Dipolar Surfaces Research Articles

Two 'braking mechanisms' for tin phthalocyanine molecular rotors on dipolar iron oxide surfaces.

Manipulation of artificial molecular rotors/motors is a key issue in the field of molecular nanomachines. Here we assemble non-planar SnPc molecules on an FeO film to form two kinds of rotors with different apparent morphologies, rotational speeds and stabilities. Both kinds of rotors can switch to each other via external field stimulation and the switch depends on the polarity of the applied bias voltage. Furthermore, we reveal that the molecular fragment has a great influence on the motions of molecules. Combining scanning tunneling microscopy and DFT calculations, two braking mechanisms are addressed for molecular rotors. One is the transformation of adsorption configurations under the external electric field stimulus that enables the molecular rotor to stop/restart its rotation. The other is the introduction of embedded molecular fragments that act as a brake pad and can stop the molecular rotation. We find that the rotation can be recovered by separating the molecule from the fragments. Our study suggests a good system for manipulating molecular rotors' properties in nanophysics and has important value for the design of controllable molecular machines.

Influence of molecular dipole orientations on long-range exponential interaction forces at hydrophobic contacts in aqueous solutions.

Strong and particularly long ranged (>100 nm) interaction forces between apposing hydrophobic lipid monolayers are now well understood in terms of a partial turnover of mobile lipid patches, giving rise to a correlated long-range electrostatic attraction. Here we describe similarly strong long-ranged attractive forces between self-assembled monolayers of carboranethiols, with dipole moments aligned either parallel or perpendicular to the surface, and hydrophobic lipid monolayers deposited on mica. We compare the interaction forces measured at very different length scales using atomic force microscope and surface forces apparatus measurements. Both systems gave a long-ranged exponential attraction with a decay length of 2.0 ± 0.2 nm for dipole alignments perpendicular to the surface. The effect of dipole alignment parallel to the surface is larger than for perpendicular dipoles, likely due to greater lateral correlation of in-plane surface dipoles. The magnitudes and range of the measured interaction forces also depend on the surface area of the probe used: At extended surfaces, dipole alignment parallel to the surface leads to a stronger attraction due to electrostatic correlations of freely rotating surface dipoles and charge patches on the apposing surfaces. In contrast, perpendicular dipoles at extended surfaces, where molecular rotation cannot lead to large dipole correlations, do not depend on the scale of the probe used. Our results may be important to a range of scale-dependent interaction phenomena related to solvent/water structuring on dipolar and hydrophobic surfaces at interfaces.

The reconstruction of dipolar surfaces: a preliminary step for adsorption modeling

In this brief review the crystal dipolar surfaces and the way to stabilize them are discussed. Two classes of crystals are interested: intrinsically polar crystals and non‐polar crystals, where the polarity along some directions is the consequence of the alternating layers of positive and negative charges. I discuss the importance of taking into account the bulk symmetry of the crystal when a surface reconstruction is performed. As examples, different reconstructions of the {111} form of halite (NaCl), and the {01.2} and {00.1} forms of calcite (CaCO3) are discussed in detail.

Experimental and theoretical aspects of the growth and equilibrium morphology of NaNO3

AbstractSodium nitrate (mineral nitratine, NaNO3, space group R $ \bar 3 $c at room temperature) is a material used in a great variety of fields, such as the molten salt technology. It is also of very interest from a crystallographic point of view, being iso‐structural with calcite (CaCO3). We performed several growth experiments in aqueous solution, near‐equilibrium, with the aim of finding the ideal conditions for investigating both face‐by‐face growth kinetics and surface characterization, as well as the influence of K+ and Li+ ions on the growth morphology. Single {10.4} rhombohedral crystals have been systematically grown from pure aqueous solution, while {00.1} truncated rhombohedra were obtained in the presence of K+ and Li+ ions in the mother solution. When determining the theoretical equilibrium morphology of NaNO3 crystals, the {00.1} and {10.4} surfaces were studied by using the 2D‐slab model and relaxed and athermal surface energies (surfaces energies at T = 0 K) were evaluated at DFT level (B3LYP Hamiltonian) by using the CRYSTAL06 package. The {00.1} dipolar surfaces were reconstructed by the octopolar Hartman‐Lacmann's model, already proposed for NaCl‐like lattices. This type of reconstruction is the only one which respects the bulk symmetry of the crystal. The octopolar Na+ terminated {00.1} form resulted to be more stable than the NO3– terminated one. However, the {00.1} form does not enter the theoretical athermal equilibrium shape of nitratine crystals, being γ(10.4)Na=160, γ(00.1)Na= 695 and γ(00.1)NO3=1535 erg/cm2. Finally, a simple 2D‐epitaxy model is proposed to explain the appearance of NaNO3{00.1} truncated rhombohedra in the presence of K+ and Li+ ions. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Surface reconstructions and relaxation effects in a centre-symmetrical crystal: the {00.1} form of calcite (CaCO3)

Reconstructing polar surfaces of crystals represents the pathway needed to obtain both surface stability and minimum of surface free energy. Knowing these equilibrium conditions is, in turn, necessary to interpret surface kinetics, especially when foreign adsorption or epitaxial phenomena are involved. Here we reconstructed the {00.1} dipolar surfaces of a centre-symmetrical crystal, calcite (CaCO3), either by removing one half of the ions in the outermost layer of the face, or by applying calcite to the octopolar Lacmann's model, already proposed for NaCl-like lattices. This second way respects the bulk symmetry of the crystal and was carried out by removing ¾ of the ions in the outermost layer and ¼ in the second to last one, respectively. Relaxed and athermal surface energies were determined at the empirical level and the octopolar CO3 terminated {00.1} form was the most stable. Moreover, calculation shows that the {00.1} form, along with the {10.4} and {01.2} rhombohedra, can enter the athermal equilibrium morphology of the calcite crystal. Finally, when recollecting our results on the reconstructed {01.2} form of calcite and {111} NaCl octahedron, it can be stressed that the bulk crystal symmetry has to be considered if one aims to achieve the self consistency of the surface reconstruction (Curie's principle).

Polarization of Water near Dipolar Surfaces: A Simple Model for Anomalous Dielectric Behavior

A model for the electrostatic interactions in water in the vicinity of a surface is suggested, which accounts, within the Poisson-Boltzmann mean field approach, for the screening of the charges and for the coupling interactions between neighboring dipoles. When the water molecules near a solid surface are assumed to be organized in icelike layers, the polarization is not a continuous function but exists only at the discrete positions of the water molecules. The particular positions of the water molecules in the icelike structure govern the manner in which the average water dipoles align with each other. On the basis of this model, one could explain the nonmonotonic behavior of the polarization and the electrical potential as well as the anomalous dielectric response of water (the nonproportionality of the polarization and the macroscopic electric field), which were obtained recently via molecular dynamics simulations.

Atomistic models for CeO(2)(111), (110), and (100) nanoparticles, supported on yttrium-stabilized zirconia.

Ceria is an important component in three-way catalysts for the treatment of automobile exhaust gases owing to its ability to store and release oxygen, a property known as the oxygen storage capacity. Much effort has been focused on increasing the OSC of ceria, and one avenue of exploration is the ability to fabricate CeO(2)-based catalysts, which expose reactive surfaces. Here we show how models for a polycrystalline CeO(2) thin film, which expose the (111), (110), and dipolar (100) surfaces, can be synthesized. This is achieved by supporting the CeO(2) thin film on an yttrium-stabilized zirconia substrate using a simulated amorphization and recrystallization strategy. In particular, the methodology generates models which reveal the atomistic structures present on the surface of the reactive faces and provides details of the grain-boundary structures, defects (vacancies, substitutionals, and clustering), and epitaxial relationships. Such models are an important first step in understanding the active sites at the surface of a catalytic material.

Modelling the effect of water on the surface structure and stability of forsterite

We describe the application of atomistic simulation techniques to investigate the effect of associative and dissociative adsorption of water on the structures and stabilities of the low-index surfaces of forsterite. All surfaces are amenable to associative adsorption of water, while dissociative adsorption is energetically favourable on all but the non-dipolar {1 0 0} surface. Often, otherwise unstable (dipolar) surfaces are stabilised to a large extent by hydration, e.g. the dipolar {0 1 0} surface. However, on thermodynamic grounds we do not expect associatively adsorbed water to dissociate on all surfaces, as the energies released for dissociative adsorption of water on the non-dipolar {0 1 0} and {1 0 0} surfaces are less than those released for associative adsorption. As such, there is no energetic incentive for the associatively adsorbed water molecules to dissociate. The stabilities of the two terminations of the {0 1 0} surface, the main cleavage plane of forsterite, are reversed when hydroxylated, indicating that some dissolution of the magnesium ions may occur upon hydration, which is shown to be an exothermic process for both surface terminations. The equilibrium morphology was calculated as a way of assessing the change in surface energies. The experimental morphology of forsterite is adequately reproduced, suggesting that the relative stabilities of the surfaces, both unhydrated and hydroxylated, are calculated correctly.

The interaction of dipolar layers. Hydration forces and image charges

The free energy of interaction between two parallel infinite dipolar surfaces is calculated using a Monte Carlo simulation technique. The surfaces and surrounding space are treated as dielectric continua, albeit with varying dielectric permittivity, while the dipoles are handled explicitly taking all interactions into account. The remaining adhesive energy between two hydrocarbon phases in water is not calculated here but obtained from experiment. Within this semi-empirical theory we find that the free energy of interaction when bringing two dipolar layers from infinity into contact, can be either repulsive or attractive depending on the system parameters. For a reasonable choice of these, the estimated free energy of interaction can be made to agree with experimental data.

Interaction between dipolar surfaces in a dielectric continuum model

The force between surfaces bearing mobile, orientable dipoles is analyzed in a dielectric continuum model that includes the difference in dielectric permittivity on either side of the surfaces. The basis for the approach is a recently derived perturbation formula for the interaction free energy. The necessary input to the perturbation expression, the dipole susceptibility for a single surface, is obtained from Monte Carlo simulations

The structure of the interface in the solvent-mediated interaction of dipolar surfaces

Interaction of two dipolar surfaces separated by a polar medium is considered within the framework of nonlocal electrostatics. The dipolar-surface layers are modelled as regular lattices with fixed orientation of dipoles which are immersed into the solvent; solvent response is characterized by nonlocal dielectric function. The model is elaborated in order to reveal the role of the dipolar-layer discreteness in the electric field produced by one surface and the interaction between two surfaces (which gives rise to the so-called «hydration» or «structural» force acting between mineral surfaces and phospholipid bilayers). The discreteness effects are present only for commensurate lattices. Their special mutual arrangement then may lead to considerable reduction of structural forces,viz. the usual repulsion regime may change at short distances to attraction. Conditions are considered when repulsion is entirely replaced by attraction,i.e. the «hydration barrier» disappears. In appended note we discuss the role of solvation of surface dipolar groups. We propose an explanation of why two modes of decay (one with oscillative fine structure) may be present in the dependence of the force upon distance, if the surface dipolar groups are immersed deep enough in the solvent, and how the long-range oscillative mode disappears when the surface is but weakly solvated.

The forces between surfaces of mobile, orientable dipoles. The method of reflection coefficients

The interaction free energy between two planar bodies interacting across a uniform dielectric, expressed as a sum over allowed electrodynamic modes, is interpreted in terms of reflection coefficients. It is proved that, within the approximations implied by this modal approach, the force between identical bodies interacting across a uniform dielectric is always attractive. Explicit electrostatic results for interacting dipolar surfaces are obtained and these are applicable to the measured long range attractive forces between polar lipid bilayers.