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Section 6.2.2). (dn/dc) can be determined in a separate experiment using a
refractive index detector, simply by measuring the refractive index at different
(known) polymer concentrations, for instance between 0 an 1.0 mg/ml. A plot
of
n
as a function of
c
provides (dn/dc) as the slope (linear regression
analysis), as the line is generally straight for low concentrations). If the solvent
is changed, for instance from aqueous solvent to DMSO, then (dn/dc) must be
determined for the latter solvent, as it can be drastically different from the
former. Some examples:
Polymer system
dn/dc (ml/g)
Na alginate and xanthan in dilute salt solution 0.150-0.154
Chitosan acetate in acetate buffer pH 4.5
0.142
Pullulan and dextran in aqueous solution
0.148
Proteins in aqueous solution
0.185
Cellulose in DMAc/LiCl (0.5%)
0.104
Strictly, (dn/dc) = (dn/dc)
µ
, the refractive index increment determined at the
same chemical potential as the solvent. This is achieved by dialyzing the
samples against the solvent before determining the refractive index, and is
particularly important for polyelectrolytes.
Returning to the scattering behavior of small molecules (R
G
< ca.
λ
/20), the
theory states that the scattering intensity will be independent of the scattering
angle. Therefore, such molecules can be studied e.g. at
θ
= 90
°
, which is the
simplest situation from a technical point of view.
A light scattering experiment then involves measuring i
θ
for a series of
different concentrations. From the measured i
θ
values (and I
0
which also must
be measured) Kc/R
θ
is calculated, and plotted as a function of c. Example:
0.0E+00
2.0E-06
4.0E-06
6.0E-06
8.0E-06
1.0E-05
1.2E-05
1.4E-05
1.6E-05
0.0000
0.0005
0.0010
0.0015
c (g/ml)
Kc/R
!