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ISRN Physical Chemistry
Volume 2012 (2012), Article ID 835397, 6 pages
Research Article

Hysteresis of Isotherms of Mixed Monolayers of N-Octadecyl-N′-phenylthiourea and Stearic Acid at Air/Water Interface

Department of Chemistry, University of Botswana, PB 00704, Gaborone, Botswana

Received 11 October 2012; Accepted 2 December 2012

Academic Editors: J. J. Lopez Cascales, G. Pellicane, and P. O. Westlund

Copyright © 2012 Siji Sudheesh et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


Surface pressure area isotherms of Langmuir monolayers formed by spreading mixed solutions of varying concentrations of N-octadecyl-N′-phenylthiourea (OPT) and octadecanoic acid or stearic acid (SA) over air-water interface are described. Examination of the hysteresis behavior and an analysis of the limiting area per molecule of the isotherms show that when the spread solution has an excess of OPT, the limiting surface area is consistent with a monolayer composed of equimolar amounts of the two components. This indicates that any excess thiourea, which on its own does not form a stable monolayer, is squeezed out and is not part of the monolayer. On the other hand, when the spreading mixture has an excess of SA over OPT, the isotherm indicates that the entire originally spread material is incorporated into the surface film. In this case, the values of area/molecule indicate that the monolayer is composed of SA : OPT complex with a ratio of 1 : 1 together with the excess SA remaining in the monolayer.