Solvent-Dependent Properties and Higher-Order Structures of Aryl Alcohol + Surfactant Molecular Gels

Molecular organogels, comprising small organic gelators in solvents, can be applied for dispersal of optical devices, such as emitters. Phenolic compounds and the surfactant bis(2-ethylhexyl) sulfosuccinate (AOT) are known examples of self-assembly organogels. However, conventional phenol + AOT gels in aromatic and acyclic alkane solvents are optically turbid, which is an obstacle for use as host materials in optical devices. In this study, a variety of aryl alcohol-AOT-solvent sets have been investigated systematically, and the correlation between the molecular architecture and optical transparency of the gels was considered. Accordingly, p-chlorophenol + AOT gels in cyclic alkane solvents were shown to form optically transparent gels. In contrast, aromatic and acyclic alkane solvents gave rise to turbid or opaque gels, even when utilizing the same gelators. AFM, NMR, SAXS, and FTIR were employed to determine the organogel structures. Consequently, we found that the gel transparency strongly depends on the size of the fibrous network of the gel, the structure of which is attributed to higher-order aggregates of the gelators. The average contour length and diameter of the fibrous network, lav and dav, respectively, were determined from AFM images. The transparent gels were shown to have lav = 4-9 μm and dav ≤ 0.3 μm, whereas the turbid gels had lav = 15 μm and dav = 0.4-0.6 μm. Such differences in the size of the fibrous network significantly affected the mechanical response of the gels, as shown by stress-strain measurements.