Analytical interfacial layer model for the capacitance and electrokinetics of charged aqueous interfaces

We construct an analytical model to account for the influence of the subnanometer-wide interfacial layer on the differential capacitance and the electro-osmotic mobility of solid–electrolyte interfaces. The interfacial layer is incorporated into the Poisson–Boltzmann and Stokes equations using a box model for the dielectric properties, the viscosity, and the ionic potential of mean force. We calculate the differential capacitance and the electro-osmotic mobility as a function of the surface charge density and the salt concentration, both with and without steric interactions between the ions. We compare the results from our theoretical model with experimental data on a variety of systems (graphite and metallic silver for capacitance and titanium oxide and silver iodide for electro-osmotic data). The differential capacitance of silver as a function of salinity and surface charge density is well reproduced by our theory, using either the width of the interfacial layer or the ionic potential of mean force as the only fitting parameter. The differential capacitance of graphite, however, needs an additional carbon capacitance to explain the experimental data. Our theory yields a power-law dependence of the electro-osmotic mobility on the surface charge density for high surface charges, reproducing the experimental data using both the interfacial parameters extracted from molecular dynamics simulations and fitted interfacial parameters. Finally, we examine different types of hydrodynamic boundary conditions for the power-law behavior of the electro-osmotic mobility, showing that a finite-viscosity layer explains the experimental data better than the usual hydrodynamic slip boundary condition. Our analytical model thus allows us to extract the properties of the subnanometer-wide interfacial layer by fitting to macroscopic experimental data

International Variability in Biofuel Trade: An Assessment of U.S. Policies

Although the United States has typically been in a position to import ethanol, corn-based ethanol exports are surging as the domestic market becomes saturated and world prices rise due to high prices for sugar, the competing global feedstock. The U.S. is now the world’s leading ethanol producer but domestic demand is constrained because of technical limitations in the current vehicle fleet. Higher ethanol blends have been approved for use (15% rather than 10%) but a limited number of vehicles that can use such higher blends. Infrastructure constraints also affect the potential supply of higher ethanol blends. As a result of these factors, U.S. biofuel policies can have significant implications for the world ethanol market. Usage mandates under the Renewable Fuel Standard, blender tax credits, and the blend wall can interact to generate excess supplies of ethanol that are likely to be diverted to the world market. This paper examines how fluctuations in corn yield and gasoline prices affect the excess supply of U.S. corn-based ethanol in the presence of alternative assumptions about the maximum amount of ethanol that can be consumed domestically. Using stochastic simulations we also explore the impact of current policies on the mean and variance of export supply. The results highlight the complex interaction between technological constraints, economic incentives, and government policies in the U.S. biofuels sector, and point to the potentially destabilizing effect of such policies in international markets.Ethanol Exports, Biofuel Policies, Variability, International Relations/Trade, Resource /Energy Economics and Policy,

The Impact of Feedstock Supply and Petroleum Price Variability on Domestic Biofuel and Feedstock Markets – The Case of the United States

The promotion of biofuel use in preference to traditional petroleum-based transportation fuel has linked agricultural commodity markets and energy markets more closely together. Biofuel policies can involve multiple policy instruments, but studies examining their effects on biofuel feedstock and energy markets are scarce. In addition, the impact of alternative policy approaches in the context of variability in petroleum prices and the supply of biofuel feedstock has received limited attention. Focusing on the current situation in the United States, in which prohibitively high duties prevent imports of ethanol, this paper examines how variability in the price of petroleum and corn supply affects domestic market variability under three types of domestic policies, inclusive of their combinations, for promoting the use of ethanol: 1) the provision of a fixed subsidy (tax credit) for blending ethanol with gasoline; 2) the use of a blending mandate; and 3) the use of a consumption mandate. Varying relative variability in petroleum price and corn supply, we analyze numerically the implications of changes in domestic biofuel policy for variability (measured by the coefficient of variation) in ethanol use and corn prices. We also provide some brief insights into the design of market stabilization policies. Results obtained from Monte Carlo simulations show that in the absence of mandates the quantity of ethanol used under a subsidy policy is highly susceptible to fluctuations in oil prices and corn supply, providing that there are no constraints to adjustment in ethanol demand. The impact of oil price fluctuations on the price of corn is large, but corn supply fluctuations have no or a small impact on the equilibrium corn price, depending on the flexibility of the use of corn in ethanol refining. This is because variations in ethanol volume absorb shocks caused by corn supply fluctuations. Consequently, high fluctuations in the price of petroleum are expected to result in high variability in the corn price in the absence of mandates. With a mandate (with or without a subsidy), as the likelihood that the mandate becomes binding increases, variability in ethanol use declines, the impact of variations in petroleum price on corn prices is reduced, and the impact of variations in corn supply on prices is accentuated. Therefore, if the mandate is likely to be binding, high fluctuations in corn supply are expected to result in high variability in the corn price. If the likelihood that ethanol use exceeds the mandated level is high, the effects are similar to those in the absence of a mandate. The effects of changes in biofuel policy, such as a reduction in the level of tax credit under a mandate and an increase in its level, on the price of corn depend on the relative magnitudes of world oil price and domestic corn supply fluctuations.biofuels, subsidies, mandates, variability, Agricultural and Food Policy, International Relations/Trade, Resource /Energy Economics and Policy,

The Implications of Alternative U.S. Domestic and Trade Policies for Biofuels

The U.S. Renewable Fuel Standard program (RFS), which involves mandates for various biofuels, is complex and has been often misinterpreted or oversimplified in previous studies. In this paper we analyze the implications of the RFS for the U.S. domestic and international ethanol markets. We demonstrate the vital role of the advanced biofuel mandate within the RFS. Impacts of changes in tariffs on imported fuel ethanol and subsidies for U.S. domestic ethanol production are examined. One of our important findings is that the RFS could result in serious misallocation of resources in both a national and international context. There is a possibility that the United States could be required to import sugarcane-based ethanol to meet the advanced biofuel mandate, simultaneously exporting corn-based ethanol, while satisfying the national overall mandate. Since the provision of subsidies for domestic ethanol production can stimulate exports of corn-based ethanol, they are equivalent to export subsidies in this situation. The removal of tariffs can reduce the burden imposed on consumers in the United States from the operation of the RFS. Our analysis shows that it is extremely important to understand the potential impact of the RFS on agricultural and energy markets.Ethanol, trade liberalization, Renewable Fuel Standard, mandate, subsidies, Industrial Organization, F13, Q18, Q42, Q48,

NaI revisited: Theoretical investigation of predissociation via ultrafast XUV transient absorption spectroscopy.

Avoided crossings can trigger abrupt changes of electronic character and redirect the outcomes of photochemical reactions. Here, we report a theoretical investigation into core-level spectroscopic probing of predissociation dynamics of sodium iodide (NaI), a prototype system for studies of avoided-crossing dynamics. The elegant femtochemistry work of Zewail and co-workers pioneered the real-time dynamics of NaI, detecting the Na atoms bursting forth from the avoided crossing and the residual NaI molecules oscillating inside the quasibound potential. The simulated results show that core-level spectroscopy not only observes these integrated outcomes but also provides a direct measure of the abrupt switching of electronic character at the avoided crossing. The valence and core-excited electronic structures of NaI are computed by spin-orbit general multiconfigurational quasidegenerate perturbation theory, from which core-level absorption spectra of the predissociation dynamics are constructed. The wave-packet motion on the covalent potential is continuously mapped as shifts in the absorption energies, and the switching between the covalent and ionic character at the avoided crossing is characterized as the sharp rise and fall of the Na+ signal. The Na+ signal is found to be insensitive to the wave-packet motion in the asymptotic part of the ionic potential, which, in turn, enables a direct measure of the nonadiabatic crossing probability excluding the effect of wave-packet broadening

Tensor factorizations of local second-order M{\o}ller Plesset theory

Efficient electronic structure methods can be built around efficient tensor representations of the wavefunction. Here we describe a general view of tensor factorization for the compact representation of electronic wavefunctions. We use these ideas to construct low-complexity representations of the doubles amplitudes in local second order M{\o}ller-Plesset perturbation theory. We introduce two approximations - the direct orbital specific virtual approximation and the full orbital specific virtual approximation. In these approximations, each occupied orbital is associated with a small set of correlating virtual orbitals. Conceptually, the representation lies between the projected atomic orbital representation in Pulay-Saeb{\o} local correlation theories and pair natural orbital correlation theories. We have tested the orbital specific virtual approximations on a variety of systems and properties including total energies, reaction energies, and potential energy curves. Compared to the Pulay-Saeb{\o} ansatz, we find that these approximations exhibit favourable accuracy and computational times, while yielding smooth potential energy curves

Macroscopic proof of the Jarzynski-Wojcik fluctuation theorem for heat exchange

In a recent work, Jarzynski and Wojcik (2004 Phys. Rev. Lett. 92, 230602) have shown by using the properties of Hamiltonian dynamics and a statistical mechanical consideration that, through contact, heat exchange between two systems initially prepared at different temperatures obeys a fluctuation theorem. Here, another proof is presented, in which only macroscopic thermodynamic quantities are employed. The detailed balance condition is found to play an essential role. As a result, the theorem is found to hold under very general conditions.Comment: 9 pages, 0 figure

Kaluza-Klein bubble like structure and celestial sphere in inflationary universe

We consider five dimensional deSitter spacetimes with a deficit angle due to the presence of a closed 2-brane and identify one dimension as an extra dimension. From the four dimensional viewpoint we can see that the spacetime has a structure similar to a Kaluza-Klein bubble of nothing, that is, four dimensional spacetime ends at the 2-brane. Since a spatial section of the full deSitter spacetime has the topology of a sphere, the boundary surface surrounds the remaining four dimensional spacetime, and can be considered as the celestial sphere. After the spacetime is created from nothing via an instanton which we describe, some four dimensional observers in it see the celestial sphere falling down, and will be in contact with a 2-brane attached on it.Comment: 5pages, 4figures, to be published in GR