Environmental Engineering Reference
In-Depth Information
Fig. 7
Neutral and ionised
structures of bio-polymer
PAA ISA 23.
a
Neutral.
b
pH
<
(a)
pKa
1
.
c
pKa
1
<
pH
<
pKa
2
.
d
pKa
2
<
pH
<
pKa
3
(b)
(c)
(d)
The molecular weight of ISA23 is 16,500 g/mol and it has three
pKa
's:
pKa
1
=
2
3. For this reason, the molecule could be in three
different ionisation forms as illustrated in Fig.
7
b-d.
Electrostatic DPD simulations have been performed (Mayoral and Nahmad-Achar
to be published
) in order to study the radius of gyration of this molecule and com-
pare with experimental data reported. The mapping used is shown in Fig.
8
.Itwas
established by taking into account the molar volume of each segment or monomeric
unit, and considering the volume of each DPD bead as 3
V
.
1,
pKa
2
=
7
.
5 and
pKa
3
=
3
.
30 Å
3
, where
V
w
=
is
w
the molar volume of one water molecule.
ISA23 could be considered as a weak poly-acid and the pH could be modelled
considering its ionisation degree over the polymeric structure. Partial charges are
introduced over the molecule considering that the B-DPD bead (see Fig.
8
) could
be neutral or have a charge of 1
−
depending on the pH in the solution. The C-DPD
beads could be neutral or have a positive charge of 1
+
or 2
+
depending also on the
pH of the medium according to the acid-base equilibrium given by
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