Mesophases of Alkylammonium Salts. V. Heat Capacities of Symmetric Halides with Alkyl Chains Longer than Ethyl

Abstract

The heat capacities of tetra-n-alkylammonium bromides and iodides in the solid state have been measured from 130 K through their respective transitions and fitted at low temperatures to two θ temperatures, using group vibrations computed from the vibrational spectra and the Tarasov analysis, as developed for linear macromolecules. The compounds analyzed were (C₃H₇)₄bNBr, (C₄H₉)₄NBr, (C₅H₁₁)₄NBr, (C₆H₁₃)₄NBr, (C₇H₁₅)₄NBr, (C₈H₁₇)₄NBr, (C₁₀H₂₁)₄NBr, (C₁₂H₂₅)₄NBr, (C₁₆H₃₃)₄NBr, (C₁₈H₃₇)₄NBr, (C₃H₇)₄NI, (C₄H₉)₄NI, (C₅H₁₁)₄NI, (C₆H₁₃)₄NI, (C₇H₁₅)₄NI, and (C₁₂H₂₅)₄NI. The total vibrational contribution to the heat capacity is thus derived and compared to the experimentally measured heat capacity over the whole temperature range. Positive deviations of the measured heat capacity from the calculation occur already below room temperature, implying the existence of large-amplitude, nonvibrational motion in the solid state, as suggested before by solid-state ¹³C NMR analysis. Quantitative information on the entropy of disordering generated outside of the transition ranges is derived and agrees with the entropy deficits predicted by the analysis of the first-order transitions. An addition scheme is developed for the heat capacity of the methylene groups in these compounds, that is similar to the paraffins.

Key Words:

• tetra-n-alkylammonium bromides and iodides
• disorder and motion
• mesophases
• thermal analysis
• heat capacity
• entropy