Department of Physical-Chemistry, Institute of Chemistry, University of Campinas, P.O. BOX 6154, 13084-862 Campinas, SP, Brazil.
Department of Physical-Chemistry, Institute of Chemistry, University of Campinas, P.O. BOX 6154, 13084-862 Campinas, SP, Brazil.
J Colloid Interface Sci. 2022 Feb;607(Pt 2):1014-1022. doi: 10.1016/j.jcis.2021.09.018. Epub 2021 Sep 6.
The unexpected formation of a lamellar structure with concomitant gelation in solutions containing high urea concentration (40 wt%) and relatively low amount of cationic surfactant (3 wt%), indicates that a hierarchically structured complex is formed by both molecules.
Gels formed by combination of aqueous solutions of urea and CTAB, CTAB or CTAB were prepared in different proportions and their structures at microscopic and mesoscopic levels were investigated using XRD and SAXS, respectively. The elastic and viscous moduli and yield stress of the samples were determined and correlated with the composition and structuration of the gels. The lamellar structure is reversibly thermically destroyed and this process was investigated using DSC.
XRD revealed that, at microscopic scale, the gels are formed through crystallization of adducts containing surfactant molecules loaded into the cavities of honeycomb-like urea assemblies. Such crystalline phase arranges itself in lamellae with interplanar distance around ∼20-30 nm, which were observed by SAXS. This hierarchical structure is independent of the chain length of the cationic surfactants. The blocks of lamellae dispersed in the continuous phase form a three-dimensional rigid particulate network structure, giving the characteristic rheological behavior of a hydrogel. DSC revealed a reversible thermal transition at around 20-25 °C, beyond which the adducts and the lamellar phase are destroyed and micelles are formed. The characteristic transition temperature is independent of the chain length of the surfactant, and thus, it is not associated with their Krafft temperatures. The structures of the gels indicate that they resemble alpha-gels formed by fatty-alcohols and surfactants, although they self-assemble by different driving forces.
在含有高浓度尿素（40wt%）和相对低浓度阳离子表面活性剂（3wt%）的溶液中，出乎意料地形成层状结构并伴随凝胶化，这表明两种分子形成了具有层次结构的复杂体系。
通过将尿素和 CTAB、CTAB 或 CTAB 的水溶液以不同比例组合，制备了形成的凝胶，并分别使用 XRD 和 SAXS 研究了其微观和介观结构。测定了样品的弹性和粘性模量以及屈服应力，并将其与凝胶的组成和结构进行了关联。层状结构可被热可逆破坏，并用 DSC 研究了这一过程。
XRD 表明，在微观尺度上，凝胶是通过包含负载在蜂窝状尿素组装体空腔中的表面活性剂分子的加合物结晶形成的。这种晶相以约 20-30nm 的层间距排列成层状结构，通过 SAXS 观察到。这种分层结构与阳离子表面活性剂的链长无关。分散在连续相中层状结构的块形成了具有刚性颗粒的三维网络结构，赋予了水凝胶的特征流变行为。DSC 揭示了约 20-25°C 的可逆热转变，超过该温度后，加合物和层状相被破坏并形成胶束。特征转变温度与表面活性剂的链长无关，因此与它们的克拉夫特点温度无关。凝胶的结构表明，它们类似于由脂肪酸醇和表面活性剂形成的α-凝胶，尽管它们通过不同的驱动力自组装。
