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.