Study of mixed Langmuir-Blodgett films of hydrocarbon and fluorocarbon amphiphilic compounds by scanning surface potential microscopy and friction force microscopy

The phase separation and orientation of molecules in mixed Langmuir-Blodget t (LB) films of hydrocarbon (HC) and fluorocarbon (FC) amphiphilic compound s were studied using scanning surface potential microscopy (SSPM) and frict ion force microscopy (FEM). The LB films were formed with a monolayer on an aqueous subphase containing calcium cations as the counter ions for the co mmon carboxylate anions of the two types of amphiphilic compounds. It was f ound that the structure of the phase-separated LB films was different from that of previously studied HC-FC mixed LB films complexed with polymer cati ons. Namely, the structure of the present LB films was a "side-by-side" str ucture, while that of the previous ones was an ''on-top'' structure. In add ition, it was found from SSPM that differences in surface potentials betwee n the two separated phases were small for all the mixed LB films prepared w ith various HC/FC molar ratios. In other words, compositions were similar b etween these two phases: the rich component of one phase was at a slightly higher concentration than that of the other phase. The similarity in the co mpositions explained the low contrast in friction between these two phases as observed by FFM. (C) 2000 Elsevier Science B.V. All rights reserved.