Water dynamics in bolaamphiphile hydrogels investigated by 1H NMR relaxometry and diffusometry

The dynamical properties of water in hydrogels formed by the bolaamphiphiles dotriacontane-1,32-diyl-bis[2-(trimethylammonio)ethylphosphate] (PC-C32-PC) and its head group derivative dotriacontane-1,32-diyl-bis[2-(dimethylammonio)ethylphosphate] (Me(2)PE-C32-Me(2)PE) were investigated by determining the transverse relaxation times and the mean diffusion coefficients of the water protons. T(2) distributions obtained for hydrogels containing only 1 mg/mL of PC-C32-PC or Me(2)PE-C32-Me(2)PE were found to be almost unchanged compared to that of pure deionized water. In a hydrogel containing 30 mg/mL PC-C32-PC, a small dynamical perturbation of the water molecules could be observed, since the obtained major peak in the T(2) distribution was found to be slightly broadened and shifted to lower T(2) values with respect to pure water. Moreover, a slightly decreased mean diffusion coefficient of the water molecules was determined for the hydrogel with 30 mg/mL PC-C32-PC. For 30 mg/mL Me(2)PE-C32-Me(2)PE at pH 5, the dynamical properties of water are significantly influenced by the presence of the bolaamphiphiles. Whereas the mean diffusion coefficient of water was again only slightly decreased, the relaxation behavior was found to be considerably changed. At room temperature, the major T(2) peak obtained for 30 mg/mL Me(2)PE-C32-Me(2)PE at pH 5 was significantly broadened and shifted to lower T(2) values compared to pure buffer. This broad monomodal peak becomes bimodal when the temperature is decreased to 5 °C. Increasing the temperature revealed that the structural changes of the bolaamphiphilic self-assemblies at 45 °C are reflected in the determined mean relaxation times (T(2m)). These results indicate the presence of a significant degree of structural heterogeneity in a hydrogel formed by 30 mg/mL Me(2)PE-C32-Me(2)PE at pH 5. By comparing the mean diffusion coefficients with the mean transverse relaxation times, the distance scale of these heterogeneities could be estimated to be in the range of 50 to 120-140 μm. They can most probably be ascribed to the existence of a considerable number of lamellar domains being formed besides the nanofibers in hydrogels with 30 mg/mL Me(2)PE-C32-Me(2)PE at pH 5. Upon storage, the phenomenon of syneresis was observed for this hydrogel, which corresponds to a growth of the lamellar domains to sizes between 140 and 190 μm.