Excess heat capacities of mixtures of two alcohols

Abstract

The molar excess heat capacity (C^E_p) has been obtained through the concentration range at 25 °C for mixtures of alkan-1-ols: methanol with butanol, hexanol and decanol and decanol with butanol and hexanol. C^E_p is negative and small, increasing in magnitude with difference in chain length of the alkan-1-ols to a maximum of –3 J mol^–1 K^–1 for methanol–decanol. C^E_p has also been measured at 25 °C for methanol and other alkanols mixed with iso-, sec- and tert-butyl alcohol (I), 2-methylbutan-2-ol (II), 3-methylpentan-3-ol (III) and 3-ethyl-pentan-3-ol (IV). With increasing steric crowding of the tertiary OH, C^E_p becomes extremely large, –18 J mol^–1 K^–1 for methanol–IV at equimolar concentration, the apparent molar C_p of methanol in IV being negative at low concentration. The negative C^E_p(x) for methanol–I is of normal positive curvature, but with II, C^E_p has a W-shaped concentration dependence exhibiting two regions of positive C^E_p(x) curvature separated by a region of negative curvature. An extension of the Treszczanowicz–Kehiaian association model has been made to alcohol–alcohol mixtures with consideration of multimers mainly confined to linear tetramers. Assuming equal enthalpies and equilibrium constants for H-bonding between like alkan-1-ols (AA and BB) and between unlike alkanols (AB) leads to positive C^E_p predictions. The experimental negative C^E_p values are associated with a lowering of the equilibrium constant for H-bond formation in the alkan-1-ol with increasing chain length coupled with relatively stronger AB bonding. The equilibrium constant, K_BB, for H-bonding between tertiary alcohol hydroxyls is found to be lowered by steric crowding of the tertiary hydroxyl, raising the C_p of the pure liquid. The constant, K_AB, for bonding between primary (A) and tertiary (B) hydroxyls in the multimer chain is less affected by the tertiary crowding resulting in lower solution C_p and large negative C^E_p. The S- and W-shape concentration dependences require modifications of K_AB owing to the proximity in the tetramer chain of either A or B alcohols resulting in an effective concentration dependence of K_AB.