Particle size analysis

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We offer different solutions from Horiba Scientific in the field of particle size analysis and zetapotential.

The LA-960 Laser Diffraction Analyser has a dynamic measuring range of 10nm and 5000µm for dry dispersions and 10nm and 3000µm for wet dispersions.

The SZ-100 Nano Partica Analyser is the solution for determining the particle size of nanoparticles between 0.3nm and 8µm. With this analyzer the zetapotential, molecular weight and Second Virial Coefficient can be determined as well as the particle size.

The theory behind these analysers is explained below.

Dynamic light scattering

Laser radiates through a polarizer and then through a sample. The scattered light passes through a second polarizer where it is captured by a PMT. The image is projected onto a ‘screen’, the so-called ‘Speckle pattern’.

All molecules in a solution are hit by the light and scatter the light in all directions. The scattered light of the molecules interferes in two ways:

1) constructive: light areas on the Speckle pattern

2) destructive: dark areas on the Speckle pattern

This process is repeated in short time intervals and results in a set of Speckle patterns. These are analyzed by an autocorrelator that compares the light intensity (I) of each spot over time.

Rayleigh scattering occurs when light passes through a transparent liquid or solid. Rayleigh scattering is the scattering of light by particles that are smaller than the wavelength of light. This can be most clearly observed with gases. This is why a cloudless sky is blue.

If the light source is a laser, and therefore monochromatic and coherent, the scattering intensity changes over time. This fluctuation is caused by the ‘Brownian motion’ of molecules and particles which causes the distance between the diffusers (particles and molecules) and the solution to change over time. This scattered light undergoes constructive or destructive interference by the particles present. This intensity fluctuation provides information about the time scale of movement of the particles or molecules.

Theory Zetapotential:

The potential difference between the dispersion medium and the stationary liquid layer (=DL: double layer) attached to the dispersed particle.

Morphology: To determine the influence of the shape and state of a molecule on chemical properties, dynamic re-configuration and interaction.

Anisotropic: The property of a material to have different properties in different directions.

Zeta potential is the effectiveness of the surface charge in solution. This depends on:

  • Fundamental surface spots: how many, what type
  • Conditions of the solution: temperature, pH, electrolyte concentration It is useless to give a zetapotential value without specifying the conditions of the suspension.
  • Relationship between ζ (=zetapotential) and UE is not linear: UE = 2εε0 ζ F(κa)/3η
  • Ex = the applied electric field
  • Vp = speed of the particle (µm/s)
  • Electrophoresis mobility = UE = Vp / Ex
  • Zetapotential is determined indirectly. The most common technique is microelectrophoresis (ELS/PALS).

Brownian movement

The random movement of particles in a liquid as a result of the collisions of the particles with the fast moving molecules and/or atoms in the liquid.

Larger particles have a lower frequency than smaller particles.

Double layer (DL) or Electric double layer (EDL)

The double layer is a structure created on the surface of an object (particle, bubble, droplet) when this object is exposed to a liquid. They are two layers of charge around the object parallel to each other.

The first layer, the ‘surface charge’, consists of ions adsorbed to the object by chemical interactions. The second layer consists of ions attracted by the ‘surface charge’ via the Coulomb force and form an electric screen around the first layer. This layer is relatively loose and consists of free ions that move in the liquid under the influence of electrical attraction and thermal movement.

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