Monolayers of stearic acid esters at the air-water interface: two-dimensional phases and miscibility

Monolayers of three stearic acid esters, methyl-stearate, propyl-stearate and butyl-stearate, were studied at the air-water interface in the 15–35 °C temperature range.To investigate the surface phases of these esters spread at the air-water interface, some state equations were fitted with the experimental data taken from the isotherms.Surface potential measurements were carried out to obtain information on the molecular orientation. The interfacial orientation and distribution was discussed in relationship to the surface phases present.Ellipsometric measurements were made to determine the thicknesses.The two-dimensional miscibility for the mixture methyl-stearate/butyl-stearate was also studied: surface free energies, enthalpies, and entropies of mixing were computed. The results obtained confirmed previous deductions about the role of the hydrophobic chains in determining the two-dimensional miscibility when they have the same interfacial orientation.