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

P5.80Wed 711:10-14:00Molecular aggregates in the aqueous solutions of bileacid saltsPál Jedlovszky, 1 Lívia Pártay, 2 and Marcello Sega 31 ELTE University, Pázmány P. stny 1/A, H-1117, Budapest, Hungary2 Cambridge University, Cambridge, United Kingdom3 University of Stuttgart, Stuttgart, GermanyThe aggregation behaviour of two bile acid salts, i.e., sodium cholate (SC) and sodium deoxycholate(SDC) has been studied in their aqueous solutions of three different concentrations, i.e.30 mM, 90 mM and 300 mM by means of molecular dynamics computer simulation. In orderto let the systems reach thermodynamic equilibrium rather long simulations were performed, duringwhich the average size of the bile aggregates already fluctuated around a constant value. Theruns were about an order of magnitude longer than the average lifetime of both the monomericbile ions and the bonds sticking two ions together to be part of the same aggregate. This allowedthe bile ions belonging to various aggregates to be in a dynamic equilibrium with the isolatedmonomers. We found that, due to their molecular structure, bile ions form rather different aggregatesthan the usual spherical micelles. At 30 mM the bile ions are only forming small oligomers,kept together mostly by hydrophobic interactions. At 300 mM the bile ions form large secondarymicelles, kept together both by hydrophobic interactions and H-bonds. In the secondary micellessmall, hydrophobic primary micelles are linked together via formation of H-bonds between theirhydrophilic outer surfaces. We also analyzed the shape of the aggregates and counterion bindingof the micelles. We found that primary micelles are of flattened, disk-like shapes, while secondarymicelles might even have irregular shapes. In order to resolve the apparent contradiction betweendifferent experimental values of the counterion binding measured we calculated its value regarding(i) only the contact, and (ii) also the solvent separated ions as bound ones. We found that contradictingexperimental results are originated in the fact that solvent separated ions are seen as boundones by some of the experimental methods, and not seen as bound ones by some other methods.80