Chemistry Reference
In-Depth Information
If the solute molecules are independent agents and contribute additively to the
observed turbidity, one can write
X
H
X
c
i
M
i
τ 5
τ
i
5
(3-55)
where
τ
i
,c
i
, and
M
i
refer to the turbidity, weight concentration, and molecular
weight of monodisperse species
i
which is one of the components of the mixture
that makes up the real polymer sample. Then
X
c
i
5
H
X
c
i
M
i
X
c
i
5
c-
0
c
5τ
lim
HM
w
(3-56)
Thus, the light-scattering method measures the average molecular weight of
the solute.
3.2.3
Scattering from Large Particles
The equations to this point assume that each solute molecule is small enough
compared to the wavelength of incident light to act as a point source of secondary
radiation, so that the intensity of scattered light is symmetrically distributed as
A
P
1
θ
2
(a)
P
2
θ
1
C
B
Scattering envelope
for larger scatterers
Incident
(b)
direction
Small scatterers
FIGURE 3.4
(a) Interference of light scattered from different regions of a scatterer with dimensions
comparable to the wavelength of the scattered light. (b) Scattering envelopes for small and
large scatterers. These scattering envelopes are cylindrically symmetrical about the
direction of the incident light.
