Self-assembly of coarse-grained ionic surfactants accelerated by graphics processing units

Abstract

Due to the relatively long time scales inherent to ionic surfactant self-assembly (>ms), an aggressive computational approach is needed to obtain converged data on micellar solutions. This work presents a study of micellization using a coarse-grained (CG) model of aqueous ionic surfactants in replicated molecular dynamics (MD) simulations run on graphics processing unit hardware. The performance of our implementation of the CG model with electrostatics into the HOOMD-Blue GPU-accelerated MD software package is comparable to that of a modest sized cluster running a highly optimized parallel CPU code. From 0.36 ms of cumulative trajectory data, we are able to predict equilibrium thermodynamic and morphological properties of ionic surfactant micellar solutions. Estimating the critical micelle concentrations (CMC) from the free monomer (r 1 ) and premicellar concentrations obtained from simulations of sodium hexyl sulfate (S6S, CMC of 460 AE 6 mM) at high (1 M) concentration, a value in good agreement with experimental results is obtained; however, the same method applied to simulations of sodium nonyl sulfate (S9S, r 1 of 2.4 AE 0.01 mM) and sodium dodecyl sulfate (SDS, r 1 of 0.02 AE 0.01 mM) at the same total concentration systematically underestimates the CMCs. An alternative method for calculating the CMC is presented, where the free monomer concentration computed from high concentration CG-MD data is used as the input to a simple theoretical model which can be used to extrapolate to a more accurate prediction of the CMC. Better agreement between the empirical and predicted CMC is obtained from this theory for S9S (28.7 AE 0.3 mM) and SDS (3.32 AE 0.04 mM), though the CMC for S6S is slightly underestimated (304 AE 3 mM). We also present statistically converged morphological data, including aggregation number distributions and the principal components of the gyration tensor. This data suggest a transition from spherical micelles to rod-like at a specific aggregation number, which increases with increasing hydrocarbon length.

Structure of phase-separated athermal colloid-polymer systems in the protein limit

“…Two related continuum models were also simulated using molecular dynamics (MD) simulations for comparison with the lattice model. We used the HOOMD (Highly Optimized Object-oriented Many-particle Dynamics) package [31,32] which takes advantage of graphics processing units (GPUs) to accelerate the simulation of large systems [33]. All athermal interactions were approximated through the use of a LennardJones potential cut off at the minimum,…”

Complex coacervate micelles formed by a C18-capped cationic triblock thermoresponsive copolymer interacting with SDS

“…39. Very recent simulation work by Lebard et al 40 reports radii of gyration that would be equivalent to a range of diameters from 33 A to well above 40 A. In any case, this shows that the SDS micelles in our system are compactly organized in the coacervate core, possibly with a small average distance between them (approx.…”

Theoretical description of 2D-cluster formation of nonionic surfactants at the air/water interface

“…The same objects were investigated in ref. [93] with the only difference that the alkyl chain lengths of the alkylsulphates were 6, 9, and 12 carbon atoms. The transition from spherical to rodlike micelles was detected at a certain value of the aggregation number, which increases with lengthening the surfactant alkyl chain.…”

“…[86,[92][93][94][95], the MD method was used for the assessment of surface tension and CMC of different amphiphiles. Modeling of the ethanol behavior, done by Tarek et al [86], gives the adequate orientation of the alcohol molecule at the interface with the hydrophilic part in the water phase and the hydrophobic in the air.…”