Estimation of the mutual orientation and intermolecular interaction of C12Ex from molecular dynamics simulations

Nonionic surfactants, such as poly(ethylene glycol) alkyl ethers (abbreviated as CyEx) show a rich phase behavior in aqueous solution, i.e., they form micellar, lamellar, cubic, and so forth phases depending on experimental parameters such as the hydrophobic and hydrophilic chain lengths, temperature, or concentration. The aim of the present study is to determine the nature of the preaggregates, which are inferred to exist before the actual self-assembly process in aqueous solution, and to assess the aptitude to their formation. The target molecules are C12E3, C12E4 and C12E5, surfactants of moderate water solubility. Coarse-grained and all-atom molecular dynamics simulations (NPT/293 K) of two molecules of each species with explicit water in periodic boundary conditions are carried out to estimate the mutual orientation and the interaction between the surfactants in their dimers. The force fields are MARTINI and Amber99, the latter with self-derived parameters for the ether groups. The change in the orientation and distance between the molecules in the dimers are discussed based on different structural parameters. In addition, the interaction between the surfactants is evaluated from quantum chemistry calculations in terms of binding energy for the average structures from the cluster analysis. The solvent-solute interaction is quantified by the mean number of hydrogen bonds formed between them. On the basis of combined analysis, a series of different structures for subsequent study of the possible self-assembly patterns of C12E3, C12E4, and C12E5 is outlined.