Structure of Phase-Separated Langmuir−Blodgett Films of Hydrogenated and Perfluorinated Carboxylic Acids Investigated by IR Spectroscopy, AFM, and FFM

Abstract

The structure of the mixed Langmuir and Langmuir-Blodgett (LB) films of arachidic acid (AA) and perfluorotetradecanoic acid (PA) are investigated. Surface pressure-area isotherms of the mixed Langmuir films of AA and PA fall between those of pure AA and pure PA, indicating that the additivity rule holds in this system. This shows that the two components are phase-separated or are ideally mixed. The results of transmission and reflection-absorption IR spectroscopic measurements of the mixed LB films reveal that the composition of the LB films is the same with that in the spreading solution and that the orientation of the two components does not change with mixing. Molecular axes of AA and PA are almost perpendicular to the film surface. AFM and FFM show that AA forms distinct monolayer domains that are surrounded by monolayer PA. These results indicate that AA and PA are phase-separated in the mixed LB films. On the other hand, rather different results were reported for the mixed Langmuir and LB films of AA and PA in the literature, where the additivity rule did not hold for the surface pressure-area isotherms and barely discernible domains were observed in the AFM images. The different behaviors should be due to the differences in the experimental conditions such as the spreading solvent.

Thermodynamic Behaviour of Mixed Films of an Unsaturated and a Saturated Polar Lipid. (Oleic Acid-Stearic Acid and POPC-DPPC)

“…Seoane et al [34] reported mixed films of cholesterol with saturated or unsaturated fatty acids, but these are not discussed here due to the peculiar characteristics of cholesterol. Several authors [23,[25][26][27]30,32] have also observed phase separation in fatty acid mixed monolayers of fluorinated and hydrogenated amphiphiles.…”

“…One of the main aspects of these mixtures is the miscibility and phase separation. Recent articles on mixed fatty acids systems are those of [23][24][25][26][27][28][29][30][31][32]. Furthermore, since fatty acids or phospholipids usually present unsaturations in the hydrocarbon chains, the study of mixtures of saturated and unsaturated compounds is of interest.…”

Characterization of domain growth kinetics in a mixed perfluorocarbon-hydrocarbon Langmuir–Blodgett monolayer

“…Many of these applications originate from the improved thermal, chemical and physical properties of fluorine-terminated surfaces in comparison with their non-fluorinated counterparts, including low friction coefficients, chemical inertness [3][4][5] and high dielectric constant [6]. Of particular recent interest is the ability of fluorinated surfactants to phase-separate from hydrocarbon surfactants at the air-water and solid-air interface.…”

“…Of particular recent interest is the ability of fluorinated surfactants to phase-separate from hydrocarbon surfactants at the air-water and solid-air interface. Under appropriate conditions, fluorinated surfactants can phase-separate from hydrocarbon amphiphiles, forming monolayer films with well-defined surface domains [3,[7][8][9][10]. We believe that the controlled patterning of solid surfaces with fluorinated molecules has important technological applications in the field of chemical and biological sensing [11], and wish to both understand and control the mechanism of phase-separation in these mixed systems.…”

Studies on the structure and miscibility of mixed Langmuir–Blodgett films of a double-armed dibenzo-18-crown-6 ether with stearic acid

“…The π-A isotherms of mixed DACE/SA monolayer show steeper surface pressure-area curves and higher collapse pressures, compared to those of pure DACE monolayer. As the content of SA increases, the shape of the π-A isotherm [21,22]. The value of A ex can be expressed as:…”