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Digitale Medien

Molecular dynamics studies of the melting of butane and hexane monolayers adsorbed on the basal-plane surface of graphite (1993)

Hansen, Flemming Y. ; Newton, J. C. ; Taub, H.

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 98 (1993), S. 4128-4141

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ISSN: 1089-7690

Quelle: AIP Digital Archive

Thema: Physik , Chemie und Pharmazie

Notizen: The effect of molecular steric properties on the melting of quasi-two-dimensional solids is investigated by comparing results of molecular dynamics simulations of the melting of butane and hexane monolayers adsorbed on the basal-plane surface of graphite. These molecules differ only in their length, being members of the n-alkane series [CH3(CH2)n−2CH3] where n=4 for butane and n=6 for hexane. The simulations employ a skeletal model, which does not include the hydrogen atoms explicitly, to represent the intermolecular and molecule–substrate interactions. Nearest-neighbor intramolecular bonds are fixed in length, but the molecular flexibility is preserved by allowing the bend and dihedral torsion angles to vary. The simulations show a qualitatively different melting behavior for the butane and hexane monolayers consistent with neutron and x-ray scattering experiments. The melting of the low-temperature herringbone (HB) phase of the butane monolayer is abrupt and characterized by a simultaneous breakdown of translational order and the orientational order of the molecules about the surface normal. In contrast, the hexane monolayer exhibits polymorphism in that the solid HB phase transforms to a rectangular-centered structure with a short coherence length in coexistence with a fluid phase. A significant result of the simulations is that they demonstrate the importance of molecular flexibility on the nature of the melting transition. The formation of gauche molecules is essential for the melting process in the hexane monolayer but unimportant for butane. The effect of molecular length on the qualitative nature of the melting process is discussed for both monolayers.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.465067

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Digitale Medien

Pathology of experimental enteric septicaemia in channel catfish, Ictalurus punctatus (Rafinesque), following immersion-exposure to Edwardsiella ictaluri (1989)

NEWTON, J. C. ; WOLFE, L. G. ; GRIZZLE, J. M. ; [weitere]

Oxford, UK : Blackwell Publishing Ltd

Journal of fish diseases 12 (1989), S. 0

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ISSN: 1365-2761

Quelle: Blackwell Publishing Journal Backfiles 1879-2005

Thema: Biologie , Medizin

Notizen: Abstract. Fingerling channel catfish, Ictalurus punctatus (Rafinesque), were exposed experimentally to Edwardsiella ictaluri by immersion for 1 h in water containing 5 × 108 colony-forming units (CFU) of the bacterium per ml. Ninety per cent of the fish developed lesions typical of enteric septicaemia of catfish (ESC), 93% of affected fish developed the acute form of ESC and 7% developed chronic ESC. Acute disease was characterized grossly by cutaneous haemorrhage and ulceration, and microscopically by enteritis, olfactory sacculitis, hepatitis and dermatitis. The earliest lesions of acute ESC, i.e. enteritis and olfactory sacculitis, were observed microscopically at 2 days post-exposure (PE); gross lesions, primarily mild subcutaneous haemorrhage at the base of fins, were first apparent at 4 days PE. Chronic ESC, seen most commonly 3–4 weeks PE, was characterized by dorsocranial swelling and ulceration, granulomatous olfactory neuritis/perineuritis, and meningoencephalitis involving the olfactory bulbs, olfactory tracts and olfactory lobes of the brain. Gross and microscopic lesions in the acute and chronic forms of experimental ESC were similar to the lesions reported in naturally occurring ESC. Definitive pathogenesis of acute and chronic ESC remains to be determined.