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Malvern 4700c Dynamic Light Scattering. For Dynamic & Static Light Scattering

 

It allows measuring of:

·         Particle size & Size distribution of colloid particles in dispersion within the range from 3 nm to 5 µm: Critical when optimizing the performance of raw materials, intermediates and finished products.

·         Zeta potential: Essential for assessing the stability of a wide variety of disperse systems.

·         Particle shape: A pivotal factor in gaining a clearer understanding of processes and process optimization.

·         Molecular weight: A fundamental determinant of many physical properties including transition temperatures from liquids to waxes and from rubbers to solids and of mechanical properties such as stiffness, strength, viscoelasticity, toughness and viscosity. Molecular mass distribution: 102-1012 Daltons.

·         Virial coefficients (static and dynamic)

·         Primarily used for higher concentration work, or work with micro-organisms

·         Rheological properties: Vital to understanding relationships between processing, end-use performance and structure of materials.

·         Chemical imaging: Enables the identification and spatial localization of chemical species, this provides a greater understanding for the development and manufacture of chemically heterogeneous materials and products.

 

 

 

 

 

Outline of a Light Scattering Experiment

 

Dynamic Light Scattering (DLS) also known as Photon Correlation Spectroscopy (PCS) works by first measuring this scattered light intensity at one angle. The intensity of light scattered in a particular direction by dispersed particles tends to periodically change with time. These fluctuations in the intensity versus time profile are caused by the constant changing of particle positions brought on by Brownian motion. DLS instruments obtain, from the intensity versus time profile, a correlation function. This exponentially decaying correlation function is analyzed for characteristic decay times, which are related to diffusion coefficients and then by the Stokes-Einstein equation, to a particle radius.

 

Light may be scattered, whenever it passes through a medium that is “polarizable” or has a dielectric constant different from unity. If the light experiences no energy loss, when interacts with the electrons bound in the material, the scattering is termed “elastic”, if not “inelastic”. Light may also interact by changing the energy state of an electron, rather than being scattered. This is referred to as absorption. Therefore, we need to separate the “incident” light from the source of the scattered light that spreads out from the scattering region. Using a source of collimated or well-focused light may do this. The former defines a parallel beam, the latter a beam that converges to a “waist of focus” (minimum size) in the sample then spreads out again. The characteristics of a laser source that are especially valuable are its well-collimated beam, stability, single wavelength (often called a “laser line”) operation (not all lasers have this property) and coherence (important for PCS but not intensity light scattering).

 

The well-collimated beam enables the direction of scattering to be established with precision, whilst stability is clearly important when comparing the scattered light intensity from different samples or under different conditions. We will see that wavelength plays a crucial part in all light scattering theories and that fixing it is almost essential in interpreting any measurement.

 

The simplest type of measurement we can make on a light scattering instrument is that of the intensity of light scattered. In general, unless we take care to calibrate the instrument in some way this will be an arbitrary or relative measurement since the exact power of the source, the collection angle of the detector, and its absolute efficiency will not be known. Also the effects of backgrounds that might be present will need to be taken into account. All of these factors can be admitted into the experiment.