Experimental binary phase diagram of bilayer compounds [n-C n H2n+1N(CH3)3]2CoCl4

Abstract

Compounds [n-CnH2n+1N(CH3)3]2CoCl4(n=16, C16C3Co; n=18, C18C3Co) containing lipid-like bilayers embedded in a crystalline matrix exist in solid-solid phase transition. The low-temperature bilayer structures of the two compounds were organized by neutralizing CoCl42− with alkylammonium ions. Alkyl chains lay parallel to each other and slightly tilted with respect to the normal of the inorganic layers. The adjacent alkyl chains interacted with each other by van der Waals interaction. When the temperature increased, the two compounds underwent a reversible solid-solid phase transformation within 310–330 K. In such a case, the chains showed a large motional freedom, and a disordered phase appeared. The structures can alternatively be viewed as a double layer of alkylammonium ions between CoCl42− sheets and be considered as crystalline models of lipid bilayers. The experimental subsolidus binary phase diagram of [n-C16H33N(CH3)3]2CoCl4-[n-C18H37N(CH3)3]2CoCl4 was constructed over the entire composition range by differential scanning calorimetry and X-ray diffraction technique. Experimental phase diagram indicates one stable intermediate phase [n-C16H33N(CH3)3][n-C18H37N(CH3)3]CoCl4 at \(w_{C_{16} C_3 Co} = 39.89\%\) and two invariant three-phase equilibria, which shows two eutectoid temperature e1 at (316±1) K for \(w_{C_{16} C_3 Co} = 27.35\%\) and e2 at (313±1) K for \(w_{C_{16} C_3 Co} = 59.76\%\). These three noticeable solid-solution ranges are α-phase at the left, β-phase at the right, and γ-phase in the middle of the phase diagram.