Subsolidus binary phase diagram of the bilayer compounds [n-C n H2n+1N(CH3)3]2ZnCl4 (n = 16, 18)

Abstract

The bilayer compounds [n-C n H2n+1N(CH3)3]2ZnCl4 (n = 16, 18) experience solid-solid phase transition within the temperature range of 310 to 340 K. The low-temperature crystal structures of the pure compounds are characteristic of the piling effect in which a 2D macro-anion ZnCl4 2− is sandwiched between two alkylammonium layers. These layers become conformationally disordered in the high-temperature phases. The structures can alternatively be viewed as a double layer of alkylammonium ions between ZnCl4 2− sheets and can be considered as crystalline models of lipid bilayers. The experimental subsolidus binary phase diagram of [n-C16H33N(CH3)3]2ZnCl4-[n-C18H37N(CH3)3]2ZnCl4 has also been established over the whole composition range by differential scanning calorimetry (DSC) and X-ray diffraction. In the phase diagram, one intermediate compound [n-C16H33N(CH3)3][n-C18H37N(CH3)3]ZnCl4 at $$W_{C_{16} C_3 Zn} \%$$ 47.50 and two eutectoid invariants points at $$W_{C_{16} C_3 Zn} \%$$ 35.10 and 75.70 were observed; the respective temperatures of the two eutectoids are 320 ± 1 and 315 ± 1 K. In addition, there are three noticeable solid solution ranges in the phase diagram: α-phase at the left, β-phase at the right, and γ-phase in the middle.