HEAT CAPACITIES OF ORGANIC COMPOUNDS IN SOLUTION .I. LOW MOLECULAR WEIGHT ALCOHOLS IN WATER

The significance of heat capacities of solvation to physicochemical processes in aqueous solution is discussed and the dearth of reliable values is documented. Heat capacities of solution (∆Cp2°) are reported herein for 21 low molecular weight alcohols from a pure liquid standard state to high dilution in water at 25°. The integral heat method of Cobble and Criss is used wherein the partial molar heat of solution is measured as a function of temperature, each ∆Cp2° value being based on 40 to 70 heat measurements over a 25° range. ∆Cp2° values are large, positive, and dependent on the size of the alcohol molecule since within a series they correlate with carbon number or molar volume. Branching, unsaturation, and ring closure reduce ∆Cp2°, but branching has the reverse effect if a gaseous standard state is used. The relatively large magnitude of ∆Cp2° is even larger when r ferred to a gaseous standard state, and it is proposed that the principal contribution is the increased order developed in the water adjacent to the hydrocarbon chain. A close correlation of the heat capacity and entropy of transfer from the vapor phase to water supports this hypothesis. © 1969, American Chemical Society. All rights reserved.