A series of molecular dynamics calculations have been carried out for spherical sodium dodecyl sulfate micelle of various sizes in order to investigate surface structure of the micelle as a function of its size. Two-dimensional surface pair correlation function of the hydrophilic head group projected on the sphere of the micelle showed a gas-like structure for small micelle, e.g. N = 31, and a liquid-like one for large micelles, e.g. N >= 61, depending on the surface density of the head group. Coordination structure of the sodium ion near the surface has been analyzed in detail, too. (c) 2006 Elsevier B.V. All rights reserved.