Origin of Anomalously Large Depletion Zones in Like-Charged Colloid-Polyelectrolyte Mixtures

Depletion zones in polyelectrolyte solutions in contact with like-charged flat surfaces are investigated. Using a coupled self-consistent field and Debye-Hückel approach, an explicit expression for the thickness δ of the depletion layer is derived. It is found that δ∼δ_{n}+cκ^{-1}, where δ_{n} is the depletion thickness at a neutral surface, c is a function of the electrostatic characteristics of the system, and κ^{-1} is the Debye length. It is argued that the theory still holds beyond the mean-field approximation, which is confirmed by quantitative agreement between our theoretical results and experiments.

Multiphase Coexistence in Binary Hard Colloidal Mixtures: Predictions from a Simple Algebraic Theory

A general theoretical framework is proposed to quantify the thermodynamic properties of multicomponent hard colloidal mixtures. This framework is used to predict the phase behavior of mixtures of rods with spheres and rods with plates taking into account (liquid) crystal phases of both components. We demonstrate a rich and complex range of phase behaviors featuring a large variety of different multiphase coexistence regions, including two five-phase coexistence regions for hard rod/sphere mixtures, and even a six-phase equilibrium for hard rod/plate dispersions. The various multiphase coexistences featured in a particular mixture are in line with a recently proposed generalized phase rule and can be tuned through subtle variations of the particle shape and size ratio. Our approach qualitatively accounts for certain multiphase equilibria observed in rod/plate mixtures of clay colloids and will be a useful guide in tuning the phase behavior of shape-disperse mixtures in general.

Depletion interaction mediated by semiflexible polymers

We present a simple mean-field theory to describe the polymer-mediated depletion attraction between colloidal particles that accounts for the polymer's chain stiffness. We find that for fixed polymer radius of gyration and volume fraction, the strength of this attraction increases with increasing chain stiffness in both dilute and semidilute concentration regimes. In contrast, the range of attraction monotonically decreases with chain stiffness in the dilute regime, while it attains a maximum in the semidilute regime. The obtained analytical expressions for the depletion interaction were compared with numerical self-consistent field lattice computations and shown to be in quantitative agreement. From the interaction potential between two spheres, we calculated the second osmotic virial coefficient B₂, which appears to be a convex function of chain stiffness. A minimum of B₂ as a function of chain stiffness was observed both in the numerical self-consistent field computations and the analytical theory. These findings help explain the general observation that semiflexible polymers are more effective depletants than flexible polymers and give insight into the phase behavior of mixtures containing spherical colloids and semiflexible polymers.

The depletion thickness in solutions of semi-flexible polymers near colloidal surfaces: analytical approximations

We derive a simple, yet accurate approximate mean-field expression for the depletion thickness δ_sf of a solution of dilute semi-flexible polymers next to a hard surface. In the case of a hard wall this equation has the simple form δ_sf = δ₀[1 - tanh(p_sf/δ₀)], where p_sf accounts for the degree of flexibility and δ₀ is the depletion thickness in the case of fully flexible polymers. For fixed polymer coil size, increasing the chain stiffness leads to a decrease in the depletion thickness. The approach is also extended to include higher polymer concentrations in the semidilute regime. The analytical expressions are in quantitative agreement with numerical self-consistent field computations. A remarkable finding is that there is a maximum in the depletion thickness as a function of the chain stiffness in the semidilute concentration regime. This also means that depletion attractions between colloidal particles reach a maximum for a certain chain stiffness, which may have important implications for the phase stability of colloid-polymer mixtures. The derived equations could be useful for the description of interactions in- and phase stability of mixtures of colloids and semi-flexible polymers.

Phase separation in mixed suspensions of bacteria and nonadsorbing polymers

The shapes of bacteria can vary widely; they may, for instance, be spherical, rod-like, string-like, or curved. In general, bacilli are highly anisotropic. For research and (bio)technological purposes, it can be useful to concentrate bacteria, which is possible by adding nonadsorbing polymers. The induced phase separation originates from a polymer-mediated depletion interaction, first understood by Asakura and Oosawa. Here, it is shown that free volume theory (FVT) can semi-quantitatively describe the phase transitions observed when adding sodium polystyrene sulfonate polymers to E. coli bacteria [Schwarz-Linek et al., Soft Matter 6, 4540 (2010)] at high ionic strength. The E. coli bacteria are described as short, hard spherocylinders. FVT predicts that the phase transitions of the mixtures result from a fluid-ABC crystal solid phase coexistence of a hard spherocylinder-polymer mixture.

Phase stability of colloidal mixtures of spheres and rods

We determined the phase boundaries of aqueous mixtures containing colloidal rod-like fd-viruses and polystyrene spheres using diffusing-wave spectroscopy and compared the results with free volume theory predictions. Excluded volume interactions in mixtures of colloidal rods and spheres lead to mediated depletion interactions. The strength and range of this attractive interaction depend on the concentrations of the particles, the length L and diameter D of the rods, and the radius R of the spheres. At strong enough attraction, this depletion interaction leads to phase separation. We experimentally determined the rod and sphere concentrations where these phase transitions occur by systematically varying the size ratios L/R and D/R and the aspect ratio L/D. This was done by using spheres with different radii and modifying the effective diameter of the rods through either the ionic strength of the buffer or anchoring a polymeric brush to the surface of the rods. The observed phase transitions were from a binary fluid to a colloidal gas/liquid phase coexistence that occurred already at very low concentrations due to the depletion efficiency of highly anisotropic rods. The experimentally measured phase transitions were compared to phase boundaries obtained using free volume theory (FVT), a well established theory for calculating the phase behavior of colloidal particles mixed with depletants. We find good correspondence between the experimental phase transitions and the theoretical FVT model where the excluded volume of the rod-like depletants was explicitly accounted for in both the reservoir and the system.

Defying the Gibbs Phase Rule: Evidence for an Entropy-Driven Quintuple Point in Colloid-Polymer Mixtures

Using a minimal algebraic model for the thermodynamics of binary rod-polymer mixtures, we provide evidence for a quintuple phase equilibrium; an observation that seems to be at odds with the Gibbs phase rule for two-component systems. Our model is based on equations of state for the relevant liquid crystal phases that are in quantitative agreement with computer simulations. We argue that the appearance of a quintuple equilibrium, involving an isotropic fluid, a nematic and smectic liquid crystal, and two solid phases, can be reconciled with a generalized Gibbs phase rule in which the two intrinsic length scales of the athermal colloid-polymer mixture act as additional field variables.

Scattering from colloidal cubic silica shells: Part II, static structure factors and osmotic equation of state

HYPOTHESIS

The shape of colloidal particles affects the structure of colloidal dispersions. The effect of the cube shape on the thermodynamics of colloidal cube dispersions has not yet been studied experimentally. Static light scattering measurements on colloidal cubic silica shells at finite concentrations allows us to measure the structure factor of colloidal cube fluids and to test theoretical predictions for the equation of state of hard convex superballs.

EXPERIMENTS

Hollow silica nanocubes of varying concentrations in N,N,-dimethylformamide were studied with static light scattering. The structure factor was extracted from the scattering curves using experimental form factors. From this experimental structure factor, the specific density of the particles, and the osmotic compressibility were obtained. This osmotic compressibility was then compared to a theoretical equation of state of hard superballs.

FINDINGS

The first experimental structure factors of a stable cube fluid are presented. The osmotic compressibility of the cube fluid can be described by the equation of state of a hard superball fluid, showing that silica cubes in N,N,-dimethylformamide with LiCl effectively interact as hard particles.

Scattering from colloidal cubic silica shells: Part I, particle form factors and optical contrast variation

HYPOTHESIS

Colloidal cubic silica shells, prepared from cuprous oxide cubes, with a typical size of 100 nm are promising model particles for scattering studies on dilute, as well as concentrated fluids, of non-spherical colloids.

EXPERIMENTS

Small angle X-ray scattering, and static light scattering are employed to determine form factors of cubic silica shells and silica covered cuprous oxide cubes. Contrast variation experiments are performed to assess the refractive index and optical homogeneity of the cubic silica shells, which is important for the extension of the scattering study to concentrated dispersions of cubic shells in Part II (Dekker, submitted for publication).

RESULTS

The experimental form factors, which compare well to theoretical form factors, manifest cubic silica shells that are dispersed as single stable colloids with a shape intermediate between a sphere and a perfect cube. Contrast variation demonstrates that the silica shells are optically homogeneous, with a refractive index that is independent of the shell thickness. The results presented here open up the possibility to extract structure factors from light scattering measurements on concentrated cube dispersions in Part II.

Phase stability of dispersions of hollow silica nanocubes mediated by non-adsorbing polymers

Although there are theoretical predictions (Eur. Phys. J. E 41, 110 (2018)) for the rich-phase behaviour of colloidal cubes mixed with non-adsorbing polymers, a thorough verification of this phase behaviour is still underway; experimental studies on mixtures of cubes and non-adsorbing polymers in bulk are scarce. In this paper, mixtures of hollow silica nanocubes and linear polystyrene in N,-N-dimethylformamide are used to measure the structure factor of the colloidal cubes as a function of non-adsorbing polymer concentration. Together with visual observations these structure factors enabled us to assess the depletion-mediated phase stability of cube-polymer mixtures. The theoretical and experimental phase boundaries for cube-depletant mixtures are in remarkable agreement, despite the simplifications underlying the theory employed.

Algebraic equations of state for the liquid crystalline phase behavior of hard rods

Based on simplifications of previous numerical calculations [H. Graf and H. Löwen, Phys. Rev. E 59, 1932 (1999)1063-651X10.1103/PhysRevE.59.1932], we propose algebraic free energy expressions for the smectic-A liquid crystal phase and the crystal phases of hard spherocylinders. Quantitative agreement with simulations is found for the resulting equations of state. The free energy expressions can be used to straightforwardly compute the full phase behavior for all aspect ratios and to provide a suitable benchmark for exploring how attractive interrod interactions mediate the phase stability through perturbation approaches such as free-volume or van der Waals theory.

Entropic patchiness drives multi-phase coexistence in discotic colloid-depletant mixtures

Entropy-driven equilibrium phase behaviour of hard particle dispersions can be understood from excluded volume arguments only. While monodisperse hard spheres only exhibit a fluid-solid phase transition, anisotropic hard particles such as rods, discs, cuboids or boards exhibit various multi-phase equilibria. Ordering of such anisotropic particles increases the free volume entropy by reducing the excluded volume between them. The addition of depletants gives rise to an entropic patchiness represented by orientation-dependent attractions resulting in non-trivial phase behaviour. We show that free volume theory is a simple, generic and tractable framework that enables to incorporate these effects and rationalise various experimental findings. Plate-shaped particles constitute the main building blocks of clays, asphaltenes and chromonic liquid crystals that find widespread use in the food, cosmetics and oil industry. We demonstrate that mixtures of platelets and ideal depletants exhibit a strikingly rich phase behaviour containing several types of three-phase coexistence areas and even a quadruple region with four coexisting phases.

Phase behaviour of colloids plus weakly adhesive polymers

The phase behaviour of a colloidal dispersion mediated by weakly adhesive polymers is considered. The polymers are depleted but are weakly adhesive and hence comprise a non-zero polymer concentration at the colloid's surface, in contrast to the classical assumption in depletion theories involving a zero polymer concentration at the surface. The theory is composed of a generalized free-volume theory for colloid-polymer mixtures and a self-consistent mean-field theory for polymers at surfaces. It is found that the weak adhesion of the polymers shifts the phase stability of the colloid-polymer mixtures to higher polymer concentrations as compared to assuming a full depletion effect. The predicted phase diagrams employing the new theory are consistent with experiments on mixtures of silica spheres coated with stearyl alcohol and polydimethylsiloxane in cyclohexane and with Monte Carlo simulation results.

Controlled nanoparticle formation by diffusion limited coalescence

Polymeric nanoparticles (NPs) have great application potential in science and technology. Their functionality strongly depends on their size. We present a theory for the size of NPs formed by precipitation of polymers into a bad solvent in the presence of a stabilizing surfactant. The analytical theory is based upon diffusion-limited coalescence kinetics of the polymers. Two relevant time scales, a mixing and a coalescence time, are identified and their ratio is shown to determine the final NP diameter. The size is found to scale in a universal manner and is predominantly sensitive to the mixing time and the polymer concentration if the surfactant concentration is sufficiently high. The model predictions are in good agreement with experimental data. Hence the theory provides a solid framework for tailoring NPs with a priori determined size.

How flow changes polymer depletion in a slit

A theoretical model is developed for predicting dynamic polymer depletion under the influence of fluid flow. The results are established by combining the two-fluid model and the self-consistent field theory. We consider a uniform fluid flow across a slit containing a solution with polymer chains. The two parallel and infinitely long walls are permeable to solvent only and the polymers do not adsorb to these walls. For a weak flow and a narrow slit, an analytic expression is derived to describe the steady-state polymer concentration profiles in a Θ-solvent. In both Θ- and good-solvents, we compute the time evolution of the concentration profiles for various flow rates characterized by the Peclet number. The model reveals the interplay of depletion, solvent condition, slit width, and the relative strength of the fluid flow.

High refractive index nanocomposite fluids for immersion lithography

The concept of using dispersions of nanoparticles as high refractive index fluids in immersion lithography is examined both from a theoretical and experimental point of view. In the theoretical part we show that gelation and demixing can be controlled in high solid dispersions, needed to achieve a high (refractive) index, by using short stabilizing brushes. We considered both fluid-fluid demixing by using statistical thermodynamics and percolation, computed using liquid-state approaches. Whenever demixing or percolation takes place, the nanoparticle dispersion is unsuited for immersion lithography. The minimum thickness of the stabilizer layer of a stable suspension is estimated assuming particles plus steric stabilizer to act as hard spheres with van der Waals attraction between the cores. Since the van der Waals attraction can be related to the optical properties of the particles and dispersion medium, it is also possible to estimate the refractive index that can be attained with composite immersion fluids. Using materials that are known to be highly transparent in the bulk at a wavelength of 193 nm, indices above 1.8 can be attained. Other materials with higher indices are expected to be transparent at 193 nm due to a blue shift of the UV absorption and enable much higher indices. In the experiment, we show that it is possible to prepare suspensions with particles of about 4 nm diameter that increase the refractive index of the continuous phase with 0.2 at a wavelength of 193 nm. The refractive index and density of such dispersions are proportional to the volume fraction of the disperse phase, and it is shown that the refractive index of the composite fluid can be predicted very well from the optical properties of the components. Furthermore, successful imaging experiments were performed through a dispersion of silica nanoparticles. These findings lead to the conclusion that immersion lithography using nanoparticle dispersions is indeed possible.

Phase behaviour of a dispersion of charge-stabilised colloidal spheres with added non-adsorbing interacting polymer chains

We present a theory for the phase behaviour of mixtures of charge-stabilised colloidal spheres plus interacting polymer chains in good and theta -solvents within the framework of free-volume theory. We use simple but accurate combination rules for the depletion thickness around a colloidal particle and for the osmotic pressure up to the semi-dilute concentration regime. Hence, we obtain expressions for the free energy for mixtures of charged colloidal particles and non-adsorbing interacting polymers. From that, we calculate the phase behaviour, and discuss its topology in dependence on the competition between the charge-induced repulsion and the polymer-induced attraction. The homogeneous mixture of colloids and polymers becomes more stabilised against demixing when increasing the electrostatic repulsion. This charge-induced stabilisation is strongest for small polymer-to-colloid size ratios and is more pronounced for charged colloids mixed with polymers in a good solvent than for polymers in a theta -solvent. For the weakly charged regime we find that the phase diagram becomes salt-concentration-independent in the protein limit for charged colloids plus polymers in a theta -solvent. The liquid window, i.e., the concentration regimes where a colloidal liquid exists, is narrowed down upon increasing the charge-induced repulsion. Also this effect is more pronounced when charged colloids are mixed with polymer chains in a good solvent. In summary, we demonstrate that the solvent quality significantly influences the phase behaviour of mixtures of charged colloids plus non-adsorbing polymers if the range of the screened electrostatic repulsion becomes of the order of the range of the depletion-induced attraction.

Phase behavior of a suspension of hard spherocylinders plus ideal polymer chains

We study isotropic-isotropic and isotropic-nematic phase transitions of fluid mixtures containing hard spherocylinders (HSC) and added non-adsorbing ideal polymer chains using scaled particle theory (SPT). First, we investigate isotropic-nematic (I -N phase coexistence using SPT in the absence of polymer. We compare the results obtained using a Gaussian form of the orientational distribution function (ODF) to minimize the free energy versus minimizing numerically. We find that formal numerical minimization gives results that are much closer to computer simulation results. In order to describe mixtures of HSC plus ideal chains we studied the depletion of ideal chains around a HSC. We analyze the density profiles of ideal chains near a hard cylinder and find the depletion thickness delta is a function of the ratio of the polymer's radius of gyration R(g) and the cylinder radius R(c). Our results are compared with a common approximation in which the depletion thickness is taken equal to the radius of gyration of the polymer chain. We incorporate the correct depletion thickness into SPT and find that for R (g)/R (c) < 1.56 using ideal chains gives phase transitions at smaller polymer concentrations, whereas for R (g)/R (c) > 1.56 , which is a common experimental situation, the phase transitions are found at larger polymer concentrations with respect to delta = R (g) . The differences are significant, especially for R (g) >> R (c) , so we can conclude it is essential to take into account the properties of ideal polymer chains and the resulting depletion near a cylinder. Finally, we present phase diagrams for rod-polymer mixtures which could be realized under experimental conditions.

Depletion-induced phase separation in colloid-polymer mixtures

Phase separation can be induced in a colloidal dispersion by adding non-adsorbing polymers. Depletion of polymer around the colloidal particles induces an effective attraction, leading to demixing at sufficient polymer concentration. This communication reviews theoretical and experimental work carried out on the polymer-mediated attraction between spherical colloids and the resulting phase separation of the polymer-colloid mixture. Theoretical studies have mainly focused on the limits where polymers are small or large as compared to the colloidal size. Recently, however, theories are being developed that cover a wider colloid-polymer size ratio range. In practical systems, size polydispersity and polyelectrolytes (instead of neutral polymers) and/or charges on the colloidal surfaces play a role in polymer-colloid mixtures. The limited amount of theoretical work performed on this is also discussed. Finally, an overview is given on experimental investigations with respect to phase behavior and results obtained with techniques enabling measurement of the depletion-induced interaction potential, the structure factor, the depletion layer thickness and the interfacial tension between the demixed phases of a colloid-polymer mixture.

Segment-sphere size ratio influence on the stability of a polymer-colloid mixture

The influence of the sphere to segment size ratio on the effective depletion layer thickness around a sphere and the resulting phase behaviour of a colloid-polymer mixture is considered. An analytical expression for the excluded volume between an ideal freely jointed chain and a sphere [L. Lue, J. Colloid Interface Sci. 202, 558 (1998)] allows the effect of polymer characteristics (chain length, segment/sphere size ratio) on the depletion interaction in polymer-colloid mixtures to be calculated. For both small and intermediate polymer-to-colloid size ratios the respectively gas-solid and gas-liquid coexistence curves shift to larger polymer coil volume fractions as the relative segment size increases. It is further demonstrated that the conditions for optimal (protein) crystallization are affected when the segment to sphere size is altered.

On the long-range attraction between proteins due to nonadsorbing polysaccharide

The effect of a nonadsorbing polysaccharide (dextran) on the structure factor of a solution of lysozyme was studied using small-angle neutron scattering (SANS) experiments. By choosing the appropriate water/deuterium ratio as solvent, we made the scattering signal from dextran invisible for the SANS measurements. Dextran induces a weak long-range attraction between the lysozyme molecules. This attraction is described using a depletion interaction potential from theory for two spheres in an ideal polymer solution. Incorporation of the theory in a mean-spherical approximation shows that the wave vector below which the structure factor increases depends on the polymer size. The theoretical prediction is in fair agreement with the measured structure factor of lysozyme, as affected by nonadsorbing dextran.

Interaction potential between two spheres mediated by excluded volume polymers

The interaction between two spheres in a solution of nonadsorbing polymers, with excluded volume interaction, is calculated from the depletion layers around the spheres using the generalized Gibbs adsorption equation. By combining the bulk correlation length with the curvature-dependent interfacial tension between a sphere and the surrounding polymer solution [Hanke, Eisenriegler, and Dietrich, Phys. Rev. E 59, 6853 (1999)], the depletion layer thickness around a sphere is obtained. The resulting contact potential agrees with a scaling prediction of de Gennes in the semidilute regime.

Electrosorption of pectin onto casein micelles

Pectin, a polysaccharide derived from plant cells of fruit, is commonly used as stabilizer in acidified milk drinks. To gain a better understanding of the way that pectin stabilizes these drinks, we studied the adsorption and layer thickness of pectin on casein micelles in skim milk dispersions. Dynamic light scattering was used to measure the layer thickness of adsorbed pectin onto casein micelles in situ during acidification. The results indicate that the adsorption of pectin onto casein micelles is multilayered and takes place at and below pH 5.0. Renneting, i.e., cleaving-off kappa-casein from the casein micelles, did not alter the adsorption pH. It did, however, show that pectin arrests the rennet-induced flocculation of casein micelles below pH 5.0. From the findings we concluded the attachment of pectin onto casein micelles is driven by electrosorption. Adsorption measurements confirmed the multilayered nature of the adsorption of pectin onto casein micelles. Both the adsorbed amount and the layer thickness increased with decreasing pH in the relevant range 3.5-5.0. The phase behavior of a casein micelles/pectin mixture was determined and could be explained in terms of thermodynamic incompatibility being relevant above pH 5.0 and adsorption, leading to either stabilization and bridging, being relevant below pH 5.0. The results confirm that electrosorption is the driving force for the adsorption of pectin onto casein micelles.

Viscoelastic properties of an exocellular polysaccharide produced by a Lactococcus lactis

The viscoelastic properties of a well-characterized exocellular polysaccharide (EPS) produced by the lactic acid bacterium Lactococcus lactis subsp. cremoris strain B40 were investigated. Dynamic rheological measurements were made as a function of frequency and EPS concentration. The bead-spring model of Rouse could reasonably describe the dynamic properties. Concentrated EPS solutions have a significant elasticity (G' > G") at high frequencies. The relatively high G' values at high concentrations and high frequencies are indicative of significant normal stress differences, and we put forward a hypothesis that suggests that these normal stresses may explain the contribution of EPSs to the ropy behavior of yogurts.

Effects of structural modifications on some physical characteristics of exopolysaccharides from Lactococcus lactis

The relationship between the primary structure and the chain stiffness of exopolysaccharides (EPSs) and modified EPSs produced by two strains of Lactococcus lactis subsp. cremoris was investigated. The molar mass and radius of gyration of these exopolysaccharides were analyzed by multiangle static light scattering after size-exclusion chromatography. From these results and the chemical structure of the repeating units of the investigated EPSs, the Kuhn lengths could be calculated. We found that the initial Kuhn lengths of the two native EPSs are similar. Modification of the EPSs by removing parts of the side groups resulted in a decrease in both the absolute value and the normalized value of the Kuhn length. It is therefore concluded that partial removal of the side groups of these polysaccharides could make them less efficient as thickeners if no specific interaction with other components occurs.

Influence of different substrate limitations on the yield, composition and molecular mass of exopolysaccharides produced by Lactococcus lactis subsp. cremoris in continuous cultures

The type of substrate limitation had a remarkable influence on the molecular mass of exopolysaccharides (EPS) produced by Lactococcus lactis subsp. cremoris NIZO B40 and NIZO B891. Under glucose/energy limitation, the molecular mass was much smaller than under leucine or phosphate limitation, resulting in a marked decrease of the intrinsic viscosity of this EPS. The sugar composition of EPS produced by both strains, and the phosphate content of EPS produced by NIZO B40, were not affected by the type of nutrient limitation. Both strains produced comparable amounts of EPS under leucine and glucose limitation, but the efficiency of EPS production was highest under glucose limitation. The EPS yields of the phosphorylated B40 EPS as well as the unphosphorylated B891 EPS were reduced, with about 40% under conditions of phosphate limitation.

Concentration and shear-rate dependence of the viscosity of an exocellular polysaccharide

The viscosity of an exocellular polysaccharide (EPS) produced by the bacterium Lactococcus lactis subsp. cremoris B40 was studied in aqueous solution at an ionic strength of 0.10M. First, the zero-shear viscosity was determined as a function of the concentration. From the data in the low concentration range, the intrinsic viscosity was determined. In addition, the shear-thinning behavior was measured at several concentrations. By combining existing theories, a new equation is proposed that describes and predicts the intrinsic viscosity and the concentration dependence of the (zero-shear) viscosity of B40 EPS solutions from the molar mass and the hydrodynamic radius of the polysaccharide. Based on the Rouse theory, the shear-rate dependence of the viscosity also could be described and predicted from the molecular characteristics, i.e., molar mass and radius of gyration. It is shown that these equations can be applied to all random coil polysaccharides. Copyright 1999 John Wiley & Sons, Inc.

Phase Separation, Creaming, and Network Formation of Oil-in-Water Emulsions Induced by an Exocellular Polysaccharide

We have investigated the effect of an exocellular polysaccharide (EPS) on the phase behavior and the rheology of oil-in-water emulsions. Already at low EPS concentrations the phase separation occurs. The phase line can be described by depletion interaction theory. At high EPS concentrations and dispersed phase volume fractions above 10% there is a stable "gel"-like region in the phase diagram. A kinetic study showed that the rate of creaming decreases with increasing oil content due to hydrodynamic effects. This rate depends strongly on the concentration of EPS, which is related to the strength of the depletion interaction and the viscosity of the continuous phase. At low EPS concentration the creaming rate strongly increases with EPS concentration because of the stronger attraction. At higher EPS concentrations creaming is slowed down by the viscosity increase of the continuous phase and of the particle network which is formed. At high EPS concentrations this network becomes so strong that the gel prevents creaming. The rheological behavior of the "gel" was studied by measuring flow curves, which could be interpreted by a theoretical model for weakly aggregating particles. Copyright 1999 Academic Press.

Exopolysaccharides produced by Lactococcus lactis: from genetic engineering to improved rheological properties?

Over the last years, important advances have been made in the study of the production of exopolysaccharides (EPS) by several lactic acid bacteria, including Lactococcus lactis. From different EPS-producing lactococcal strains the specific eps gene clusters have been characterised. They contain eps genes, which are involved in EPS repeating unit synthesis, export, polymerisation, and chain length determination. The function of the glycosyltransferase genes has been established and the availability of these genes opened the way to EPS engineering. In addition to the eps genes, biosynthesis of EPS requires a number of housekeeping genes that are involved in the metabolic pathways leading to the EPS-building blocks, the nucleotide sugars. The identification and characterisation of several of these housekeeping genes (galE, galU, rfbABCD) allows the design of metabolic engineering strategies that should lead to increased EPS production levels by L. lactis. Finally, model development has been initiated in order to predict the physicochemical consequences of the addition of a EPS to a product.

Depletion interaction of casein micelles and an exocellular polysaccharide

Casein micelles become mutually attractive when an exocellular polysaccharide produced by Lactococcus lactis subsp. cremoris NIZO B40 (hereafter called EPS) is added to skim milk. The attraction can be explained as a depletion interaction between the casein micelles induced by the nonadsorbing EPS. We used three scattering techniques (small-angle neutron scattering, turbidity measurements, and dynamic light scattering) to measure the attraction. In order to connect the theory of depletion interaction with experiment, we calculated structure factors of hard spheres interacting by a depletion pair potential. Theoretical predictions and all the experiments showed that casein micelles became more attractive upon increasing the EPS concentration.

Isolation and physical characterization of an exocellular polysaccharide

The physical properties of a polysaccharide produced by the lactic acid bacterium Lactococcus lactis subsp. cremoris strain NIZO B40 were investigated. Separation of the polysaccharide from most low molar mass compounds in the culture broth was performed by filtration processes. Residual proteins and peptides were removed by washing with a mixture of formic acid, ethanol, and water. Gel permeation chromatography (GPC) was used to size fractionate the polysaccharide. Fractions were analyzed by multiangle static light scattering in aqueous 0.10 M NaNO3 solutions from which a number- (Mn) and weight-averaged (Mw) molar mass of (1.47 +/- 0.06).10(3) and (1.62 +/- 0.07).10(3) kg/mol, respectively, were calculated so that Mw/Mn approximately 1.13. The number-averaged radius of gyration was found to be 86 +/- 2 nm. From dynamic light scattering an apparent z-averaged diffusion coefficient was obtained. Upon correcting for the contributions from intramolecular modes by extrapolating to zero wave vector a hydrodynamic radius of 86 +/- 4 nm was calculated. Theoretical models for random coil polymers show that this z-averaged hydrodynamic radius is consistent with the z-averaged radius of gyration, 97 +/- 3 nm, as found with GPC.

Effective Viscosity of Polymer Solutions: Relation to the Determination of the Depletion Thickness and Thickness of the Adsorbed Layer of Cellulose Derivatives

The diffusion of silica particles with radii ranging from 12 to 510 nm in dilute solutions of carboxymethyl cellulose (Mw = 180 to 1200 kg mol-1, cCMC = 5 to 1000 mg l-1) was investigated by means of dynamic light scattering at pH 5 in 0.01 mol l-1 NaCl. The viscosity of the polymer solution as experienced by the silica probes (the "microscopic" or effective viscosity, etaeff) differs from the viscosity as determined by capillary viscometry (etap). For small particles etaeff nearly equals the viscosity of the solvent (eta0). The effective viscosity increases with the size of the probe particles and the polymer concentration but remains less than etap. The effective viscosity is interpreted in terms of a model in which the particle is surrounded by a layer of polymer free solution (eta = eta0). The thickness of the polymer-free layer is assumed to be equal to the thickness of the depletion layer (lambdad). Applying this model, a decrease in lambdad as a function of CMC concentration is observed. At low concentration lambdad equals the radius of gyration. The hydrodynamic layer thickness (deltah) of cellulose derivatives (carboxymethyl cellulose and hydroxyethyl cellulose) adsorbed on inorganic oxide surfaces (alpha-Fe2O3 and SiO2) is also investigated by dynamic light scattering. Upon using etap maxima in deltah are found. However, these maxima are a consequence of an incorrect choice of the viscosity. When the viscosity is used as obtained from inert probe diffusion, no anomalies are observed. Copyright 1998 Academic Press.