655 research outputs found
Studies of the influence of physical properties, in particular particle size, on the flow behaviour of emulsions
The physical properties of concentrated solid–liquid emulsion systems typical of materials important in the pharmaceutical and cosmetics industries are investigated. [Continues.
Gelatin behaviour in dilute aqueous solution : designing a nanoparticulate formulation
Although it has been claimed that nanoparticles can be produced from gelatin, a naturally occurring polypeptide, the commercial conversion of animal collagen to gelatin results in a heterogeneous product with a wide molecular-weight range. This is probably responsible for the widely observed variation in the experimental conditions required for nanoparticle formation.
In this study, 0.2% w/v aqueous B225 gelatin solutions were incubated under various conditions of time, temperature, pH and ethanol concentration and characterized by both size-exclusion high-performance liquid chromatography (HPLC) and dynamic light scattering. Gelatin was shown to be denatured when the temperature was increased to 37°C (approx.) and the rate of renaturation was optimized over the temperature range 7–20°Cat pH5.0, equivalent to the isoelectric point (IEP). The molecular-weight profile remained unchanged at 37°C (approx.) in the pH range 5–7. When the gelatin solutions were mixed with ethanol, higher-molecular-weight fractions (microgel, δ and ζ fractions, all with molecular weights > 700 kDa) precipitated at ethanol concentrations lower than those required to precipitate the lower molecular weight material (< 700 kDa), with maximum precipitation occurring close to the isoelectric point (pH 5.0).
The molecular weight profile of gelatin in solution is evidently critically affected in a time-dependent manner by both pH and temperature. These two factors influence the noncovalent interactions responsible for the molecular structure of gelatin. The molecular weight profiles, in turn, affect the phase behaviour of gelatin in hydroalcoholic solutions. Systematically investigating the effect of time, temperature, pH and ethanol concentration on the molecular-weight-distribution profile of a gelatin solution enabled a robust method to be developed for the preparation of colloidal dispersions of non-aggregated gelatin nanoparticles 220–250 nm in diameter. This contrasts with the multiparticulate aggregates produced by earlier literature methods.peer-reviewe
Separate compilation of structured documents
This paper draws a parallel between document preparation and the traditional processes of compilation and link editing for computer programs. A block-based document model is described which allows for separate compilation of various portions of a document. These portions are brought together and merged by a linker program, called dlink, whose pilot implementation is based on ditroff and on its underlying intermediate code. In the light of experiences with dlink the requirements for a universal object-module language for documents are discussed. These requirements often resemble the characteristics of the intermediate codes used by programming-language compilers but with interesting extra constraints which arise from the way documents are executed
Comparison of the molecular weight distribution of gelatin fractions by size-exclusion chromatography and light scattering
Commercial gelatin is a heterogeneous proteinaceous product with a broad range of molecular weights. The use of gelatin to prepare insoluble nanoparticles depends largely on the presence of high molecular weight fractions that can be separated by size-exclusion chromatography. The purpose of this study was to determine the molecular fractions of gelatins from three commercial types (B225, B60 and A60), measure their molecular weight and compare these with data obtained from an absolute light scattering method.
The mean molecular weight of the gelatins decreased in the order B225 > B60 > A60. All samples were polydisperse, with fractions varying from 2 MDa. Each sample was divided into eight fractions based on the molecular weight distribution and using a paired t-test the two methods were shown to be in substantial agreement.
The light scattering method would appear to provide an absolute quality control procedure for commercial gelatin, depending on its application and requirements.peer-reviewe
A physically-based model of the ionizing radiation from active galaxies for photoionization modeling
We present a simplified model of Active Galactic Nucleus (AGN) continuum
emission designed for photoionization modeling. The new model {\sc oxaf}
reproduces the diversity of spectral shapes that arise in physically-based
models. We identify and explain degeneracies in the effects of AGN parameters
on model spectral shapes, with a focus on the complete degeneracy between the
black hole mass and AGN luminosity. Our re-parametrized model {\sc oxaf}
removes these degeneracies and accepts three parameters which directly describe
the output spectral shape: the energy of the peak of the accretion disk
emission , the photon power-law index of the non-thermal
emission , and the proportion of the total flux which is emitted in the
non-thermal component . The parameter is
presented as a function of the black hole mass, AGN luminosity, and `coronal
radius' of the {\sc optxagnf} model upon which {\sc oxaf} is based. We show
that the soft X-ray excess does not significantly affect photoionization
modeling predictions of strong emission lines in Seyfert narrow-line regions.
Despite its simplicity, {\sc oxaf} accounts for opacity effects where the
accretion disk is ionized because it inherits the `color correction' of {\sc
optxagnf}. We use a grid of {\sc mappings} photoionization models with {\sc
oxaf} ionizing spectra to demonstrate how predicted emission-line ratios on
standard optical diagnostic diagrams are sensitive to each of the three {\sc
oxaf} parameters. The {\sc oxaf} code is publicly available in the Astrophysics
Source Code Library.Comment: 14 pages, 9 figures, 1 table. Accepted for publication in Ap
The mass-metallicity relation of local active galaxies
We systematically measure the gas-phase metallicities and the
mass-metallicity relation of a large sample of local active galaxies for the
first time. Observed emission-line fluxes from the Sloan Digital Sky Survey
(SDSS) are compared to a four-dimensional grid of photoionization models using
the Bayesian parameter estimation code NebulaBayes. For the first time we take
into account arbitrary mixing between HII region and narrow-line region (NLR)
emission, and the models are also varied with metallicity, ionization parameter
in the NLR, and the gas pressure. The active galactic nucleus (AGN) oxygen
abundance is found to increase by dex as a function
of host galaxy stellar mass over the range .
We also measure the metallicity and ionization parameter of 231000 star-forming
galaxies for comparison with the sample of 7670 Seyfert 2 galaxies. A
systematic offset in oxygen abundance of 0.09 dex is observed between the
mass-metallicity relations of the star-forming and active galaxies. We
investigate potential causes of the offset, including sample selection and the
treatment in the models of diffuse ionized gas, pressure, and ionization
parameter. We cannot identify the major cause(s), but suspect contributions due
to deficiencies in modeling the ionizing spectra and the treatment of dust
physics. Optical diagnostic diagrams are presented with the star-forming and
Seyfert data colored by the inferred oxygen abundance, ionization parameter and
gas pressure, clearly illustrating the trends in these quantities.Comment: 12 pages, 4 figures and 1 table; accepted for publication in Ap
What Drives the Redshift Evolution of Strong Emission Line Ratios?
We study the physical mechanisms that cause the offset between low-redshift
and high-redshift galaxies on the [OIII]/H versus [NII]/H
``Baldwin, Phillips & Terlevich'' (BPT) diagram using a sample of local
analogues of high-redshift galaxies. These high-redshift analogue galaxies are
selected from the Sloan Digital Sky Survey. Located in the same region on the
BPT diagram as the ultra-violet selected galaxies at , these
high-redshift analogue galaxies provide an ideal local benchmark to study the
offset between the local and high-redshift galaxies on the BPT diagram. We
compare the nitrogen-to-oxygen ratio (N/O), the shape of the ionising radiation
field, and ionisation parameters between the high-redshift analogues and a
sample of local reference galaxies. The higher ionisation parameter in the
high-redshift analogues is the dominant physical mechanism driving the BPT
offset from low- to high-redshift, particularly at high {\nii/\ha}.
Furthermore, the N/O ratio enhancement also plays a minor role to cause the BPT
offset. However, the shape of the ionising radiation field is unlikely to cause
the BPT offset because the high-redshift analogues have a similar hard ionising
radiation field as local reference galaxies. This hard radiation field cannot
be produced by the current standard stellar synthesis models. The stellar
rotation and binarity may help solve the discrepancy.Comment: 7 pages, 5 figures. accepted for publication in MNRA
Interrogating Seyferts with NebulaBayes: Spatially probing the narrow-line region radiation fields and chemical abundances
NebulaBayes is a new Bayesian code that implements a general method of
comparing observed emission-line fluxes to photoionization model grids. The
code enables us to extract robust, spatially resolved measurements of
abundances in the extended narrow line regions (ENLRs) produced by Active
Galactic Nuclei (AGN). We observe near-constant ionization parameters but
steeply radially-declining pressures, which together imply that radiation
pressure regulates the ENLR density structure on large scales. Our sample
includes four `pure Seyfert' galaxies from the S7 survey that have extensive
ENLRs. NGC2992 shows steep metallicity gradients from the nucleus into the
ionization cones. An {\it inverse} metallicity gradient is observed in
ESO138-G01, which we attribute to a recent gas inflow or minor merger. A
uniformly high metallicity and hard ionizing continuum are inferred across the
ENLR of Mrk573. Our analysis of IC5063 is likely affected by contamination from
shock excitation, which appears to soften the inferred ionizing spectrum. The
peak of the ionizing continuum E_peak is determined by the nuclear spectrum and
the absorbing column between the nucleus and the ionized nebula. We cannot
separate variation in this intrinsic E_peak from the effects of shock or HII
region contamination, but E_peak measurements nevertheless give insights into
ENLR excitation. We demonstrate the general applicability of NebulaBayes by
analyzing a nuclear spectrum from the non-active galaxy NGC4691 using a HII
region grid. The NLR and HII region model grids are provided with NebulaBayes
for use by the astronomical community.Comment: Accepted for publication in ApJ; 29 pages with 10 figures and 3
table
The MAPPINGS III Library of Fast Radiative Shock Models
We present a new library of fully-radiative shock models calculated with the
MAPPINGS III shock and photoionization code. The library consists of grids of
models with shock velocities in the range v=100-1000 km/s and magnetic
parameters B/sqrt(n) of 10^-4 - 10 muG cm^(3/2) for five different atomic
abundance sets, and for a pre-shock density of 1.0 cm^(-3). Additionally, Solar
abundance model grids have been calculated for densities of 0.01, 0.1, 10, 100,
and 1000 cm^(-3) with the same range in v and B/sqrt(n). Each model includes
components of both the radiative shock and its photoionized precursor, ionized
by the EUV and soft X-ray radiation generated in the radiative gas. We present
the details of the ionization structure, the column densities, and the
luminosities of the shock and its precursor. Emission line ratio predictions
are separately given for the shock and its precursor as well as for the
composite shock+precursor structure to facilitate comparison with observations
in cases where the shock and its precursor are not resolved. Emission line
ratio grids for shock and shock+precursor are presented on standard line ratio
diagnostic diagrams, and we compare these grids to observations of radio
galaxies and a sample of AGN and star forming galaxies from the Sloan Digital
Sky Survey. This library is available online, along with a suite of tools to
enable the analysis of the shocks and the easy creation of emission line ratio
diagnostic diagrams. These models represent a significant increase in parameter
space coverage over previously available models, and therefore provide a unique
tool in the diagnosis of emission by shocks.Comment: 39 pages, 34 figures, accepted for publication in ApJS, April 200
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