Organogels based on 12-hydroxy stearic acid as a leitmotif: Dependence of gelation properties on chemical modifications

Various compounds based on the structural leitmotif of 12-hydroxy stearic acid (HSA) were studied with respect to their ability to form organogels. They were modified by ethoxylation in order to avoid the acid group of HSA, which is unwanted for many of the applications of organogels. In this paper, it is shown that the rheological performance of organogels depends strongly on the extent of ethoxylation, exhibiting an optimum at intermediate degrees of ethoxylation. Furthermore, we reveal that the ability for gelation as well as the mechanical properties are substantially reduced by the presence of stearic acid (SA) in the original reaction mixture, which is a typical contamination of HSA. This is quantified by the amount of gelator required for gelation and the elastic moduli observed for the gels. At the same time the mesoscopic structure, as probed by small-angle neutron scattering (SANS), is almost unchanged for different degrees of ethoxylation or the addition of SA--and similarly thick fibres are observed, while the viscoelastic parameters evolve. Accordingly the elastic efficiency of the individual structural units is responsible for the observed changes in the gelation properties. These findings are relevant for the application of such low molecular weight organogelators in practical formulations, as one can optimise the rheological properties of organogelators by appropriately choosing the degree of ethoxylation.