The extended law of corresponding states when attractions meet repulsions.

Short-range attractive colloids show well-defined phase behaviour in the absence of repulsions, and highly intriguing equilibrium gelation in the presence of long-range repulsions. We present the state diagram of short-range attractive colloids with repulsions that range from fully screened to intermediately ranged, i.e. longer-ranged than the attractions, but shorter ranged than the colloid size. We demonstrate that although the macroscopic phase behaviour does not change perceptibly, there is a dramatic increase of inhomogeneities once the repulsions become longer-ranged than the attractions. The interaction potentials are characterized with small angle neutron scattering, and used to renormalize the state diagram with the minimum in the interaction potential, min[U(r)], and with the reduced second virial coefficient, B2*. We find that the extended law of corresponding states captures the onset of phase separation for shorter ranged repulsions, but fails for longer ranged repulsions. Instead, for a given model of U(r), the transition from visually homogeneous fluid to phase separation and/or gelation can be rescaled with min[U(r)] over the full range of repulsions. Finally, we suggest a generic state diagram to describe the effect of repulsions on short-range attractive systems.

Erratum: Structure and Dynamics of Loosely Cross-Linked Ionic Microgel Dispersions in the Fluid Regime [Phys. Rev. Lett. 109, 048302 (2012)].

This corrects the article DOI: 10.1103/PhysRevLett.109.048302.

Dielectric spectroscopy of ionic microgel suspensions.

The determination of the net charge and size of microgel particles as a function of their concentration, as well as the degree of association of ions to the microgel backbone, has been pursued in earlier studies mainly by scattering and rheology. These methods suffer from contributions due to inter-particle interactions that interfere with the characterization of single-particle properties. Here we introduce dielectric spectroscopy as an alternative experimental method to characterize microgel systems. The advantage of dielectric spectroscopy over other experimental methods is that the polarization due to mobile charges within a microgel particle is only weakly affected by inter-particle interactions. Apart from electrode polarization effects, experimental spectra on PNIPAM-co-AA [poly(N-isopropylacrylamide-co-acrylic acid)] ionic microgel particles suspended in de-ionized water exhibit three well-separated relaxation modes, which are due to the polarization of the mobile charges within the microgel particles, the diffuse double layer around the particles, and the polymer backbone. Expressions for the full frequency dependence of the electrode-polarization contribution to the measured dielectric response are derived, and a theory is proposed for the polarization resulting from the mobile charges within the microgel. Relaxation of the diffuse double layer is modeled within the realm of a cell model. The net charge and the size of the microgel particles are found to be strongly varying with concentration. A very small value of the diffusion coefficient of ions within the microgel is found, due to a large degree of chemical association of protons to the polymer backbone.

Structure and dynamics of loosely cross-linked ionic microgel dispersions in the fluid regime.

We report a comprehensive experimental-theoretical study of the temperature- and concentration-dependent swelling behavior of weakly cross-linked PNiPAm ionic microgel particles in the deionized fluid phase. The particles swell reversibly when the dispersion is cooled from the collapsed state to lower temperatures. While the collapsed state shows no dependence on the microgel number density, the swelling at lower T is more pronounced at lower concentrations. The static pair correlations and short-time diffusion functions, and the concentration and temperature dependence of the microgel radius and effective charge, are studied using static and dynamic light scattering in combination with state-of-the-art analytical theoretical schemes based on a Yukawa-type effective pair potential and a core-shell model. We show that only such a combined, simultaneous fit of static and dynamic scattering functions allows for an unambiguous determination of the microgel radius and effective charge.

Magnetic properties of silica coated spindle-type hematite particles.

Magnetic properties of particles are generally determined from randomly oriented ensembles and the influence of the particle orientation on the magnetic response is neglected. Here, we report on the magnetic characterization of anisotropic spindle-type hematite particles. The easy axis of magnetization is within the basal plane of hematite, which is oriented perpendicular to the spindle axis. Two standard synthesis routes are compared and the effects of silica coating and particle orientation on the magnetic properties are investigated. Depending on the synthesis route we find fundamentally different magnetic behavior compatible with either single domain particles or superparamagnetic sub-units. Furthermore, we show that silica coating reduces the mean blocking temperature to nearly room temperature. The mechanical stress induced by the silica coating appears to reduce the magnetic coupling between the sub-units.

Inorganic-organic elastomer nanocomposites from integrated ellipsoidal silica-coated hematite nanoparticles as crosslinking agents.

We report on the synthesis of nanocomposites with integrated ellipsoidal silica-coated hematite (SCH) spindle type nanoparticles which can act as crosslinking agents within an elastomeric matrix. Influence of the surface chemistry of the hematite, leading either to dispersed particles or crosslinked particles to the elastomer matrix, was studied via swelling, scattering and microscopy experiments. It appeared that without surface modification the SCH particles aggregate and act as defects whereas the surface modified SCH particles increase the crosslinking density and thus reduce the swelling properties of the nanocomposite in good solvent conditions. For the first time, inorganic SCH particles can be easily dispersed into a polymer network avoiding aggregation and enhancing the properties of the resulting inorganic-organic elastomer nanocomposite (IOEN).

Colloidal characterization and thermodynamic stability of binary eye lens protein mixtures.

We present a study of binary mixtures of eye lens crystallin proteins. A coarse-grained model of aqueous alpha- and gamma-crystallin mixtures based on molecular dynamics simulations and SANS experiments is proposed. Thermodynamic perturbation theory is implemented to obtain the stability boundaries, or spinodal surface, of the binary mixture in the full parameter space. The stability of these high-concentration crystallin mixtures was found to depend on the alpha-gamma attraction in a manner that is both extremely sensitive and nonmonotonic; stronger or weaker attraction resulted in a spectacularly enhanced instability. The relevance of these mechanisms as possible sources of the alteration of the spatial distribution of the lens proteins encountered in cataract disease is discussed.

Renormalization in charged colloids: non-monotonic behaviour with the surface charge.

The static structure factor S(q) is measured for a set of deionized latex dispersions with different numbers of ionizable surface groups per particle and similar diameters. For a given volume fraction, the height of the main peak of S(q), which is a direct measure of the spatial ordering of latex particles, does not increase monotonically with the number of ionizable groups. This behaviour cannot be described using the classical renormalization scheme based on the cell model. We analyse our experimental data using a renormalization model based on the jellium approximation, which predicts the weakening of the spatial order for moderate and large particle charges.

The interaction of saccharides with lipid bilayer vesicles: stabilization during freeze-thawing and freeze-drying.

The fusion of small unilamellar vesicles of phosphatidylcholines during freeze-thawing and freeze-drying/rehydration, and the suppression of fusion under these conditions by various saccharides, was investigated by gel filtration on Sepharose 4B, quasielastic light scattering, high-resolution 1H-NMR, ESR spin labeling, and differential scanning calorimetry. Freeze-thawing and freeze-drying of aqueous small unilamellar vesicle suspensions in the presence of sufficient sucrose had no significant effect on the average size and size distribution of small unilamellar vesicles. In the presence of sucrose the structural integrity and the permeability properties of the phosphatidylcholine bilayers were retained during freeze-thawing and freeze-drying. A comparison of the stabilizing effect of sucrose with that of trehalose and glucose showed that the stabilization is not sugar-specific but is a general property of saccharides. The fraction of small unilamellar vesicles recovered after freeze-thawing depended on the saccharide/phosphatidylcholine molar ratio. The mechanism of the cryoprotective effect involves binding of the sugar to the phospholipid polar group, probably through hydrogen bonding.

The effect of phosphorylation on the structure of alpha-crystallin.

Homopolymers were constructed from the highly purified phosphorylated (A1 and B1) and non-phosphorylated (A2 and B2) polypeptides of alpha-crystallin. These were examined using electron microscopy, light scattering, fluorescence spectroscopy and sulphydryl probing to determine the effects of phosphorylation on the structure of alpha-crystallin. Each reconstituted aggregate consisted of uniform particles with circular cross-sections and diameters of 9.3-9.5 nm. Some of these were associated in chain-like structures. The molecular mass of the homopolymers varied from 360-507 kDa; for a single particle, it was estimated to be 216 +/- 10 kDa. Tryptophan residues in the alpha B2 homopolymers were more accessible to the solvent and to quenchers than those in alpha A2. Phosphorylation increased this accessibility in the alpha A homopolymer but decreased it in alpha B. This decrease could be attributed to phosphorylation of the serine adjacent to tryptophan 60 in the B chain. The kinetics of the reaction with DTNB indicated that the single cysteine in the A chain was buried in the alpha A2 homopolymer (k1' = 0.013 min-1) but more accessible in alpha A1 (k1' = 0.046 min-1). It was concluded that phosphorylation significantly alters the conformation of the alpha A subunits but probably has little effect on the B subunits. The alterations do not affect the ability of the subunits to associate into alpha-crystallin-like particles.

Vesicle formation in aqueous solutions of bile salt and monoacylglycerol.

We have investigated by means of quasielastic light scattering and freeze-fracture electron microscopy the aggregation behavior of aqueous solutions of taurocholate and monoolein. A spontaneous micelle-to-vesicle transition has been observed upon dilution of a mixed micellar solution. The size and stability of the micelles and vesicles present in these solutions has been studied as a function of the concentration and incubation time.

Preparation of monodisperse vesicles with variable size by dilution of mixed micellar solutions of bile salt and phosphatidylcholine.

We have investigated by means of quasi-elastic light scattering the aggregative behavior of aqueous mixed micellar solutions of glycocholate and phosphatidylcholine. Upon dilution with buffer the micellar size and the polydispersity increases dramatically, and, as the system is diluted beyond the mixed micellar phase boundary, a spontaneous transition from polydisperse micelles to monodisperse vesicles occurs. The radius of the vesicles formed upon dilution depends strongly upon the final composition of the solution and can be varied between 120 and 550 A. In contrast to the thermodynamically stable mixed micelles these vesicle solutions can be brought into a metastable state in which it is possible to remove by dialysis the bile salt molecules from the mixed vesicles without changing their radius by more then 10%. The combination of dilution and dialysis thus represents a method for the preparation of unilamellar, monodisperse and detergent-free vesicles with a desired radius that can be chosen between 120 and 500 A.

Size analysis of biological membrane vesicles by gel filtration, dynamic light scattering and electron microscopy.

Biological membrane vesicles are analysed in terms of size and size distribution using gel filtration on Sephacryl S-1000, electron microscopy and quasi-elastic light scattering. The agreement between the three methods is satisfactory particularly for homogeneous dispersions. Gel filtration on Sephacryl S-1000 is a quick and convenient method for the routine size analysis of membrane vesicles up to a diameter of about 250 nm.

The effect of monoacylglycerol on the phase behavior of egg phosphatidylcholine.

Phosphatidylcholine bilayers can accommodate large quantities of monoacylglycerol. Incorporating up to 40% monoacylglycerol has little effect on the orientation and motion of the phosphatidylcholine polar group. Briefly heating mixed dispersions of 1-monooleoylglycerol/egg phosphatidylcholine (1:1, weight ratio; 2.1:1, mole ratio) to 50-60 degrees C induced spontaneous vesiculation: unilamellar and some oligolamellar vesicles bud off the large multilamellar particles. The size of the resulting vesicles ranges from 100 to 1000 nm, with the bulk of the vesicles having diameters between 100 and 500 nm. The spontaneous vesiculation process is reflected in the visual clearance of the mixed lipid dispersion and in the collapse of the 31P powder NMR spectrum to a sharp, asymmetric peak. The narrowing of the 31P-NMR spectrum is explained in terms of additional molecular and/or segmental motion of the lipid polar groups. In mixed dispersions of 1-monooleoylglycerol/egg phosphatidylcholine containing an excess of 1-monooleoylglycerol (greater than or equal to 50%) domain formation takes place, i.e., the formation of local clusters enriched in either of the two lipids. As a result the mechanical properties of these mixed lipid bilayers seem to be quite different from those of pure egg phosphatidylcholine.

Changes in structure of alpha 2-macroglobulin upon reaction with trypsin as assessed by light scattering and differential scanning calorimetry.

Employing a combination of static and dynamic light scattering, as well as differential scanning calorimetry (DSC), the structural changes which appear in alpha 2-macroglobulin (alpha 2M) upon trypsin binding have been further characterized. Light-scattering measurements suggest that a 15% reduction in both the hydrodynamic radius and radius of gyration occurs when two molecules of trypsin complex to alpha 2M. Approx. 85% of this trypsin-induced compaction results from the binding of the first proteinase. A complementary result was obtained from DSC measurements in which the major fraction of the trypsin-induced conversion of alpha 2M to a single more thermally stable form results from interaction with the first proteinase molecule. These observations support a functionally asymmetric model of trypsin binding to alpha 2M in which the significant reduction in size of the complex is primarily due to the initial interaction of alpha 2M with a single proteinase molecule.

The solution structure of functionally active human proliferating cell nuclear antigen determined by small-angle neutron scattering.

The function of proliferating cell nuclear antigen (PCNA) in DNA replication and repair is to form a sliding clamp with replication factor C (RF-C) tethering DNA polymerase delta or epsilon to DNA. In addition, PCNA has been found to interact directly with various proteins involved in cell cycle regulation. The crystal structure of yeast PCNA shows that the protein forms a homotrimeric ring lining a hole through which double-stranded DNA can thread, thus forming a moving platform for DNA synthesis. Human and yeast PCNA are highly conserved at a structural and functional level. We determined the solution structure of functionally active human PCNA by small-angle neutron scattering. Our measurements strongly support a trimeric ring-like structure of functionally active PCNA in solution, and the data are in good agreement with model calculations based on the crystal structure from yeast PCNA. The human PCNA used in the small-angle neutron scattering experiments was active before and after the measurements in a RF-C independent and a RF-C dependent assay suggesting that the trimeric structure is the in vivo functional form.

Off-lattice Monte Carlo simulations of irreversible and reversible aggregation processes.

Monte Carlo simulations are used to get an insight into the formation of fractal aggregates from diluted to concentrated colloidal particle dispersions. Using irreversible conditions, we investigate the aggregation size distribution, architecture of the resulting fractal aggregates, possible transitions from simple aggregation to percolation and from percolation to the homogeneous aggregation regime, and discuss the fractal dimension determination from the radial distribution function. In particular the effects of the particle concentration on the aggregate fractal dimensions are considered. Reversibility is also introduced in the model so as to consider more realistic systems. The effects of aggregate fragmentation and internal reorganization are then investigated by adjusting the interparticle interaction potential. Important results dealing with the concomitant effect of aggregate break-up and internal reorganization on the aggregate local structure and stability with regards to phase separation are discussed.

Characterizing nanoparticles in complex biological media and physiological fluids with depolarized dynamic light scattering.

Light scattering is one of the few techniques available to adequately characterize suspended nanoparticles (NPs) in real time and in situ. However, when it comes to NPs in multicomponent and optically complex aqueous matrices - such as biological media and physiological fluids - light scattering suffers from lack of selectivity, as distinguishing the relevant optical signals from the irrelevant ones is very challenging. We meet this challenge by building on depolarized scattering: Unwanted signals from the matrix are completely suppressed. This approach yields information with an unprecedented signal-to-noise ratio in favour of the NPs and NP-biomolecule corona complexes, which in turn opens the frontier to scattering-based studies addressing the behaviour of NPs in complex physiological/biological fluids.

Sol-Gel transition of concentrated colloidal suspensions

Diffusing-wave spectroscopy (DWS) was used to follow the sol-gel transition of concentrated colloidal suspensions. We present a new technique based on a sandwich of two scattering cells aimed to overcome the problem of nonergodicity in DWS of solidlike systems. Using this technique we obtain quantitative information about the microscopic dynamics all the way from an aggregating suspension to the final gel, thereby covering the whole sol-gel transition. At the gel point a dramatic change of the particle dynamics from diffusion to a subdiffusive arrested motion is observed. A critical-power-law behavior is found for the time evolution of the maximum mean square displacement delta(2) probed by a single particle in the gel.