Micro-Phase Separation in Binary Mixed Langmuir Monolayers of n-Alkyl Fatty Acids and a Perfluoropolyether Derivative

Micro-phase separation in binary mixed Langmuir monolayers of cadmium salts of n -alkyl fatty acids (CH 3 (CH 2 ) n - 2 COOH; C n ( n = 18, 20, 22, 24)) and a perfluoropolyether surfactant (F(CF(CF 3 )CH 2 O) 3 CF(CF 3 )COOH, PFPE) is studied by film balance measurement and atomic force microscopy (AFM). At different temperatures, mixtures of C n /PFPE in chloroform were spread onto the aqueous Cd 2+ subphase and deposited on silicon wafers. AFM images showed that C n and PFPE separate into microscopic domains of condensed phase and a surrounding matrix of expanded phase, respectively, in their mixed monolayers. The morphological feature of phase-separated structures was characterized by characteristic length ( λ ) expressing the periodicity of two-phase distribution and fractal dimension ( D ) of C n domains reflecting the complexity of domain shape, which were determined through AFM image analyses. It was found that the monolayer morphologies systematically vary with alkyl chain length of C n and temperature of the water surface; circular-shaped condensed phase microdomains with D of about 1.1 are formed at λ of 7−9 μm when the mixed monolayers are prepared using a shorter alkyl chain fatty acid and/or at a higher temperature, whereas branched narrow domains with D close to 2 are formed at λ of approximately 4 μm when a longer chain fatty acid is used at a lower temperature.