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In this study we present a unified theoretical description of neutral, saturated, phospholipid monolayers at air–water interfaces. This model encompasses the liquid-condensed (LC) to liquid-expanded (LE) and liquid-expanded to surface gas (SG) phase transitions observed in such systems. The model is a lattice model of lipid hydrocarbon chains which allows for the introduction of free volume. The lipid chains can be upright in a ground or excited state or collapsed relative to the substrate. Furthermore, the chains interact via short range potentials due to steric, van der Waals, and dipolar forces. We show that the LC/LE phase transition is to be understood as a chain melting transition and through the growth of lipid domains across the transition. We further show that the LE/SG transition involves the creation of large amounts of free volume into which the lipid chains can collapse. [ABSTRACT FROM AUTHOR]

A new theory is developed for a polar electrolyte adjacent to a charged surface. This model explicitly includes the polar nature of the solvent by modeling the electrolyte as a system of point charges and point dipoles, with the number density of ions plus dipoles constrained to be constant. The free energy of the model electrolyte is derived in the continuum limit to yield two coupled differential equations describing the electrolyte. Values of the potential V, polarization P, dielectric constant ε, and local mole fraction of dipoles f are evaluated as functions of the distance x from the charged surface. These are compared with the predictions of standard Gouy–Chapman–Debye–Hückel (GCDH) theory. A significant supression of ε(x) is observed close to the surface. This model electrolyte is then used to investigate the behavior of charged lipid bilayers. In particular the pressure between two identical charged bilayers is calculated from the appropriate free energy derivative. GCDH-like behavior is found for separations greater than 15 Å. However, for separations below 15 Å an exponentially decaying short range repulsive force is predicted, with a decay length of [bar_over_tilde:_approx._equal_to]1.4 Å. This decay length is of the same order of magnitude as those observed in measurements of short range forces between phospholipid bilayers. [ABSTRACT FROM AUTHOR]

Presents a study that examined the transverse spin freezing in a Heisenberg model using Monte Carlo simulations. Background on the Heisenberg model; Analysis of the temperature dependence of transverse spin length of spin glasses; Examination of the paramagnetic and ferromagnetic spin-glass phase.