Gelling Lyotropic Liquid Crystals with the Organogelator 1,3:2,4-Dibenzylidene‑d‑sorbitol Part II: Microstructure

This study deals with the gelation of lyotropic liquid crystals (LLCs) of the binary system H2O–heptaethylene glycol monododecyl ether (C12E7). The Lα and H1 phases are gelled with the organogelator 1,3:2,4-dibenzylidene-d-sorbitol (DBS). The microstructure of the gelled LLCs is compared to those of the binary counterparts, i.e., the pure LLCs and the binary gel ethylene glycol–DBS. We present the first examples of gelled lyotropic liquid crystals (LLCs) formed by two different ways upon cooling: (1) At a DBS mass fraction of η = 0.015, the gel is formed first, followed by LLC formation. (2) At η = 0.0075, the LLC is formed first, followed by gel formation. Addressing LLC and gel formation in different orders, the influence of the LLC on the gel network and vice versa can be examined. Independent of which structure is formed first, the interlayer spacing dLLC of the LLCs is only slightly larger in the presence of the gel network compared to the nongelled counterparts. Likewise, the influence of the LLCs on the gel fibers is independent of the chronology of the gel and LLC formation. For both ways, the gel fibers are twisted and arranged in bundles parallel to the bilayers of the Lα phase and the cylindrical micelles of the H1 phase. Whereas the twisted structure of the gel fibers in ethylene glycol is retained in the presence of the LLCs, the arrangement in bundles is not observed in the binary gels. In the latter case, randomly distributed single fibers which are also slightly thinner are detected. However, we observed the fiber bundles independent of whether the gel network is formed in the isotropic phase or in the LLC and argue that the difference is caused by different interactions of organogelator DBS with the system H2O–C12E7 than with ethylene glycol. In summary, we found that both the surfactant and the gelator molecules self-assemble in the presence of each other, leading to the coexistence of an LLC and a gel network. This is what is called orthogonal self-assembly

Gelled Lyotropic Liquid Crystals

In our previous work we were able to prove that gelled bicontinuous microemulsions are a novel type of orthogonal self-assembled system. The study at hand aims at complementing our previous work by answering the question of whether gelled lyotropic liquid crystals are also orthogonal self-assembled systems. For this purpose we studied the same system, namely, water–<i>n</i>-decane/12-hydroxy­octa­decanoic acid (12-HOA)–<i>n</i>-decyl tetra­oxy­ethylene glycol ether (C₁₀E₄). The phase boundaries of the nongelled and the gelled lyotropic liquid crystals were determined visually and with ²H NMR spectroscopy. Oscillating shear measurements revealed that the absolute values of the storage and loss moduli of the gelled liquid crystalline (LC) phases do not differ very much from those of the binary organogel. While both the phase behavior and the rheological properties of the LC phases support the hypothesis that gelled lyotropic liquid crystals are orthogonal self-assembled systems, freeze–fracture electron microscopy (FFEM) seems to indicate an influence of the gel network on the structure of the L_α phase and vice versa

How Cellulose Nanofibrils Affect Bulk, Surface, and Foam Properties of Anionic Surfactant Solutions

We study the generation and decay of aqueous foams stabilized by sodium dodecyl sulfate (SDS) in the presence of unmodified cellulose nanofibrils (CNF). Together with the rheology of aqueous suspensions containing CNF and SDS, the interfacial/colloidal interactions are determined by quartz crystal microgravimetry with dissipation monitoring, surface plasmon resonance, and isothermal titration calorimetry. The results are used to explain the properties of the air/water interface (interfacial activity and dilatational moduli determined from oscillating air bubbles) and of the bulk (steady-state flow, oscillatory shear, and capillary thinning). These properties are finally correlated to the foamability and to the foam stability. The latter was studied as a function of time by monitoring the foam volume, the liquid fraction, and the bubble size distribution. The shear-thinning effect of CNF is found to facilitate foam formation at SDS concentrations above the critical micelle concentration (cSDS ≥ cmc). Compared with foams stabilized by pure SDS, the presence of CNF enhances the viscosity and elasticity of the continuous phase as well as of the air/water interface. The CNF-containing foams have higher liquid fractions, larger initial bubble sizes, and better stability. Due to charge screening effects caused by sodium counter ions and depletion attraction caused by SDS micelles, especially at high SDS concentrations, CNF forms aggregates in the Plateau borders and nodes of the foam, thus slowing down liquid drainage and bubble growth and improving foam stability. Overall, our findings advance the understanding of the role of CNF in foam generation and stabilization