Investigation of counterion effects on the stability of ionic membranes studied by computer simulations

• Applicant: Dr. Arben Jusufi
• Subject Area: Theoretical Condensed Matter Physics
• Term: from 2006 to 2008
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 22505276

Recent experiments from computer simulation studies [1, 2, 3] have demonstrated that different counterion species have specific effects on the stability of micelles and surfactant layers. Micelles and surfactant bilayers belong to the class of self-assembling molecules like membranes. They usually consist of a solvophobic part, e.g. a surfactant hydrocarbon tail (hydrophobic), and a solvent-friendly part, e.g. a surfactant headgroup (hydrophilic). The driving force for self-assembly above a certain surfactant concentration, the critical micelle concentration, is based on these properties, i.e., the tails effectively attract each other and form aggregates. Counterion effects on the shape of the micelles are often attributed to some ¿specific interaction of the counterions with ionic headgroups. It is the aim of this project to help identify these specific effects in membrane systems by simulating surfactant bilayer surfaces on a microscopic length scale and to investigate systematically the stability of the membrane surface by exchanging the counterion species. A sub-task is the study of the fluidity of bound hydration layers according to recent experiments of U. Raviv and J. Klein [4]. It was found that surface-attached hydration layers (via the condensed counterions) retain a shear fluidity characteristic of the bulk liquid, even when two of such layers are compressed down to films of approx. 1 nm thickness. It is claimed that this effect can be attributed to ready exchange of water molecules within the hydration layers, which was recently confirmed by molecular dynamics simulations of potassium-water solutions between mica surfaces [5]. It is the aim to extend this study on charged surfactant bilayers with different counterions in order to observe specific counterion influences on the bound water fluidity. Molecular simulations can help to understand these effects, which are directly implicated in areas ranging from clay plasticity [6] and biolubrication [7] to gating of charge migration in DNA [8].

• DFG Programme: Research Fellowships
• International Connection: USA
• Host: Professor Dr. Athanassios Panagiotopoulos