Equilibrium spreading pressure and Langmuir-Blodgett film formation of omega-substituted palmitic acids

Langmuir-Blodgett isotherms and equilibrium spreading pressures were measured for compounds of the series X-(CH2)15COOH, X = CH3, SH, OH, F, Cl, Br. Only the CH3 and F terminated compounds formed monolayers with sufficient stability for accurate isotherm measurement, film transfer and X-ray photoelectron spectroscopic analysis. The presence of the terminal heteroatom substituents significantly diminished the stability of the L-B film and depressed the equilibrium spreading pressures (20 °C) from 15.4 mN/m for the CH3 terminated compound to a range of 0.95 to 0.08 mN/m for the other members of the series. These characteristics are attributed to the monolayer film being in a metastable state and the dipole moment of the heteroatom terminal group increasing the monolayer film kinetic instability by facilitating the formation of three-dimensional structures.