8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

Fri 911:10-14:00P6.7Formation of solid metal stearate layers at thedecane/water interfaceRiëlle de Ruiter, 1 R. Willem Tjerkstra, 2 Michèl H. G. Duits, 2 and Frieder Mugele 31 University of Twente, P. O. Box 217, 7500 AE, Enschede, Netherlands2 University of Twente, Enschede, Netherlands3 University of Twente, Enschede, AustriaWe study how solid layers are formed at the interface between aqueous salt solutions and decanewith stearic acid, by monitoring the evolution of their mechanical, optical and chemical propertiesfrom the moment that the two liquids are exposed to each other. Pendant and oscillating dropmethods are used to measure the time dependence of the interfacial tension and dilatational surfacemoduli. X-ray photoelectron and infrared spectroscopy are used to determine the elemental andchemical composition of the ultimately formed layers. Since the composition of the aqueous phaseis expected to play an important role in the formation of interfacial stearate layers, we examinethis for a range of pH values and for different combinations of mono- and/or divalent cations.Variation of the cationic composition (at sufficiently high pH) is found to have strong effects.Layer formation is particularly pronounced in artificial seawater (ASW), in which the constituentgroups of mono- and divalent cations interact synergistically. Also individual types of cations havedistinct contributions: layers formed in the presence of Ca + 2 or Mg + 2 develop a large transientE’, whereas for Na + such behavior is hardly observable. In layers obtained from ASW, almostexclusively Ca + 2 cations are found, in spite of its much lower abundance in ASW than Na + andMg + 2 . Our findings suggest that layer formation in ASW is initiated by monovalent cations, whichare subsequently replaced by Ca + 2 .7