601 research outputs found
Solid mesostructured polymer-surfactant films at the air-liquid interface
Pioneering work by Edler et al. has spawned a new sub-set of mesostructured materials. These are solid, self-supporting films comprising surfactant micelles encased within polymer hydrogel; composite polymer-surfactant films can be grown spontaneously at the air-liquid interface and have defined and controllable mesostructures. Addition of siliconalkoxide to polymer-surfactant mixtures allows for the growth of mesostructured hybrid polymer-surfactant silica films that retain film geometry after calcinations and exhibit superior mechanical properties to typically brittle inorganic films. Growing films at the air-liquid interface provides a rapid and simple means to prepare ordered solid inorganic films, and to date the only method for generating mesostructured films thick enough (up to several hundred microns) to be removed from the interface. Applications of these films could range from catalysis to encapsulation of hydrophobic species and drug delivery. Film properties and mesostructures are sensitive to surfactant structure, polymer properties and polymer-surfactant phase behaviour: herein it will be shown how film mesostructure can be tailored by directing these parameters, and some interesting analogies will be drawn with more familiar mesostructured silica materials.</p
On the Estimation of Bivariate Return Curves for Extreme Values
In the multivariate setting, defining extremal risk measures is important in
many contexts, such as finance, environmental planning and structural
engineering. In this paper, we review the literature on extremal bivariate
return curves, a risk measure that is the natural bivariate extension to a
return level, and propose new estimation methods based on multivariate extreme
value models that can account for both asymptotic dependence and asymptotic
independence. We identify gaps in the existing literature and propose novel
tools for testing and validating return curves and comparing estimates from a
range of multivariate models. These tools are then used to compare a selection
of models through simulation and case studies. We conclude with a discussion
and list some of the challenges.Comment: 41 pages (without supplementary), 11 figures, 2 table
Improving estimation for asymptotically independent bivariate extremes via global estimators for the angular dependence function
Modelling the extremal dependence of bivariate variables is important in a
wide variety of practical applications, including environmental planning,
catastrophe modelling and hydrology. The majority of these approaches are based
on the framework of bivariate regular variation, and a wide range of literature
is available for estimating the dependence structure in this setting. However,
this framework is only applicable to variables exhibiting asymptotic
dependence, even though asymptotic independence is often observed in practice.
In this paper, we consider the so-called `angular dependence function'; this
quantity summarises the extremal dependence structure for asymptotically
independent variables. Until recently, only pointwise estimators of the angular
dependence function have been available. We introduce a range of global
estimators and compare them to another recently introduced technique for global
estimation through a systematic simulation study, and a case study on river
flow data from the north of England, UK
Sulfosuccinate and Sulfocarballylate Surfactants As Charge Control Additives in Nonpolar Solvents
A series
of eight sodium sulfonic acid surfactants with differently
branched tails (four double-chain sulfosuccinates and four triple-chain
sulfocarballylates) were studied as charging agents for sterically
stabilized poly(methyl methacrylate) (PMMA) latexes in dodecane. Tail
branching was found to have no significant effect on the electrophoretic
mobility of the latexes, but the number of tails was found to influence
the electrophoretic mobility. Triple-chain, sulfocarballylate surfactants
were found to be more effective. Several possible origins of this
observation were explored by comparing sodium dioctylsulfosuccinate
(AOT1) and sodium trioctylsulfocarballylate (TC1) using identical
approaches: the inverse micelle size, the propensity for ion dissociation,
the electrical conductivity, the electrokinetic or ζ potential,
and contrast-variation small-angle neutron scattering. The most likely
origin of the increased ability of TC1 to charge PMMA latexes is a
larger number of inverse micelles. These experiments demonstrate a
small molecular variation that can be made to influence the ability
of surfactants to charge particles in nonpolar solvents, and modifying
molecular structure is a promising approach to developing more effective
charging agents
A global climatological model of extreme geomagnetic field fluctuations
This paper presents a multi-parameter global statistical model of extreme horizontal geomagnetic field fluctuations (dBH/dt), which are a useful input to models assessing the risk of geomagnetically induced currents in ground infrastructure. Generalised Pareto (GP) distributions were fitted to 1-min measurements of |dBH/dt| from 125 magnetometers (with an average of 28 years of data per site) and return levels (RL) predicted for return periods (RP) between 5 and 500 years. Analytical functions characterise the profiles of maximum-likelihood GP model parameters and the derived RLs as a function of corrected geomagnetic latitude, λ. A sharp peak in both the GP shape parameter and the RLs is observed at |λ| = 53° in both hemispheres, indicating a sharp equatorward limit of the auroral electrojet region. RLs also increase strongly in the dayside region poleward of the polar cusp (|λ| > 75°) for RPs > 100 years. We describe how the GP model may be further refined by modelling the probability of occurrences of |dBH/dt| exceeding the 99.97th percentile as a function of month, magnetic local time, and the direction of the field fluctuation, dBH, and demonstrate that these patterns of occurrence align closely to known patterns of auroral substorm onsets, ULF Pc5 wave activity, and (storm) sudden commencement impacts. Changes in the occurrence probability profiles with the interplanetary magnetic field (IMF) orientation reveal further details of the nature of the ionospheric currents driving extreme |dBH/dt| fluctuations, such as the changing location of the polar cusp and seasonal variations explained by the Russell-McPherron effect
Pd-η3-C6H 9 complexes of the Trost modular ligand: high nuclearity columnar aggregation controlled by concentration, solvent and counterion
Pd-η3-C6H9 cations bearing the Trost ligand (2) undergo two-stage oligomerisation-aggregation to form high nuclearity aggregates (up to 56 Pd centres), with aggregation strongly modulated by concentration, solvent and counter-anion.</p
Low-surface energy surfactants with branched hydrocarbon architectures
International audienceSurface tensiometry and small-angle neutron scattering have been used to characterize a new class of low-surface energy surfactants (LSESs), "hedgehog" surfactants. These surfactants are based on highly branched hydrocarbon (HC) chains as replacements for environmentally hazardous fluorocarbon surfactants and polymers. Tensiometric analyses indicate that a subtle structural modification in the tails and headgroup results in significant effects on limiting surface tensions γcmc at the critical micelle concentration: a higher level of branching and an increased counterion size promote an effective reduction of surface tension to low values for HC surfactants (γcmc 24 mN m-1). These LSESs present a new class of potentially very important materials, which form lamellar aggregates in aqueous solutions independent of dilution
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