Particle Sizing

Particle size and shape are two of the most important physical properties of particulate samples. Even small differences can significantly affect material properties such as chemical reactivity, bioavailability, dissolution and crystallization rates, stability in suspension, texture of the material, flow ability and handling, packing density and porosity, to name a few.

Three Malvern instruments are currently available at MCL that are capable of measuring size and shape for particles over a wide size range:

  • Morphologi G3 SE  (Automated Imaging)
  • Mastersizer 3000 (Laser Diffraction)
  • Zetasizer  Nano ZS (Dynamic Light Scattering)

In addition to particle size, the Zetasizer Nano ZS can provide zeta potential values estimated from electrophoretic mobility in aqueous and non-aqueous dispersions using Laser Doppler Micro-Electrophoresis.  Zeta potential is a physical property which measures electrostatic interaction among particles in suspension. Measurements of zeta potential are commonly used to assess the stability of colloidal systems.

Malvern Mastersizer 3000 - Laser Diffraction (LD)

In the case of laser diffraction systems, the particle size reported is the diameter of a sphere that yields an equivalent light scattering pattern as the particle being measured. Laser diffraction works by measuring the angular variation in intensity of light scattered as a laser beam passes through a well dispersed sample. The angular scattering intensity received is then analyzed to calculate the size of the particles responsible for creating the scattering pattern. In laser diffraction a volume distribution is obtained, which shows the volume percentage of particles at a given size obtained. This technique provides rapid measurements (several minutes) and repeatability.

Applications

Coatings, Ceramics, Cosmetics, Cement, Food, Clays

Sample Requirements
  • Size range: 0.05 to 900 microns (wet)
  • Refractive index of sample and solvent must be known.
  • 1-10 grams of sample needed.
  • Samples need to be well dispersed

Malvern Zetasizer Nano ZS

(Figure right: Schematic showing changes in light scattering based on particle size; Source: https://en.wikipedia.org/wiki/Dynamic_light_scattering)

For Particle and Molecular Size - Dynamic Light Scattering (DLS)

This technique is usually used to measure particle size of materials in the submicron region down to below 1nm. Particles that are in suspension undergo Brownian motion caused by thermally induced collisions between solvent molecules and the material particles. When the particles are illuminated with a laser, the intensity of the scattered light fluctuates over time at a rate dependent upon the particle size; smaller particles are displaced further by the solvent molecules and move more rapidly. Analysis of these intensity fluctuations yields the velocity of the Brownian motion and hence the particle size using the Stokes-Einstein relationship. The diameter measured in Dynamic Light Scattering is called the hydrodynamic diameter and refers to the way a particle diffuses within a fluid. Non-invasive backscatter optics (NIBS) are used to measure particle size for suspensions with concentrations ranging from 0.1 ppm to 40 w/v%.

For Mobility and Zeta Potential – Electrophoretic Light Scattering

In addition to measuring the size and molecular weight of particles in suspension, the Zetasizer ZS can also measure the mobility and zeta potential of particles in suspension. Laser Doppler micro-electrophoresis is used to measure zeta potential by applying an electric field to the solution causing the particles to move in suspension. The velocity of the particles is measured and the electrophoretic mobility is calculated. From this value the zeta potential is determined.

For Molecular Weight – Static Light Scattering
Static light scattering is used to determine the molecular weight of proteins and polymers using several solutions at different concentrations.

schematic of charged particles(Figure right: Figure 3: Schematic of charged nanoparticle.  The charge acquired by a particle or molecule in a medium is called the zeta potential.  It arises from the surface charge and the concentration and types of ions in the solution.)

See Malvern ZetaSizer Nano Series Manual [pdf]

Applications

Proteins, Polymers, Micelles, Carbohydrates, Nanoparticles, Colloidal dispersions, Emulsions

Sample Requirements

Particle Size:

  • Measurement range: 0.3nm – 10.0 microns (diameter)
  • Minimum sample volume is 12 microliters
  • Measurement principle: Dynamic Light Scattering
  • Refractive index of particles must be known
  • Refractive index and viscosity of solvent is needed for measurement
  • Minimum sample volume: 12µL
  • Accuracy: Better than +/-2% on NIST traceable latex standards
  • Precision/Repeatability: Better than +/-2% on NIST traceable latex standards

Zeta Potential:

  • Measurement range: 3.8nm – 100 microns (diameter)
  • Measurement principle: Electrophoretic Light Scattering
  • Dielectric constant and viscosity must be known.
  • Minimum sample volume: 150µL (20µL using diffusion barrier method)
  • Accuracy: 0.12µm.cm/V.s for aqueous systems using NIST SRM1980 standard reference material
  • Sensitivity: 10mg/mL

See Zeta Potential characterization technique.

Molecular Weight:

  • Measurement range: 980Da – 20M Da*
  • Measurement principle: Static Light Scattering using Debye plot
  • Refractive index must be known (to the third decimal place) for each concentration.
  • Minimum sample volume: 12µL (3-5 sample concentrations required)
  • Accuracy: +/- 10% typical