Phase behaviour of stearic acid-stearonitrile mixtures. A thermodynamic study in bulk and at the air-water interface

Ana Cristina Tavares; Amélia M. P. S. Gonçalves da Silva; Anabela Catarino

Key information

Authors:

Ana Cristina Tavares (Ana Cristina Tavares Teixeira); Amélia M. P. S. Gonçalves da Silva (Amélia Maria Pina Soares Gonçalves da Silva); Anabela Catarino (Anabela Catarino Fernandes)

Published in

November 2006

Abstract

The solidliquid phase behaviour of stearic acid (SA) and stearonitrile (SN) in binary mixtures was investigated by differential scanning calorimetry (DSC), and the formation of SASN mixed monolayers at the airwater interface was followed by surface pressurearea (πA) measurements and by Brewster angle microscope (BAM) observation. The solidliquid phase diagram is a eutectic type phase diagram, with the eutectic composition 0.90 < XSN < 0.95 and Teut = 40.9 ◦C. The DSC results also suggest that the two components are immiscible in the solid phase but form a liquid mixture with positive deviations to the ideal behaviour. At the airwater interface, the two components form liquid condensed monolayers in the entire range of compositions, at low surface pressures, while solid mixed monolayers only form at high surface pressures for XSN < 0.8. Thermodynamic analysis indicates that SA and SN are miscible in the liquid condensed phase, with negative deviations from the ideal behaviour. The variation of the collapse surface pressure of mixed monolayers also indicates miscibility at the airwater interface.