Mixing Behavior of the Binary Monolayers of Fatty Acids Based on Their Cohesive Energy Differences

Abstract

The morphology involving the height difference and the surface roughness of the binary monolayers of saturated fatty acids were evaluated using atomic force microscopy (AFM) to investigate the mixing behavior of their monolayers. AFM observations revealed that the mixed monolayers of (palmitic acid/arachidic acid) and (arachidic acid/lignoceric acid), which had four methylene group differences between fatty acids, were in a molecularly mixed state. Further, the mixed monolayer of (stearic acid/lignoceric acid), which had six methylene group differences, was in a phase-separated state. From the results of the present and previous studies, it became clear that the difference in the cohesive energy between fatty acids, which corresponds to the enthalpy difference, was an important factor in determining whether the molecular aggregation state of a fatty acid mixed monolayer was in a molecularly mixed or phase-separated state. Moreover, the boundary value of cohesive energy difference was approximately 2.5 kJ mol–1 at a subphase temperature of 293 K.

Interfacial Enrichment of Lauric Acid Assisted by Long-Chain Fatty Acids, Acidity and Salinity at Sea Spray Aerosol Surfaces

“…Phase separation of binary FA mixtures is dependent on the difference in alkyl chain length and is generally not predicted to occur for FAs with differences in chain length less than 6 carbons. 83,84 These results suggest, however, that the incomplete phase separation of this mixture is due, not to the difference in chain length, but primarily to the solubility of LA, which precludes LA−LA aggregation and leads to dissolution of the molecules from the interface entirely. Surface pressure may similarly facilitate phase separation in binary mixtures of FAs with differences of fewer than 6carbons, provided that both FAs are insoluble.…”

Evaluation of Temperature-Dependent Boundary Lubrication Behavior of Stearic Acid Using a Force-Controlled Tribometer

“…Analyses of binary monolayers of saturated fatty acids using AFM provide additional insights into their mixing behaviour. 21 This body of research collectively establishes a comprehensive foundation for examining the influence of fatty acid properties and operating parameters on frictional properties in tribological systems. However, since researchers have mainly conducted studies in displacement mode to investigate the effects of fatty acid properties on friction in boundary lubrication, conducting force-controlled measurement tests is essential for a comprehensive understanding.…”

“…An increase in temperature also influences the kinetics of the reactions of the tribofilm formation. Analyses of binary monolayers of saturated fatty acids using AFM provide additional insights into their mixing behaviour . This body of research collectively establishes a comprehensive foundation for examining the influence of fatty acid properties and operating parameters on frictional properties in tribological systems.…”