Molecular dynamics simulation of C₈E₅ micelle in explicit water: Structure and hydrophobic solvation thermodynamics

Results are presented from a large scale molecular dynamics (MD) simulation of a non-ionic micelle comprising 80 C₈E₅ surfactant molecules in 9798 explicit TIP3P waters. The results are consistent with the conventional static picture of a simple spherical micelle ('hydrophilic heads out, hydrophobic tails in'). In addition, the MD simulation reveals structural details that show clearly the dynamic nature of the micellar aggregate. The micelle is roughly spherical with thermal fluctuations leading to instantaneous shapes that are significantly non-spherical. The individual surfactant molecules adopt various nonlinear conformations, predominately in the hydrophilic segment, thereby leading to the overall compact globular shape of the micelle aggregate. Atomic distribution functions and dihedral angle distributions show that the micelle interior is similar to liquid n-octane. Although no water penetration in the micelle hydrophobic core is observed, there is considerable exposure of the hydrophobic tails of the surfactant molecules to water in the interracial region. The excess chemical potentials of hydrophobic Lennard-Jones (LJ) solutes and their WCA analogs show a preference for the michelle core relative to bulk water. Interestingly, the solvation energies of LJ solutes show a minimum in the interfacial region. A qualitative explanation for this behaviour based on different packing tendencies of water and chain-like molecules is presented.