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Natascha Schelero, Regine von Klitzing Stranski-Laboratorium, Institut für Chemie, TU Berlin, Str. d. 17.Juni 124, D-10623 Berlin, klitzing@mailbox.tu-berlin.de The (de)stability and functionality of thin liquid films plays an important role in processes and technical applications like foaming, emulsification and flotation. The first part of the presentation will address the question about charges at a air/water interface. Therefore results of water wetting films on differently charged substrates will be presented. The interactions between film surfaces are determined in a thin film pressure balance (TFPB). By this method equilibrium measurements of a disjoining pressure isotherm (disjoining pressure vs. film thickness) are determined. The disjoining pressure is a quantitative measure for the interactions between the two opposing film interfaces, including repulsive electrostatic, attractive van der Waals (both together known as DLVO forces) and steric contributions [1]. There is a clear evidence for negative charges at the free air/water interface [2,3]. The second part focuses on the stability of single films, tuned by the addition of different electrolytes. The films are either free-standing (foam films) or supported by a solid substrate (wetting films) and they are formed from aqueous surfactant solutions. Usually the addition of low molecular salt like NaCl leads to a thinning of the aqueous film due to screening of the electrostatic interactions. But not only the ionic strength, also the type of added ions has a pronounced effect on the film thickness and stability. For instance a “simple” water wetting film or a foam film with negativley charged surfaces becomes thicker and more stable by adding NaI instead of NaF at a fixed ionic strength [3-5]. It is explained by a stronger adsorption of I- ions at the film surface in comparison to F- ions due to differences in the hydration shell. [1] R. von Klitzing, Adv. Coll. Interf. Sci. (2005) 114/115, 253. [2] K. Ciunel, M. Armelin, G. H. Findenegg, R. v. Klitzing, Langmuir 21 4790. [3] K. Ciunel, N. Schelero, R. von Klitzing, Faraday Disc. (2009) 141, 41–53. [4] N. Schelero, R. von Klitzing, Soft Matter (2011) 7, 2936. [5] N. Schelero, G. Hedicke, P. Linse, R. von Klitzing, J. Phys. Chem. B, (2010) 114, 15523. |
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