Table of contents 1 Why is particle size important?
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| PARTICLE SIZE
Particle size can be determined by measuring the random changes in the intensity of light scattered from a suspension or solution. Small particles in suspension undergo random thermal motion known as Brownian motion. This random motion is measured to calculate particle size using the process described below. A top view of the optical setup for particle size measurements in the SZ-100 is shown in Figure 24. DYNAMIC LIGHT SCATTERING TECHNIQUE PARTICLE SIZE ZETA POTENTIAL MOLECULAR WEIGHT SZ-100 RANGE IN MICRONS 0.3nm - 8µm OPTIMAL APPLICATIONS NANOSUSPENSIONS AND EMULSIONS UNDER 8µm, ZETA POTENTIAL AND MOLECULAR WEIGHT WEIGHT 25kG (55 lbs) FOOTPRINT WIDTH 528mm (21”) DEPTH 385mm (18”) HEIGHT 273mm (11”) figure 24 | DYNAMIC LIGHT SCATTERING LAYOUT FOR THE SZ-100 Light from the laser light source illuminates the sample in the cell. The scattered light signal is collected with one of two detectors, either at a 90 degree (right angle) or 173 degree (back angle) scattering angle. The obtained optical signal shows random changes due to the randomly changing relative position of the particles. This is shown schematically in Figure 25. The signal can be interpreted using an autocorrelation function. Incoming data is processed in real time with a digital signal processing device known as a correlator and the autocorrelation function, shown in Figure 26 as a function of delay time, Τ, is extracted. The autocorrelation function from dynamic light scattering in Figure 26 shows a sample where all of the particles are the same size, the baseline subtracted auto- correlation function, C, is simply an exponential decay of the following form: EQUATION 1 C = exp(-2 Γ) Γ is readily derived from experimental data by a curve fit. The diffusion coefficient is obtained from the relation Γ=D t q 2 where q is the scattering vector, given by q= (4 πn/λ) sin ( θ/2). The refractive index of the liquid is n. The wavelength of the laser light is λ, and scattering angle , θ. Inserting D t into the Stokes-Einstein equa- tion then solves for particle size D h is the final step. EQUATION 2 D h = k B T Where: D h = the hydrodynamic diameter D t = the translational diffusion coefficient k B = Boltzmann’s constant T = temperature η = dynamic viscosity figure 25 | tải về 7.54 Mb. Chia sẻ với bạn bè của bạn:
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