AXISYMMETRICAL 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. Result s suggest that collapse pressure is not uniquely dependent on the comp ression rate; it depends on the amount of surfactant spread on the dro p surface. It was also found that the shape of the surface pressure-ar ea (pi-A) isotherms changes with the rate of molecular area compressio n; increasing the compression rate shifts the lower part of the isothe rm to the right. It was also found that the values of the molecular ar ea at collapse are more reproducible and consistent than the conventio nal extrapolation of the limiting molecular area; the former was found to be 19.6 +/- 0.1 Angstrom(2)/molecule, independent of both the amou nt of surfactant and motor speed, whereas the latter was found to be d ependent on both the amount of surfactant and motor speed. All of thes e findings were explained by a morphological study of an octadecanol m onolayer using Brewster angle microscopy.