Non-Isothermal Nucleation
In the development of the non-isothermal nucleation kinetics by
spectrophotometry, the values of TCS,
TCO and TC, as well as the cooling ramp
used in DSC, were taken as a starting point. Figure 4 shows the kinetics
obtained for each system. It can be observed that there is a higher
absorbance response in the systems with higher MY concentration (SAT),
this as a response to the higher formation of MY crystals (solid
fraction). It was also observed that there is a higher absorbance
response in CN systems compared to CA. The nucleation kinetics initially
present a time interval where the system is completely melted
(baseline), followed by an almost exponential growth. The point where
the increase from the baseline starts is the beginning of nucleation
(García-Andrade et al., 2020). Subsequently, the absorbance values
present an almost constant trend (Fig. 4). However, it was found that
for the CNMGC system there is a second increase after
this usual trend. This was not observed in CNSAT because
the high concentration of MY in conjunction with the nature of CN gives
higher absorbance values in response, which overlies the second
increase. This second increase in absorbance values in the
CNMGC system corresponds to the crystallization of the
oil, which although it has a high crystallization point, it is at a
lower temperature than the crystallization of MY (Fig. 2).
From the graphs shown it was possible to determine the most relevant
kinetic and thermal parameters during nucleation (Table 3). Shorter
nucleation times were obtained in SAT systems compared to MGC, and
consequently, higher nucleation rates (J) were obtained. No significant
differences were found between the oil variable with high MY
concentrations, however, at MGC, the shortest tn was for CN. Shorter
nucleation times have been reported with the use of saturated oils, in
systems with high concentrations of structurants (Garcia-Andrade et al.,
2020), so it was necessary to evaluate the effect of concentration
additionally. The differences observed by oil type in MGC systems may be
due to the unsaturation of the oils, and the content of free fatty acids
(FFA), since FFA require lower activation energy in the crystallization
process with respect to TAGs. These compositional differences may be
more representative in systems with low gelator concentrations. Due to
the inverse correlation between tn andTn , higher values of Tnand lower values of δ were obtained at shorter times, indicating
that a lower thermodynamic inductive force is needed in oleogels with
higher MY concentration.
The response exhibited by SAT systems is mainly due to a higher amount
of attraction and hydrogen bonding interactions of the amphiphilic
molecule with the TAGs of the oil. A higher amount of hydrogen bonds
promotes a faster orientation of the polar moiety of the MY by aligning
its hydrocarbon chains (attractive and hydrophobic interactions) and
generating a faster first solid core (Contreras-Ramírez et al., 2021).
Regarding the ΔGn values, the values closest to
equilibrium were obtained in the SAT systems. Except for theΔGn values that were smaller with CN oil, no
significant differences were found by oil type at SAT concentrations.
This is due to a higher interaction of SFA content and high MY
concentration.