Journal
COLLOIDS AND SURFACES A-PHYSICOCHEMICAL AND ENGINEERING ASPECTS
Volume 325, Issue 1-2, Pages 81-85Publisher
ELSEVIER
DOI: 10.1016/j.colsurfa.2008.04.037
Keywords
Langmuir monolayer; fatty acid; monolayer collapse; isotherms; subphase pH; cations
Categories
Funding
- Marie Curie European network [MRTN CT 2003-504932]
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Surface pressure versus specific molecular area (pi-A) isotherms of stearic acid monolayers have been studied by varying the pH of the aqueous subphase containing monovalent (Na) or different divalent (Mg, Ca, Ba, Zn and Cd) cations. The lowering of the tilted to untilted phase transition pressure (pi(t)) with increasing pH of the subphase is correlated with an increase of the headgroup dissociation of the acid monolayer. On pure water, the surface pressure drops after the collapse point (pi(c)) to a lower pressure value (pi(l)). The pressure-gap Delta pi = pi(c) - pi(l) changes with increasing dissociation, depending upon the particular cation. After a certain pH, no pressure gap is observed and pi remains constant or increases slowly after pi(c). This transition from constant area collapse to constant pressure collapse of the monolayer is observed for all monovalent and divalent cations near total monolayer dissociation. (C) 2008 Elsevier B.V. All rights reserved.
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