Surface Tension Anisotropy and Relaxation in Uniaxially Compressed Langmuir Monolayers

We have performed capillary wave measurements, both parallel and perpendicular to the direction of compression, on uniaxially compressed Langmuir films. We observed surface tension anisotropy only in the A and B (CS and L₂″) phases of heneicosanoic acid. These are also the phases whose reported X-ray diffraction peak widths are resolution limited. When surface tension anisotropy developed, it decayed only slightly over 30 min. Surface pressure measurements are ambiguous for phases in which the surface tension is anisotropic. At room temperature, where uniaxially compressed acid monolayers are isotropic, calcium ions caused anisotropy only at the highest pH values (≈11), even though the films are known to be stiff at moderate pH values (≈6). We observed a large change in surface tension anisotropy with a small change in pH, signaling a phase transition.