A new coarse-grained (CG) model is developed for water. Each CG unit consists of three charged sites, and there is an additional nonelectrostatic soft interaction between central sites on different units. The interactions are chosen to mimic the properties of 4-water clusters in atomistic simulations: the nonelectrostatic component is modeled using a modified Born−Mayer−Huggins potential, and the charges are chosen to reproduce the dipole moment and quadrupole moment tensor of 4-water clusters from atomistic simulations. The parameters are optimized to reproduce experimental data for the compressibility, density, and permittivity of bulk water and the surface tension and interface potential for the air−water interface. This big multipole water (BMW) model represents a qualitative improvement over existing CG water models; for example, it reproduces the dipole potential in membrane−water interface when compared to experiment, with modest additional computational cost as compared to the popular MARTINI CG model.

The effect of salt on the dynamics of water molecules follows the Hofmeister series. For some "structure-making" salts, the self-diffusion coefficient of the water molecules, D, decreases with increasing salt concentration. For other "structure-breaking" salts, D increases with increasing salt concentration. In this work, the concentration and temperature dependence of the self-diffusion of water in electrolyte solutions is studied using molecular dynamics simulations and pulsed-field-gradient NMR experiments; temperature-dependent viscosities are also independently measured. Simulations of rigid, nonpolarizable models at room temperature show that none of the many models tested can reproduce the experimentally observed trend for the concentration dependence of D; that is, the models predict that D decreases with increasing salt concentration for both structure-breaking and structure-making salts. Predictions of polarizable models are not in agreement with experiment either. These results suggest that many popular water models do not accurately describe the dynamic nature of the hydrogen bond network of water at room temperature. The simulations are in qualitative agreement, however, with experimental results for the temperature dependence of water dynamics; simulations and experiment show an Arrhenius dependence of D with temperature, T, with added salt, that is, ln D ∼ 1/T, over a range of temperatures above the freezing point of water.

The effect of solvent on the collapse dynamics of polymers is studied using computer simulation. Two cases are investigated, one where the solvent is incorporated through a pairwise additive attraction between the polymer beads and a random force on each polymer bead, and another where the solvent is incorporated in an explicit fashion as a second component. Brownian dynamics and molecular dynamics simulations are used in the former and latter model, respectively, with intermolecular interactions chosen so that the equilibrium size of the polymer is similar in both models at similar conditions. In the Brownian dynamics simulations, at short times local blobs of monomers are found separated by linear segments. With time the blobs grow in size and coalesce to form sausage like shapes. These sausages gradually become thicker and shorter until the final shape of a spherical globule is reached. The first stage is rapid whereas the second sausage-sphere stage is slow. In this stage the polymer often gets trapped in local minima and the change in size with time occurs through discrete jumps, and the equilibrium conformation is often not reached. In contrast, in the molecular dynamics simulations with explicit solvent, the size of the polymer changes smoothly with time, and the polymer does not get trapped in local minima for the cases investigated, although the sequence of polymer shapes is similar. This suggests that incorporating solvent molecules explicitly is important in the computer simulations of collapse and folding of polymers.

Supramolecular self-assembly enables access to designer soft materials that typically exhibit high-symmetry packing arrangements, which optimize the interactions between their mesoscopic constituents over multiple length scales. We report the discovery of an ionic small molecule surfactant that undergoes water-induced selfassembly into spherical micelles, which pack into a previously unknown, low-symmetry lyotropic liquid crystalline Frank-Kasper σ phase. Small-angle X-ray scattering studies reveal that this complex phase is characterized by a gigantic tetragonal unit cell, in which 30 sub-2-nm quasispherical micelles of five discrete sizes are arranged into a tetrahedral close packing, with exceptional translational order over length scales exceeding 100 nm. Varying the relative concentrations of water and surfactant in these lyotropic phases also triggers formation of the related Frank-Kasper A15 sphere packing as well as a common body-centered cubic structure. Molecular dynamics simulations reveal that the symmetry breaking that drives the formation of the σ and A15 phases arises from minimization of local deviations in surfactant headgroup and counterion solvation to maintain a nearly spherical counterion atmosphere around each micelle, while maximizing counterion-mediated electrostatic cohesion among the ensemble of charged particles.self-assembly | liquid crystals | surfactants | Frank-Kasper phases | lyotropic phase M olecular self-assembly provides a facile means of constructing a plethora of multifunctional soft materials, with mesoscopic structures that dictate their tailored properties and performance applications. Driven by noncovalent interactions between constituents, block polymers (1), giant shape amphiphiles (2), thermotropic liquid crystals (LCs) (3), lyotropic liquid crystals (LLCs) (4), and colloids (5) exemplify soft matter systems that spontaneously form periodic 1D lamellar phases, 2D columnar structures, and 3D packings of spherical particles. Columnar and spherical phases are useful as templates for mesoporous heterogeneous catalysts (6) and as microscale photonic bandgap materials (7). Manipulating supramolecular self-assembly to achieve specific materials morphologies and functions requires a fundamental understanding of the interplay between the structure and symmetry of the constituents and their multibody interactions.Although the packing of spherical objects (e.g., oranges and billiard balls) seems intuitively simple, point particles form a dizzying array of periodic crystals, quasicrystals (QCs), and structurally disordered glasses. Metallic elements typically form high-symmetry body-centered cubic (BCC), hexagonally closest-packed, and facecentered cubic (FCC) structures, due to the isotropy of metallic cohesion mediated by itinerant electrons (8). A few pure elements (e.g., Mn and U) form low-symmetry crystals with large and complex unit cells that maximize metallic cohesion against local constraints, such as maximization of Fermi surface sphericity (9).Sphere-forming soft mater...

The isotropic–nematic phase transition in semiflexible hard chain fluids is investigated via an Onsager type density functional theory. The angle-dependent excluded volume of two chains required in the theory is obtained via Monte Carlo simulations. The theory predicts an isotropic to nematic phase transition at lower densities than those predicted by previous theories. These results compare favorably with available simulation data.