Generation of Well-Relaxed All-Atom Models of Large Molecular Weight Polymer Melts: A Hybrid Particle-Continuum Approach Based on Particle-Field Molecular Dynamics Simulations

A procedure based on Molecular Dynamics (MD) simulations employing soft potentials derived from self-consistent field (SCF) theory (named MD-SCF) able to generate well-relaxed all-atom structures of polymer melts is proposed. All-atom structures having structural correlations indistinguishable from ones obtained by long MD relaxations have been obtained for poly­(methyl methacrylate) (PMMA) and poly­(ethylene oxide) (PEO) melts. The proposed procedure leads to computational costs mainly related on system size rather than to the chain length. Several advantages of the proposed procedure over current coarse-graining/reverse mapping strategies are apparent. No parametrization is needed to generate relaxed structures of different polymers at different scales or resolutions. There is no need for special algorithms or back-mapping schemes to change the resolution of the models. This characteristic makes the procedure general and its extension to other polymer architectures straightforward. A similar procedure can be easily extended to the generation of all-atom structures of block copolymer melts and polymer nanocomposites

Migration of Phospholipid Vesicles Can Be Selectively Driven by Concentration Gradients of Metal Chloride Solutions

We have investigated the migrations of phospholipid vesicles under the concentration gradients of metal ions. We microinjected metal chloride solutions, monovalent (NaCl and KCl), divalent (CaCl₂ and MgCl₂), and trivalent (LaCl₃) salts, toward phospholipid giant vesicles (GVs) composed of 1,2-dioleoyl-<i>sn</i>-glycero-3-phosphocholine (DOPC). For NaCl, CaCl₂, and MgCl₂ solutions, the GVs migrated straight toward the tip of the micropipette in response to the concentration gradients, whereas for KCl and LaCl₃, GVs moved to the opposite direction. Our motion tracking of lipid domains in a vesicle membrane showed no unidirectional flow in the membrane during the vesicle migration, indicating that the Marangoni mechanism is not responsible for the observed vesicle migration. We calculated the diffusiophoretic velocities for symmetric and asymmetrical electrolytes by solving the Stokes’ equation numerically. The theoretical diffusiophoretic velocities described the observed migration velocities well. Thus, we can control the migration of vesicle in response to the concentration gradient by adapting the electrolytes and the lipids

Hybrid Particle-Field Molecular Dynamics Simulations of Charged Amphiphiles in Aqueous Environment

<p>We develop and test specific coarse-grained models for charged amphiphilic systems such as palmitoyloleoyl phosphatidylglycerol (POPG) lipid bilayer, and sodium dodecyl sulphate (SDS) surfactant in aqueous environment, to verify the ability of the hybrid particle-field method to provide a realistic description of polyelectrolyte soft-matter systems. The intramolecular interactions are treated by a standard molecular Hamiltonian and the non-electrostatic intermolecular forces are described by density fields. Electrostatics is introduced as an additional external field obtained by a modified particle-mesh Ewald procedure. Molecular dynamics simulations indicate that the methodology is robust with respect to the choice of the relative dielectric constant, yielding the same correct qualitative behavior for a broad range of dielectric values. In particular, our methodology reproduces well the organization of the POPG bilayer, as well as the SDS concentration-dependent change in the morphology of the micelles from spherical to microtubular aggregates. </p

Hybrid Particle-Field Molecular Dynamics Simulations of Charged Amphiphiles in an Aqueous Environment

We develop and test specific coarse-grained models for charged amphiphilic systems such as palmitoyloleoylphosphatidylglycerol (POPG) lipid bilayer and sodium dodecyl sulfate (SDS) surfactant in an aqueous environment, to verify the ability of the hybrid particle-field method to provide a realistic description of polyelectrolytes. According to the hybrid approach, the intramolecular interactions are treated by a standard molecular Hamiltonian, and the nonelectrostatic intermolecular forces are described by density fields. Electrostatics is introduced as an additional external field obtained by a modified particle-mesh Ewald procedure, as recently proposed [Zhu et al. Phys. Chem. Chem. Phys. 2016, 18, 9799]. Our results show that, upon proper calibration of key parameters, electrostatic forces can be correctly reproduced. Molecular dynamics simulations indicate that the methodology is robust with respect to the choice of the relative dielectric constant, yielding the same correct qualitative behavior for a broad range of values. In particular, our methodology reproduces well the organization of the POPG bilayer, as well as the SDS concentration-dependent change in the morphology of the micelles from spherical to microtubular aggregates. The inclusion of explicit electrostatics with good accuracy and low computational cost paves the way for a significant extension of the hybrid particle-field method to biological systems, where the polyelectrolyte component plays a fundamental role for both structural and dynamical molecular properties