Electrostatics of heterogeneous monolayers. Constitutive equations and mathematical models

Molecular dynamics study of the electric and dielectric properties of model DPPC and dicaprin insoluble monolayers: size effect

Atomistic modeling of insoluble monolayers is currently used to inspect their organization and electric characteristics, providing a link between theory and experiment. Extensive molecular dynamics simulations at 300 K were carried out for model films of the lipids dipalmitoylphosphatidylcholine (DPPC) and dicaprin (DC) at the air/water interface. Surface concentrations corresponding to a set of points along the surface pressure/area isotherms of the surfactants were considered. The models contained 25 or 81 lipid molecules in hexagonal arrangement and explicit aqueous media (TIP3P) treated in periodic boundary conditions. Molecular dynamics simulations based on a classical force field (CHARMM27) were carried out and key characteristics of the studied films were estimated. The dielectric properties of the films in normal and tangential direction were quantified by means of dipole moment magnitude and orientation analysis and by monolayer dielectric permittivity. The contributions of lipids and interfacial water to each component of the considered characteristics were assessed and their variations upon film compression were discussed and compared for the two monolayers and to earlier results. The dielectric permittivity tensors were analyzed. Electrostatic potential profiles across the layers and surface pressure values were used for more detailed clarification of experimental measurements. The results show dissimilar behavior of the two lipids at the air-water interface. While the average electric and dielectric properties of DPPC monolayers result from opposite surfactant and water contributions, the two subsystems are synergetic in the DC films. The anisotropy of the monolayer dipole moment and dielectric permittivity is explained by domination of a different subsystem in the various components. Tangential characteristics turn out to be more sensitive to the size of the model and to the degree of film compression.

Effect of the surface polarizability on electrostatic screening in semiconductors

Following the Gibbs approach a general electrostatic model of heterogeneous systems with non-homogeneous interfaces is proposed. The intrinsic surface polarization is taken into account through the introduction of a surface dielectric constant and the electrostatic boundary conditions are generalized as two-dimensional Poisson equations. This model is applied to analysis of the electrostatic potential of charged defects on a semiconductor surface. As a result, a good theoretical fit of the experimental data is obtained. The fitting value of the surface dielectric constant is in good agreement with its theoretical estimation in the framework of the Gibbs approach.