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<middle dot>4H2O and C12E10 displayed lyotropic (H1 and I1) and micellar phases, in contrast to CaCl2<middle dot>xH2O-C12E10 or CaBr2<middle dot>xH2O-C12E10 mixtures where mesostructurally ordered salt-surfactant complexes were observed. The Ca(NO3)2<middle dot>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<middle dot>xH2O-C12E10 system exhibited an electrical conductivity of up to 1.0 x 10-3 S cm-1 with 4 water molecules per salt, making it a promising medium for electrochemical applications.