Structure of the n-alkane binary solid n-C₁₉H₄₀/n-C₂₁H₄₄ by infrared spectroscopy and calorimetry

The n-alkane binary solid n-C₁₉/n-C₂₁ has been studied by differential scanning calorimetry (DSC) and infrared spectroscopy. Measurements on the low-temperature ordered solid (phase I) and the high-temperature disordered solid (phase II) have been made as a function of temperature and composition. From the DSC measurements, the phase diagram has been determined as well as the transition enthalpies for the phase I to phase II transition and the phase II to liquid transition. A plot of the excess enthalpy of mixing vs. composition is symmetric for both solid phases. For the 1:1 mixture, the excess enthalpies are 2.4 ± 0.2 and 1.5 ± 0.1 kcal/mol for phase I (275 K) and II (306 K), respectively. The infrared spectra of C₁₉/C₂₁ mixtures incorporating one n-alkane that is selectively deuterated, n-C₁₉H₃₈D₂^(2,2) or n-C₂₁H₄₂D₂^(2,2), have been measured, and, from these spectra, the concentrations of gauche bonds at the chain ends of both components have been determined. Phase I of the C₁₉/C₂₁ mixtures consists primarily of all-trans chains with some end-gauche C₂₁. In general, the concentration of gauche bonds, which are mostly at the chain ends, is higher for the mixture than for the neat components. The concentration of end-gauche C₂₁ is highly dependent on concentration, going to 100% at infinite C₂₁ dilution. The concentration dependence of end-gauche C₂₁ can be accounted for in terms of a model based on nearest-neighbor chain interaction. The conformational disorder in phase II mixtures appears to be separable into two types: (i) One type of conformational disorder is associated with the lamellar surface; this end-chain disorder arises from the chain-length mismatch between the two n-alkane components, and therefore its concentration is highly dependent on the composition of the mixture. (ii) The second type of conformational disorder is that inherent to phase II; therefore, the concentration of this type of disorder is nearly independent of composition.