Reversible Phase Transitions within Self-Assembled Fibrillar Networks of (R)-18-(n-Alkylamino)octadecan-7-ols in Their Carbon Tetrachloride Gels

The CCl4 gel phases of a series of low-molecular-mass organogelators, (R)-18-(n-alkylamino)octadecan-7-ols (HSN-n, where n = 0–5, 18 is the alkyl chain length), appear to be unprecedented in that the fibrillar networks of some of the homologues undergo thermally reversible, gel-to-gel phase transitions, and some of those transitions are evident as opaque–transparent changes in the appearance of the samples. The gels have been examined at different concentrations and temperatures by a wide variety of spectroscopic, diffraction, thermal, and rheological techniques. Analyses of those data and data from the neat gelators have led to an understanding of the source of the gel-to-gel transitions. IR and SANS data implicate the expulsion (on heating the lower-temperature gel) or the inclusion (on cooling the higher-temperature gel) of molecules of CCl4 that are interspersed between fibers in bundles. However, the root cause of the transitions is a consequence of changes in the molecular packing of the HSN-n within the fibers. This study offers opportunities to design new gelators that are capable of behaving in multiple fashions without entering the sol/solution phase, and it identifies a heretofore unknown transformation of organogels.