A Study of the Mechanical Behavior of Surface Monolayers Using Orthogonal Wilhelmy Plates

We have studied the stress-strain behavior of three types of carboxylic acid monolayers on the surface of water using orthogonal Wilhelmy plate measurements. Whether a monolayer supports shear (i.e., whether it is a solid) can be determined from these measurements, which were made with commonly available apparatus used for II-a isotherm determination. We found that pentadecanoic acid monolayers do not support measurable shear at any pressure, while octadecanoic and tetracosanoic acid monolayers support shear at all pressures studied. The changes in the slope of the II-a curves of the latter two materials may be solid-solid transitions or kinetic effects unrelated to phases; stress-relaxation results show the II-a kinks to be the onset of new relaxation regimes.