Melting behaviour of molecular mixed crystalline materials: measurement with adiabatic calorimetry and modelling usingultracal

Abstract

The melting behaviour of binary mixed crystalline materials was measured with adiabatic calorimetry. The experimental results (heat capacity against temperature graphs) are explained in terms of a model that is based on the assumption that the inhomogeneity of the materials can be described by a Gaussian distribution function. Although this model, called ULTRACAL, has only one adjustable parameter, it is capable of reproducing the experimental data with a good degree of accuracy, as demonstrated for various samples of (1,4-dichlorobenzene + 1,4-dibromobenzene). ULTRACAL thus provides a link between the adiabatic calorimetry data and the solid-liquid Tx phase diagram. It is shown that the model can be used for the determination of phase diagrams. By using ULTRACAL to assess the heat capacity against temperature graphs of five samples, the solid-liquid Tx phase diagram of (1,3,5-trichlorobenzene + 1,3,5-tribromobenzene) was determined.

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

“…The nature of these forms depends on chain length. In C 15 OH and C 16 OH the stable form before melting is R 0 II . [4][5][6] For C 17 OH, C 18 OH, C 19 OH and C 20 OH the stable form before melting is the monoclinic R 0 IV .…”

“…The mean difference between the experimental temperatures and the ones calculated, taken as abs (T obs À T calc ), is 0.4 K. The value of 0.4 K is representative of the precision that is realizable by differential thermal analysis. 16 For the individual systems abs (T obs À T calc ) ranges from 0.11 (C 18 OH-C 20 OH) to 0.9 (C 15 OH-C 17 OH).…”

“…This paper reports the melting behaviour of ten binary systems, C 15 OH-C 16 20 OH with Dn ¼ 4, the majority of which have not been published before. For these systems, we also crystallographically characterized the rotator forms that melt: R 0 II (R3 ¯m, Z ¼ 6) and R 0 IV (C2/m, Z ¼ 4), and determined the thermodynamic mixing properties, excess enthalpy and excess entropy.…”

Modelling Phase Equilibria in Systems with Organic Solid Solutions

“…Reports on the formation of alloys, or solid solutions, of organic compounds, although still rare, have been appearing regularly in the last few years for aromatic compounds, such as substituted benzenes and naphtalenes [1][2][3], pyrene and anthracene [4][5][6], for carbohydrates [7], terpenes [8], neopentane derivatives [9-10] and fulerenes, besides solid solutions of C 60 with C 70 [11][12] solid solutions of C 60 in sulphur have been reported [13]. Nevertheless it is among the molecules with long n-alkyl chains such as n-alcohols Although the materials based on n-alkane alloys have been used for long as candles, waterproof coatings, pharmaceutical and cosmetics, and in spite of paraffin waxes being a valuable by-product of petroleum refining, the research on n-alkane solid solutions has been mainly prompted by its dark side: The damages and losses caused by wax precipitation from petroleum fluids.…”

“…Reports on the formation of alloys, or solid solutions, of organic compounds, although still rare, have been appearing regularly in the last few years for aromatic compounds, such as substituted benzenes and naphtalenes [1][2][3], pyrene and anthracene [4][5][6], for carbohydrates [7], terpenes [8], neopentane derivatives [9][10] and fulerenes, besides solid solutions of C 60 with C 70 [11][12] solid solutions of C 60 in sulphur have been reported [13]. Nevertheless it is among the molecules with long n-alkyl chains such as n-alcohols [14], fats [15], soaps [16] and alkanes [17][18][19][20] that solid solutions appear more frequently.…”