Inside and out: Surface thermodynamics from positive to negative curvature.

To explore the curvature dependence of solid-fluid interfacial thermodynamics, we calculate, using Grand Canonical Monte Carlo simulation, the surface free energy for a 2d hard-disk fluid confined in a circular hard container of radius R as a function of the bulk packing fraction η and wall curvature C̄=-1/R. (The curvature is negative because the surface is concave.) Combining this with our previous data [Martin et al., J. Phys. Chem. B 124, 7938-7947 (2020)] for the positive curvature case (a hard-disk fluid at a circular wall, C̄=+1/R), we obtain a complete picture of surface thermodynamics in this system over the full range of positive and negative wall curvatures. Our results show that γ is linear in C̄ with a slope that is the same for both positive and negative wall curvatures, with deviations seen only at high negative curvatures (strong confinement) and high density. This observation indicates that the surface thermodynamics of this system is consistent with the predictions of so-called morphometric thermodynamics at both positive and negative curvatures. In addition, we show that classical density functional theory and a generalized scaled particle theory can be constructed that give excellent agreement with the simulation data over most of the range of curvatures and densities. For extremely high curvatures, where only one or two disks can occupy the container at maximum packing, it is possible to calculate γ exactly. In this limit, the simulations and density functional theory calculations are in remarkable agreement with the exact results.

Surface Free Energy of a Hard-Disk Fluid at Curved Hard Walls: Theory and Simulation.

In this work, we examine the surface thermodynamics of a hard-disk fluid at curved hard walls using Monte Carlo (MC) simulation and a generalized scaled particle theory (gSPT). The curved walls are modeled as hard disks of varying radii, R. The surface free energy, γ, and excess surface volume, vex, for this system are calculated as functions of both the fluid packing fraction and the wall radius. The simulation results are used to test, for this system, the assumptions of morphometric thermodynamics (MT), which predicts that both γ and vex are linear functions of the surface curvature, 1/R, for a two-dimensional system. In addition, we compare the simulation results to the gSPT developed in this work, as well as with virial expansions derived from the known virial coefficients of the binary hard-sphere fluid. At low to intermediate packing fractions, the non-MT terms (terms of higher order than 1/R in a expansion of γ and vex) of γ are zero within the simulation error; however, at the highest densities, deviations from MT become significant, similar to what was seen in our earlier simulation work on the three-dimensional hard-sphere/hard-wall system. In addition, the new gSPT gives improved results for both γ and vex over standard scaled particle theory (SPT) but underestimates the deviations from MT at high density.

Thermodynamics of the hard-disk fluid at a planar hard wall: Generalized scaled-particle theory and Monte Carlo simulation.

A generalized scaled-particle theory for the uniform hard-disk mixture is derived in the spirit of the White Bear II free energy of the hard-sphere fluid [H. Hansen-Goos and R. Roth, J. Phys. C: Condens. Matter 18, 8413 (2006)]. The theory provides a very simple result for the interfacial free energy γ of the hard-disk fluid at a planar hard wall (which in d = 2 is a line) in terms of the equation of state. To complement and assess the theory, we perform Monte Carlo simulations from which we obtain γ using Gibbs-Cahn integration. While we find excellent overall agreement between theory and simulation, it also becomes apparent that the set of scaled-particle variables available in d = 2 is too limited, prohibiting a quasi-exact result for γ. Furthermore, this is reflected in the mixture equation of state resulting from our theory, which, similar to a previous attempt by Santos et al. [Mol. Phys. 96, 1 (1999)], displays a small but systematic deviation from simulations.

Electronic Coupling for Donor-Bridge-Acceptor Systems with a Bridge-Overlap Approach.

Understanding the modulation of the electronic coupling in donor-acceptor systems connected through an aliphatic bridge is crucial from a fundamental point of view as well as for the development of organic electronics. In this work, we present a first-principles approach for the calculation of the electronic coupling (or transfer integrals) in such systems via a block-diagonalization of the Fock/Kohn-Sham matrix of the supersystem, followed by a projection on the basis of the fragment orbitals of the donor and acceptor groups. The strength of the approach is that the bridge is shared by the donor and acceptor blocks in the diagonalization step, so that through-space and through-bond couplings are obtained simultaneously. The method is applied to two test sets: a series of fused-ring bridged systems and G(T)nG DNA oligomers. The results for the first set are compared to experiment and show an average error lower than 10%. For the DNA set, we show that the coupling may be significantly larger (and the decay with length slower) when the entire backbone is included.