Maximum withdrawal speed for Langmuir–Blodgett film deposition of arachidic acid

The maximum withdrawal speed of Langmuir–Blodgett (LB) film deposition of arachidic acid (AA) was investigated. The quality of LB deposited film was determined by the transfer ratio (TR), together with measurements of surface roughness using atomic force microscopy (AFM). A Langmuir mini-trough was used to provide the surface pressure versus molecular area (π–Aπ–A) curves and a flow visualization technique was applied to estimate the dynamic contact angles and to observe the fluid motion. The effects of hydrophobic and hydrophilic substrates, pH and the addition of four different ions, i.e., K+, Ba2+, Cd2+, and Al3+, on the withdrawal speed were examined. The “transition point” from liquid to solid states on the π–Aπ–A curve provided a clear indication of the maximum withdrawal speed. The lower the transition point, the higher was the maximum withdrawal speed. Stable deposition was possible only if the pH of the solution was maintained in a narrow range. The observation of dynamic contact angles and fluid motion, particularly the movement of air–liquid interface, was consistent with previous findings. Owing to the “soap effect” of the divalent ions Ba2+ and Cd2+, the maximum speed for successful LB film deposition without significant water entrainment could be extended substantially with the addition of divalent ions.

Graphical abstractAFM images of LB film deposition for the AA/Ba system, the maximum coating speed V=5 mm/minV=5 mm/min. Ten layers of molecules were coated on the substrate.Figure optionsDownload full-size imageDownload high-quality image (51 K)Download as PowerPoint slide