Dynamic Light Scattering and Static Light Scattering

Abstract: This lab will use a multiangle static/dynamic light scattering instrument. The goal of the lab is to measure the radius of gyration and for polymers the molecular weight using static light scattering (SLS) and the hydrodynamic radius using dynamic light scattering (DLS) . Culminant analysis and the use of Laplace Transforms will be explored for complex samples. Three samples will be explored: colloidal latex spheres (either colloidal silica or polystyrene latex), polymer in solution, and different types of milk (whole, skim, 2%). Sodium laureth sulfate (surfactant in Head and Shoulders) with variable NaCl content and concentrations (NaCl causes a transition from spherical to cylindrical to worm-like micelles). CTAB (cetyl trimethyl amonium bromide) and TMB (trimethylbenzene) in water (TMB swells the CTAB micelles).

Procedure: We will closely follow sample preparation schemes outlined by Paul Russo at LSU (now at Georgia Tech) in his lab on DLS/SLS (and old lab). Particular care must be made to exclude all dust from the samples. Some example data sets are available if we are not able to obtain suitable data for analysis.

Follow Russo's procedure on pages 14-15 to determine Rh for your samples.

Obtain <I> versus q by taking DLS patterns at different angles and measure Rg.

Analysis:

1) For all data import data into a program like Igor or Origin so that you can perform some rudimentary fits to the data.

2) Plot the data on a log-log plot and place the cursors on the region to fit if you are using Igor. Then fit with a single exponential to obtain the relaxation time.

3) Use the relaxation time and temperature to obtain the diffusion coefficient, report this value.

4) Use the Diffusion coefficient, and viscosity of water to obtain the hydrodynamic radius.

5) Repeat some of the samples using the Igor code of Schaefer. Compare the results.

6) For some of the surfactant samples you may need to use two relaxation times, so a double exponential decay function.

7) For the different angles measured confirm that the diffusion coefficient and hydrodynamic radius do not change.

8) Plot the static light scattering data on a log-log plot versus q and determine if you can measure the radius of gyration from the data (look for a knee in the data). Report Rg using Guinier's Law if it is possible to fit a radius of gyration. Report also Rh/Rg and compare with the value for monodisperse spheres.

9) Compare the results for D and Rh at different temperatures and comment on the behavior. Plot D versus T and RH versus T.

10) Find the average intensity at each angle for a given sample and temperature. Plot the time averaged intensity versus scattering vector q in a log-log plot. Are you able to observe Guinier's Law or Porod's Law in this data? Repeat this for each sample that multiple angle data was obtained.

11) For anisotropic samples there may be two discrete relaxation times associated with the two size scales of the sample. For instance, for a cylinder lateral diffusion will be slower than diffusion along the cylinder axis. (SELS can display cylinder like morphologies with a cylinder diameter of about 4 nm and length close to a micron.)

12) For decay curves measured at different angles plot G(2) versus (lag time * q^2) for comparison. Do the curves collapse on a master curve? Why or why not.

13) What is the difference between Rayleigh and Mie Scattering. What is the Rayleigh-Gans Approximation. Is it appropriate for your samples? Why or why not.

Questions:

1) Explain when and why you might use dynamic light scattering. Give an example.

2) What is a speckle pattern and how does it relate do DLS?

3) What is a time auto correlation function?

4) How is the diffusion coefficient obtained in DLS?

5) Why is the relaxation time proportional to q-2?

6) What is the Stokes-Einstein relationship?

7) Define the hydrodynamic radius. How was it calculated by Kirkwood? What moment of size does it reflect in a polydisperse sample?

8) What is a Laplace transform and how does it relate to DLS?

9) Describe multiculminate fitting of DLS data. Why is it used and what are the drawbacks?

10) What is the expected ratio of Rh/Rg for spheres, polymers, polydisperse spheres, other samples?

11) For a material, such as tobacco mosaic virus, that is cylindrical in shape with a high aspect ratio, explain how the DLS decay pattern will appear. How will the static light scattering pattern appear?

12) DLS is associated with quasielastic light scatteirng (QELS) and photon correlation spectroscopy (PCS). Explain, giving a historical reference, why there are three names for this analytic technique. Are there different ways to perform the DLS measurement? Why is one better than the others. (Consider that a randomly moving particle could move in the direction of a scattered photon (+ or - doppler effect) or normal to the direction of the scattered photon, causing flickering.)

13) If you observe two speckle patterns from two samples of the same material and one has twice as many spots at the other, what can you say about the two samples?

14) How is static light scattering related to DLS? How can you obtain the average scattered intensity at an angle from the DLS raw data?

15) Explain Guinier's law, a Guinier plot, Porod's law, the Zimm equation, and the Zimm Plot.

Analysis: