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.