Thermotropic phase transitions of catanionic dodecylsulfates with multi-charged and multi-tailed quaternary ammonium centers

• Thermal behavior of quaternary ammonium n-dodecylsulfates is characterized.
• Typical layered structure dominates in all examined compounds at room temperature.
• The dimeric didodecylsulfate crystallizes in monoclinic space group P21/m.
• Thermotropic phases formation characteristic for multi-tailed n-dodecylsulfates.
• Multi-charged n-dodecylsulfates are soft crystals at higher temperatures.

Five novel anhydrous catanionic dodecylsulfates containing multi-charged and multi-tailed quaternary ammonium centers were synthesized and examined with light microscopy, differential scanning calorimetry and X-ray powder diffraction. This study is an attempt to explain the relationship between chemical structure, molecular architecture, phase transition characteristics and thermodynamics and the nature of intermolecular interactions of the individual amphiphiles that ultimately lead to different mesomorphic product. All examined compounds are of typical layered structure at room temperature. The long spacing decreases linearly with the increase of either ionic head or n-dodecyl chain number. The thermal analysis of the examined multi-charged catanionics indicates thermotropic mesomorphism, whereas multi-tailed dodecylsulfates show only properties of soft crystals. Maltese crosses, oily streaks textures, stepped drops and fan-shaped textures affirmed the existence of various smectic mesophases at room and higher temperatures. Thermodynamically the most ordered compound is dimeric didodecylsulfate, and the most disordered is three-tailed dodecylsulfate. The addition of the new quaternary ammonium center or alkyl chain causes the increment of the lattice energy first, and it decreases by further changes in the structure. The temperatures of crystallization decrease by any mentioned addition.

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