Enthalpy Variation of the Nine Solid Phases of the Binary Molecular Alloys (n-Tricosane:n-Pentacosane) vs. Temperature

The enthalpy variations of pure n-tricosane (n-C₂₃H₄₈), pure n-pentacosane (n-C₂₅H₅₂) and sixteen binary mixtures were determined from 282 to 360 K. The differential enthalpy analyses were carried out on the pure components on the four terminal solid solutions, denoted β_o(C₂₃), β′_o(C₂₃), β′_o(C₂₅), β_o(C₂₅) and on the three intermediate phases, called β″₁, β′₁, β″₂of the binary system (C₂₃:C₂₅) using a calorimeter of the Tian Calvet type. These variations can be represented by an analytical expression, which is derived from Einstein‘s model. The two Rotator phases β-RI and α-RII were also studied. This revised version was published online in July 2006 with corrections to the Cover Date.     

Structural evolutions of the n-heneicosane and n-tricosane molecular alloys at 293 K

A structural study of odd-numbered n-alkane (C_n) binary mixtures (C₂₁ : C₂₃) was carried out on powder samples using a Guinier-de Wolff camera with increasing concentration of n-C₂₃ at 293 K.

Despite the reports in the literature, these molecular alloys do not form an orthorhombic continuous homogeneous solid solution to C₂₁ from C₂₃ at “low temperature”. Instead, as already observed in two even-numbered C_n systems, X-ray diffraction results show the existence of seven solid solutions as the molar concentration of C₂₃ increases: four terminal solid solutions, denoted β₀(C₂₁)β₀(C₂₃), isostructural with the “low temperature” phase of pure C₂₁ and C₂₃ (Pbcm), β′₀(C₂₁) and β′₀(C₂₃), identical to the phase β′₀ which appears in pure C₂₃ above the δ transition, and three orthorhombic intermediate solid solutions, designated β″₁, β′₁ and β″₂.

On the basis of powder X-ray photographs, the phases β″₁ and β″₂ (C₂₁ : C₂₃) are indistinguishable, and they are isostructural with the intermediate solid solution β″ of the even-numbered C_n binary systems (C₂₂ : C₂₄) and (C₂₄ : C₂₆). The phase β′₁(C₂₁ : C₂₃) is also isostructural with the two indistinguishable intermediate solid solutions β′₁ and β′₂ of the molecular alloys (C₂₂ : C₂₄) and (₂₄ : C₂₆).

From this study and our other laboratory results, the sequences of appearance of the solid solutions and the structural identities between these phases are established at “low temperature” for all the binary molecular alloys of consecutive C_n (odd-odd, even-even or odd-even: 19 < n < 27) when increasing the solute concentration.