Surfactant Micelles

micelles

The Martini force field is ideally suited to study formation of micelles, especially because it allows long simulation times required for the equilibration of surfactant solutions. Still, the ability to obtain an equilibrated micellar size distribution depends on the type of surfactant - exchange of surfactants between micelles, and fusion and fission of small micelles is only observed for surfactants with a relative high CMC. For instance, relatively long tail surfactants such as DPC (dodecyl-phosphatidylcholine) can be observed to self-assemble into micelles, but convergence of the size distribution is very slow [1]. Recent Martini simulations studies on less hydrophobic surfactants, including acyl-trimethylammonium chloride [2,4], SDS [3], and C12E5 [5, see figure], however, are able to obtain equilibrium, and reveal interesting phenomena such as the sphere-to-rod transition [4,5].

In a comparative study [6] it is concluded that the Martini model can reproduce experimental CMCs and aggregation numbers for nonionic surfactants reasonably well. The temperature dependence, however, is shown to be incorrect, a known shortcoming of CG models in general. Martini simulations of micelles can also be used to provide starting sturctures for fine-grained models, as shown in a recent study of lyso-lipids [7], and to provide details on the absorption and desorption process of non-ionic surfactants [8].

[1]  S.J. Marrink, A.H. de Vries, A.E. Mark. Coarse grained model for semi-quantitative lipid simulations. JPC-B, 108:750-760, 2004. abstract

[2] S.V. Burov, N.P. Obrezkov, A.A. Vanin, E. M. Piotrovskaya. Molecular dynamic simulation of micellar solutions: A coarse-grain model. Colloid J. 70:1-5, 2008.

[3] S. Jalili, M. Akhavan. A coarse-grained molecular dynamics simulation of a sodium dodecyl sulfate micelle in aqueous solution. Coll. Surfaces A., 352:99-102, 2009.

[4]  A.V. Sangwai, R. Sureshkumar. Coarse-grained molecular dynamics simulations of the sphere to rod transition in surfactant micelles. Langmuir, 27:6628–663, 2011.

[5] M. Velinova, D. Sengupta, A. Tadjer, S.J. Marrink. Sphere-to-rod transitions of nonionic surfactant micelles in aqueous solution modeled by molecular dynamics simulations, Langmuir, 27:14071–14077, 2011. abstract

[6] S.A. Sanders, A.Z. Panagiotopoulos. Micellization behavior of coarse grained surfactant models. J. Chem. Phys. 132:114902, 2010.

[7] P. Brocos, P. Mendoza-Espinosa, R. Castillo, J. Mas-Oliva, A. Pineiro. Multiscale molecular dynamics simulations of micelles: coarse-grain for self-assembly and atomic resolution for finer details. Soft Matter, ASAP 2012. DOI: 10.1039/c2sm25877c

[8] Y.N. Ahn, G. Mohan, D.I. Kopelevich. Collective degrees of freedom involved in absorption and desorption of surfactant molecules in spherical non-ionic micelles. J. Chem. Phys. 137:164902, 2012.