Characterisation of Colloidal Suspensions

Each point corresponds to nmnerous measurands and even more characterisation techniques. In practice, however, only a few parameters are important for evaluating process performance or product quality. This chapter will focus on characterisation techniques that allow for a quantification of particle size and aggregate structure, which are of fundamental importance in describing any colloidal suspension. Besides this, relevant techniques for the quantification of interfacial properties are presented. 

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Light microscopy is the oldest and the simplest imaging method. However, the lower resolution limit for conventional light microscopy lies above 200 nm. Its application to colloidal particles is, therefore, restricted to rather large colloids and aggregates of them. Even though recent developments, like stimulated emission depletion (STED) microscopy, have shifted the optical resolution limit below 100 nm (Hell 2007), light microscopy is not really relevant for the characterisation of colloidal suspensions. 

Scattering experiments can be conducted with any kind of radiation (e.g. sound, electromagnetic waves, neutron radiation). This book will be confined to the scattering of light and X-rays, as these two types are most frequentiy used for the characterisation of colloidal suspensions. Both belong to electromagnetic radiation, yet the mechanisms of interaction with matter are completely dUferent. This difference becomes manifest in the refractive indices, which deviate qualitatively. For this reason, both types of radiation are separately discussed. 

When colloidal suspensions are probed by SAXS, the X-ray absorption in the solvent has to be considered because it attenuates the scattered intensities. Maximum scattering intensities are achieved for a sample transmission of 37 %, which is approximately the transmission through a 1 mm sheet of water when a typical copper anode (wavelength 1.54 nm) is used. Because of this intrinsic background turbidity, SAXS characterisation of colloidal suspensions has to be conducted in relatively small measurement volumes. 

First, DLS measurements were conducted in the 1960s by analysing the intensity flucmations in terms of a frequency spectrum (frequency analysis— FA Cummins et al. 1964 Arrechi et al. 1967 Chu and Schones 1968 Dunning and Angus 1968). The width of the frequency spectrum is a measure of the relaxation time of the microstructural processes and can be employed for the determination of the particle diffusion coefficients (Pecora 1964). An alternative for evaluating the fluctuation of scattered light intensity is photon correlation spectroscopy (PCS), which has been used for the characterisation of colloidal suspensions since the end of the 1960s (lakeman and Pike 1969 lakeman 1970 Foord et al. 1970). PCS requires a different hardware than FA, but it can be shown that the results of both techniques are equivalent (lakeman 1970 Xu 2000, pp. 86-89). 

Static and dynamic scattering techniques are spectroscopic characterisation methods in the sense of Sect. 2.2. These techniques evaluate the functional dependency of measurement signals on a spectral parameter, i.e. on time, space, or classically on wavelength or frequency. The major advantage of spectroscopic methods is the reduced sample preparation (no fractionation), but they involve the inversion problem. That is, the spectrum is a—most frequently incomplete and discrete— nonlinear projection of the size distribution. Beside the scattering techniques, there are further spectroscopic methods which are based on the extinction of radiation or on any other response of the particle system to an external field. This section describes optical, acoustic, and electroacoustic methods that have gained relevance for the characterisation of colloidal suspensions. 

In order attain measurable SHG signals, pulsed femtosecond lasers with large intensities are usually employed (Yan et al. 1998 Schneider et al. 2007). It was possible to show that the SHG scales with surface potential and the independently measured zeta-potentials can be reproduced by adopting appropriate models for the electric double layer (Yan et al. 1998). More generally, SHG is directly related to the surface excess of adsorbate as shown for malachite green on polystyrene (Eckenrode et al. 2005). This technique offers the opportunily for online and in situ characterisation of colloidal suspensions with particle sizes considerably larger than 5 nm (Schneider and Peukert 2007 Schiirer and Peukeit 2010). 

R. J. Hunter, Recent developments in the electroacoustic characterisation of colloidal suspensions and emulsions. CoUoids Surf. A 141(1), 37-65 (1998). doi 10.1016/S0927-7757(98)00202-7 R.J. Hunter, Measuring zeta potential in concentrated industrial slurries. Colloids Surf. A 195(1-3), 205-214 (2001). doi 10.1016/S0927-7757(01X)0844-5 R.W. O Brien, Electro-acoustic effects in a dilute suspension of spherical particles. J. Fluid Mech. 

The (Theodor) Svedberg (1884—1971). .. was a Swedish chemist who early worked on colloidal materials and developed the ultracentrifuge, which became an important analytical technique for the characterisation of colloidal suspensions. He himself employed this technique to distinguish pure proteins. In 1926 he obtained the Nobel Prize in Chemistry for his work on disperse systems . 

Hunter, R.J. (1998) Recent developments in the electroacoustic characterisation of colloidal suspensions and emulsions. Colloids and Surfaces, 141, 37-65. 

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