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Solvent mediated interactions between model nano-particles and interfaces: A microscopic approach

Paul Hopkins1, Andrew J. Archer2 and Robert Evans1

1Department of Physics, University of Bristol, UK

2Department of Mathematical Sciences, University of Loughborough, UK

We determine the solvent mediated contribution to the effective potentials for model colloidal or nano-particles dispersed in a binary solvent that exhibits fluid-fluid phase separation. The interactions between the solvent particles are taken to be purely repulsive point Yukawa pair potentials. Using a simple density functional theory we calculate the density profiles of both solvent species in the presence of the `colloids', which are treated as external potentials, and determine the solvent mediated (SM) potentials. Specifically, we calculate SM potentials between (i) two colloids, (ii) a colloid and a planar fluid-fluid interface, and (iii) a colloid and a planar wall with an adsorbed wetting film. We consider three different types of colloidal particles: colloid A which prefers the bulk solvent phase rich in species 2, colloid C which prefers the solvent phase rich in species 1, and `neutral' colloid B which has no strong preference for either phase, i.e. the free energies to insert the colloid into either of the coexisting bulk phases are almost equal. When a colloid which has a preference for one of the two solvent phases is inserted into the disfavoured phase at statepoints close to coexistence a thick adsorbed `wetting' film of the preferred phase may form around the colloids. The presence of the adsorbed film has a profound influence on the form of the SM potentials. In case (i) reducing the separation between the two colloids of type A leads to a bridging transition whereby the two adsorbed films connect abruptly and form a single fluid bridge. The SM potential is strongly attractive in the bridged configuration. A similar phenomenon occurs in case (iii) whereby the thick adsorbed film on colloid A and that at the planar wall, which prefers the same phase as colloid A, connect as the separation between the colloid and the wall is reduced. In both cases the bridging transition is accompanied, in this mean-field treatment, by a discontinuity of the SM force. On the other hand, for the same wall, and a colloid of type C, the SM potential is strongly repulsive at small separations. For case (ii), inserting a single colloidal particle near the planar fluid-fluid interface of the solvent, the density profiles of the solvent show that the interface distortion depends strongly on the nature of the colloid-solvent interactions. When the interface disconnects from the colloid, once again there is a discontinuity in the SM force.

Computer simulations of nanoparticles at fluid interfaces

Fernando Bresme

Department of Chemistry, Imperial College London, UK

We have investigated using computer simulations and thermodynamic theories the behaviour of nanoparticles adsorbed at liquid-liquid and liquid-vapour interfaces. Using this approach we have quantified the stability of nanoparticles at fluid interfaces in terms of the of the line tension, particle size and particle geometry. The influence of line tension on nanoparticle adsorption is particularly strong in very anisotropic nanoparticles. We have extended the thermodynamic model to describe the behaviour of anisotropic nanoparticles in the presence of external fields (e.g., electrostatic or magnetic). We find that these simple models quantitatively predict the behaviour of the particles as a function of the field strength. Interestingly, the nanoparticles feature orientational transitions, characterized by a discontinuous change of the particle orientation at a specific field strength.

I will also discuss the relevance of the interfacial degrees of freedom in determining the interactions between nanoparticles adsorbed at fluid interfaces. The solvent mediated interactions between the nanoparticles are very different from the interactions in the bulk phase. We interpret these results in terms of the line tension acting at short distances, and capillary wave interactions acting at long interparticle separations.

Polypeptide aggregation in the vicinity of a lipid membrane - insights from simulations

Andrey Brukhno

Centre for Molecular NanoScience, School of Chemistry, University of Leeds, UK

We report on discontinuous molecular dynamics simulations of polypeptides in the vecinity of a phospholipid bilayer. Aggregation of peptides and their influence on the membrane, and vice versa, are under study. A coarse-grained peptide model is used, based on a so-called tube polymer capable of folding into alpha-helical motives and aggregating into beta-strands and bundles, while a similar three-bead model of a lipid is employed. We find that the conditions for templated peptide aggregation on the membrane comprise a very narrow window in terms of temperature and peptide-peptide and peptide-lipid interactions. We observe three main scenarios are possible: (i) adsorption and dissolution of separate peptides within the membrane, where no polypeptide complexes are formed; (ii) adsorption and aggregation of peptides on the membrane surface, followed by insertion of small aggregates into the bilayer environment resulting in a pore formation; (iii) aggregation of polypeptides in the bulk followed by incorporation of the preformed structured complexes into the membrane while pores do not emerge.

Amphiphilic dendrimer at the air-water interface: a molecular dynamic study

Paola Carbone

School of Chemical Engineering and Analytical Science, University of Manchester, UK

Hydrophilic dendrimers can be functionalized with long hydrocarbon chains forming amphiphilic soft particles that at air-water interface self-assemble in film of monolayer thickness. The film formation and its stability depend on a combination of both dispersive and hydrogen bonds interactions.

By means of atomistic molecular dynamic simulations we investigate the behaviour of single molecule and aggregates of polyamidoamine (PAMAM) dendrimers of generation 3 and 4 functionalized with C10 chains when placed at the air-water interface. The shape of the molecules and their dynamics as well as the role that the inter and intra dendrimer hydrogen bonds and dispersive interactions have on their self assembly process is investigated with the aim to gain a more generic picture of the behaviour of these amphiphilic particles.

T. Zhang, P. R. Dvornic, S. N. Kaganove, Langmuir, 23, 10589, (2007)

B. M. Mladek, G. Kahl, C. N. Likos, PRL 100, 028301 (2008)

P. Carbone, F. Müller-Plathe, Soft Matter, 5, 2638, (2009)

Monte Carlo simulation of polydisperse spheres on a spherical droplet

Sara fortuna

Department of Chemistry, University of Warwick, UK

We studied the packing pattern of silica nanoparticles on the surface of spherical poystyrene latex droplets with Monte Carlo simulations [1]. The experimental system has been modelled as a set of interacting spheres on a spherical surface. The information supplied by this model has been complementary to the experimental data, to determine whether thermodynamic equilibrium has been reached and to study the effect of the polydispersity of the spherical nanoparticles on the self-assembled structure.

[1] S. Fortuna, C. A. L. Colard, A. Troisi and S. A. F. Bon, Langmuir, 2009, 25 (21), pp 12399-12403

Two dimensional colloidal alloys

Adam Law1, Tommy S. Horozov2 & Martin Buzza1

1Department of Physics, University of Hull, UK

2Department of Chemistry, University of Hull, UK

We study both experimentally and theoretically the structure of binary colloidal monolayers at a fluid-fluid interface. Specifically the experimental system consists of large and small hydrophobic silica particles at an octane/water interface which readily form an ordered lattice due to strong long range forces between the particles through the oil phase. For small to large particle ratios of up to 2:1, the small particles are observed to occupy the energy minima between the hexagonally ordered large particles, in accordance with the predictions of energy minimization schemes. However for small to large particle ratios between 2:1 and 5:1, the additional small particles, instead of filling the secondary energy minima created by the 2:1 structure, prefer instead to form local regions of either 2:1 or 6:1 minimum energy structures. We postulate that this is an entropic effect due to the energy landscape of 5:1 monolayers being much rougher than that of either 2:1 or 6:1 monolayers. This confirmed by Monte Carlo simulations where we find that the small particle order-disorder transition for 5:1 monolayers occurs at much higher interaction strengths (i.e., lower reduced temperatures) compared to either 2:1 or 6:1 monolayers.

Saddle-splay modulus of a particle-laden fluid interface

Sergey Lishchuk

Materials and Engineering Research Institute, Sheffield Hallam University, UK


Lattice Boltzmann simulations for particles at interfaces, and other problems

Kevin Stratford

Edinburgh Parallel Computing Centre, University of Edinburgh, UK

I will provide a brief overview of work on lattice Boltzmann in the soft matter physics group at The University of Edinburgh. Lattice Boltzmann provides a coarse-grained numerical approach that is well-suited to the study of colloidal systems. One area of particular interest at Edinburgh (in both simulation and experiment) has been that of particle jamming at interfaces, and the resulting 'bijel' structures. If time permits, I will also mention more recent work on the interaction between motile bacteria and interfaces.

Do colloidal interfaces have rigidity, as well as tension?

Matthew Turner

Department of Physics, University of Warwick, UK

Fluid interfaces have surface tension. Very general arguments imply that the physics of soft interfaces must depend on a length scale that is the square root of some rigidity divided by this tension. What is this rigidity? Can it be measured? We discuss how it controls (i) forces exerted by the interface on adsorbed particles and (ii) the fluctuation spectrum of interfacial undulations.