Conclusion
In this contribution, the oleogelation of WEs in MCT-oil is discussed.
Prefatorily to the experimental work, the relevance of WEs for the
subject of oleogelation by natural waxes was established and the
knowledge on WE crystallization was reviewed. It was shown that
significantly different cooling rates can induce the same crystal habit
of either pure WEs or natural waxes in oleogels. Oleogels formed by
MCT-oil and 10 % (w/w) of different WEs or binary mixtures of WEs were
studied. The combination of literature and newly gathered data allowed
to map the crystallization behavior of WEs.
The review of crystallographic data revealed that WEs crystallize in a
mono molecular chain packing, either in an orthogonal or a tilted
orientation of the alkyl chains within the layer. The preferred
polymorph seems to be a feature of the position of the ester bond: Only
for WEs with a FaOH moiety two carbon atoms longer (\(CN=\ +2\)) or
four carbon atoms shorter than the FA moiety (\(CN=\ -4\)), the
orthogonal chain arrangement seems to occur. The thermal properties of
the pure components show systematic behavior. While the melting
temperature increases asymptotically with increasing CN, the molar heat
of fusion shows a linear increase. The position of the ester bond
strongly affects the thermal properties as increasing asymmetry of the
WE relates to reduced melting points and molar heats of fusion at
constant CN.
Due to solubility effects, the gel-sol transitions of mono-ester gels
occur at lower temperatures than the melting of the pure WEs. The
overall behavior corresponds to the melting behavior of the pure WEs.
Further analysis of more data will show in how far engineering
thermodynamics can predict dissolution temperatures and solid wax
content. Analysis of the heat of fusion data indicates a systematic
development of the WE solubilities, suggesting that larger WEs are less
dissolved than shorter ones. The viscoelastic behavior showed that all
prepared WE-in-oil solutions were able to form gels
(G’>G”) under the conditions studied. These gels at
inclusion levels of 10 % (w/w) of structurant exhibited the highest
rigidity (G* values) when smaller WEs were used, being in line with a
recent study on the gelling behavior of pure WEs .
The mixed oleogels, fabricated with binary mixtures of WEs as
structurants, behave at first sight according to the expectations. When
two WEs of significantly different CNs are applied in combination, DSC
and rheology data clearly indicate a formation of two separate crystal
phases, both relating to either of the two constituting WEs. This is in
line with a eutectic behavior for binary mixtures of WEs. The fact that
the lower transition temperatures do not arrange isothermally, despite
representing three-phase equilibria, is owed to the fact that the system
studied is at least ternary in nature. In contrast, applying mixtures of
WEs with similar CN (two carbon atoms) resulted in a single thermal
event for the gel-sol transitions. Interestingly, all of these mixed
systems seem to assume a tilted chain arrangement despite different
structures (C36(18_18): tilted; C38 (20_18): orthogonal) as single WE
systems. This is in line with earlier observations . Mixing two WEs of
same CN but different symmetry (WE C36 (14_22) and C36 (22_14)) yields
almost identical gel-sol transition signals, except for the 1:1 mixture.
For this particular system, a high-temperature shoulder at the peak
appears. Even though thus observation may be assigned to compound
formation, further research is required. The rheological
characterization by temperature tests confirmed the DSC data. Amplitude
strain tests revealed that the strongest gels were produced from
structurants that crystallize in two steps. This is the case for
mixtures of WEs that crystallize separately. This behavior has been
observed earlier in other oleogel systems and fat crystal networks and
is designated as sintering.
The compilation of own data and literature data presented here
illustrates the basic rules of WE crystallization. As the oleogels
investigated in this work are of low complexity, especially when
comparing to natural waxes, it is obvious that a correlation between
natural waxes and these synthetic systems still remains difficult.
However, the relevance of WE crystallization for wax-based oleogels
could be illustrated by on one hand illustrating that depending on the
cooling rate, similar crystal habits can be generated in either WE or
wax-based oleogels. Once the structuring of MCT-oil is based on at least
two substantially different wax esters, resulting in sequential
crystallization, structural characteristics of the gel resembled those
of SFX-gels. This illustrates that the study of pure wax esters in
oleogels can deliver detailed information necessary to better understand
wax-based oleogelation.
Tab. 1 Composition of
different natural waxes regarding the content and average CN of the WE
fraction
1: ; 2: ; 3: ; 4: ; 5: