Langmuir—Blodgett Films

Abstract

It was discovered by Katherine Blodgett and Irving Langmuir, in the 1930s, that it was possible to transfer monolayers of fatty acid salts at the air-water interface onto a solid substrate.⁽¹⁾ A monolayer of the fatty acid salt is obtained by depositing an accurately known volume of a solution of the fatty acid, such as stearic acid, in a volatile solvent such as chloroform or n-hexane, onto an aqueous subphase containing appropriate divalent cations such as cadmium, calcium, or barium. The molar concentration of the fatty acid solution is also accurately known and the deposition at the air-water interface achieved by touching drops of the solution onto the surface from a syringe. The solution spreads rapidly across the surface as the solvent evaporates leaving a known number of molecules of the fatty acid within an area of the surface defined by a set of movable barriers. The polar carboxyl headgroups of the fatty acids are immersed in the subphase and their alkyl chains remain in the gaseous phase (see Fig. 1). If the subphase has a neutral or mildly alkaline pH the fatty acids will ionize and adsorb the divalent cations to form a salt.