Dusan Bratko

Monte Carlo simulation for the potential of mean force between ionic colloids in solutions of asymmetric salts

A new technique for Monte Carlo sampling of the hard-sphere collision force has been applied to study the interaction between a pair of spherical macroions in primitive-model electrolyte solutions with valences 1:2, 2:1, and 2:2. Macroions of the same charge can attract each other in the presence of divalent counterions, in analogy with earlier observations for planar and cylindrical geometries. The attraction is most significant at intermediate counterion concentrations. In contrast to the entropic depletion force between neutral particles, attraction between macroions is of energetic origin. The entropic contribution to the potential of mean force is generally repulsive at conditions corresponding to aqueous colloids with or without salt. For systems with divalent counterions, the potentials of mean force predicted by mean-field approximations like the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory or the Sogami–Ise (SI) theory are qualitatively different from those observed in the simulations. However,...

Interaction between hydrophobic surfaces with metastable intervening liquid

Molecular simulation is used to elucidate hydrophobic interaction at atmospheric pressure where liquid water between apolar walls is metastable with respect to capillary evaporation. The steep increase of the estimated activation barrier of evaporation with surface–surface separation explains the apparent stability of the liquid at distances more than an order of magnitude below the thermodynamic threshold of evaporation. Solvation by metastable liquid results in a short-ranged oscillatory repulsion which gives rise to an irreversible potential barrier between approaching walls. The barrier increases with external pressure in accord with measured pressure-induced slowing of conformational transitions of biopolymers with strong hydrophobic interactions. At a sufficiently small separation, the force abruptly turns attractive signaling nucleation of the vapor phase. This behavior is consistent with the cavitation-induced hysteresis observed in a number of surface–force measurements for strongly hydrophobic s...

Polyelectrolyte solutions containing mixed valency ions in the cell model: A simulation and modified Poisson–Boltzmann study

Monte Carlo simulations of linear polyelectrolyte solutions containing mixed valency simple ions in the cylindrical cell model are reported. The equilibrium distributions of the simple ions and the osmotic pressure of the solution are calculated at various concentrations of the monomer units of the polyelectrolyte. Specifically, the following systems are studied—monovalent counterions with added 2:2 salt, divalent counterions with added 1:1 salt, and systems containing mono- and divalent counterions only, and mono- and trivalent counterions only. The simulation results are compared with the corresponding predictions from the Poisson–Boltzmann and modified Poisson–Boltzmann theories applied to the cell model. It is seen that upto moderate concentrations of the polyion, the modified Poisson–Boltzmann theory provides a very good description of the systems with deviations occurring at higher concentrations. The theory also reproduces the charge reversal observed in the simulations when strongly correlated cou...

Probing surface tension additivity on chemically heterogeneous surfaces by a molecular approach

Surface free energy of a chemically heterogeneous surface is often treated as an approximately additive quantity through the Cassie equation [Cassie ABD (1948) Discuss Faraday Soc 3:11–16]. However, deviations from additivity are common, and molecular interpretations are still lacking. We use molecular simulations to measure the microscopic analogue of contact angle, θc, of aqueous nanodrops on heterogeneous synthetic and natural surfaces as a function of surface composition. The synthetic surfaces are layers of graphene functionalized with prototypical nonpolar and polar head group: methyl, amino, and nitrile. We demonstrate positive as well as negative deviations from the linear additivity. We show the deviations reflect the uneven exposure of mixture components to the solvent and the linear relation is recovered if fractions of solvent-accessible surface are used as the measure of composition. As the spatial variations in polarity become of larger amplitude, the linear relation can no longer be obtained. Protein surfaces represent such natural patterned surfaces, also characterized by larger patches and roughness. Our calculations reveal strong deviations from linear additivity on a prototypical surface comprising surface fragments of melittin dimer. The deviations reflect the disproportionately strong influence of isolated polar patches, preferential wetting, and changes in the position of the liquid interface above hydrophobic patches. Because solvent-induced contribution to the free energy of surface association grows as cos θc, deviations of cos θc from the linear relation directly reflect nonadditive adhesive energies of biosurfaces.

Competition between protein folding and aggregation: A three-dimensional lattice-model simulation

Aggregation of protein molecules resulting in the loss of biological activity and the formation of insoluble deposits represents a serious problem for the biotechnology and pharmaceutical industries and in medicine. Considerable experimental and theoretical efforts are being made in order to improve our understanding of, and ability to control, the process. In the present work, we describe a Monte Carlo study of a multichain system of coarse-grained model proteins akin to lattice models developed for simulations of protein folding. The model is designed to examine the competition between intramolecular interactions leading to the native protein structure, and intermolecular association, resulting in the formation of aggregates of misfolded chains. Interactions between the segments are described by a variation of the Go potential [N. Go and H. Abe, Biopolymers 20, 1013 (1981)] that extends the recognition between attracting types of segments to pairs on distinct chains. For the particular model we adopt, t...

Water-mediated ordering of nanoparticles in an electric field

Interfacial polar molecules feature a strongly anisotropic response to applied electric field, favoring dipole orientations parallel to the interface. In water, in particular, this effect combines with generic orientational preferences induced by spatial asymmetry of water hydrogen bonding under confined geometry, which may give rise to a Janus interface. The two effects manifest themselves in considerable dependence of water polarization on both the field direction relative to the interface and the polarity (sign) of the field. Using molecular simulations, we demonstrate strong field-induced orientational forces acting on apolar surfaces through water mediation. At a field strength comparable to electric fields around a DNA polyion, the torques we predict to act on an adjacent nanoparticle are sufficient to overcome thermal fluctuations. These torques can align a particle with surface as small as 1 nm2. The mechanism can support electrically controlled ordering of suspended nanoparticles as a means of tuning their properties and can find application in electro-nanomechanical devices.

Multivalent ion–DNA interaction: Neutron scattering estimates of polyamine distribution

The partial structure factors pertaining to DNA–DNA, DNA–polyamine, and polyamine–polyamine density correlations in DNA fragment (contour length 54 nm) solutions have been measured with small angle neutron scattering and contrast matching in water. The effect of the polyamines putrescine and spermidine on the DNA molecular structure is gauged from the limiting behavior of the DNA–DNA partial structure factor at high values of momentum transfer. The double layer structure and the extent to which the polyamines can approach the DNA are derived from the DNA–polyamine and polyamine–polyamine partial structure factors. For this purpose, the structure factors are interpreted with the correlation functions derived from the classical Poisson–Boltzmann and the modified Poisson–Boltzmann equations and/or Monte Carlo simulation. For simple salt free DNA with tetramethylammonium or putrescine counterions, spatial fluctuations in the charge density are discussed in terms of the charge structure factor. The structural ...

Forces between aqueous nonuniformly charged colloids from molecular simulation

NVT Monte Carlo simulation results are presented for the forces between charged colloids within the primitive model for electrolytes. The calculations show that when charged colloids have a net dipole moment, a strong attraction can arise at short separations. The attractive force is not purely electrostatic; significant contributions follow from hard-sphere collisions between the electrolyte ions and the colloidal particles. In divalent electrolyte solutions, nonuniformly charged colloids show an oscillatory force profile as a function of separation, due to layering of electrolyte ions around the interacting colloids. Simulation results are compared to two analytical models derived from classical Debye–Huckel screened potentials. In the first model, contributions from charge–charge, dipole–dipole, and charge–dipole interactions are independently angle-averaged and then added to obtain the colloid–colloid potential. In the second model, the pair potential is obtained by simultaneously angle-averaging all ...

The influence of molecular-scale roughness on the surface spreading of an aqueous nanodrop

We examine the effect of nanoscale roughness on spreading and surface mobility of water nanodroplets. Using molecular dynamics, we consider model surfaces with sub-nanoscale asperities at varied surface coverage and with different distribution patterns. We test materials that are hydrophobic, and those that are hydrophilic in the absence of surface corrugations. Interestingly, on both types of surfaces, the introduction of surface asperities gives rise to a sharp increase in the apparent contact angle. The Cassie-Baxter equation is obeyed approximately on hydrophobic substrates, however, the increase in the contact angle on a hydrophilic surface differs qualitatively from the behavior on macroscopically rough surfaces described by the Wenzel equation. On the hydrophobic substrate, the superhydrophobic state with the maximal contact angle of 180 degrees is reached when the asperity coverage falls below 25%, suggesting that superhydrophobicity can also be achieved by the nanoscale roughness of a macroscopically smooth material. We further examine the effect of surface roughness on droplet mobility on the substrate. The apparent diffusion constant shows a dramatic slow down of the nanodroplet translation even for asperity coverage in the range of 1% for a hydrophilic surface, while droplets on corrugated hydrophobic surfaces retain the ability to flow around the asperities. In contrast, for smooth surfaces we find that the drop mobility on the hydrophilic surface exceeds that on the hydrophobic one.

A simple theory and Monte Carlo simulations for recognition between random heteropolymers and disordered surfaces

We study the adsorption of random heteropolymers (RHPs) on disordered multifunctional surfaces. Recent replica calculations suggest that when the statistics describing the sequence distribution and the surface site distributions are related in a special way, a first-order adsorption transition occurs in such systems. This phenomenon of recognition between RHPs and disordered surfaces due to statistical pattern matching is studied via Monte Carlo simulations and a simple nonreplica theory. The theory serves to elucidate the simulation results, and suggests that the origin of the first-order adsorption transition is the suppression of loop fluctuations due to competing interactions and the quenched disorder (i.e., frustration).