Adhesional stability of cadmium arachidate Langmuir-Blodgett layers

Two-layer Langmuir-Blodgett films of cadmium arachidate were deposited onto hydrophobic glass plates. The adhesional stability of the films has been studied by wetting tension measurements, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). Partial detachment of the Langmuir-Blodgett layer while immersing the plate repeatedly into water or aqueous electrolyte solutions was deduced from the increase in the wettability and the decrease in the surface tension of the immersion liquid. Surface coverages obtained from XPS data and shown by AFM images are in agreement with the expectation based on tensiometric measurements of arachidate release from the substrate. AFM investigations revealed a distinct difference in the structure of the layer depending on whether water or an aqueous solution of NaCl and CaCl₂ was applied in the stability test, providing evidence for the dissolution or the detachment type of the removal of the Langmuir-Blodgett film. The contact of a sessile droplet with the Langmuir-Blodgett film induced no detectable change in the contact angle or the surface tension of the liquid, demonstrating the role of a moving contact line in the degradation of the Langmuir-Blodgett film.