Experimental binary phase diagram of bilayer compounds [n-C _n H_2n+1N(CH₃)₃]₂CoCl₄

Abstract

Compounds [n-C _n H_2n+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 CoCl ²⁻₄ 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 CoCl ²⁻₄ 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.