EXPERIMENTAL-ANALYSIS OF THE THERMODYNAMIC MECHANISM OF LANGMUIR-BLODGETT-FILM TRANSFER

An experimental investigation was made on the mechanism for the transfer of a Langmuir-Blodgett (LB) film. The transfer ratio (ρ) and the interfacial force (f) were observed for stearyl alcohol and cadmium stearate monolayers on silicon substrates, by systematically varying both the surface pressures (π) of the monolayer and the hydrophilicities of the substrate characterized by contact angle (θ0). It was found that the ratios ρ observed both for up- and down-stroke depositions changed discretely within zero and unity, divided at the critical values of π and θ0. To examine the film-transfer characteristics, on the basis of thermodynamics, the interfacial forces applied to the moving substrate were measured during the deposition process. In the case of ρ = 0, the force showed little dependence upon π. On the contrary, in the case of ρ = 1, the forces applied to the hydrophilic substrates in the up stroke linearly increased with increasing π, while those applied to hydrophobic substrates in the down stroke decreased with increasing π. These results reveal that the dynamic interfacial force is a substantial factor in the LB film transfer. The force is ascribable to the changes in the surface energies caused by the vertical displacement of the related interfaces. The mechanism of the LB film transfer is consequently considered as a subject of thermodynamics, and the success or failure in the monolayer transfer can be quantitatively interpreted by taking account of the dynamic force changes during the processes. © 1990 American Chemical Society.