Search

Bilayers of amphiphiles can organize into spherical vesicles, nanotubes, planar, undulating, and helical nanoribbons, and scroll-like cochleates. These bilayer-related architectures interconvert under suitable conditions. Here, a charged, chiral amphiphile (palmitoyl-lysine, C16-K1) is used to elucidate the pathway for planar nanoribbon to cochleate transition induced by salt (NaCl) concentration. In situ small- and wide-angle X-ray scattering (SAXS/WAXS), atomic force and cryogenic transmission electron microscopies (AFM and cryo-TEM) tracked these transformations over angstrom to micrometer length scales. AFM reveals that the large length (L) to width (W) ratio nanoribbons (L/W > 10) convert to sheets (L/W → 1) before rolling into cochleates. A theoretical model based on electrostatic and surface energies shows that the nanoribbons convert to sheets via a first-order transition, at a critical Debye length, with 2 shallow minima of the order of thermal energy at L/W >> 1 and at L/W = 1. SAXS shows that interbilayer spacing (D) in the cochleates scales linearly with the Debye length, and ranges from 13 to 35 nm for NaCl concentrations from 100 to 5 mM. Theoretical arguments that include electrostatic and elastic energies explain the membrane rolling and the bilayer separation-Debye length relationship. These models suggest that the salt-induced ribbon to cochleate transition should be common to all charged bilayers possessing an intrinsic curvature, which in the present case originates from molecular chirality. Our studies show how electrostatic interactions can be tuned to attain and control cochleate structures, which have potential for encapsulating, and releasing macromolecules in a size-selective manner. [ABSTRACT FROM AUTHOR]

By combining molecular dynamics simulations and analytical arguments, we investigate the elastic properties of charged lipid bilayers. We show that electrostatic interactions between the head groups can lead to solidification of the lipid bilayer that would otherwise be in a liquid state if the charges were absent. All elastic parameters of the bilayer such as the bending rigidity κ and the two-dimensional bulk modulus λ and Young's modulus Y are found to depend on the values of the charges assigned to the lipid head groups. To extract κ and λ, we fit the molecular dynamics data to a standard elastic model for lipid bilayers. Moreover, we analytically obtain the dependence of the Young modulus Y on the relative strengths of electrostatic and van der Waals interactions in the zero temperature limit. [ABSTRACT FROM AUTHOR]

Motivated by recent experimental findings, we present here a minimal analytical model illustrating that the steric interactions among the ionic components can provide a simple, generic mechanism for like-charge crystallization in prototypical nanoparticle systems with counterions in polar solvents. In particular, the underlying steric interactions among these ionic components arise from the structural organization of the polar solvent molecules surrounding these ions as molecular dipole moments that may cooperatively enhance or counteract existing entropic depletion and electrostatic forces. Phenomenologically capturing these steric effects, we assume only the existence of a short-range pairwise Gaussian interaction, which has already been employed usefully for nanoparticles with hydrophillic surfaces or grafted-polymer coatings, among these ionic components (nanoparticles and counterions). The corresponding Gaussian interaction parameters characterize tunable interaction strengths. Making use of an analytically obtained effective pairwise potential between two nanoparticles, upon the contraction of counterions, we derive phase diagrams for nanoparticle systems of varying charge- and size-ratios as a function of particle densities, and observe crystallization for a range of parameters. We further demonstrate that our minimal model is compatible with the phenomenon of charge asymmetry. [ABSTRACT FROM AUTHOR]

We investigate the equilibrium properties and the underlying dynamics of emulsions formed in asymmetric A-B copolymers in matrices of immiscible B and C molecular fluids using coarse-grained molecular dynamics simulations. The emulsions are generated by introducing net attractions among the A units of the copolymers and the C molecules. They coexist with an absorbed copolymer monolayer. We determine the interfacial properties as the emulsions are forming. In general, highly asymmetric copolymers self-assemble within the B-matrix phase into swollen micelles; the cores of which are composed of C-component material. Less asymmetric copolymers, however, after initially budding and eventually fissioning from the interfacial copolymer monolayer, generate emulsified “inverse swollen micelles” within the C-matrix phase. These stable inverse (crew-cut) swollen micelles, which form under the inward bending of the saturated or oversaturated interfaces toward the longer B-block due to the attraction between the A and C units, can encapsulate large amounts of B-matrix component in their cores. This monolayer collapse mechanism can be exploited to generate nanoreactors or containers that enhance the delivery of molecular components into immiscible molecular fluid environments. [ABSTRACT FROM AUTHOR]

The swelling of polyelectrolyte (PE) gels by 1:1 and 2:1 salts is studied via osmotic ensemble Monte Carlo simulations at constant osmotic pressure and electrolyte chemical potential of a reservoir phase in equilibrium with a model PE network. Large molecular weight gels exhibit a remarkable swelling response to small changes in 2:1 salt concentration. Gel collapse is accompanied by the formation of a previously unknown heterogeneous nanostructure, predicted by theory and observed in simulations, consisting of regions dense in monomers coexisting with regions rich in mono- and divalent ions. [ABSTRACT FROM AUTHOR]

We study the equilibrium thermodynamics between two ternary ionic systems in immiscible solvents characterized by different dielectric constants. We consider system geometries wherein the two phases of immiscible solvents occupy, respectively, semi-infinite regions of space separated by neutral and charged planar interfaces. Specifically we analyze the case where the ternary system is composed of a pair of symmetric ions plus a minority charged component of high valence. We describe the system by means of a nonlinear mean-field theory. We first obtain exact analytical solutions for the electrostatic potentials, as well as density profiles for a symmetric binary system, and then extend these results to the ternary case using the perturbation theory. We show that the corresponding adsorption and depletion of multivalent macroions at the interface are highly enhanced when compared with the monovalent case. [ABSTRACT FROM AUTHOR]

The structure of soft matter systems at interfaces is of utmost importance in the fields of nanopatterning and self-assembly. It has been shown that lamellar and hexagonal patterns can form on interfaces, for a wide variety of systems. The asphericity of charged domains is considered here for different strengths of the electrostatics, determined by the interface media, relative to the short range van der Waals interactions between the molecular components. The phase behavior of the surface structure is explored by using molecular dynamics simulations, including some dynamical aspects of the interaction between neighboring domains, using the Lindemann criterion [F. Lindemann, Z. Phys. 11, 609 (1910)]. The charge ratio of the electrostatic components influences the shape of the domains, as well as the degree of local order in the interdomain structure. [ABSTRACT FROM AUTHOR]

The microphase separation transition for asymmetric triblock and (A–B)m star copolymer melts is investigated following a mean-field approach proposed by Leibler for diblock copolymer melts. Continuous transitions are found in all triblock architectures and in stars for any m at some composition fc. The phase diagram for the equilibrium morphologies is notably altered by varying the architecture of triblock copolymers. [ABSTRACT FROM AUTHOR]