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.

The influence of lead (II) ions introduced into the subphase on the stability of mixed "polyamic acid + surfactant" monolayers and manufacturing of dye-containing Langmuir-Blodgett polymeric films

“…5,32 Besides, in addition to the alterations in the state of PA carboxylic groups in acid region the (partial) release of Pb 2+ ions can occur, analogous to the well-known effect of decrease of the fractions of doubly charged metal cations from the Langmuir monolayers of fatty acids obtained at relatively low pH. 19 Thus, the results of preliminary measurements with indicators do not contradict the knowledge about the state of PA carboxylic groups. To obtain further, more definite results with indicators of various types it is necessary to take into account the (possible) ion association of anionic dye species with Pb 2+ and ODP + cations as well as of cationic ones with COO -groups of PA.…”

“…After 5 min soaking in aqueous buffer solutions with pH = 1.68 and 9.18, the C Pb /C N ratio decreases to 0.036 and 0.127, correspondingly. The probable reason is protonation of COO -groups in acidic region, leading, as usual, [17][18][19][20][21] to decay of their complex with metal ions, and hydroxo complexes formation at high pHs (see above).…”

“…[16][17][18] The phenomenon was re-examined and studied in details within the past decade. [17][18][19][20][21] The structure of a Langmuir -Blodgett film, transferred onto the substrate, depends on the quality of the monolayer on the interface and conditions of multilayer structures 657 The Influence of Lead (II) Ions Introduced into the Subphase Vol. 17, No.…”

Polymer−Surfactant Layered Heterostructures by Electropolymerization of Phenosafranine in Langmuir−Blodgett Films

“…Thus complexation with the PS dye gives stability to the AA monolayer as rendered on complexation with metal ions. 42,47 LB Film Formation. The monolayers of AA-PS complex were transferred from the air-water interface to glass substrates at a surface pressure of 35 mN/m.…”

“…42,47 During deposition of cadmium arachidate on mica at a pH 5.5, circular holes with 10-100 nm diameter were reported. However, upon increasing the pH, the number density of holes decreased and a uniform film was obtained at pH 6.5 as revealed from AFM measurements.…”