328 research outputs found
A van der Waals free energy in electrolytes revisited
A system of three electrolytes separated by two parallel planes is
considered. Each region is described by a dielectric constant and a Coulomb
fluid in the Debye-H\"uckel regime. In their book Dispersion Forces, Mahanty
and Ninham have given the van der Waals free energy of this system. We rederive
this free energy by a different method, using linear response theory and the
electrostatic Maxwell stress tensor for obtaining the dispersion force.Comment: 7 pages. PACS numbers updated. References update
Retardation turns the van der Waals attraction into Casimir repulsion already at 3 nm
Casimir forces between surfaces immersed in bromobenzene have recently been
measured by Munday et al. Attractive Casimir forces were found between gold
surfaces. The forces were repulsive between gold and silica surfaces. We show
the repulsion is due to retardation effects. The van der Waals interaction is
attractive at all separations. The retardation driven repulsion sets in already
at around 3 nm. To our knowledge retardation effects have never been found at
such a small distance before. Retardation effects are usually associated with
large distances
Polarization and Charge Transfer in the Hydration of Chloride Ions
A theoretical study of the structural and electronic properties of the
chloride ion and water molecules in the first hydration shell is presented. The
calculations are performed on an ensemble of configurations obtained from
molecular dynamics simulations of a single chloride ion in bulk water. The
simulations utilize the polarizable AMOEBA force field for trajectory
generation, and MP2-level calculations are performed to examine the electronic
structure properties of the ions and surrounding waters in the external field
of more distant waters. The ChelpG method is employed to explore the effective
charges and dipoles on the chloride ions and first-shell waters. The Quantum
Theory of Atoms in Molecules (QTAIM) is further utilized to examine charge
transfer from the anion to surrounding water molecules.
From the QTAIM analysis, 0.2 elementary charges are transferred from the ion
to the first-shell water molecules. The default AMOEBA model overestimates the
average dipole moment magnitude of the ion compared with the estimated quantum
mechanical value. The average magnitude of the dipole moment of the water
molecules in the first shell treated at the MP2 level, with the more distant
waters handled with an AMOEBA effective charge model, is 2.67 D. This value is
close to the AMOEBA result for first-shell waters (2.72 D) and is slightly
reduced from the bulk AMOEBA value (2.78 D). The magnitude of the dipole moment
of the water molecules in the first solvation shell is most strongly affected
by the local water-water interactions and hydrogen bonds with the second
solvation shell, rather than by interactions with the ion.Comment: Slight revision, in press at J. Chem. Phy
The influence of ion binding and ion specific potentials on the double layer pressure between charged bilayers at low salt concentrations
Measurements of surface forces between double-chained cationic bilayers adsorbed onto molecularly smooth mica surfaces across different millimolar salt solutions have revealed a large degree of ion specificity [Pashley et al., J. Phys. Chem. 90, 1637 (1986)]. This has been interpreted in terms of highly specific anion binding to the adsorbed bilayers. We show here that inclusion in the double layer theory of nonspecific ion binding and ion specific nonelectrostatic potentials acting between ions and the two surfaces can account for the phenomenon. It also gives the right Hofmeister series for the double layer pressure.M.B. thanks the Swedish Research Council and the German
Arbeitsgemeinschaft industrieller Forschungvereinigungen
Otto von Guericke e.V. AiF for financial support.
E.R.A.L. and F.W.T. thank FAPERJ and CNPq the Brazilian
Agencies for financial support
Ultrathin Metallic Coatings Can Induce Quantum Levitation between Nanosurfaces
There is an attractive Casimir-Lifshitz force between two silica surfaces in
a liquid (bromobenze or toluene). We demonstrate that adding an ultrathin
(5-50{\AA}) metallic nanocoating to one of the surfaces results in repulsive
Casimir-Lifshitz forces above a critical separation. The onset of such quantum
levitation comes at decreasing separations as the film thickness decreases.
Remarkably the effect of retardation can turn attraction into repulsion. From
that we explain how an ultrathin metallic coating may prevent
nanoelectromechanical systems from crashing together.Comment: 4 pages, 5 figure
Surface van der Waals Forces in a Nutshell
Most often in chemical physics, long range van der Waals surface interactions
are approximated by the exact asymptotic result at vanishing distance, the well
known additive approximation of London dispersion forces due to Hamaker.
However, the description of retardation effects that is known since the time of
Casimir is completely neglected for lack of a tractable expression. Here we
show that it is possible to describe surface van der Waals forces at arbitrary
distances in one single simple equation. The result captures the long sought
crossover from non-retarded (London) to retarded (Casimir) interactions, the
effect of polarization in condensed media and the full suppression of retarded
interactions at large distance. This is achieved with similar accuracy and the
same material properties that are used to approximate the Hamaker constant in
conventional applications. The results show that at ambient temperature,
retardation effects significantly change the power law exponent of the
conventional Hamaker result for distances of just a few nanometers.Comment: 6 pages + 4 figures + supplementary materia
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