Two-dimensional Thermodynamic Properties of Sterols.

Abstract

5α-cholestan-3β-ol (II) is similar to 5-cholesten-3β-ol (cholesterol) (I) except the absence of the Δ5 double bond, whereas 5β-cholestan-3α-ol (III) is remarkably different from I in its skeletal structure and in the direction of its OH moiety. Two-dimensional thermodynamic properties such as surface pressure-molecular area (π-A) relationship, equilibrium spreading pressure (ESP) and enthalpy of compression of monolayer at air-water interface were studied for these sterols. II shows similar π-A relationships to I, whereas III gives the different ones. ESP value, increasing with increasing temperature, followed the order of II>I>III. Enthalpy of elution, ΔHESP, from the crystals onto water surface at ESP and also enthalpy of compression, ΔHcomp, of the monolayers from π=0 to ESP were obtained from the temperature dependence of ESP and of π-A relationship, respectively. The sum of ΔHESP and −ΔHcomp, which means the two-dimensional enthalpy of fusion, is approximately proportional to the ordinary enthalpy of fusion. II and III molecules gave a 1:1 compound in the mixed monolayer on water surface as well as in solid state. The ESP value for the 1:1 compound was also determined.