Free energy of water permeation into hydrophobic core of sodium dodecyl sulfate micelle by molecular dynamics calculation

Revealing the Influence of Salts on the Hydration Structure of Ionic SDS Micelles by Contrast-Variation Small-Angle Neutron Scattering

The influence of lithium chloride (LiCl) on the hydration structure of anionic micelles of sodium dodecyl sulfate (SDS) in water was studied using the contrast-variation small-angle neutron scattering (SANS) technique. In the past, extensive computational studies have shown that the distribution of invasive water plays a critical role in the self-organization of SDS molecules and the stability of the assemblies. However, in past scattering studies the degree of the hydration level was not examined explicitly. Here, a series of contrast-variation SANS data was analyzed to extract the intramicellar radial distributions of invasive water and SDS molecules from the evolving spectral lineshapes caused by the varying isotopic ratios of water. By addressing the intramicellar inhomogeneous distributions of water and SDS molecules, a detailed description of how the counterion association influences the micellization behavior of SDS molecules is provided. The extension of our method can be used to provide an in-depth insight into the micellization phenomenon, which is commonly found in many soft matter systems.

Micelle Formation in Alkyl Sulfate Surfactants Using Dissipative Particle Dynamics

We use dissipative particle dynamics (DPD) to study micelle formation in alkyl sulfate surfactants, with alkyl chain lengths ranging from 6 to 12 carbon atoms. We extend our recent DPD force field [ J. Chem. Phys. 2017 , 147 , 094503 ] to include a charged sulfate chemical group and aqueous sodium ions. With this model, we achieve good agreement with the experimentally reported critical micelle concentrations (CMCs) and can match the trend in mean aggregation numbers versus alkyl chain length. We determine the CMC by fitting a charged pseudophase model to the dependence of the free surfactant on the total surfactant concentration above the CMC and compare it with a direct operational definition of the CMC as the point at which half of the surfactant is classed as micellar and half as monomers and submicellar aggregates. We find that the latter provides the best agreement with experimental results. Finally, with the same model, we are able to observe the sphere-to-rod morphological transition for sodium dodecyl sulfate (SDS) micelles and determine that it corresponds to SDS concentrations in the region of 300-500 mM.

Self-Assembly of Lysine-Based Dendritic Surfactants Modeled by the Self-Consistent Field Approach

Implementing a united atom model, we apply self-consistent field theory to study structure and thermodynamic properties of spherical micelles composed of surfactants that combine an alkyl tail with a charged lysine-based dendritic headgroup. Following experiments, the focus was on dendron surfactants with varying tail length and dendron generations G0, G1, G2. The heads are subject to acetylation modification which reduces the charge and hydrophilicity. We establish a reasonable parameter set which results in semiquantitative agreement with the available experiments. The critical micellization concentration, aggregation number, and micelle size are discussed. The strongly charged dendronic surfactants micelles are stable for generation numbers G0 and G1, for progressively higher ionic strengths. Associates of G2 surfactants are very small and can only be found at extreme surfactant concentration and salt strengths. Micelles of corresponding weaker charged acetylated variants exist up to G2, tolerate significantly lower salt concentrations, but lose the spherical micelle topology for G0 at high ionic strengths.

Free energy calculation using molecular dynamics simulation combined with the three dimensional reference interaction site model theory. I. Free energy perturbation and thermodynamic integration along a coupling parameter

This article proposes a free energy calculation method based on the molecular dynamics simulation combined with the three dimensional reference interaction site model theory. This study employs the free energy perturbation (FEP) and the thermodynamic integration (TDI) along the coupling parameters to control the interaction potential. To illustrate the method, we applied it to a complex formation process in aqueous solutions between a crown ether molecule 18-Crown-6 (18C6) and a potassium ion as one of the simplest model systems. Two coupling parameters were introduced to switch the Lennard-Jones potential and the Coulomb potential separately. We tested two coupling procedures: one is a "sequential-coupling" to couple the Lennard-Jones interaction followed by the Coulomb coupling, and the other is a "mixed-coupling" to couple both the Lennard-Jones and the Coulomb interactions together as much as possible. The sequential-coupling both for FEP and TDI turned out to be accurate and easily handled since it was numerically well-behaved. Furthermore, it was found that the sequential-coupling had relatively small statistical errors. TDI along the mixed-coupling integral path was to be carried out carefully, paying attention to a numerical behavior of the integrand. The present model system exhibited a nonmonotonic behavior in the integrands for TDI along the mixed-coupling integral path and also showed a relatively large statistical error. A coincidence within a statistical error was obtained among the results of the free energy differences evaluated by FEP, TDI with the sequential-coupling, and TDI with the mixed-coupling. The last one is most attractive in terms of the computer power and is accurate enough if one uses a proper set of windows, taking the numerical behavior of the integrands into account. TDI along the sequential-coupling integral path would be the most convenient among the methods we tested, since it seemed to be well-balanced between the computational load and the accuracy. The numerical results reported in this article qualitatively agree with the experimental data for the potassium ion recognition by the 18C6 in aqueous solution.

All-atom molecular dynamics study of a spherical micelle composed of N-acetylated poly(ethylene glycol)-poly(gamma-benzyl L-glutamate) block copolymers: a potential carrier of drug delivery systems for cancer

An all-atom molecular dynamics simulation of a spherical micelle composed of amphiphilic N-acetylated poly(ethylene glycol)-poly(gamma-benzyl L-glutamate) (PEG-PBLG-Ac) block copolymers was performed in aqueous solution at 298.15 K and 1 atm. Such copolymers have received considerable attention as carriers in drug delivery systems. In this study, we used copolymers consisting of 11 EG units and 9 BLG units as models. Starting from the copolymers arranged spherically, the calculation predicted an equilibrium state consisting of a slightly elliptical micelle structure with a hydrophobic PBLG inner core and a hydrophilic PEG outer shell. The micelle structure was dynamically stable during the simulation, with the PEG blocks showing a compact helical conformation and the PBLG blocks an alpha-helix form. Multiple hydrogen bonds with solvent water molecules stabilized the helical conformation of the PEG blocks, leading to their hydration as shown by longer residence times of water molecules near the PEG ether oxygen atoms compared with that of bulk water. Some water molecules have also been found distributed within the hydrophobic core; they showed continuous exchange with bulk water during the simulation. Those molecules existed mostly as a cluster in spaces between the copolymers, forming hydrogen bonds among themselves as well as with the hydrophobic core through hydrophilic groups such as esters and amides. The water molecules forming hydrogen bonds with the micelle may play an important role in the stabilization of the micelle structure.

Statistical thermodynamics of biomembranes

An overview of the major issues involved in the statistical thermodynamic treatment of phospholipid membranes at the atomistic level is summarized: thermodynamic ensembles, initial configuration (or the physical system being modeled), force field representation as well as the representation of long-range interactions. This is followed by a description of the various ways that the simulated ensembles can be analyzed: area of the lipid, mass density profiles, radial distribution functions (RDFs), water orientation profile, deuterium order parameter, free energy profiles and void (pore) formation; with particular focus on the results obtained from our recent molecular dynamic (MD) simulations of phospholipids interacting with dimethylsulfoxide (Me(2)SO), a commonly used cryoprotective agent (CPA).