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

The solid-liquid phase behaviour of stearic acid (SA) and stearonitrile (SN) in binary mixtures was investigated by differential scanning calorimetry (DSC), and the formation of SA-SN mixed monolayers at the air-water interface was followed by surface pressure-area (π-A) measurements and by Brewster angle microscope (BAM) observation. The solid-liquid phase diagram is a eutectic type phase diagram, with the eutectic composition 0.90 < X_SN < 0.95 and T^eut = 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 air-water 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 X_SN < 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 air-water interface.