40,121 research outputs found
Ultrafiltration modeling of non-ionic microgels
Membrane ultrafiltration (UF) is a pressure driven process allowing for the
separation and enrichment of protein solutions and dispersions of nanosized
microgel particles. The permeate flux and the near-membrane
concentration-polarization (CP) layer in this process is determined by
advective-diffusive dispersion transport and the interplay of applied and
osmotic transmembrane pressure contributions. The UF performance is thus
strongly dependent on the membrane properties, the hydrodynamic structure of
the Brownian particles, their direct and hydrodynamic interactions, and the
boundary conditions. We present a macroscopic description of cross-flow UF of
non-ionic microgels modeled as solvent-permeable spheres. Our filtration model
involves recently derived semi-analytic expressions for the
concentration-dependent collective diffusion coefficient and viscosity of
permeable particle dispersions [Riest et al., Soft Matter, 2015, 11, 2821].
These expressions have been well tested against computer simulation and
experimental results. We analyze the CP layer properties and the permeate flux
at different operating conditions and discuss various filtration process
efficiency and cost indicators. Our results show that the proper specification
of the concentration-dependent transport coefficients is important for reliable
filtration process predictions. We also show that the solvent permeability of
microgels is an essential ingredient to the UF modeling. The particle
permeability lowers the particle concentration at the membrane surface, thus
increasing the permeate flux.Comment: 19 pages, 11 figures (Electronic Supplementary Information included:
2 pages, 1 figure
Kinetics of non-ionic surfactant adsorption at a fluid-fluid interface from a micellar solution
The kinetics of non-ionic surfactant adsorption at a fluid-fluid interface
from a micellar solution is considered theoretically. Our model takes into
account the effect of micelle relaxation on the diffusion of the free
surfactant molecules. It is shown that non-ionic surfactants undergo either a
diffusion or a kinetically limited adsorption according to the characteristic
relaxation time of the micelles. This gives a new interpretation for the
observed dynamical surface tension of micellar solutions.Comment: 4 page
Numerical modelling of non-ionic microgels: an overview
Microgels are complex macromolecules. These colloid-sized polymer networks
possess internal degrees of freedom and, depending on the polymer(s) they are
made of, can acquire a responsiveness to variations of the environment
(temperature, pH, salt concentration, etc.). Besides being valuable for many
practical applications, microgels are also extremely important to tackle
fundamental physics problems. As a result, these last years have seen a rapid
development of protocols for the synthesis of microgels, and more and more
research has been devoted to the investigation of their bulk properties.
However, from a numerical standpoint the picture is more fragmented, as the
inherently multi-scale nature of microgels, whose bulk behaviour crucially
depends on the microscopic details, cannot be handled at a single level of
coarse-graining. Here we present an overview of the methods and models that
have been proposed to describe non-ionic microgels at different length-scales,
from the atomistic to the single-particle level. We especially focus on
monomer-resolved models, as these have the right level of details to capture
the most important properties of microgels, responsiveness and softness. We
suggest that these microscopic descriptions, if realistic enough, can be
employed as starting points to develop the more coarse-grained representations
required to investigate the behaviour of bulk suspensions
Removal of chromium (III) by two-aqueous phases extraction
Two-aqueous phase extraction of chromium (III) as a solute from their aqueous solutions was investigated using polyethoxylated alcohols (CiEj) as a biodegradable non-ionic surfactant in the presence of anionic sodium dodecylbenzene sulfonate (SDBS). First, the combined effects of chromium and surfactants mixture (anionic and non-ionic) on the cloud point temperature were determined. After this, the phase diagrams of binary systems water–surfactant (NW342 and C10E3) were traced. According to the given surfactants concentration, the extracted solute reached 98.5% and 60% for NW342 and C10E3, respectively at pH equal to 3. The addition of sodium chloride lowers the cloud point temperature of surfactant where the phenomenon of salting-out was pronounced. Under the optimal extraction conditions, the suggested extraction mechanism is based on chromium species-NW342 non-ionic surfactant micelles solvatation. Since, the prevalence species (93.82%) were Cr(III)4(OH)66+, given by a theoretical calculation using CHEAQS V. L20.1. The first stage regeneration of NW342 surfactant was 27.82% at pH equal to 4.23 factorial designs were employed for screening the factors that would influence the overall optimization of a batch procedure of sorption
Shear and longitudinal viscosity of non-ionic C12E8 aqueous solutions
We present measurements of the steady shear viscosity, the longitudinal
elastic modulus and the ultrasonic absorption in the one-phase isotropic liquid
region of the nonionic surfactant C12E8 aqueous solutions. The overall results
support the presence of two separated intervals of concentration corresponding
to different structural properties. In the surfactant-rich region the
temperature dependence of the steady shear viscosity follows an equation
characteristic of glass-like systems. The ultrasonic absorption spectra show
unambiguous evidence of viscoelastic behaviour described by a Cole-Cole
relaxation formula. In the water-rich region the behaviour of the measured
quantities are more complex and reflect the presence of dispersed aggregates
whose size increases with temperature and concentration. An additional low
frequency contribution is also observed, which is ascribed to the exchange of
water molecules and/or surfactant monomers between the aggregates and the bulk
solvent region.Comment: 23 Pages, 7 Figures, 1 Table, submitted to J. Phys. Chem B, accepted
for publicatio
Kinetics of Surfactant Adsorption at Fluid/Fluid Interfaces: Non-ionic Surfactants
We present a model treating the kinetics of adsorption of soluble
surface-active molecules at the interface between an aqueous solution and
another fluid phase. The model accounts for both the diffusive transport inside
the solution and the kinetics taking place at the interface using a free-energy
formulation. In addition, it offers a general method of calculating dynamic
surface tensions. Non-ionic surfactants are shown, in general, to undergo a
diffusion-limited adsorption, in accord with experimental findings.Comment: 6 pages, 3 figures, see also cond-mat/960814
Membrane synthesis by microemulsion polymerisation stabilised by commercial non-ionic surfactants
Earlier works had demonstrated that microemulsion polymerisation is a well suited technique to produce nanostructured membranes if surfmers (polymerisable surfactants) are used to stabilise the primary template (sponge phase microemulsion). Up to now, however; same hadn't been done using common surfactants. Present work aims to show this is possible if proper surfactants are selected. Specific formulation selection was done by means of phase diagram. Phase diagram was obtained by conductivity, surface tension and QELS measurements through several dilution lines. Polymerisation region was selected from phase diagram were no globular system was observed. Membranes were characterised by SEM, DSC and permeation experiment
Self-Diffusion and Collective Diffusion of Charged colloids Studied by Dynamic Light Scattering
A microemulsion of decane droplets stabilized by a non-ionic surfactant film
is progressively charged by substitution of a non-ionic surfactant molecule by
a cationic surfactant. We check that the microemulsion droplets remain
identical within the explored range of volume fraction (0.02 to 0.18) and of
the number of charge per droplets (0 to 40) . We probe the dynamics of these
microemulsions by dynamic light scattering. Despite the similar structure of
the uncharged and charged microemulsions the dynamics are very different . In
the neutral microemulsion the fluctuations of polarization relax, as is well
known, via the collective diffusion of the droplets. In the charged
microemulsions, two modes of relaxation are observed. The fast one is ascribed
classically to the collective diffusion of the charged droplets coupled to the
diffusion of the counterions. The slow one has, to our knowledge, not been
observed previously neither in similar microemulsions nor in charged spherical
colloids. We show that the slow mode is also diffusive and suggest that its
possible origine is the relaxation of local charge fluctuations via local
exchange of droplets bearing different number of charges . The diffusion
coefficient associated with this mode is then the self diffusion coefficient of
the droplets
Structural Relaxations in a Simple Model Molten Salt
The structural relaxations of a dense, binary mixture of charged hard spheres
are studied using the Mode Coupling Theory (MCT). Qualitative differences to
non--ionic systems are shown to result from the long--range Coulomb interaction
and charge ordering in dense molten salts. The presented non--equilibrium
results are determined by the equilibrium structure, which is input using the
well studied Mean Spherical Approximation.Comment: 6 pages, 4 Postscript figures, uses epsfig.sty, rotate.sty, here.st
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