Partial molar enthalpies of solution as indicators of interactions in mixtures of 2-butoxyethanol and 2-butanol with water

Calorimetric measurements have been made of differential enthalpies of solution of both components in the binary system 2-butoxyethanol-water and of 2-butanol in the system 2-butanol-water as a function of composition at three different temperatures. The heat capacity changes for dissolution were calculated from the temperature variation of the solution enthalpies. Drastic changes of the solution properties are seen with increasing solute concentration in water-rich solutions. In the 2-butoxyethanol-water system, which could be studied over the whole composition range, four different regions can be identified. At extreme dilution in water, the solute is fully hydrated with a primary hydration layer of monolayer thickness involved in long-range secondary hydration. In dilute solutions the primary hydration layer is unchanged but the secondary hydration diminishes with increasing solute concentration. In semi-dilute solution the primary hydration layer breaks down and the particular hydrophobic characteristics of hydrocarbon groups in aqueous solution disappear. At higher solute content the mixtures show no hydrophobic character but the behavior of regular mixtures of polar solutes.