Structures of Langmuir-Gibbs Films Consisting of Long-Chain Fatty Acid and Water-Soluble Surfactants

The structures of Langmuir-Gibbs (LG) films at the air-solution interface were studied using surface tensiometry, Brewster angle microscopy (BAM), and infrared external reflection spectroscopy (IR-ERS). The LG films were fabricated by forming Langmuir films of deuterated arachidic acid (D19A) and then injecting a water-soluble surfactant, sodium dodecyl sulfate or octaethylene glycol monododecyl ether, into the subphase. The presence of the LG films at the air-solution interface affected the surface tension of water. BAM observations revealed the formation of phase-separated monolayers consisting of D19A monolayers and expanded monolayers of water-soluble surfactant at low concentrations of the water-soluble surfactant. The presence of phase-separated structures was supported by the IR-ERS results. At high concentrations, the water-soluble surfactants adsorbed to the domains of D19A monolayers, forming bilayers under the monolayers. The IR-ERS results suggest the penetration of the water-soluble surfactants into the domains of D19A monolayers and the diffusion of D19A molecules into the bilayer regions of the water-soluble surfactants. At concentrations greater than the critical aggregate concentration and/or critical micelle concentration, D19A molecules were solubilized in the solution. In some cases, the multilayers were kinetically stabilized and were present even 24 h after the injection of the water-soluble surfactants into the subphase.