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Measurement System of Particles Dimensions by
Dynamic Light Scattering, Molecular Weight and Z Potential



     A typical system comprises the Zetasizer instrument and a computer with the Zetasizer software. The software is used to control the measurement of the sample. There are two basic ways to make a measurement:
- SOP measurements. A standard operating procedure (SOP) is like a template that pre-defines all the measurement settings. This ensures that measurements made on the same type of sample are made in a consistent way. SOPs are ideal if the same type of sample is regularly measured, inputting the same parameters each time a measurement is made.
- Manual measurement. A manual measurement is where all the measurement parameters are set immediately before the measurement is performed. This is ideal for measuring different types of sample, or experimenting with the measurement parameters.

- Particle size measurements

     The Zetasizer Nano series performs size measurements using a process called DLS - Dynamic Light Scattering (also known as PCS - Photon Correlation Spectroscopy). DLS measures Brownian motion and relates this to the size of the particles. It does this by illuminating the particles with a laser and analyzing the intensity fluctuations in the scattered light.

     The particles suspended in a liquid are constantly moving due to Brownian motion. Brownian motion is the movement of particles due to the random collision with the molecules of the liquid that surrounds the particle. An important feature of Brownian motion for DLS is that small particles move quickly and large particles move more slowly. The relationship between the size of a particle and its speed due to Brownian motion is defined in the Stokes-Einstein equation.
     The Zetasizer Nano system measures the fluctuation in scattering intensity and uses this to calculate the size of particles within the sample. If large particles are being measured, then, as they are moving slowly, the intensity of the speckle pattern will also fluctuate slowly. And similarly, if small particles are being measured, then, as they are moving very quickly, the intensity of the speckle pattern will also quickly fluctuate.

- Zeta potential measurements

     The Zetasizer Nano series calculates the zeta potential by determining the Electrophoretic Mobility and then applying the Henry equation. The electrophoretic mobility is obtained by performing an electrophoresis experiment on the sample and measuring the velocity of the particles using "Laser Doppler Velocimetry" (LDV).

     The zeta potential of the sample will determine whether the particles within a liquid will tend to flocculate (stick together) or not. The development of a net charge at the particle surface affects the distribution of ions in the surrounding interfacial region, resulting in an increased concentration of counter ions (ions of opposite charge to that of the particle) close to the surface. Thus an electrical double layer exists around each particle. The liquid layer surrounding the particle exists as two parts:an inner region, called the Stern layer, where the ions are strongly bound and an outer, diffuse, region where they are less firmly attached. Within the diffuse layer there is a notional boundary inside which the ions and particles form a stable entity. When a particle moves (e.g. due to gravity), ions within the boundary move with it, but any ions beyond the boundary do not travel with the particle. This boundary is called the surface of hydrodynamic shear or slipping plane.
     The potential that exists at this boundary is known as the Zeta potential. The general dividing line between stable and unstable suspensions is generally taken at either +30 mV or 30 mV. Particles with zeta potential more positive than +30 mV or more negative than 30 mV are normally considered stable.

- Molecular weight measurements

     Nano series performs Molecular weight measurements using a process called Static Light Scattering (SLS).

      Static Light Scattering is a non-invasive technique used to characterize the molecules in solution. In a similar way to "Dynamic light scattering", the particles in a sample are illuminated by a light source such as a laser, with the particles scattering the light in all directions. But, instead of measuring the time dependent fluctuations in the scattering intensity, "Static light scattering" makes use of the time-averaged intensity of scattered light instead. From this we can determine the Molecular weight (MWt) and the 2nd Virial Coefficient (A2).
     Coefficient (A2) is a property describing the interaction strength between the particles and the solvent or appropriate dispersant medium.
- For samples where A2>0, the particles "like" the solvent more than itself, and will tend to stay as a stable solution.
- When A2<0, the particle "likes" itself more than the solvent, and therefore may aggregate.
- When A2=0, the particle-solvent interaction strength is equivalent to the molecule-molecule interaction strength the solvent can then be described as being a solvent.