Transcript Contents Design - Chemical Engineering:IIT Madras

Dynamic Light Scattering
S. Ramanathan, Dept. Chem. Engg., IIT-Madras
Theory
Issues
Soln
Simplified theoretical background
Problems faced during experiments
Suggested solutions
Theory
Motion
Scatter
Small, spherical
particles (less than 3
micron size) in liquid undergo brownian motion.
Smaller particle move faster.
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
Intensity vs angle is called static
Static light scattering. Here, light detector is
Scattering placed at some angle, and the intensity of
scattered light is measured for some time
and the average value is used
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
Intensity vs time is measured at
Dynamic
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
Measurement
Measured
Correlation curve
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
Particle Size Distribution
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.
Model Assumptions
Single
Single particle scattering effects can
be compard with analytical solutions. Multi
particle scattering (i.e. high concentration)
give incorrect results
Non spherical particles can not be
Spherical
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.
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.
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
‘unstable’ suspension. The correlation
curves may look OK, but the diameter
measured over time will change
Issues
Dust
Correlation curve
(good sample)
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
(BAD sample)
Solution
Conc
Soln
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.
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.
Solution
Spherical
Soln
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.
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
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,
Soln
perhaps by centrifugation. One can use
a more sensitive detector (avalance
photo diode) for low concentrations, but
we don’t have it at present.
Solution
Repeat the experiments. A general
Unstable
rule of thumb is “1 min” experiments,
repeated 5 times, at a given
concentration. If data (dia and counts)
change over time, then…
No Solution If your sample is
Soln
inherently unstable. Our DLS can’t
handle it.
Solution
Dust
Soln
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)
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