3.3. Fourier Transform Infrared Spectroscopy (FTIR)
FTIR was used to investigate potential interactions among components of all phases; the oleogel, hydrogel and MDG. Fig. 3 shows the spectra for the 60:40 and 70:30 OG:HG formulations. The broad peak at ~3370 cm-1 is associated with O-H, N-H stretching and hydrogen bonding. This peak can mostly be attributed to water molecules (Sagiri et al., 2015) and hydroxyl groups on gelatin amino acid residues and MDG (Mohammed et al., 2018). Peaks at ~2920 cm-1 and ~2850 cm-1 are associated with C-H stretching in RBW, soybean oil and MDG (Mohammed et al., 2018; Wakhet et al., 2015; Ishaka et al., 2014). Peaks ~2100 cm-1 and ~1635 cm-1 are associated with amino acid groups in gelatin and were confirmed by a spectrum of pure 7% (w/w) gelatin (not shown). Peaks ~1740 cm-1 and in the region of below 1600 cm-1 are associated with carbonyl groups stretching and C-H bending, respectively, in RBW, soybean oil and MDG (Mohammed et al., 2018; Wakhet et al., 2015; Ishaka et al., 2014).
There were noticeable changes in peak intensity which can be attributed to the presence of MDG in the samples. In general, in both 60:40 and 70:30 OG:HG ratios, the peaks attributed to MDG and the oleogel phase increased in intensity as the MDG concentration increased. Simultaneously, peaks associated with the aqueous phase decreased in intensity with increasing MDG concentration. From these observations, it appears that interactions within the aqueous phase are decreasing in strength while interactions within the lipid phase are increasing in strength with the addition of MDG. For 60:40 OG:HG formulations, there was a gradual decline in peak intensity at 3368 cm-1which may indicate a decrease in hydrogen bonding or diminishing strength in hydrogen bonds (Yilmaz & Ogutcu, 2014). This is an unexpected result because monoglycerides, when crystallized, have an inverse bilayer structure stabilized by hydrogen bonds established by its secondary and primary -OH groups. (Aguilar-Zarate et al., 2019). It would be expected that the peak at 3368 cm-1 would increase in intensity due to the presence of additional -OH groups from MDG or the establishment of new hydrogen bonds from MDG crystallization. This decrease in this peak’s intensity may indicate that MDG weakened or reduced the density of hydrogen bonds in the hydrogel matrix, eventually leading to a bigel phase inversion shown in the 60:40(3) formulation in Fig. 2. MDG crystallization could still be happening at the same time, but the hydrogen bonding associated with this process may be lesser in comparison to the dissociation of the hydrogel’s hydrogen bonding, leading to a net decrease in the peak intensity. The same phenomenon is also seen for the 70:30 OG:HG formulations, but the decrease in hydrogel associated peak intensities is most prominent in the 70:30(2) formulation. Unlike the 60:40 OG:HG ratio, the decrease in hydrogel peak intensities did not lead to a phase inversion in 70:30 OG:HG, but it is notable that 70:30(3) could not be stabilized for reasons that are not yet clear.
In contrast, an increase in peak intensities associated with the oleogel phase was observed for both 60:40 and 70:30 OG:HG ratios, with more prominent changes noticeable in 60:40 OG:HG formulations. This increase in intensity may just indicate that MDG is acting as an organogelator, strongly associating with the oleogel phase and/or strengthening the interactions of the oleogel matrix. This would agree with the formation of a continuous matrix in the 60:40(3) formulation seen in Fig. 2. The exception to this pattern is the 60:40(0.5) formulation where, not only is there a decrease in oleogel associated peaks, but there is a decrease in oleogel region size in Fig. 2. When the regions are smaller, there is less interaction within the oleogel phase because a larger area of the lipid interface is surrounded by the continuous hydrogel matrix. The data from Figs. 2 and 3 suggest that the emulsification capacity for MDG in this bigel system is greatest at 0.5% (w/w) MDG and a OG:HG ratio of 60:40. Beyond 0.5% (w/w), MDG appears to have greater interactions in the oleogel phase, as an organogelator, than at the lipid-aqueous interface as an emulsifier.