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Dynamic Light Scattering Theory Simplified theoretical background Issues Problems faced during experiments Soln Suggested solutions S. Ramanathan, Dept. Chem. Engg., IIT-Madras

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Theory Motion Small, spherical particles (less than 3 micron size) in liquid undergo brownian motion. Smaller particle move faster. Scatter Light passing through the suspension (particle + liquid) will be scattered by the particles. This happens when the refractive index of the particles are different from that of the liquid StaticScattering Intensity vs angle is called static light scattering. Here, light detector is placed at some angle, and the intensity of scattered light is measured for some time and the average value is used

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Theory Static Many detectors are placed at various angles and the “intensity vs angle” data is analysed. This is useful for particles over 1 micron size Dynamic Intensity vs time is measured at a fixed angle (usually 90 degrees). The scattered light fluctuates because of movement of particle scattering it. Think of doppler effect. This is not the same, but somewhat similar in idea. A fast response detector, at a fixed angle, is used. This is useful for particles less than 3 micron size

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Measurement Measured Auto correlation vs time is meaured. Don’t ask what this is (i.e. if you want to know, please google). Using a model, the particle size distribution is calculated Correlation curve Particle Size Distribution

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Measurement Counts The number of signals (counts per second) is also given. Usually it will be in kcps (kilo cps) or Mcps. More is better.

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Model Assumptions Single Single particle scattering effects can be compard with analytical solutions. Multi particle scattering (i.e. high concentration) give incorrect results Spherical Non spherical particles can not be compared with analytical solutions. So only a ‘sort of’ diameter can be calculated. This is the diameter of spherical particle that will give similar fluctuations.

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Issues Single Too high a concentration gives very nice looking correlation curves, but incorrect (usually less than actual) diameter. If particle dia is 200 nm, the diameter given by the instrument may be 20 or 50 nm! Spherical Non spherical particles can not be compared with analytical solutions. So only a ‘sort of’ diameter can be calculated. This is the diameter of spherical particle that will give similar fluctuations.

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Issues Single Too low a concentration gives very noisy correlation curves, and possibly incorrect diameter. Counts will be low and decrease (or remain stable) with dilution Particles settling over time give ‘unstable’ suspension. The correlation curves may look OK, but the diameter measured over time will change Unstable

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Issues Dust Correlation curve should go to zero on the right side, for good samples. If not, it means that the solution is dirty (i.e.large particles are present) Correlation curve (good sample) Correlation curve (BAD sample)

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Solution Conc Identify concentration effects. A general rule of thumb is “dilute 5 times and measure”. Repeat (i.e. dilute 25 times and measure). Counts should DECREASE with dilution. If they increase, you have too much concentration. Note : Counts may appear to be low to begin with! Dilute and remeasure. Soln Dilute Until “counts vs dilution” remains stable OR counts decrease with dilution. If you can’t dilute (e.g. some sample property of interest depends on dilution), then you can’t use our DLS. You can try other DLS instrument which claim to handle concentrated solutions, but I don’t have any experience with them.

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Solution Spherical Non spherical particles can not be compared with analytical solutions. So only a ‘sort of’ diameter can be calculated. This is the diameter of spherical particle that will give similar fluctuations. Soln No Solution. The best that can be done is to estimate the spherical particle(s) that will give similar correlation curve. There is no DLS model for non spherical particles, in our software

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Solution Single Too low a concentration gives very noisy correlation curves, and possibly incorrect diameter. Counts will be very low (10 or 20 kcps) and will decrease (or remain stable) with dilution. Concentrate the suspension, perhaps by centrifugation. One can use a more sensitive detector (avalance photo diode) for low concentrations, but we don’t have it at present. Soln

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Solution Unstable Repeat the experiments. A general rule of thumb is “1 min” experiments, repeated 5 times, at a given concentration. If data (dia and counts) change over time, then… Soln No Solution If your sample is inherently unstable. Our DLS can’t handle it.

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Solution Dust Correlation curve should go to zero on the right side, for good samples. If not, it means that the solution is dirty (i.e.large particles are present) Soln Use clean water to make the samples. Millipore water with 0.2 micron filter is good. If sample inherently has large particles, you can’t use DLS. If other liquids (e.g. acetone) are present, then use semiconductor grade materials

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