A qualitative and quantitative description (exact diagonalization of finite clusters) of the doping dependence of one-electron-removal and -addition spectral weights is given. A half-filled Hubbard system in the localized limit has two low-energy electron-addition (-removal) states for each hole (electron) created by doping. In contrast, a charge-transfer system in the localized limit is fundamentally asymmetric between hole and electron doping. However, when the hybridization is increased this asymmetry disappears quickly and both holes and electrons introduced by doping show a strongly correlated behav-PACS numbers: 74.70.Vy A surprising feature of the high-r^: materials is a strong doping dependence of high-energy spectral distributions, and a shift of spectral intensity from high to low energies. Two of the nicest examples are the electronenergy-loss study [1] and O \s x-ray-absorption study [2] of the La2-xSrxCu04 system. These spectra show a strong decrease with x in the intensity of the upper Hubbard band as the lower-energy structure develops due to holes in the O 2p band. Such behavior has previously been found in Li-doped NiO as well [3].Also, in optical-absorption experiments [4], a transfer of spectral weight from the band-gap transition at about 2 eV in insulating La2Cu04 to the low-energy scale (< 1.0 eV) in the superconductor Lai.8Sro.2Cu04 is observed. The integral of the optical conductivity up to 1 eV increases much more rapidly than the doping concentration would suggest. At the same time the high-energy-scale structure around 2 eV decreases rapidly, whereas the total integrated spectral weight up to 4 eV remains nearly constant. Similar behavior has also been found recently for the electron-doped system Pr2-xCejc04-5 by Cooper et at. [5].That these kinds of effects can be related to strong correlations is evidenced by exact diagonalization studies. A convincing description of the O 1^ spectra is given using the Mott-Hubbard (MH) model [2]. Also, the calculated optical spectra for the MH and t-J models show features quantitatively similar to what is seen experimentally [6][7][8][9].At first glance one would not expect to see such spectral-weight transfers in the hole-doped high-r^ materials, which are generally accepted to be charge-transfer (CT) systems, because the additional holes mainly occupy oxygen 2p states. In a previous paper [10] we found, however, using a multiband cluster (CU2O7) calculation and parameters suitable for the high-r^ compounds and for 50% doping, a substantial transfer of spectral weight. In this paper we describe the physical origin of the observed spectral-weight transfer and its doping dependence. In addition we study both the MH and CT systems and describe the effects of hybridization. Special attention is paid to the asymmetry in electron-and hole-doped systems expected in a CT model. We show that if the hybridization in a CT system is large enough, the spectralweight transfer becomes similar to that of a MH model, but the physical origin is really quite diffe...

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a b s t r a c tBiochar application to soil is currently being widely posited as a means to improve soil quality and thereby increase crop yield. Next to beneficial effects on soil nutrient availability and retention, biochar is assumed to improve soil water retention. However, evidence for such an effect in the primary literature remains elusive. Therefore, we studied the effect of biochar on soil hydrological characteristics in two separate field experiments on a sandy soil in The Netherlands. In Experiment I, biochar produced through slow pyrolysis of herbaceous feedstock at two temperatures (400°C and 600°C) was applied to soil at a rate of 10 t ha −1 . In Experiment II, the 400°C biochar was applied at rates of 1, 5, 20 and 50 t ha −1 . Soils were analysed for soil water retention, aggregate stability and other soil physical parameters after three growing seasons and one growing season for Experiment I and Experiment II, respectively. We characterised the pore structure of the biochar using X-ray computed micro-tomography (XRT) and hydrophobicity using contact angle measurements. We found no significant effects of biochar application on soil water retention in either experiment. Aggregate stability was also not significantly affected, nor was field saturated hydraulic conductivity. XRT analysis of the biochars showed that they were highly porous, with 48% and 57% porosity for the 400°C and 600°C biochar respectively. More than 99% of internal pores of the biochar particles were connected to the surface, suggesting a potential role for biochars in improving soil water retention. However, the biochars were highly hydrophobic. We postulate that this strong hydrophobicity prevented water from infiltrating into the biochar particles, prohibiting an effect on soil water retention. Our results suggest that, in addition to characterising pore space, biochars should be analysed for hydrophobicity when assessing their potential for improving soil physical properties.

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Using native and caprylated ovalbumin, the role of exposed hydrophobicity on the kinetics of protein adsorption to the air-water interface is studied. First, changes in the chemical properties of the protein upon caprylation were characterized followed by measurement of the changes in adsorption kinetics. No change in the molecular structure of ovalbumin was observed upon caprylation. However, aggregation of the protein was observed when more than three capryl chains were coupled per protein. A batch of caprylated ovalbumin with an average coupling of four capryl chains per protein was separated into a monomeric and an aggregated protein fraction. The exposed hydrophobicity of the monomeric and the aggregated species was measured using 8-anilino-1-naphthalenesulfonic acid fluorescence. The exposed hydrophobicity of the monomeric fraction was significantly higher than that of the nonmodified protein. The changes in adsorption kinetics were studied by measuring the increase in surface load (Γ) and in surface pressure (Π) as a function of time (t) using an ellipsometer and a Wilhelmy plate, respectively. It was found that the increase of surface load in time (even at low surface coverage) is much lower than the value that was calculated from diffusional transport. This shows that the adsorption of native ovalbumin is barrier limited. The adsorption kinetics of the caprylated protein follow the calculations from diffusional transport more closely, which shows that the energy barrier for adsorption of caprylated ovalbumin is much lower than for the native protein. The surface pressure at a certain surface load (Π-Γ) was not affected by the modification, indicating that the effect of increased hydrophobicity is limited to the adsorption process.