Polyelectrolyte Solutions with Multivalent Salts

We investigate the thermodynamic properties of a polyelectrolyte solution in a presence of {\it multivalent} salts. The polyions are modeled as rigid cylinders with the charge distributed uniformly along the major axis. The solution, besides the polyions, contain monovalent and divalent counterions as well as monovalent coions. The strong electrostatic attraction existing between the polyions and the counterions results in formation of clusters consisting of one polyion and a number of associated monovalent and divalent counterions. The theory presented in the paper allows us to explicitly construct the Helmholtz free energy of a polyelectrolyte solution. The characteristic cluster size, as well as any other thermodynamic property can then be determined by an appropriate operation on the free energy

The performance of forecast-based monetary policy rules under model uncertainty

We investigate the performance of forecast-based monetary policy rules using five macroeconomic models that reflect a wide range of views on aggregate dynamics. We identify the key characteristics of rules that are robust to model uncertainty: such rules respond to the one-year-ahead inflation forecast and to the current output gap and incorporate a substantial degree of policy inertia. In contrast, rules with longer forecast horizons are less robust and are prone to generating indeterminacy. Finally, we identify a robust benchmark rule that performs very well in all five models over a wide range of policy preferences

Variation in a host-epiphyte relationship along a wave exposure gradient

The red alga Polysiphonia lanosa (L ) Tandy is an obligate epiphyte that primarily occurs on the fucoid brown algal basiphyte Ascophyllum nodosum (L) Le Jolis In the present study we examine how epiphytic interactions between P lanosa and A nodosum vary along a wave exposure gradient within the southern Gulf of Maine, USA P lanosa was most dense on protected shores, however because the stature of P lanosa was greater on exposed than on sheltered shores, greater biomass occurred In exposed habitats Epiphytlc P lanosa pnmanly attached to inlured vegetative bssue at exposed sites, while ~ t osc currence was primarily receptacular at sheltered sites A significantly stronger correlation was found between host receptacle abundance and epiphyte abundance at a protected low than an exposed site As a result, the distribution of epiphytes along the host S stlpe vanes at different sites We suggest that changes in the distribution and abundance of P lanosa across this wave exposure gradient are highly influenced by vanations in the distribution and persistence of suitable attachment sites on the host plant Because both the quantity and quality of attachment sites vanes w t h exposure, we hypothesize that d~fferenpt rocesses limit or de t e rm~neP lanosa populations in different locations In protected sites P lanosa may be limited by the presence of adequate substrata (inlured bssue and lateral pits) where successful recruitment may occur By contrast at exposed sites the supply of P lanosa sporelings, rather than quantity of appropnate substrata, may limlt population size

Improved magnesia for thermal control coatings

Formation of radiation-generated color centers using single crystals of magnesium oxide is discussed. Crystal structure of magnesium oxide is described. Chemical processes used to produce magnesium oxide with desired color center kinetics are presented. Proton irradiation of magnesium oxide crystals was conducted to determine lattice defects

The Performance of Forecast-Based Monetary Policy Rules under Model Uncertainty

In this paper, we consider whether monetary policymakers should adjust short-term nominal interest rates in response to inflation and output forecasts rather than to recent outcomes of these variables. The use of forecast-based rules has been advocated on the basis of transmission lags and other considerations, and such rules also provide a reasonably good description of the policy strategies of several inflation-targeting central banks. We address these issues using four different macro-econometric models of the U.S. economy (the Fuhrer-Moore model, the MSR model of Orphanides and Wieland, Taylor's Multi-Country Model, and the FRB/US staff model); all four models incorporate rational expectations and nominal inertia, but differ in many other respects. We begin by evaluating the performance of various forecast-based rules that have been proposed in the literature. We find that some of these rules yield relatively poor performance, and that a number of such rules fail to yield determinacy (that is, a unique rational expectations equilibrium) in at least one of the four models. Next, we determine the optimal set of forecast-based rules for each model (that is, the rules that trace out the inflation-output volatility frontier subject to an upper-bound on interest rate volatility). We find that even optimized forecast-based rules yield very small benefits compared with optimized outcome-based rules that respond to current inflation, the current output gap, and the lagged interest rate. In the case of rules that respond directly to inflation forecasts but not to the output gap, we find a substantial deterioration in performance, even as measured by a policymaker whose sole objective is to minimize inflation variability. Finally, rules that involve relatively short forecast horizons (less than one year ahead) are reasonably robust to model uncertainty; that is, when such a rule is optimized for one model, the rule also performs reasonably well in the other three models. However, rules that respond to longer-horizon forecasts are not robust to model uncertainy (and in some cases yield indeterminacy), mainly because of the sharp differences in output and inflation persistence across the four models considered here.

Rod-like Polyelectrolytes in Presence of Monovalent Salt

We investigate the properties of rigid polyelectrolyte solutions in presence of monovalent salt. The free energy within the Debye-H\"uckel-Bjerrum (DHBj) theory [M. E. Fisher and Y. Levin, {\it Phys. Rev. Lett.} 71, 3826 (1993)] is constructed. It is found that at thermodynamic equilibrium the polyelectrolyte solution consists of clusters composed of one polyion and various counterions. The distribution of the cluster densities is determined by finding the minimum of the Helmholtz free energy. The osmotic pressure and the average charge of the cluster are found and their dependence on Manning parameter $\xi$ is elucidated. A good agreement with the experimental results is obtained.Comment: 11 pages, Revtex (using twocolumn style), 15 figures and postscript file. Submitted to Macromomelecule

The one-component plasma: a conceptual approach

The one-component plasma (OCP) represents the simplest statistical mechanical model of a Coulomb system. For this reason, it has been extensively studied over the last forty years. The advent of the integral equations has resulted in a dramatic improvement in our ability to carry out numerical calculations, but came at the expense of a physical insight gained in a simpler analytic theory. In this paper we present an extension of the Debye-Hueckel (DH) theory to the OCP. The theory allows for analytic calculations of all the thermodynamic functions, as well as the structure factor. The theory explicitly satisfies the Stillinger-Lovett and, for small couplings, the compressibility sum rules, implying its internal self consistency.Comment: 28 pages, 5 Postscript figures, uses elsart.sty, accepted for publication in Physica