477 research outputs found
Crystallization of dense binary hard-sphere mixtures with marginal size ratio
Molecular dynamics simulations are performed for binary hard-sphere mixtures with a size ratio of =0.9 and a volume fraction of =0.58 over a range of compositions. We show how, at this high volume fraction, crystallization depends sensitively on the composition. Evidence is presented that crystallization in these mixtures does not proceed by the standard nucleation and growth paradigm. Rather, some crystallite forms almost immediately and then an interplay between compositional fluctuations and crystal growth is able to dramatically extend the time scale on which further crystallization occurs. This can be seen as a form of geometric frustration
Variability in the α and β anomer content of commercially available lactose
Lactose, a disaccharide is a ubiquitous excipient in many pharmaceutical formulations which exists in two anomeric forms; either as - or -lactose. The anomers have different properties which can affect their application. Nevertheless, batches of lactose products are widely produced by many manufacturers, and is available in many grades. However, the anomeric content of these batches has not been accurately characterized and reported previously. Therefore, the aim of this study was to analyse a set of 19 commercially available samples of lactose using a novel H1-NMR technique to establish a library showing the anomeric content of a large range of lactose products. The lactose samples were also analysed by DSC. The anomeric content of the -lactose monohydrate samples were found to vary by more than 10%, which might in_uence bioavailability from ^nal formulations. The data showed that there is a need to determine and monitor the anomeric content of lactose and this should be a priority to both manufacturers and formulators of medicines
Evaluating thermogravimetric analysis for the measurement of drug loading in mesoporous silica nanoparticles (MSNs)
In this study, a thermogravimetric analysis (TGA) method for measuring the drug loading in mesoporous silica nanoparticles (MSNs) has been developed and evaluated in comparison with the drug loading quantification by high-performance liquid chromatography (HPLC). Indapamide was loaded into two different types of MSNs, namely Mobile Crystalline Material (MCM-41, pore size = 1.2 nm) and Santa Barbara Amorphous (SBA-15, pore size = 4.1 nm). Physical mixtures of the drug and silica gave a linear correlation between the observed and expected drug content for both TGA and HPLC, which were used for calibration purposes. The limit of detection (LOD) for the TGA method obtained from the physical mixture calibration curve was 0.77 % (w/w) and the r² value was 0.9936, whereas the HPLC had a LOD of 0.06 % (w/w) and an r² value of 0.9933. The sensitivity of the TGA method was well established using the drug loading studies, as it can detect the low loading of MCM-41 at 2.2 ± 0.21 % (w/w), compared to 5.1 ± 0.12 % (w/w) with the SBA-15. In all samples applied, the multiple comparison analysis showed an insignificant difference between the two methods (p > 0.05). The TGA data presented good evidence for using this technique as a sensitive, cost-effective, and low-variable quantitative analysis in the drug loading determination of the MSNs. TGA is not a selective method of quantification, but optimising the method using the pure and blank samples of MSNs and drug can significantly improve the sensitivity. This work provides a unique approach to apply TGA as a selective and more favourable method to characterise MSNs to do early formulation developments
Temperature as an external field for colloid-polymer mixtures : "quenching" by heating and "melting" by cooling
We investigate the response to temperature of a well-known colloid-polymer
mixture. At room temperature, the critical value of the second virial
coefficient of the effective interaction for the Asakura-Oosawa model predicts
the onset of gelation with remarkable accuracy. Upon cooling the system, the
effective attractions between colloids induced by polymer depletion are
reduced, because the polymer radius of gyration is decreases as the
theta-temperature is approached. Paradoxically, this raises the effective
temperature, leading to "melting" of colloidal gels. We find the Asakura-Oosawa
model of effective colloid interactions with a simple description of the
polymer temperature response provides a quantitative description of the
fluid-gel transition. Further we present evidence for enhancement of
crystallisation rates near the metastable critical point.Comment: 13 page
Sedimentation of binary mixtures of like- and oppositely charged colloids: the primitive model or effective pair potentials?
We study sedimentation equilibrium of low-salt suspensions of binary mixtures
of charged colloids, both by Monte Carlo simulations of an effective
colloids-only system and by Poisson-Boltzmann theory of a colloid-ion mixture.
We show that the theoretically predicted lifting and layering effect, which
involves the entropy of the screening ions and a spontaneous macroscopic
electric field [J. Zwanikken and R. van Roij, Europhys. Lett. {\bf 71}, 480
(2005)], can also be understood on the basis of an effective colloid-only
system with pairwise screened-Coulomb interactions. We consider, by theory and
by simulation, both repelling like-charged colloids and attracting oppositely
charged colloids, and we find a re-entrant lifting and layering phenomenon when
the charge ratio of the colloids varies from large positive through zero to
large negative values
Dark Solitons in High Velocity Waveguide Polariton Fluids
We study exciton-polariton nonlinear optical fluids in the high momentum waveguide regime for the first
time. We demonstrate the formation of dark solitons with the expected dependence of width on fluid
density for both main classes of soliton-forming fluid defects. The results are well described by numerical
modeling of the fluid propagation. We deduce a continuous wave nonlinearity more than ten times that on
picosecond time scales, arising due to interaction with the exciton reservoir
Dynamical density functional theory for dense atomic liquids
Starting from Newton's equations of motion, we derive a dynamical density
functional theory (DDFT) applicable to atomic liquids. The theory has the
feature that it requires as input the Helmholtz free energy functional from
equilibrium density functional theory. This means that, given a reliable
equilibrium free energy functional, the correct equilibrium fluid density
profile is guaranteed. We show that when the isothermal compressibility is
small, the DDFT generates the correct value for the speed of sound in a dense
liquid. We also interpret the theory as a dynamical equation for a coarse
grained fluid density and show that the theory can be used (making further
approximations) to derive the standard mode coupling theory that is used to
describe the glass transition. The present theory should provide a useful
starting point for describing the dynamics of inhomogeneous atomic fluids.Comment: 14 pages, accepted for publication in J. Phys.: Condens. Matte
Experimental determination of configurational entropy in a two-dimensional liquid under random pinning
A quasi two-dimensional colloidal suspension is studied under the influence of immobilisation (pinning) of a random fraction of its particles. We introduce a novel experimental method to perform random pinning and, with the support of numerical simulation, we find that increasing the pinning concentration smoothly arrests the system, with a cross-over from a regime of high mobility and high entropy to a regime of low mobility and low entropy. At the local level, we study fluctuations in area fraction and concentration of pins and map them to entropic structural signatures and local mobility, obtaining a measure for the local entropic fluctuations of the experimental system
The Physics of the Colloidal Glass Transition
As one increases the concentration of a colloidal suspension, the system
exhibits a dramatic increase in viscosity. Structurally, the system resembles a
liquid, yet motions within the suspension are slow enough that it can be
considered essentially frozen. This kinetic arrest is the colloidal glass
transition. For several decades, colloids have served as a valuable model
system for understanding the glass transition in molecular systems. The spatial
and temporal scales involved allow these systems to be studied by a wide
variety of experimental techniques. The focus of this review is the current
state of understanding of the colloidal glass transition. A brief introduction
is given to important experimental techniques used to study the glass
transition in colloids. We describe features of colloidal systems near and in
glassy states, including tremendous increases in viscosity and relaxation
times, dynamical heterogeneity, and ageing, among others. We also compare and
contrast the glass transition in colloids to that in molecular liquids. Other
glassy systems are briefly discussed, as well as recently developed synthesis
techniques that will keep these systems rich with interesting physics for years
to come.Comment: 56 pages, 18 figures, Revie
A frequentist framework of inductive reasoning
Reacting against the limitation of statistics to decision procedures, R. A.
Fisher proposed for inductive reasoning the use of the fiducial distribution, a
parameter-space distribution of epistemological probability transferred
directly from limiting relative frequencies rather than computed according to
the Bayes update rule. The proposal is developed as follows using the
confidence measure of a scalar parameter of interest. (With the restriction to
one-dimensional parameter space, a confidence measure is essentially a fiducial
probability distribution free of complications involving ancillary statistics.)
A betting game establishes a sense in which confidence measures are the only
reliable inferential probability distributions. The equality between the
probabilities encoded in a confidence measure and the coverage rates of the
corresponding confidence intervals ensures that the measure's rule for
assigning confidence levels to hypotheses is uniquely minimax in the game.
Although a confidence measure can be computed without any prior distribution,
previous knowledge can be incorporated into confidence-based reasoning. To
adjust a p-value or confidence interval for prior information, the confidence
measure from the observed data can be combined with one or more independent
confidence measures representing previous agent opinion. (The former confidence
measure may correspond to a posterior distribution with frequentist matching of
coverage probabilities.) The representation of subjective knowledge in terms of
confidence measures rather than prior probability distributions preserves
approximate frequentist validity.Comment: major revisio
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