Molecular Dynamics Simulation of the Kinetics of Spontaneous Micelle Formation

Abstract

Using an atom based force field, molecular dynamics (MD) simulations of 54 dodecylphosphocholine (DPC) surfactant molecules in water at two different concentrations above the critical micelle concentration have been performed. Starting from a random distribution of surfactants, we observed the spontaneous aggregation of the surfactants into a single micelle. At the higher DPC concentration (0.46 M) the surfactants aggregated into a worm-like micelle within 1 ns, whereas at lower concentration (0.12 M) they aggregated on a slower time scale (∼12 ns) into a spherical micelle. The difference in the final aggregate is a direct consequence of the system achieving the lowest free energy configuration for a given quantity of surfactant within the periodic boundary conditions. The simulation at low surfactant concentration was repeated three times in order to obtain statistics on the rate of aggregation. It was found that the aggregation occurs at a (virtually) constant rate with a rate constant of k ) 1 × 10 -4 ps -1 . This is an unexpected result. On the basis of Monte Carlo simulations of a stochastic description of the system, using diffusion rates and cluster radii as determined by separate MD simulations of single DPC clusters, a lower rate constant which diminishes in the course of the aggregation process had been predicted. Neglect of hydrodynamic interactions, of long-range hydrophobic interactions, or of spatial correlations in the stochastic approach might account for the descrepancies with the more accurate MD simulations.

Molecular Dynamics Simulations of Mixed Micelles Modeling Human Bile

“…Starting from random initial configurations has proven to be an effective way to avoid kinetically trapped intermediate states, however. Spontaneous aggregation into the thermodynamic equilibrium states has been successfully simulated in the case of dodecylphosphocholine [into simple micelles (16)] and of phospholipids [into bilayers (17)]. The fact that we obtain very similar structures for three independent simulations provides a strong indication that the observed structures represent the thermodynamic ground state.…”

“…To avoid biasing the final structure of the micelles, the simulations were initiated from random mixtures of the constituents from which micelles spontaneously self-assemble. This approach follows from recent studies in which we have shown that surfactants spontaneously aggregate into micelles (16) and phospholipids into bilayers (17) on time scales currently accessible to MD simulations. The simulations presented in this paper are nevertheless still very timeconsuming, requiring about half a year of computer power on the currently fastest multiprocessor machines.…”

Modelling the interfacial behaviour of dilute light-switching surfactant solutions

“…It is not to say that "heroic" atomistic simulations have not been attempted to study a variety of surfactant micellar systems [9][10][11][12][13][14][15][16][17][18][19][20][21] and insights have been gained regarding structural properties of post-cmc regions through calculation of density distributions, micelle size and shapes, average micellar population, etc. The challenge remains in tackling the computationally demanding calculations in the pre-cmc region.…”

Ionic Surfactant Aggregates in Saline Solutions: Sodium Dodecyl Sulfate (SDS) in the Presence of Excess Sodium Chloride (NaCl) or Calcium Chloride (CaCl2)

“…[20][21][22][23][24][25][26][27][28] More recently, atomistic studies with and without solvent have emerged. [29][30][31][32][33][34][35][36][37][38][39] Since characteristic equilibration times for micelle formation are on the order of microseconds or longer, most of these simulations start from a spherical or rodlike micelle with a predetermined number of detergent molecules. 26,30,33,35,36,40 Such an approach provides valuable insight into detergent dynamics and micelle structure within that particular initial setup, but the results implicitly contain the assumption that the global minimum energy configuration, that is, micellar shape and aggregation number, is close to the initial configuration.…”