Journal of Physical Chemistry, Vol.100, No.26, 11113-11119, 1996
Morphology of Microphase Separation in Arachidic Acid/Cadmium Arachidate Langmuir-Blodgett Multilayers
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 less than or equal to 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.
Keywords:SCANNING FORCE MICROSCOPY;AIR-WATER-INTERFACE;X-RAY-DIFFRACTION;PHYSICAL-CHEMISTRY;CADMIUM ARACHIDATE;LIPID MONOLAYERS;FATTY-ACIDS;FTIR-ATR;FILMS;SURFACE