Non-ionic surfactant self-assembly in calcium nitrate tetrahydrate and related salts

Self-assembly of amphiphilic molecules can take place in extremely concentrated salt solutions, such as inorganic molten salt hydrates or hydrous melts. The intermolecular interactions governing the organization of amphiphilic molecules under such extreme conditions are not yet fully understood. In this study, we investigated the specific effects of ions on the self-assembly of the non-ionic surfactant C₁₂H₂₅(OCH₂CH₂)₁₀OH (C₁₂E₁₀) under extreme salt concentrations, using calcium nitrate tetrahydrate as a reference. The mixtures of Ca(NO₃)₂·4H₂O and C₁₂E₁₀ displayed lyotropic (H₁ and I₁) and micellar phases, in contrast to CaCl₂·xH₂O-C₁₂E₁₀ or CaBr₂·xH₂O-C₁₂E₁₀ mixtures where mesostructurally ordered salt-surfactant complexes were observed. The Ca(NO₃)₂·4H₂O-C₁₂E₁₀ system was thoroughly investigated by constructing its binary phase diagram and performing thermal and spectral comparisons with other salt hydrates. The Ca(NO₃)₂ system displayed significantly higher isotropization temperatures than zinc, aluminium, and lithium nitrate systems. ATR-FTIR analysis revealed that Ca²⁺ primarily interacts with the surfactant head groups through ion-dipole interactions, while these interactions were less pronounced with other cations. The results show that an intermediate hydration/coordination energy of the metal ion can lead to stronger metal-surfactant interactions and thermally more stable liquid crystals. Comparison between the Ca(NO₃)₂, CaCl₂, and CaBr₂ systems suggests that reduced ion pair formation enhances the interactions between Ca²⁺ and oxyethylene groups, leading to the salting-out of salt-surfactant complexes. Despite its low water content and strong intermolecular interactions, the Ca(NO₃)₂·xH₂O-C₁₂E₁₀ system exhibited an electrical conductivity of up to 1.0 × 10^-3 S cm^-1 with 4 water molecules per salt, making it a promising medium for electrochemical applications.