Mixed crystals of n-alkane pairs A global view of the thermodynamic melting properties

Abstract

The excess thermodynamic melting properties of the mixed crystalline rotator form of binary mixtures of neighbours and nextnearest neighbours of n-alkanes in the range to are characterized by a uniform temperature of enthalpy/entropy C 11 H 24 C 26 H 54 compensation of 320 K. Excess enthalpy di †erences have been obtained by microcalorimetry and excess Gibbs energy di †erences by phase diagram analysis and the two properties were combined to give the excess entropy di †erences. The excess enthalpy and excess entropy di †erences are negative and their absolute values, which are virtually equal to the excess properties of the rotator form, increase with increasing relative mismatch between the crystallographic unit cells of the two components involved.

Phase‐Transition Regularities in Critical Constants, Fusion Temperatures and Enthalpies of Chemically Similar Chainlike Structures

“…2, the crystal structure remains the "same", though different solid rotator phases are stable just preceding melting. [16] The different crystal structures, with different rotator phases, evidently relieve different degrees of "molecular freedom" prior to melting, so that rather different entropies of fusion and consequent melting points arise (Figure 2).…”

Melting behaviour in the n-alkanol family. Enthalpy–entropy compensation

“…In fundamental terms, the n-alkanols family offers the opportunity of correlation of their properties with those of other families of mixed crystals, such as the n-alkanes. 1,2 Research into families of mixed crystals rather than isolated systems has a number of advantages. It allows correlation of results with one or more exo-thermodynamic parameters.…”

Isothermal Transformations in Alkane Alloys

“…In the present case is a rotator form, for which the excess Gibbs energy is already known. 16,17 The excess Gibbs energy of R, which is one of the Φ ord , is subsequently calculated from ∆ R G E and G E . Finally, the individual G ER and G E are used, along with the transition data of the components A and B, to calculate the equilibrium phase diagram, that is to say, the upper solvus and lower one.…”

“…16,17 For the family of binary mixed crystals of which the components are n-alkanes and the form RI, the parameter θ has a common, systemindependent value of 320 K. The parameter A, on the other hand, is system-dependent, and such that its value is related to the relative difference in chain length between the molecules of the two components of thesystem Cn + Cn′. The relationship is 19 where ∆n ) |(n′ -n)| and n j ) (1/2)(n + n′) With eq 10, and putting, in eq 9, T ) 290 K (i.e., the mean temperature of the transition range), and, moreover, taking for the parameter B the ratio of the two numerical values in eq 6 (i.e., B ) 6/23), eq 7 is obtained.…”