Axisymmetric drop shape analysis as a film balance: Rate dependence of the collapse pressure and molecular area at close packing of 1-octadecanol monolayers

The rate dependence of collapse pressure was studied systematically by axisymmetric drop shape analysis for an octadecanol monolayer. Results suggest that collapse pressure is not uniquely dependent on the compression rate; it depends on the amount of surfactant spread on the drop surface. It was also found that the shape of the surface pressure-area (pi-A) isotherms changes with the rate of molecular area compression; increasing the compression rate shifts the lower part of the isotherm to the right. It was also found that the values of the molecular area at collapse are more reproducible and consistent than the conventional extrapolation of the limiting molecular area; the former was found to be 19.6 +/- 0.1 Angstrom(2)/molecule, independent of both the amount of surfactant and motor speed, whereas the latter was found to be dependent on both the amount of surfactant and motor speed. All of these findings were explained by a morphological study of an octadecanol monolayer using Brewster angle microscopy.