Gelling Lyotropic Liquid Crystals with the Organogelator 1,3:2,4-Dibenzylidene-d-sorbitol Part I: Phase Studies and Sol-Gel Transitions

Langmuir. 2019 Dec 31;35(52):17132-17141. doi: 10.1021/acs.langmuir.9b01688. Epub 2019 Jul 29.

Abstract

Gelled lyotropic liquid crystals (gelled LLCs) are the combination of an LLC and a gel network. One method for obtaining gelled LLCs is the addition of a low molecular weight gelator, which forms gels via self-assembled fibrillar networks, to an aqueous surfactant solution. A potent gelator for LLCs is the LMW organogelator 1,3:2,4-dibenzylidene-d-sorbitol (DBS). This gelator gels the lamellar Lα phase, the bicontinuous cubic V1 phase, and the hexagonal H1 phase of the system H2O-C12E7 (heptaethylene glycol monododecyl ether) without influencing the phase boundaries as visual phase studies and rheometry show. Varying the DBS mass fraction η, one can adjust the sol-gel transition temperature Tsol-gel of the gelled LLCs. At η = 0.0075, all Tsol-gel values are below the LLC-to-isotropic phase transition temperatures TLLC-iso, that is, the LLCs are formed first while cooling down, followed by gel formation. At η = 0.015, however, Tsol-gel > TLLC-iso, that is, the gel is formed in an isotropic solvent, which becomes an LLC while cooling down. The system H2O-C12E7 is the first where an adjustment of the gelator concentration allowed us to decouple gel and LLC formation for all three LLCs, that is, gel and LLC formation happen one after the other and not simultaneously. This allows us to study whether the structure and thus the properties of gelled LLCs can be manipulated by the order of gel and LLC formation. We discuss our findings in light of the following question: are our gelled LLCs truly orthogonal self-assembled systems, that is, do the LLCs and the gel network form and coexist independently?