Molecular Dynamics simulation of the sodium octanoate micelle in aqueous solution: comparison of force field parameters and molecular topology effects on the micellar structure

We have performed a series of 10 ns Molecular Dynamics simulations of the sodium octanoate micelle in aqueous solution in the constant NpT ensemble, at p = 1 bar and T = 300 K. Two molecular topologies were studied, one with all internal degrees of freedom and the other constraining bond stretching and angle bending degrees of freedom. Two Lennard-Jones parameters for sodium ions, namely the OPLS and ° Aqvist parameters, were used. The results show an artificial enhancement of stable sodium bridges between octanoate anions when the OPLS parameters for sodium are used. The ° Aqvist parameters give a micellar structure in good agreement with experimental and thermodynamical evidences. It is also observed that the aggregation of monomers is strongly dependent on the molecular topology. When the ° Aqvist parameters were employed, the model system without constraining geometry had one dissociated monomer after 10 ns, while the model system with bond length and bond angle constraining had five dissociated monomers after a 10 ns trajectory.