is used to generate the image. In order to visualize different components, different dyes can be used, e.g. a fat soluble one for lipids and a water soluble one for e.g. proteins. The fluorescent dye is added during sample preparation for the CSLM, and thus sample preparation is critical as is the need for suitable dyes. CRM, on the other hand, has no need for labeling, but autofluorescence is a problem with some materials. When the Raman spectra for different components are very similar, it can be difficult to generate images clearly showing where the components are located. To some extent the software for the analysis deals with this as the whole spectrum of the references and the sample are used to generate the images. Figure 5 shows a CRM image of an aspirin tablet, where the different materials are clearly observed.
Figure 5. CRM image as an optical, vertical cut in the particle, which is composed of itraconazole and PVP. The itraconazole signal is shown. In b) the spectra in different colour-coded positions are shown, illustrating the relative change in peak height for itraconazole (itra) and PVP with changing distance from the particle surface.