Conformational and thermal behavior of a pH-sensitive bolaform hydrogelator

The phase behavior of the symmetric long-chain bolaamphiphile dotriacontan-1,1'-diyl-bis[2-(dimethylammonio)ethyl phosphate] (MePE-C32-MePE) was investigated in aqueous suspensions of different pH by transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and Fourier transform infrared spectroscopy (FT-IR). At pH 5 the compound exhibits excellent properties as a hydrogelator by forming a dense network of helically structured nanofibrils with a diameter of 3-4 nm. The phase behavior of MePE-C32-MePE is compared with the previously examined trimethylammonio analogue PC-C32-PC, whose self-assembly process seems to be exclusively driven by hydrophobic interactions. At pH 5, MePE-C32-MePE can form intermolecular hydrogen bonds between the headgroups, which causes a significantly higher stability of the nanofibrils up to at least 75 °C. In contrast, no pronounced gelling properties are observed at pH 10. Obviously, the negatively charged headgroups seem to prevent the suspension from gelling. Nevertheless, nanofibrils are formed, but at 75 °C a fragmentation into smaller aggregates occurs due to the lack of stabilizing effect of intermolecular hydrogen bonds and the destabilizing effect of the negatively charged headgroups.