Environmental Engineering Reference
In-Depth Information
packing density rather than the formation of a three-dimensional gelation
which involves three-dimensional links among APTES molecules.
To determine the degree of hydrolyzation of ethoxy groups and subsequent
cross-linking, which have significant impact on the density and stability of the
film, DRIFT spectra were therefore collected. In contrast to silanization in
water, DRIFTS spectra of the APTES film deposited from toluene solutions
showed vibrational bands of methyl group at 2,974 and 2,887 cm
1
, indicating
the presence of small fraction of unhydrolyzed ethoxy groups in the film. The
unhydrolyzed ethoxy groups on the surface are anticipated to inhibit the lateral
polymerization of APTES, thus resulting in a poorer packing density and lower
surface coverage films of lower stability than those of the film silanized from
water where the hydrolysis is more effective. This observation is consistent with
the findings from XPS.
To further confirm the orientation of APTES on g-Fe
2
O
3
, the electrokinetics
(zeta potentials) of the coated particles were measured. The isoelectric point
(IEP), the point where zeta potential is zero, of the particles silanized with
APTES from water and toluene was found at about pH 8.5 and 9.2, respec-
tively, in contrast to pH 4.5 for uncoated g-Fe
2
O
3
. The similar electrokinetics
confirms the condensation of APTES on g-Fe
2
O
3
from both water and toluene
with amine groups facing the environment and remaining reactive. The differ-
ence in the measured zeta potentials using the particles silanized in water and
toluene reflects mainly the variation of APTES orientation on the particles.
A slightly lower IEP value observed for the particles silanized in water suggests
that more amine groups were hidden inside the film compared to the silanation
in toluene. As a result, the contribution from amine groups to the number of
positive surface sites decreases while that from silanol groups to the number of
negative sites increases. These two effects result in a lower IEP of g-Fe
2
O
3
particles silanized in water than in toluene as experimentally observed. This
finding is consistent with XPS analysis, which showed a higher degree of
protonation of amine groups considered to bind with the surface.
The stability of the silanized APTES films on g-Fe
2
O
3
was investigated by
leaching the particles in pH 2 HCl solutions for 20 h. It was found that the
amount of iron leached out reduced from 60 mg/g for unsilanized g-Fe
2
O
3
to
34 mg/g for silanized g-Fe
2
O
3
in water and toluene. Compared to the particles
coated with MHA using the SA method, the amount of iron leached out is
significant, suggesting that the original particles were not fully protected by
APTES, compounded with some degree of detachment of APTES from the
surface in acid solutions, for direct silanation from water and toluene.
Detachment of silanized APTES was confirmed by zeta-potential measure-
ments as shown in Fig. 6.5. After leaching, the zeta potentials shifted back
towards the zeta-potential values of unsilanized particles. Figure 6.5 also shows
a much greater shift in IEP by base attack, suggesting that the APTES coating is
less stable in basic than in acidic environments. Similar trends in electrokinetics
were observed for particles silanized from toluene. In the base environment, an
IEP shift from pH 9 to 4.3 was observed. However, in acidic solution, the films
