Temperature-dependent IR spectroscopic and structural study of 18-crown-6 chelating ligand in the complexation with sodium surfactant salts and potassium picrate

• Synthesis of novel 18C6 ether coordination complexes with different guests.
• Baseline analysis and temperature-dependent IR spectra obtained phase transitions.
• Temperature-dependent IR spectroscopy gave thermodynamic decomplexation parameters.
• 18C6-sodium 4-(1-pentylheptyl)benzenesulfonate is a compound with low melting point.
• 18C6-potassium picrate has a high thermal stability with the two-step distortion.

18-crown-6 ether (18C6) complexes with the following anionic surfactants: sodium n-dodecylsulfate (18C6-NaDS), sodium 4-(1-pentylheptyl)benzenesulfonate (18C6-NaDBS); and potassium picrate (18C6-KP) were synthesized and studied in terms of their thermal and structural properties. Physico-chemical properties of new solid 1:1 coordination complexes were characterized by infrared (IR) spectroscopy, thermogravimetry and differential thermal analysis, differential scanning calorimetry, X-ray diffraction and microscopic observations. The strength of coordination between Na+ and oxygen atoms of 18C6 ligand does not depend on anionic part of the surfactant, as established by thermodynamical parameters obtained by temperature-dependent IR spectroscopy. Each of these complexes exhibit different kinds of endothermic transitions in heating scan. Diffraction maxima obtained by SAXS and WAXS, refer the behavior of the compounds 18C6-NaDS and 18C6-NaDBS as smectic liquid crystalline. Distortion of 18C6-NaDS and 18C6-KP complexes occurs in two steps. Temperature of the decomplexation of solid crystal complex 18C6-KP is considerably higher than of mesophase complexes, 18C6-NaDS, and 18C6-NaDBS. The structural and liquid crystalline properties of novel 18-crown-ether complexes are function of anionic molecule geometry, type of chosen cation (Na+, K+), as well as architecture of self-organized aggregates. A good combination of crown ether unit and amphiphile may provide a possibility for preparing new functionalized materials, opening the research field of ion complexation and of host–guest type behavior.

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