Dissolution States of Normal Alkane Derivatives with a Polar Group in Water

Abstract

Abstract This paper proposes a method by means of which the heat of solution at an infinite dilution of an organic compound in water, directly measured by means of calorimetry at a given temperature, is extended over a wide range of temperatures by the use of a phase diagram with water. The heat of solution at an infinite dilution in water over a temperature range from 290 to 350 K, which is estimated by this method for some alkanols and alkanoic acids, increases with a rise in the temperature and has a transition temperature from the exothermic region to the endothermic region which is close to the minimum dissolution temperature in the solubility curve in water for each substance. The increase in the heat of solution might be due to the breakdown of the iceberg formed around the alkyl group. The difference between the partial molar heat capacity at an infinite dilution and the molar heat capacity at the pure state, ΔScp\overset\ominus2, has its maximum at a certain temperature. The higher homolog shows a sharper curve with a lower maximum temperature. The curve of ΔScp\overset\ominus2vs. the temperature for 2- or 3-alkanol corresponds to that for the lower 1-alkanol. These facts reveal that a polar group, such as OH and COOH, would preferentially break the weaker structure of the iceberg within its breaking sphere, which structure should break down at a lower temperature.