Figure 3. Thermo-gravimetric analysis showing the weight losses
of VG (green), ALG (red), CB8 (dark red), VGALG (blue), and VGCB8ALG
(dark cyan).
The weight loss percentage values at different temperatures were
calculated from the TGA curves for all the samples to estimate the
weight % of VG and CB8 in the modified alginates (Tables S2 in the
Supporting Information). We divided the weight % by the molecular
weight, giving us several moles. We then concluded the percentage by
moles of VG (~20 %) in both alginate polymers. We also
compared the number of moles of VG with CB8, which allowed us to
conclude to 2:1 VG: CB8 interaction on the alginate surface, which
agrees with other supramolecular architectures that contain
CB8-complexed viologen dyes.[18] Table S2 also
includes the percentage of humidity, decomposition, complete organic
degradation mass losses, and residues for ALG, VG, VGALG, CB8, and
VGCB8ALG. It was observed that VGCB8ALG samples released a higher
percentage of absorbed water than the ALG sample. This result indicates
a more robust immobilization of water molecules because of the linking
of ALG with VGCB8. The thermal stability of materials is manifested by a
significant weight loss in the degradation stage when complete thermal
degradation starts.
DSC data characterize the thermal behavior of individual ALG, VG, and
CB8 in terms of their simple structure, hydrophilic properties, and
association rate.[29] The results confirm the
linking of VG to ALG and its host-guest complexation to CB8 while linked
to ALG. For the unmodified ALG, the DSC curve in Figure S3 in the
Supporting Information shows an endothermic peak at 94.8°C followed by a
strong exothermic peak around 288°C. Endothermic peaks were correlated
with the loss of water associated with hydrophilic groups of the
biopolymer. In contrast, exothermic peaks resulted from the
decomposition of ALG, and subsequently, the decomposition of
carbonaceous material occurred above 400°C. The DSC thermogram of VG
unfolded an endothermic peak at 117°C attributed to the water release
and an exothermic peak at 354°C due to the decomposition of viologen.
Upon linking VG to ALG, the two exothermic peaks for ALG and VG were
shifted to 232°C and 360°C, respectively. Table S3 in the Supporting
Information presents the exothermic and endothermic peaks and the heat
flow changes associated with each peak. The thermogram of CB8 shows an
endothermic peak at 96°C, which is associated with the water release,
and an exothermic peak at 387°C because of the decomposition of CB8. In
contrast, the DSC of VGCB8ALG indicates the guest-host association as
evidenced by the evolution of an endothermic peak at 387°C instead of an
exothermic one in VGALG.
The particle size distribution (z-average hydrodynamic diameter (nm))
and surface charge (ζ potential (mV)) of VG, ALG, VGALG, and VGCB8ALG
were studied by dynamic light scattering (DLS) and electrophoretic light
scattering (ELS) as shown in Figure 4.