Enthalpic Behaviour of Sodium Deoxycholate in Pure Water and in Water‐Urea Solutions at 25°C

The heats of dilution and apparent molal heat contents of sodium deoxycholate have been measured in water and in 2.5 molal urea aqueous solution at 25°C by microcalorimetry. The critical micellar region of the bile salt in water‐urea, has been identified by conductometric methods. The solute concentrations ranged from zero to about 0.4 mol · 1−1. A strict analogy has been found between the binary system, deoxycholate‐water, and the ternary system, deoxycholate‐urea‐water, except for a general shift of the critical phenomena toward the higher concentrations observed for the solutions containing urea.—Below the critical region, the enthalpic behaviour of deoxycholate conforms to the features expected for 1:1 organic electrolytes. Approaching the critical concentration, additional enthalpic effects, parallel to the dilution ones, influence the calorimetric trend of the investigated bile salt. These have been correlated with processes of molecular self‐association of the solute, similar to the micellization phenomena occurring for other surfactants.—Above the first critical concentration range, the concentration dependence of the relative molal heat content of deoxycholate, both in water and in water‐urea, is very peculiar and can be assumed as a confirmation of a multistage aggregation of the solute.