| 1. |
- Cardinaux, Frederic, et al.
(författare)
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Cluster-Driven Dynamical Arrest in Concentrated Lysozyme Solutions
- 2011
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Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 115:22, s. 7227-7237
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Tidskriftsartikel (refereegranskat)abstract
- We present a detailed experimental and numerical study of the structural and dynamical properties of salt-free lysozyme solutions. In particular, by combining small-angle X-ray scattering (SAXS) data with neutron spin echo (NSE) and rheology experiments, we are able to identify that an arrest transition takes place at intermediate densities, driven by the slowing down of the cluster motion. Using an effective pair potential among proteins, based on the combination of short-range attraction and long-range repulsion, we account remarkably well for the peculiar volume fraction dependence of the effective structure factor measured by SAXS. We show that a transition from a monomer to a cluster-dominated fluid happens at volume fractions larger than phi greater than or similar to 0.05 where the close agreement between NSE measurements and Brownian dynamics simulations confirms the transient nature of the clusters. Clusters even stay transient above the geometric percolation found in simulation at phi > 0.15, though NSE reveals a cluster lifetime that becomes increasingly large and indicates a divergence of the diffusivity at phi greater than or similar to 0.26. Macroscopic measurements of the viscosity confirm this transition where the long-lived-nature of the clusters is at the origin of the simultaneous dynamical arrest at all length scales.
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| 2. |
- Dorsaz, Nicolas, et al.
(författare)
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Phase separation in binary eye lens protein mixtures
- 2011
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Ingår i: Soft Matter. - : Royal Society of Chemistry (RSC). - 1744-6848 .- 1744-683X. ; 7:5, s. 1763-1776
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Tidskriftsartikel (refereegranskat)abstract
- Liquid-liquid phase separation occurs in young mammalian eye lenses and in concentrated solutions of isolated eye lens proteins, and has been linked to some forms of cataract. Here we study theoretically the protein compositions and cloud temperatures of two separated equilibrium phases that form out of concentrated mixtures of model proteins, chosen to have properties similar to those which reproduce experimental data on mixtures of two of the prevalent mammalian eye lens proteins, gamma- and alpha-crystallin. We use a thermodynamic perturbation theory that has previously been shown to provide a quantitative model for key features of the experimentally observed neutron scattering, phase boundary and tie line data, and that is also consistent with corresponding model, coarse-grained molecular dynamics simulations. In so doing we find an extremely sensitive dependence of protein partitioning on mutual attraction that is likely to have implications for many other protein, colloid, and other soft condensed matter systems. Previously, we found that a model square well attraction between the proteins of well depth u(alpha gamma) approximate to 0.5 k(B)T protects concentrated gamma-alpha mixtures against thermodynamic instability and is thus essential for their transparency. Furthermore, the dependence of the mixture phase separation on u(alpha gamma) was found to be highly non-monotonic, in that either weakening or increasing u(alpha gamma) by 0.5 k(B)T can lead to considerably enhanced phase separation that occurs at much higher temperatures. In the present work we show that the compositions of the separated protein phases are even more dramatically sensitive to the magnitude of u(alpha gamma). Specifically, increasing u(alpha gamma) by just 0.2 k(B)T can change the phase separation of alpha-gamma mixtures from one that is primarily compositional in nature to one of protein density separation, in which the two phases in equilibrium differ principally in overall protein concentration. Further, for the square-well widths investigated, we find that the phase separation properties change relatively rapidly in response to changes in square well depth, in comparison with their response to changes in the diameter ratio of the model proteins. We discuss potential ways in which sensitive connections between changes in molecular attraction and their macroscopic consequences, a hallmark of concentrated liquid mixtures, can lead to potential molecular mechanisms for hereditary and other forms of cataract, and can be applied to other colloidal and physiological systems.
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| 3. |
- Foffi, Giuseppe, et al.
(författare)
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Hard sphere-like glass transition in eye lens α-crystallin solutions.
- 2014
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Ingår i: Proceedings of the National Academy of Sciences. - : Proceedings of the National Academy of Sciences. - 1091-6490 .- 0027-8424. ; 111:47, s. 16748-16753
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Tidskriftsartikel (refereegranskat)abstract
- We study the equilibrium liquid structure and dynamics of dilute and concentrated bovine eye lens α-crystallin solutions, using small-angle X-ray scattering, static and dynamic light scattering, viscometry, molecular dynamics simulations, and mode-coupling theory. We find that a polydisperse Percus-Yevick hard-sphere liquid-structure model accurately reproduces both static light scattering data and small-angle X-ray scattering liquid structure data from α-crystallin solutions over an extended range of protein concentrations up to 290 mg/mL or 49% vol fraction and up to ca. 330 mg/mL for static light scattering. The measured dynamic light scattering and viscosity properties are also consistent with those of hard-sphere colloids and show power laws characteristic of an approach toward a glass transition at α-crystallin volume fractions near 58%. Dynamic light scattering at a volume fraction beyond the glass transition indicates formation of an arrested state. We further perform event-driven molecular dynamics simulations of polydisperse hard-sphere systems and use mode-coupling theory to compare the measured dynamic power laws with those of hard-sphere models. The static and dynamic data, simulations, and analysis show that aqueous eye lens α-crystallin solutions exhibit a glass transition at high concentrations that is similar to those found in hard-sphere colloidal systems. The α-crystallin glass transition could have implications for the molecular basis of presbyopia and the kinetics of molecular change during cataractogenesis.
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| 4. |
- Gibaud, Thomas, et al.
(författare)
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New routes to food gels and glasses
- 2012
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Ingår i: Faraday Discussions. - : Royal Society of Chemistry (RSC). - 1364-5498 .- 1359-6640. ; 158, s. 267-284
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Tidskriftsartikel (refereegranskat)abstract
- We describe the possibility to create solid-like protein samples whose structural and mechanical properties can be varied and tailored over an extremely large range in a very controlled way through an arrested spinodal decomposition process. We use aqueous lysozyme solutions as a model globular protein system. A combination of video microscopy, small-angle neutron and X-ray scattering and reverse Monte Carlo modeling is used to characterize the structure of the bicontinuous network with two coexisting phases of a dilute protein solution and a glassy or arrested dense protein backbone at all relevant length scales. Rheological measurements are then used to determine the complex mechanical response of these protein gels as a function of protein concentration and quench temperature. While in particular the origin of the dependence of the mechanical properties on quench depth and concentration is not well understood currently, it seems ultimately connected to the particular bicontinuous structure of the arrested spinodal network created by the interplay between the early stage of a spinodal decomposition and the position of the glass line. We then generalize this behavior and discuss how this could open up new routes to prepare gel-like food systems with adjustable structural and mechanical properties. We present results from a first feasibility study where we use a depletion interaction caused by the addition of small non-adsorbing polymers to suspensions of casein micelles in order to create food gels with tunable structural and mechanical properties through an arrested spinodal decomposition process.
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| 5. |
- Gibaud, Thomas, et al.
(författare)
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Phase separation and dynamical arrest for particles interacting with mixed potentials—the case of globular proteins revisited
- 2011
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Ingår i: Soft Matter. - : Royal Society of Chemistry (RSC). - 1744-683X .- 1744-6848. ; 7:2, s. 857-860
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Tidskriftsartikel (refereegranskat)abstract
- We examine the applicability of the extended law of corresponding states (ELCS) to equilibrium and non equilibrium features of the state diagram of the globular protein lysozyme. We provide compelling evidence that the ELCS correctly reproduces the location of the binodal for different ionic strengths, but fails in describing the location of the arrest line. We subsequently use Mode Coupling Theory (MCT) to gain additional insight into the origin of these observations. We demonstrate that while the critical point and the connected binodal and spinodal are governed by the integral features of the interaction potential described by the normalized second virial coefficient, the arrest line is mainly determined by the attractive well depth or bond strength.
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| 6. |
- Moitzi, Christian, et al.
(författare)
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The pH Induced Sol-Gel Transition in Skim Milk Revisited. A Detailed Study Using Time-Resolved Light and X-ray Scattering Experiments
- 2011
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Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 27:6, s. 2195-2203
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Tidskriftsartikel (refereegranskat)abstract
- We present a detailed study of the evolution of the size, structure and stability of casein micelles upon acidification of skim milk typically applied in yogurt-making processes using a combination of time-resolved light and small-angle X-ray scattering experiments. While most of the available light scattering studies on casein acidification have been restricted to transparent and therefore highly diluted samples, we now profit from a newly developed multiangle 3D light scattering instrument, which allows for time-resolved measurements in highly turbid samples. Our experiments clearly demonstrate the presence of two parallel pH-dependent processes, micellar reassembly and aggregation. Using a systematic investigation of the effect of casein concentration, acidification rate, and ionic strength, we are able to decouple these two processes and obtain detailed information about the pH-induced restructuration of the casein micelle structure that occurs prior to destabilization. Moreover, our experiments also unambiguously demonstrate that these micellar reassembly processes are highly concentration dependent, and that typical light scattering studies conducted under highly diluted conditions are resulting in findings that may not be relevant for the situation encountered in industrial processes at higher concentrations. Experiments conducted with covalently cross-linked micelles, where the pH-induced reassembly has been suppressed, further confirm our findings.
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| 7. |
- van Gruijthuijsen, Kitty, et al.
(författare)
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Direct comparison of SESANS and SAXS to measure colloidal interactions
- 2014
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Ingår i: Europhysics Letters. - : IOP Publishing. - 0295-5075. ; 106:2
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Tidskriftsartikel (refereegranskat)abstract
- We present a systematic comparison of conventional small-angle X-ray scattering (SAXS) with the relatively new real-space technique, spin-echo small-angle neutron scattering (SESANS). We apply both techniques to colloidal suspensions where the particles interact as hard spheres, with electrostatic repulsions, or with polymer-induced depletion attractions. We successfully model the data sets with the appropriate colloid-colloid interaction potentials to determine the colloid radius, effective volume fraction, and surface charge, while illustrating the different representations of colloid structuring related to both techniques. Copyright (C) EPLA, 2014
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| 8. |
- van Gruijthuijsen, Kitty, et al.
(författare)
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Light and Neutron Scattering Study of PEG-oleate and Its Use in Emulsion Polymerization
- 2012
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Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 28:28, s. 10381-10388
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Tidskriftsartikel (refereegranskat)abstract
- Steric stabilization of colloids forms a robust mechanism to obtain colloids that are stable in a variety of environments, and that can be used to study the phase behavior of hard or soft spheres. We report the synthesis of sterically stabilized colloids in an aqueous environment using readily dissolvable surfactants, with an unsaturated hydrophobic tail. We synthesized a new surfactant by esterification of a poly(ethylene glycol) (PEG) chain of 4.1 kg/mol with oleic acid, called PEG4OA. The micellization of PEG4OA was characterized by light and neutron scattering, which yielded values for the aggregation number and the overall size that are in excellent agreement with a comparable surfactant with a saturated octadecane chain, Brij 700. We successfully used PEG4OA in the emulsion polymerization of polystyrene colloids. In comparison with the smaller surfactant Tween 80, PEG4OA yielded smaller colloids with radii around 50 nm, and the addition of 1-dodecanethiol reduced the formation of aggregates during the synthesis. A contrast variation study with small angle neutron scattering (SANS) showed that a dense PEG layer was grafted to the colloid surface.
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| 9. |
- van Gruijthuijsen, Kitty, et al.
(författare)
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Origin of suppressed demixing in casein/xanthan mixtures
- 2012
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Ingår i: Soft Matter. - : Royal Society of Chemistry (RSC). - 1744-6848 .- 1744-683X. ; 8:5, s. 1547-1555
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Tidskriftsartikel (refereegranskat)abstract
- We explore the properties of casein/xanthan mixtures for xanthan concentrations beyond those inducing phase separation. Previous work has successfully described the onset of demixing by depletion theory in the protein limit, where the xanthan polysaccharides, the polymers, are larger than the caseins from skim milk powder, the colloids (S. Bhat et al., J. Phys.: Condens. Matter, 2006, L339). We now extend these studies to xanthan concentrations in a range of c/c* = 13-88, aiming to arrest the phase separation via a (transiently) arrested casein-rich phase. Surprisingly, we find that the casein-rich phase remains fluidic deep into the two-phase region, with an equilibrium composition that significantly differs from predictions for mixtures of hard spheres plus flexible polymer chains in a good solvent. Furthermore, we show that macroscopic phase separation is arrested for c/c* > 60. Our investigations reveal that the rheological properties of the mixtures are fully determined by the xanthan-rich phase with characteristic long relaxation times that depend remarkably strongly on the xanthan concentration.
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| 10. |
- van Gruijthuijsen, Kitty, et al.
(författare)
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Phase behaviour of colloids with short-range repulsions plus nonadsorbing polymer chains
- 2013
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Ingår i: Soft Matter. - : Royal Society of Chemistry (RSC). - 1744-6848 .- 1744-683X. ; 9:42, s. 9977-9982
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Tidskriftsartikel (refereegranskat)abstract
- The colloidal gas-liquid phase behaviour has been studied for aqueous mixtures of well-defined spherical particles with short-ranged repulsions mixed with relatively monodisperse poly(ethylene oxide) polymers. We show that our set of experimental phase diagrams are in quantitative agreement with theoretical predictions using generalized free volume theory [G. J. Fleer and R. Tuinier, Phys. Rev. E, 2007, 76, 041802]. The determination of the equilibrium composition of coexisting phases reveals qualitative deviations between the two, for which we propose a tentative explanation.
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