Binary Mixtures of n-Alkanes. Phase Diagram Generalization: Intermediate Solid Solutions, Rotator Phases

Abstract

At “low temperatures”, the binary mixtures of the consecutive [even-numbered: even-numbered]; [even-numbered: odd-numbered] and [odd-numbered: odd-numbered] n-alkanes (19 < n < 27) do not form a continuous homogeneous solid solution. This study shows the existence of terminal solid solutions which have the pure n-alkanes structures and many intermediate solid solutions, denoted β′ _n and β″ _n (n = 1 or 2): these intermediate phases have an orthorhombic cell as the odd-numbered n-alkanes, but they are not isostructural with the pure n-alkanes nor with their terminal solid solutions. On the basis of powder X-ray diffraction photographs, the phases and β'₁ and β'₂ are indistinguishable in all the binary systems. The X-ray behavior of the phases β″ _n is also identical. The succession of the solid solutions is described when varying the concentration. With increasing temperature, the intermediate phases undergo the same solid-solid transitions as the odd-numbered n-alkanes (n-tricosane), and the rotator phases appear at “high temperatures” below the solidus line. The phase diagrams of n-C₂₂: n-C₂₃ n-C₂₃: n-C₂₄ n-C₂₂: n-C₂₄ and n-C₂₄: n-C₂₆ are presented.

Key Words:

• Alkanes
• binary mixtures
• phase diagrams
• phase transitions
• solid phase sequence rules
• rotator