Lateral force microscopy study of Langmuir–Blodgett films of a macrocyclic compound

Langmuir–Blodgett films of 4-phenyl-4-sulfide-11-(1-oxodecyl)-1,7-dithia-11-aza-4-phosphacyclotetradecane, a thiomacrocyclic compound used as a Cu(II) ions sensor, were extracted over mica at several surface pressure values from two subphases: pure water and a 0.01 M Cu(II) aqueous solution. Atomic Force Microscopy and Force Spectroscopy (Lateral Force Microscopy) were used to study both the morphology and the nanomechanical response of Langmuir–Blodgett films. A correlation between extraction pressure and monolayer mechanical properties was observed, so an increase in the extraction pressure of the monolayers corresponds with an increase in the vertical force at which the monolayer breaks while doing lateral force experiments. Experimental data proves that Langmuir–Blodgett extraction technique truly obtains monolayers with different nanotribological properties as a function of the extraction surface pressure. The formation of islands on top of the monolayers was studied and a model mechanism of formation is proposed. A higher friction value was measured on the islands than on the monolayer and friction asymmetry was observed in the latest stage of island formation.