Sources of error in Langmuir trough measurements: Wilhelmy plate effects and surface curvature

• Published in: Colloids and surfaces. A, Physicochemical and engineering aspects Vol. 144; no. 1; pp. 229 - 234
• Main Authors: Welzel, Petra B., Weis, Ingrid, Schwarz, Gerhard
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 20-12-1998
• Subjects: Air/water interface; Fatty acids; Lipids; Molecular area; Pressure–area isotherms; Spreading technique; Pressure–area isotherms; Lipids; Spreading technique; Fatty acids; Air/water interface; Molecular area
• ISSN: 0927-7757; 1873-4359
• DOI: 10.1016/S0927-7757(98)00649-9

When characterising so-called insoluble monolayers at the air/water interface by means of a film balance, it is customary to calculate the surface area solely from the geometric trough dimensions. In this paper we show that use of a Wilhelmy plate made of filter paper gives rise to an additional surface area. Ignoring this area increment may lead to a non-negligible systematic error which results in underrating the area per molecule, with an error becoming more severe upon higher compression. We also consider an increase of the surface area due to the curvature of its meniscus at the periphery of the trough. The practical significance of these errors will be assessed and appropriate remedies to eliminate fallacious data are suggested. Finally we discuss possible consequences regarding previously published results.