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: