Morphology of Microphase Separation in Arachidic Acid/Cadmium Arachidate Langmuir-Blodgett Multilayers

Abstract

It is well-known that the fraction of cadmium salt incorporated into arachidic acid Langmuir−Blodgett (LB) films deposited from a dilute CdCl2 subphase increases from 0 to 1 over the pH range of about 4.8 to 6.2. We report a systematic change in the surface morphology of such LB multilayers over this pH range using atomic force microscopy (AFM). At pH ≤ 5.0 (low pH) the surface displays increasing coverage of stripes (ridges), 0.6 ± 0.2 nm above the surrounding area, aligned in the dipping direction. At pH = 5.8 (high pH), the surface is pockmarked with irregular but compact indentations about 1.2 ± 0.3 nm deep (in addition to numerous monolayer and bilayer deep holes). At intermediate pH values, the surface is covered by alternating bands (perpendicular to the dipping direction) of the low- and high-pH textures. These bands represent an example of “substrate-mediated condensation”. When films are soaked in benzene, dissolving away the free acid, ridges 2−2.5 nm high remain in the low-pH films; however, structures 5−6 nm high remain in the high-pH structure. Also, the low-pH structure is easily damaged by AFM scanning, while the high-pH structure is more robust and molecular resolution images are obtained. This implies that, at low pH, correlations are only within the top monolayer; however, at high pH, structures are correlated with those in the neighboring layer.