Macroscopic properties and microstructure of HSA based organogels: sensitivity to polar additives

Organogels can be formed by a large variety of different low molecular weight gelators. In our investigations we employed 12-hydroxy stearic acid (HSA) and studied its ability to gel various organic solvents, such as dodecane, paraffin oil, triglycerides, octyl palmitate, and silicone oil. Gelation typically occurs above a concentration of 0.1-0.8 wt% and the gelation concentration is related to the polarity of the oil. In a second part we investigated the influence of polar additives which can affect the gelation process due to their polar character. By doing so we varied largely the polarity conditions under which the gelation of the HSA occurs. The macroscopic gel properties were characterised by means of oscillatory rheology experiments, differential scanning calorimetry (DSC), and optical microscopy. The microstructure of the samples was studied by means of small-angle neutron scattering (SANS). The gelation of HSA mainly depends on the availability of free acidic and hydroxy groups for the formation of H-bonds. Accordingly the elastic properties and finally the gelation process become suppressed by the addition of polar additives such as alcohols. This effect depends just on the molar ratio of added alcohol and HSA, being independent of the length of the added alcohol. Evidently the added alcohol is competing for the H-bonding with the HSA and thereby effectively destroying the H-bonding necessary for the formation of the network-forming fibres. This finding is of importance for potential applications of such molecules as rheological modifiers in oil-based systems.