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The aggregation behavior and rheological properties of aqueous suspensions of the bolaamphiphile dotriacontane-1,32-diyl-bis[2-(dimethylammonio)ethylphosphate] (Me(2)PE-C32-Me(2)PE) were investigated depending on the pH value and the salinity by means of differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and oscillatory rheology. This bolaamphiphile self-assembles into helical fibers of approximately 5 nm thickness with an all-trans conformation of the alkyl chains. These nanofibers can gel water very effectively by forming a three-dimensional network. The headgroups' protonation depends on the pH value of the solution and influences the ability of the molecules to aggregate into fibers. At low pH values the headgroups are zwitterionic and stable hydrogels are formed, whereas at high pH values the headgroups are negatively charged and the length of the fiber aggregates diminishes as does the stability of the gel structure. We can show that by the addition of cations it is possible to shield the repulsive interaction between the molecules at high pH values so that the formation of the fiber aggregates and the gelation of the system occur. By addition of salt at high pH values the viscous flow behavior of Me(2)PE-C32-Me(2)PE suspensions could be transformed into the viscoelastic behavior of a gel. The gels show characteristics that are common in systems of wormlike micelles. However, there are also significant differences that arise from the unique bolaamphiphile fiber structure with highly ordered alkyl chains that render the properties similar to crystalline nanofibers.

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.

The formation of temperature-, concentration-, and pH-responsive hydrogels composed of the symmetric long-chain bolaamphiphile dotriacontane-1,1'-diyl bis[[2-(dimethylammonio)ethyl]phosphate] (Me(2)PE-C32-Me(2)PE) was investigated by rheological, scattering, and spectroscopic techniques. At pH 5, this bolaamphiphile is known to form a dense network of helically structured nanofibers (Köhler et al. Soft Matter 2006, 2, 77-86). Rheological measurements and dynamic light scattering were used to describe the macroscopic behavior of the hydrogels. Small-angle neutron scattering (SANS) and time-resolved static light scattering were applied to get information about the morphology of the self-assembled aggregates. Finally, solid-state 31P NMR spectroscopy was used to gain insight into the mobility of the bolaamphiphile molecules within the fiber aggregates. In comparison with the previously examined trimethylammonio analogue PC-C32-PC, which forms temperature-dependent hydrogels, Me(2)PE-C32-Me(2)PE exhibits additional concentration- and pH-dependent gelling properties. The significantly higher stability of the Me(2)PE-C32-Me(2)PE hydrogel is supported by the SANS data, which indicate the presence of fiber aggregates up to 50 degrees C.

The temperature-dependent self-assembly of the single-chain bolaamphiphile dotriacontan-1,1'-diyl-bis[2-(trimethylammonio)ethyl phosphate] (PC-C32-PC) was investigated by transmission electron microscopy (TEM), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FT-IR), X-ray scattering, rheological measurements, and dynamic light scattering (DLS). At room temperature this compound, in which two phosphocholine headgroups are connected by a C(32) alkyl chain, proved to be capable of gelling water very efficiently by forming a dense network of nanofibers (Kohler et al. Angew. Chem., Int. Ed. 2004, 43, 245). A specific feature of this self-assembly process is that it is not driven by hydrogen bonds but solely by hydrophobic interactions of the long alkyl chains. The nanofibers have a thickness of roughly the molecular length and show a helical superstructure. A model for the molecular structure of the fibrils which considers the extreme constitution of the bolaamphiphile is proposed. Upon heating the suspensions three different phase transitions can be detected. Above 49 degrees C, the temperature of the main transition where the alkyl chains become "fluid", a clear low-viscosity solution is obtained due to a breakdown of the fibrils into smaller aggregates. Through mechanical stress the gel structure can be destroyed as well, indicating a low stability of these fibers. The gel formation is reversible, but as a drastic rearrangement of the molecules takes place, metastable states occur.