Analysis and Characterization of Manufactured Nanoparticles in Aquatic Environments - Environmental and Human Health Impacts of Nanotechnology - page 222

For fast diffusing particles, the light intensity loses its autocorrelation rapidly
(microseconds), while for larger slow diffusing particles the intensity can be
correlated to the initial for substantial timescales (many milliseconds).
If the NPs are not affected by their adjacent neighbours (particle- particle inter-
actions), then the scattered light fl uctuations can be correlated to the self-diffusion
of the particles (their diffusion coeffi cient) (Schurtenberger and Newman, 1993).
For such dilute systems, the hydrodynamic diameter, d H , of a particle can be
derived from the Stokes- Einstein relationship:

kT
D

d

B

(6.2)

= 3

H

πη

where D is the diffusion coeffi cient, is the viscosity of the medium and k B is

Boltzmann's constant. But the scattering effi ciency of the light varies as the sixth
power of d H (or proportional to d 6 ) for particles smaller than one twentieth of the
wavelength and as the square of d H (or proportional to d 2 ) for larger particles.
This size dependence of the scattered light intensity will skew the result towards
the larger particles in the measurement. DLS consequently delivers an intensity-
weighted correlation function, which is usually converted to an intensity weighted
(z - average) diffusion coeffi cient (Table 6.3). If the size distribution of the sample
contains several particle types or a broad size distribution, the deconvolution of
several diffusion coeffi cients from the autocorrelation from such a sample is an ill
posed mathematical problem and the obtained results are usually not very robust
(Filella et al. , 1997; Finsy, 1994; Schurtenberger and Newman, 1993).
The advantage of DLS is rapid analysis time, along with simple operation, which
can even be used in the fi eld (Ledin et al. , 1994), and is suitable to qualitatively
monitor agglomeration (Viguie et al. , 2007). The drawbacks are the complicated
data interpretation for polydisperse samples and the diffi culty of using intensity
weighted results for any size measurements that are not very monodisperse samples
(Filella et al. , 1997). The conversion from diffusion coeffi cient to hydrodynamic
diameter also involves a spherical particle assumption.

η

6.2.3.2 Static (Classical) Light Scattering

For particles much smaller than the wavelength (d /20) of the incident light, the

scattering intensity in the plane perpendicular to the polarization of the laser light
is equal in all directions (the particles are isotropic scatterers). In the size range
(

< λ

) destructive interferences on all angles but forward favour forward
scattering intensity, and the larger particles in this size range have the higher
forward scattering intensity. When the particle sizes are in a similar size range as
the wavelength of the laser light, then the electromagnetic interactions of the light
with the matter within one particle will cause constructive or destructive light at
certain angles which will be dependent on the particle size. When particle sizes are
similar to or larger than the wavelength, the scattering angular dependency becomes
complex with maxima and minima at certain angles, which is described by the Mie
theory (van de Hulst, 1981). For these size ranges there is consequently an angular
dependency of the scattered light. These phenomena are used in static light

λ /20 < d < λ