Atomic force microscopy/friction force microscopy study on phase separation of mixed LB films containing amphiphiles with bulky probe units

Polyion complexed mixed LB films of amphiphiles with long FC chains (PFECA [CF3(CF2)8(CH2)2O(CH2)2COOH] or perfluorodecanoic acid (PFDA) [CF3(CF2)8COOH]) and those with long hydrocarbon (HC) chains, 3-, 6-, 9- or 12-(9-anthroyloxy) stearic acids (3-, 6-, 9- or 12-ANSTAs) deposited on Si substrates were characterized by atomic force microscopy/friction force microscopy (AFM/FFM). AFM/FFM images of 1:1 mixed LB films of PFDA and 6-, 9-, or 12-ANSTA showed phase separated structures, while those of PFDA and 3-ANSTA did not. AFM/FFM images of 1:1 mixed LB films of PFECA and 3-, 6-, or 9-ANSTA did not show phase separated structure. These results show that the amphiphile molecules with long HC chain which have a bulky probe unit at the position in the methylene chain close to head group (e.g. 3-ANSTA) can mix well with the amphiphile molecules with long fluorocarbon (FC) chains (PFDA or PFECA). Similar results were also observed in the mixed LB films of PFDA (or PFECA) and doxylstearic acids. According to the π–A isotherms for these amphiphiles with bulky probe unit, the molecular sectional area in the monolayers for these amphiphile molecule increased as the position of the probe unit approached to the head group. Since the cohesion energy for molecules decrease with the increase in the inter-molecular distance, the molecular mixing between the amphiphile with FC chain and that with HC chain which have a bulky probe unit at the position closer to head group can be attributed to the decrease of cohesion energy due to the increase in the inter-molecular distance. In the AFM/FFM image for 1:1 mixed LB films of PFDA and 12-ANSTA, the following interesting phenomenon were observed. After a scanning with large load over the part of an island composed of 12-ANSTA, which is higher than the sea area before these scanning, the height of the whole area of the island became lower than the sea area. From this result, it was confirmed that this mixed LB film has a ‘side by side’ structure.