The use of scanning polarization force microscopy to study the miscibility of a molecular wire candidate and an insulating fatty acid in mixed LB films

Abstract

Mixed films containing a conjugated “molecular wire” candidate and an “insulating” fatty acid have been prepared by the Langmuir–Blodgett technique. Specifically, this paper reports the fabrication of mixed films as well as miscibility studies of 4-[4-(4-hexyloxyphenylethynyl)phenylethynyl]benzoic acid (HBPEB) and docosanoic (or behenic) acid (BA). Surface pressure vs. area per molecule isotherms were recorded, with excess area and excess Gibbs energy of mixing calculated. Surface potential-area per molecule isotherms were also recorded for mixtures over the whole range of mole fractions, with negative deviations from the additivity rule revealing orientational changes induced in the HBPEB molecules. The Langmuir films were transferred onto solid supports and characterized by SPM techniques, with atomic force microscopy (AFM) revealing that well-ordered, defect-free films are obtained. The use of scanning polarization force microscopy (SPFM), which provides non-contact imaging based on differences in surface charge distribution, i.e., surface potential, provides complimentary information regarding distribution of the components within the mixed films. From the comprehensive miscibility study performed, which includes thermodynamic and imaging methods, it can be concluded that the wire-like molecule and the fatty acid are miscible over the 0–0.1 and 0.8–1 ranges of HBPEB mole fraction while phase separation occurs for HBPEB mole fractions over the 0.1–0.8 range.