Computer Simulations of Surfactant Self Assembly

Abstract

In this letter we present the results of computer simulations of a simple water/surfactant model, from which we obtained a complete micellar size distribution. We are able to observe (equilibrium) dynamical processes such as the entering of single surfactants into micelles, single surfactants leaving micelles, the fusion of two micelles, and the slow breakdown of a micelle. We use our results to answer some controversial questions concerning the interactions that are essential for surfactant self-assembly.The dynamics of self-assembled surfactant structures are of importance in a variety of processes ranging from the transport of molecules through cell membranes to the removal of stains in a washing machine. Even in the simplest assemblies, micelles in water, the time scales of dynamical processes may vary from to s. Here we present the results of molecular dynamics simulations of a simple surfactant/water model. Together with the equilibrium properties, these simulations yield the dynamical processes that are observed experimentally.In computer simulations on "realistic" models of surfactants the assembly is constructed a priori and this structure is studied for several tenths of a These simulations yielded important information about the structure of the aggregate. However, the study of the dynamics is limited to only very short time scales.5 Furthermore, it is clearly impossible to study the collective behaviour of several assemblies using these models. This collective behavior turns out to be essential in the dynamics of micelles. An alternative approach, using simplified models, shows the spontaneous formation of monolayers, micelles,6v7 and also a membrane.8 An intriguing question is whether it is possible to observe in a computer simulation of a simple molecular model the dynamics that is observed experimentally in these ~y s t e m s .~ That this is far from obvious becomes clear if we recall that even in the simplest case, micelles in water, the relevant time scales range from s, which is the time it takes a surfactant to leave or enter a micelle, to s, the time scale of the fusion of micelles.In our model, two simple observationslo constituted our starting point: oil and water do not mix, and a surfactant is an amphiphilic molecule, i.e., a molecule of which one side is hydrophilic and dislikes oil and the other side is hydrophobic and likes oil. We assume the existence of three types of particles: w particles, h particles, and t particles. These particles are used to model two types of molecules, namely, water molecules and surfactant molecules. A water molecule consists of a single w particle. A surfactant molecule is made up o f t particles and h particles, joined by harmonic potentials. The particles interact with truncated and shifted Lennard-Jones potentials with energy parameter q, distance parameter aij, and the cut-off radius Rijc. We have assumed that for all interactions eij = 6 and uij = u. In order to make the interactions different, the truncation of the potential (Rif) is made depending ...

Biological and synthetic membranes: What can be learned from a coarse-grained description?

“…Clearly these models allow for much flexibility in describing the molecular geometry and they can be studied by Molecular Dynamics. A generic off-lattice model has been utilized by Smit and co-workers [ 168] to elucidate micelle formation. Water and oil molecules are modeled by Lennard-Jones particles while the amphiphile is represented by a collection of particles bonded together via harmonic springs.…”

Coarse grained molecular dynamics and theoretical studies of carbon nanotubes entering cell membrane

“…By reducing the number of degrees of freedom together with the use of short range potentials, CGMD models can gain four to five orders of magnitude in speed compared with full-atom models, achieving length scales on the order of micrometers and timescales on the order of milliseconds. Encouraged by such computational efficiency, a number of CGMD models have been developed and used to study lipids and surfactants systems, including micelles [17,18], bilayers [19,20], micelle aggregation [21] and vesicle formation [22]. However, it should be noted that the computational efficiency of CGMD models is usually gained at the cost of loss in accuracy.…”

Self-assembly of amphiphilic molecules: A review on the recent computer simulation results

“…There have been several simulation studies [113][114][115][116][117][118], including atomistic or coarse grained models, where micelle fusion and fission events were observed. For the fusion mechanism, more extensive reviews can be found in literature [77,119,120].…”