Dieterich Sonja, Sottmann Thomas, Giesselmann Frank
Institute of Physical Chemistry , University of Stuttgart , Pfaffenwaldring 55 , 70569 Stuttgart , Germany.
Langmuir. 2019 Dec 24;35(51):16793-16802. doi: 10.1021/acs.langmuir.9b02621. Epub 2019 Oct 31.
We present a systematical investigation of gelled lyotropic liquid crystals (LLCs). This new class of soft materials combines the anisotropy of LLCs with the mechanical stability of a physical gel. The studied LLC system consists of sodium dodecyl sulfate as a surfactant, -decanol as a cosurfactant, and water as a solvent. At room temperature, four liquid crystalline phases (lamellar L, nematic N and N, and hexagonal H) are formed depending on the composition. We were successful in gelling the lyotropic lamellar phase with the low-molecular-weight organogelator 12-hydroxyoctadecanoic acid (12-HOA). The obtained gelled lamellar phase shows optical birefringence, elastic response, and no macroscopic flow. However, we were not able to obtain gels with hexagonal or nematic structure. These findings can be explained twofold. When gelling the hexagonal phase, the long-range hexagonal order was destroyed and an isotropic gel was formed. The reason might be the incompatibility between the gel fiber network and the two-dimensional long-range translational order of the cylindrical micelles in the hexagonal phase. Otherwise, the lyotropic nematic phase was transformed into an anisotropic gel with the lamellar structure during gelation. Evidently, the addition of the gelator 12-HOA to the lyotropic system considerably widens the lamellar regime because the integration of the surface-active 12-HOA gelator molecules into the nematic micelles flattens out the micelle curvature. We further investigated the successfully gelated L phase to examine the impacts of the gel network and the remaining monomeric gelator on both the structure and properties of the gelled lamellar phase. Small-angle X-ray scattering results showed an arrested lamellar layer spacing in the gelled state, which indicates a higher translational order for the gelled lamellar phases in comparison with their gelator-free counterparts.
我们对凝胶化溶致液晶(LLCs)进行了系统研究。这类新型软材料将溶致液晶的各向异性与物理凝胶的机械稳定性结合在一起。所研究的溶致液晶体系由十二烷基硫酸钠作为表面活性剂、癸醇作为助表面活性剂以及水作为溶剂组成。在室温下,根据组成会形成四种液晶相(层状L相、向列相N和N′相以及六方相H相)。我们成功地用低分子量有机凝胶剂12 - 羟基十八烷酸(12 - HOA)使溶致层状相凝胶化。所得到的凝胶化层状相表现出光学双折射、弹性响应且无宏观流动。然而,我们未能获得具有六方或向列结构的凝胶。这些发现可以从两方面来解释。当使六方相凝胶化时,长程六方有序被破坏并形成各向同性凝胶。原因可能是凝胶纤维网络与六方相中圆柱形胶束的二维长程平移有序之间不相容。否则,溶致向列相在凝胶化过程中转变为具有层状结构的各向异性凝胶。显然,向溶致体系中添加凝胶剂12 - HOA极大地拓宽了层状区域,因为表面活性的12 - HOA凝胶剂分子融入向列胶束使胶束曲率变平。我们进一步研究了成功凝胶化的L相,以考察凝胶网络和剩余单体凝胶剂对凝胶化层状相的结构和性质的影响。小角X射线散射结果表明在凝胶状态下层状层间距停滞,这表明与无凝胶剂的对应物相比,凝胶化层状相具有更高的平移有序度。