The molecular organogel n-decane/12-hydroxyoctadecanoic acid: Sol–gel transition, rheology, and microstructure

• In-depth rheological study of the organogel n-decane/12-HOA.
• Unique SANS and FFEM study of the organogel n-decane/12-HOA.
• Sense of the helical twist and fine structure of the individual gel fibers.
• Proof that commercially available is (R)-12-HOA and no racemat.

The paper at hand deals with the properties of a particular organogel, namely the binary system n-decane/12-hydroxyoctadecanoic acid (12-HOA). To provide a complete characterisation we firstly determined the sol–gel transition temperatures as a function of the gelator concentration with DSC and rheometry. Moreover, we calculated thermodynamic parameters from the DSC measurements such as the sol–gel transition enthalpy. Secondly, we studied the rheological properties with an oscillating shear rheometer. Thirdly, we investigated the structure of the gel network and of the gelator fibers. For this purpose, we carried out SANS measurements from which we obtained structural parameters such as the mean radius of the fibers and the mean thickness of the fiber nodes, but also the mole fraction of gelator involved in building up the gelator network. Additionally we present electron microscopy (EM) pictures of a 12-HOA gel prepared with the freeze-fracture method (FFEM). FFEM allows imaging a sample in its original state due to cryofixation while other EM methods require modifications like evaporation of the gel solvent. From our pictures we were able to determine the sense of the helical twist of the gelator fibers and we also observed a layered fine structure, which – to the best of our knowledge – has not been seen in EM pictures of 12-HOA gels yet. Our study allows an in-depth understanding of the gelator network in the model system n-decane/12-HOA. Moreover, our results are in perfect agreement with all data published for binary organogels consisting of an oil and 12-HOA, which, in turn, will help to understand many of the unanswered questions that still exist in relation to gels.

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