Formation of stable singularities in mixed monolayers of porphyrins and tetracosanoic acid upon SFM tapping

Langmuir films of tetracosanoic acid have been transferred at 10 mN/m from water to mica surfaces and were characterized by scanning force microscopy (SFM) in the tapping mode at first as a smooth monolayer. Then, upon repeat ed tapping cycles, many singularities appeared in form of 2.5 nm high pairs of peaks in a plane, which stretched only 1.8 nm above the mica subphase. These peaks are attributed to islands of upright-standing molecules in a la yer of molecules tilted at an angle of 35 degrees. Transferring the films t hus results first in a nonequilibrated film on mica, which undergoes relaxa tion upon tapping to a nonhomogeneous equilibrium phase. The same phenomeno n was observed in films made of rigid porphyrin and tetracosanoic acid doma ins at 20 mN/m. The formation of fluid fatty acid structures at pressures w here strong ordering prevails in pure fatty acid films was related to a reo rientation in the rigid porphyrin domains after the transfer to mica. SFM p hase shift images were applied to different hard and soft parts of the mixe d monolayer, and scanning near-field optical microscopy was used to confirm the assignment of the porphyrin domains on the basis of their fluorescence .