Heat capacity and corresponding states in alkan-1-ol–n-alkane systems

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Abstract
The apparent molar heat capacities, ΦC, and excess heat capacities, CpE, have been obtained at 25 °C for the following alkan-1-ols, Cm H2m+1 OH at low mole fraction, x1 < 0.1, in n-alkanes, Cn H2n+2, (m,n): (1,6), (1,7), (2,10), (4,10), (6,6), (6,9), (6,10), (6,12), (6,16), (7,10), (10,6), (10,10), (10,16), (11,10), (12,10), (14,6), (16,6) and (16,16). The Treszczanowicz–Kehiaian (TK) association theory was applied to the associative part of the apparent heat capacity, ΦC(assoc), obtaining ΔH° for hydrogen-bond formation and the volume-fraction equilibrium constant, K4 Φ, for formation of tetramers which are the dominant multimers. K4 Φ are related to K4, an equilibrium constant for hydrogen-bond formation in tetramers, which is found to be independent of alkan-1-ol chain length from m= 4 to 16, while K4 for m= 2 and 1 are larger. A common K4 implies a single corresponding states curve (CSC) of ΦC(assoc) either against ψ1, the number of moles of alcohol per mole of segments in the solution, as introduced by Pouchly, or against molarity. The CSC is obeyed by the dilute solution data, with some deviation for methanol and ethanol systems, and also by ΦC(assoc) measured throughout the whole concentration range at 25 °C for m= 6 with n= 7, 8, 12 and 16 and for m= 1 (high and low concentration only), 4, 6 and 10 with n= 7 and m= 2, 4, 6 and 10 with n= 12 (again with m= 2 as exception). The CSC in reduced form allows the determination of the size of the dominant multimer from experimental ΦC data and should be followed by alkan-1-ol–inert solvent data at any temperature. CpE for constant m increases with n, and this is a consequence of the adherence to a CSC as well as being predicted by the TK theory. Together with literature results for other systems it is found that for n= constant, CpE increases as m decreases, reaching a maximum at m= 3, then decreasing again for m= 2 and 1. The behaviour of m= 10–3 is a consequence of the CSC, whereas m= 2 and 1 represent deviations from the CSC.

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