Urea induces (unexpected) formation of lamellar gel-phase in low concentration of cationic surfactants

Hypothesis: 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. Experiments: Gels formed by combination of aqueous solutions of urea and C(12)TAB, C(14)TAB or C(16)TAB 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. Findings: 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 similar to 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 degrees 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. (C) 2021 Elsevier Inc. All rights reserved.

Automated summary

Experiments revealed the formation of gels with a lamellar structure in solutions containing high urea concentration and low cationic surfactant amount, independent of the chain length of the surfactants. The crystallization of surfactant molecules loaded into urea assemblies forming adducts at a microscopic scale was observed through XRD and SAXS analysis, indicating a hierarchical structure of the gels.