Surface induced disorder in body-centered cubic alloys

We present Monte Carlo simulations of surface induced disordering in a model of a binary alloy on a bcc lattice which undergoes a first order bulk transition from the ordered DO3 phase to the disordered A2 phase. The data are analyzed in terms of an effective interface Hamiltonian for a system with several order parameters in the framework of the linear renormalization approach due to Brezin, Halperin and Leibler. We show that the model provides a good description of the system in the vicinity of the interface. In particular, we recover the logarithmic divergence of the thickness of the disordered layer as the bulk transition is approached, we calculate the critical behavior of the maxima of the layer susceptibilities, and demonstrate that it is in reasonable agreement with the simulation data. Directly at the (110) surface, the theory predicts that all order parameters vanish continuously at the surface with a nonuniversal, but common critical exponent. However, we find different exponents for the order parameter of the DO3 phase and the order parameter of the B2 phase. Using the effective interface model, we derive the finite size scaling function for the surface order parameter and show that the theory accounts well for the finite size behavior of the DO3 ordering but not for that of B2 ordering. The situation is even more complicated in the neighborhood of the (100) surface, due to the presence of an ordering field which couples to the B2 order.Comment: To appear in Physical Review

Intramolecular and Lattice Melting in n-Alkane Monolayers: An Analog of Melting in Lipid Bilayers

URL:http://link.aps.org/doi/10.1103/PhysRevLett.83.2362 DOI:10.1103/PhysRevLett.83.2362Molecular dynamics (MD) simulations and neutron diffraction experiments have been performed on n-dotriacontane ( n-C32D66) monolayers adsorbed on a graphite basal- plane surface. The diffraction experiments show little change in the crystalline monolayer structure up to a temperature of ~350K above which a large thermal expansion and decrease in coherence length occurs. The MD simulations provide evidence that this behavior is due to a phase transition in the monolayer in which intramolecular and translational order are lost simultaneously. This melting transition is qualitatively similar to the gel-to-fluid transition found in bilayer lipid membranes.Acknowledgment is made to the U.S. National Science Foundation under Grants No. DMR-9314235 and No. DMR-9802476, the Missouri University Research Reactor, and to the donors of The Petroleum Research Fund, administered by the ACS, for partial support of this research. We thank L. Criswell for assistance with the figures

Assimilation of phytate-phosphorus by the extracellular phytase activity of tobacco (Nicotiana tabacum) is affected by the availability of soluble phytate

Phytate, the major organic phosphorus in soil, is not readily available to plants as a source of phosphorus (P). It is either complexed with cations or adsorbed to various soil components. The present study was carried out to investigate the extracellular phytase activities of tobacco (Nicotiana tabacum variety GeXin No.1) and its ability to assimilate external phytate-P. Whereas phytase activities in roots, shoots and growth media of P i-fed 14-day-old seedlings were only 1.3-4.9% of total acid phosphatase (APase) activities, P starvation triggered an increase in phytase secretion up to 914.9 mU mg -1 protein, equivalent to 18.2% of total APase activities. Much of the extracellular phytase activities were found to be root-associated than root-released. The plants were not able to utilize phytate adsorbed to sand, except when insoluble phytate salts were preformed with Mg 2+ and Ca 2+ ions for supplementation. Tobacco grew better in sand supplemented with Mg-phytate salts (31.9 mg dry weight plant -1; 0.68% w/w P concentration) than that with Ca-phytate salts (9.5 mg plant -1; 0.42%), presumably due to its higher solubility. We conclude that insolubility of soil phytate is the major constrain for its assimilation. Improving solubility of soil phytate, for example, by enhancement of citrate secretion, may be a feasible approach to improve soil phytate assimilation. © Springer 2006.postprin

Structural characterization of water and ice in mesoporous SBA-15 silicas: II. The 'almost-filled' case for 86 angstrom pore diameter

Neutron diffraction measurements for D2O in SBA-15 silica of pore diameter 86 angstrom have been made in a temperature range from 300 to 100 K. The pore-filling factor for the liquid phase is 0.95, resulting in an `almost-filled' sample. The nucleation and transformation of the ice phase were determined for cooling and warming cycles at two different rates. The primary nucleation event at 258 K leads to a defective form of ice-I with predominantly cubic ice features. For temperatures below the main nucleation event, the data indicate the formation of an interfacial layer of disordered water/ice that varies with temperature and is reversible. The main diffraction peak for the water phase shows similar features to those observed in earlier studies, indicating enhanced hydrogen bonding and network correlations for the confined phase as the temperature is decreased. A detailed profile analysis of the triplet peak is presented in the accompanying paper (Seyed-Yazdi et al 2008 J. Phys.: Condens. Matter 20 205108)

Neutron diffraction and NMR relaxation studies of structural variation and phase transformations for water/ice in SBA-15 silica: I. The over-filled case

Neutron diffraction and NMR relaxation measurements have been made of water/ice in SBA-15, a mesoporous silica constituting an ordered array of cylindrical mesopores of pore diameter similar to 86 angstrom, over the temperature range 180-300 K in a cooling and heating cycle. The over-filled sample shows the initial formation of hexagonal ice on the outside of the silica grains, followed by the nucleation of cubic ice inside the pores at a lower temperature. Neutron scattering profiles for the cubic ice peaks are significantly broadened and indicate a defective structure, as observed in previous experiments on ice formation in sol-gel and MCM-type silicas. Below the pore freezing temperature the intensity of the cubic ice peaks exhibit a significant increase, down to the lowest experimental temperature, indicating a reversible conversion of defective ice to ordered ice crystals. The peak profile analysis for the two ice patterns indicates a systematic variation in the position as a function of temperature, giving values of the expansion coefficients that are slightly lower than other measurements for the bulk phase. NMR results on proton relaxation as a function of temperature indicate the presence of a mobile phase for temperatures below pore freezing that supports the view that there is interconversion between brittle and plastic phases of ice

Ellipsometric study of the liquid/liquid interface in a phase-separated colloid-polymer suspension

An investigation of the interface in a phase-separated colloid-polymer suspension with ellipsometry is presented. The coefficient of ellipticity at the Brewster angle of light reflected at the interface between the coexisting colloid-rich and polymer-rich phase was measured along two trajectories through the phase diagram. Interpreting the data in terms of a diffuse interface model, an interfacial thickness in the order of the diameter of the colloidal particles is found

Aggregation of an amphiphilic poly(p-phenylene) in micellar surfactant solutions. Small-angle neutron scattering

Futterer T, Hellweg T, Findenegg GH, Frahn J, Schluter AD. Aggregation of an amphiphilic poly(p-phenylene) in micellar surfactant solutions. Small-angle neutron scattering. Macromolecules. 2005;38(17):7451-7455.Mixed aggregates of a short-chain nonionic amphiphilic poly(p-phenylene), in which each benzene ring of the aromatic backbone is 2,5-disubstituted with a hydrophobic and a hydrophilic group, and of the nonionic surfactant C8E4, C12E6, or C(10)G(2) were studied in aqueous solutions. Cryo-transmission electron microscopy and light scattering experiments revealed the existence of fiberlike aggregates of mean length greater 200 nm and a diameter of ca. 6 nm in an excess of small surfactant micelles in these solutions. In the present contribution small-angle neutron scattering (SANS) is used to characterize the fiber aggregates in these complex liquids with respect to their composition and their radial scattering length density distribution sigma(r). The composition of the fiber aggregates (polymer-to-surfactant number ratio) was determined using perdeuterated surfactant and contrast variation of the solvent (H2O/D2O)Moreover, the diameter of the mixed aggregates is determined using form factors for descriptions of the scattering curves and additionally numerical approaches to treat the data. The ratio of surfactant molecules to PPP was found to be 3:1 (20 wt %:80 wt %)