HOW TO DEVELOP A WET METHOD FOR LASER DIFFRACTION PARTICLE SIZE MEASUREMENT?
The wet dispersion method is a common method for particle size distribution analysis using the laser diffraction technique. It is useful for samples containing fine particles below a few microns in size. Because the wet method significantly reduces adhesion forces between particles through the process of surface wetting. It helps agglomerated particles get dispersed with relatively little energy during stirring.
In this article, we are going to discuss the role of a few important parameters, as below:
1. Dispersion
2. Sampling.
3. Measurement conditions (obscuration range, measurement time, and stirring speed),
Reproducible results for particle size distribution measurement depend on the above three factors. Depending on the size of the particles, the significance of each of these variables varies: sampling is more significant for coarse materials, whereas dispersant and dispersion energy are more critical for fine particles.
1. Dispersion:
a. Definition: The process of dispersion involves mixing a dry powder substance with a liquid so that all of the dry powder's individual molecules and particles become separated and are then uniformly distributed and thoroughly mixed in the liquid medium.
b. Selection of Dispersant Media: Water is the most widely used dispersion, however, it is not appropriate for every sample due to poor wetting or sample dissolution. Using a dispersant of opposite polarity to the sample can reduce the solubility of the sample in the dispersant. However, this is balanced by the requirement for particle wetting, which becomes more challenging without the aid of surfactants because of significant polarity differences. Table 1 displays a list of possible dispersants in decreasing order of polarity.
The selection of an appropriate dispersant is the initial step in this procedure. In order to quantify laser diffraction, the dispersion should:
c. Preparation of Dispersant:
There are three main steps to dispersing a powder in a liquid:
Step No. 1: Wetting the sample
Step No. 2: Adding energy to improve dispersion
Step No. 3: Stabilizing the dispersion.
Step No. 1: Wetting the sample
A beaker test is used to determine how well different dispersants wet the sample while evaluating new samples. This visual evaluation is faster than measuring with different dispersants.
A homogeneous suspension of the particles in the liquid between the dispersant and the particles indicates good wetting. In contrast, poor wetting may be indicated by liquid droplets on top of the powder or by substantial agglomeration and sedimentation.
Using a surfactant to lower the surface tension can enhance the wetting of the sample. One to two drops of a surfactant are usually sufficient. If the surfactant quantity is more then it causes foaming and bubbles, and it may be interpreted as large particles.
Step No. 2: Adding energy to improve dispersion
You must determine the sample's level of dispersion in the instrument after selecting an appropriate dispersant to moisten it. The role of sample stirring, sonication, and stability of dispersion after sonication should be evaluated to achieve a reproducible result.
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When measuring in an organic solvent, the sonication must be applied in a stepwise manner. Apply sonication for about one minute, then wait for the dispersant's temperature to stabilize before measuring (otherwise, false peaks at large particle sizes may be observed due to temperature variation in the dispersant). This procedure should be repeated until no further reduction in size is observed as a result of sonication.
Step No. 3: Stabilizing the dispersion
If the particle size begins to rise as a result of particle re-agglomeration, an additive may be essential to stabilize the dispersion. By imparting a charge to the particle surface, additives such as sodium hexametaphosphate, ammonium citrate, and sodium pyrophosphate may help stabilize a suspension. Additives are often used in concentrations of less than 1 w/v%.
2. Sampling
For any particle characterization technique, it is essential to make sure the sample you insert into the machine is an accurate representation of the majority of your material. When measuring coarse particles or samples with a wide variety of sizes, sampling becomes the most likely source of inaccuracy. You must measure a certain minimum number of particles in order to obtain a representative result.
The sampling technique must be suitable for a sample of a suitable volume for the particle size measurement. Sample splitting techniques, for example, the spinning riffler or the cone and quartering method, may be applied.
3. Measurement conditions:
Setting the right measuring conditions is important for obtaining a reliable result from a laser diffraction measurement. Sample concentration, obscuration range, measurement duration, and stirring speed are a few of them.
a. Sample concentration:
The most suitable sample concentration for a laser diffraction measurement is a balance between adding enough sample to provide a satisfactory signal-to-noise ratio or a representative sample of the bulk material and not adding too much sample such that multiple scattering affects the measurement.
In a laser diffraction system, the concentration of the sample is quantified by obscuration, which is the percentage loss of laser light through the sample. The obscuration is nothing but related to the concentration of particles in the system.
b. Obscuration range: The lower obscuration limit for coarse particles will be set by sampling rather than signal-to-noise ratio. If measurements of numerous subsamples of a coarse material indicate a large degree of variability, try increasing the mass of the subsample and hence the obscuration at which the observations are taken.
Multiple scattering is an effect that establishes the upper limit of obscuration for a laser diffraction measurement. The theory utilized to explain the scattering data in a diffraction system makes the assumption that only one particle has scattered the laser light that hits the detector. There is a greater chance that more than one particle has scattered the laser light before it reaches the detector if the particle concentration in the cell is too high.
The laser light is scattered at higher angles as a result of these multiple scattering events. Multiple scattering results in an underestimation of the particle size since higher angle scattering is linked to fine particles.
Fine particle measurements are more likely to be affected by multiple scattering, whereas coarse particle measurements are more likely to be affected by sampling. As a result, appropriate obscuration ranges based on particle size are provided in Table 2.
Table 2: Ranges of obscuration recommended based on particle size
Particle siz Obscuration range
Fine particle ~ 5 to 10% (less than 5% may be required for <1μm).
Coarse particles 5 to 12%.
Polydisperse samples 15 to 20%.
c. Measurement duration
In a wet laser diffraction measurement, the measurement duration must be long enough to allow a representative sample of the particles in the dispersion unit to circulate through the measurement cell. The required time will be determined based on the particle size and polydispersity of the sample.
A short dispersion time for fine monodisperse samples is sufficient, while coarse particles or broader distributions require a longer measurement time. Increase the measurement time for the analysis of large particles or broad distributions, to improve repeatability.
d. stirrer speed
A wet dispersion unit's stirrer is responsible for ensuring homogeneous dispersion and representative sample passage through the measurement cell. A stirrer speed titration is required for larger or denser materials to ensure that all of the particles in the sample are suspended. A stirrer speed titration for emulsion samples can tell the speed at which the stirrer's motion starts to break up the droplets.
Conclusions
Reproducible results from wet laser diffraction measurements are dependent on three major factors: obtaining a representative sample of the bulk material, achieving a stable dispersion condition, and setting proper measuring conditions. You will be able to improve the reproducibility of your particle size data by considering the above points.
well thought out and narrated, Pritam Surkar