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–Blodgett (LB) films of hydrocarbon (HC) and fluorocarbon (FC) amphiphilic compounds were studied using scanning surface potential microscopy (SSPM) and friction force microscopy (FFM). The LB films were formed with a monolayer on an aqueous subphase containing calcium cations as the counter ions for the common carboxylate anions of the two types of amphiphilic compounds. It was found that the structure of the phase-separated LB films was different from that of previously studied HC–FC mixed LB films complexed with polymer cations. Namely, the structure of the present LB films was a “side-by-side” structure, while that of the previous ones was an “on-top” structure. In addition, it was found from SSPM that differences in surface potentials between the two separated phases were small for all the mixed LB films prepared with various HC/FC molar ratios. In other words, compositions were similar between 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 compositions explained the low contrast in friction between these two phases as observed by FFM.