Taco Nicolai

Publications of Taco Nicolai

• Particles Trapped at the Droplet Interface in Water-in-Water Emulsions.

  Authors: Gireeshkumar Balakrishnan, Taco Nicolai, Lazhar Benyahia, Dominique Durand

  Langmuir : the ACS journal of surfaces and colloids. 03/2012; 28(14):5921-6.

  Water-in-water emulsions were formed by mixing incompatible aqueous solutions of dextran and poly(ethylene oxide) (PEO) in the presence of latex or protein particles. It was found that particles withWater-in-water emulsions were formed by mixing incompatible aqueous solutions of dextran and poly(ethylene oxide) (PEO) in the presence of latex or protein particles. It was found that particles with a radius as small as 0.1 μm become trapped at the interface between the PEO- and dextran-rich phases with interfacial tensions down to 10(-6) N/m. The particles were visualized at the interface of the emulsion droplets using confocal laser scanning microscopy (CLSM) allowing determination of the contact angle. Various degrees of coverage with particles could be observed. On densely covered droplets, the particles had a hexagonal crystalline order. At intermediate coverage, transient clustering of the particles was observed. The diffusion coefficient of the particles at the interface was determined using multiparticle tracking. Fusion of droplets was observed in all cases leading eventually to macroscopic phase separation.

• On the crucial importance of the pH for the formation and self-stabilization of protein microgels and strands.

  Authors: Tuan Phan-Xuan, Dominique Durand, Taco Nicolai, Laurence Donato, Christophe Schmitt, Lionel Bovetto

  Langmuir : the ACS journal of surfaces and colloids. 11/2011; 27(24):15092-101.

  Stable suspensions of protein microgels are formed by heating salt-free β-lactoglobulin solutions at concentrations up to about C = 50 g·L(-1) if the pH is set within a narrow range between 5.75 andStable suspensions of protein microgels are formed by heating salt-free β-lactoglobulin solutions at concentrations up to about C = 50 g·L(-1) if the pH is set within a narrow range between 5.75 and 6.1. The internal protein concentration of these spherical particles is about 150 g·L(-1) and the average hydrodynamic radius decreases with increasing pH from 200 to 75 nm. The formation of the microgels leads to an increase of the pH, which is a necessary condition to obtain stable suspensions. The spontaneous increase of the pH during microgel formation leads to an increase of their surface charge density and inhibits secondary aggregation. This self-stabilization mechanism is not sufficient if the initial pH is below 5.75 in which case secondary aggregation leads to precipitation. Microgels are no longer formed above a critical initial pH, but instead short, curved protein strands are obtained with a hydrodynamic radius of about 15-20 nm.

• Self-diffusion of non-interacting hard spheres in particle gels.

  Authors: Jean-Christophe Gimel, Taco Nicolai

  Journal of physics. Condensed matter : an Institute of Physics journal. 06/2011; 23(23):234115.

  Different kinds of particle gels were simulated using a process of random aggregation of hard spheres. The mean square displacement of Brownian spherical tracer particles through these rigid gels wasDifferent kinds of particle gels were simulated using a process of random aggregation of hard spheres. The mean square displacement of Brownian spherical tracer particles through these rigid gels was monitored and the average diffusion coefficient, normalized with the free diffusion coefficient (D), was obtained. For each gel structure the effect of the gel volume fraction (φ) and size ratio of the tracer (d) on the relative diffusion coefficient was investigated systematically. The volume fraction that is accessible to the tracers (φ(a)) was determined in each case. D was found to be approximately the same if φ(a) was the same, independent of φ, d and the gel structure. However a different behaviour is found if the tracers can penetrate the strands of the gel. A state diagram of d versus φ is given that shows the critical values (d(c), φ(c)) at which all tracers become trapped. Different values are found for different gel structures. The dependence of D on φ/φ(c) is independent of d, while the dependence of D on d/d(c) is independent of φ.

• Particle diffusion in globular protein gels in relation to the gel structure.

  Authors: Gireeshkumar Balakrishnan, Dominique Durand, Taco Nicolai

  Biomacromolecules. 02/2011; 12(2):450-6.

  Globular protein gels with a variety of structures were prepared by heating β-lactoglobulin solutions at different concentrations and different ionic strengths. The structure was analyzed in terms ofGlobular protein gels with a variety of structures were prepared by heating β-lactoglobulin solutions at different concentrations and different ionic strengths. The structure was analyzed in terms of the pair correlation function of the protein concentration, and the volume fraction of the gels was determined. A strong coarsening of the gel structure was observed upon increasing the NaCl concentration between 0.1 and 0.25 M. The mean square displacement of spherical particles with diameters between 0.2 and 2 μm was determined in solutions and in gels by multiparticle tracking of confocal laser scanning microscopy images. Brownian diffusion or trapping of spheres with different sizes was observed, depending on the gel structure. In few cases the diffusion was anomalous. The relationship between gel structure and particle mobility is discussed.

• Salt-induced gelation of globular protein aggregates: structure and kinetics.

  Authors: Komla Ako, Taco Nicolai, Dominique Durand

  Biomacromolecules. 03/2010; 11(4):864-71.

  Aggregates of the globular protein beta-lactoglobulin were formed by heating solutions of native proteins at pH 7, after which gels were formed by the addition of salt. The second step does notAggregates of the globular protein beta-lactoglobulin were formed by heating solutions of native proteins at pH 7, after which gels were formed by the addition of salt. The second step does not necessitate elevated temperatures and is therefore often called cold gelation. The structure of the gels was studied during their formation using light scattering and turbidity. Complementary confocal laser scanning microscopy measurements were done. We compared the structure with that of gels formed by heating native beta-lactoglobulin under the same conditions. Whereas in the latter case, microphase separation occurs above 0.2 M NaCl, no microphase separation was observed during cold gelation up to at least 1 M NaCl. The dependence of the kinetics and the final gel structure on the protein concentration, the temperature, the salt concentration, and the aggregate size was quantified. A few measurements on gels formed by adding CaCl(2) confirmed the higher efficiency of this bivalent cation but revealed no qualitative differences with gels formed by adding NaCl.

• Transient Gelation and Glass Formation of Reversibly Cross-linked Polymeric Micelles.

  Authors: Frédéric Renou, Taco Nicolai, Lazhar Benyahia, Erwan Nicol

  The journal of physical chemistry. B. 03/2009;

  Poly(ethylene oxide) (PEO) end-capped with a hexadecyl group at one (alpha-PEO) or both ends (alpha,omega-PEO) are highly asymmetric diblock or triblock copolymers that form spherical micelles inPoly(ethylene oxide) (PEO) end-capped with a hexadecyl group at one (alpha-PEO) or both ends (alpha,omega-PEO) are highly asymmetric diblock or triblock copolymers that form spherical micelles in aqueous solution. alpha,omega-PEO can bridge between two micelles leading to reversible association of the micelles. At a first critical concentration (C(p)), the micelles percolate and a transient network is formed with an elastic modulus determined by the concentration of alpha,omega-PEO. C(p) increases with increasing fraction of alpha-PEO and is insensitive to the temperature. At a second critical concentration (C(c)), a liquid-solid transition occurs. C(c) is independent of the fraction of alpha-PEO and increases with increasing temperature. There are indications that the solid state is formed by nucleation and growth of domains of dynamically arrested micelles. The properties of the transient network are almost the same in the liquid and in the solid state.

• Crystallization and dynamical arrest of attractive hard spheres.

  Authors: Sujin Babu, Jean Christophe Gimel, Taco Nicolai

  The Journal of chemical physics. 03/2009; 130(6):064504.

  Crystallization of hard spheres interacting with a square well potential was investigated by numerical simulations using so-called Brownian cluster dynamics. The phase diagram was determined over aCrystallization of hard spheres interacting with a square well potential was investigated by numerical simulations using so-called Brownian cluster dynamics. The phase diagram was determined over a broad range of volume fractions. The crystallization rate was studied as a function of the interaction strength expressed in terms of the second virial coefficient. For volume fractions below about 0.3 the rate was found to increase abruptly with increasing attraction at the binodal of the metastable liquid-liquid phase separation. The rate increased until a maximum was reached after which it decreased with a power law dependence on the second virial coefficient. Above a critical percolation concentration, a transient system spanning network of connected particles was formed. Crystals were formed initially as part of the network, but eventually crystallization led to the breakup of the network. The lifetime of the transient gels increased very rapidly over a small range of interaction energies. Weak attraction destabilized the so-called repulsive crystals formed in pure hard sphere systems and shifted the coexistence line to higher volume fractions. Stronger attraction led to the formation of a denser, so-called attractive, crystalline phase. Nucleation of attractive crystals in the repulsive crystalline phase was observed close to the transition.

• Structure and Viscoelasticity of Mixed Micelles Formed by Poly(ethylene oxide) End Capped with Alkyl Groups of Different Length.

  Authors: Frédéric Renou, Taco Nicolai, Erwan Nicol, Lazhar Benyahia

  Langmuir : the ACS journal of surfaces and colloids. 01/2009;

  Poly(ethylene oxide) (PEO) end capped with an alkyl group is a highly asymmetric diblock copolymer that forms spherical micelles in aqueous solution resembling multiarm star polymers. The effect ofPoly(ethylene oxide) (PEO) end capped with an alkyl group is a highly asymmetric diblock copolymer that forms spherical micelles in aqueous solution resembling multiarm star polymers. The effect of varying the length of the alkyl end group on the structure and viscoelasticity was investigated for pure and mixed micelle suspensions. The aggregation number (p) of the micelles increased and the critical association concentration (CAC) decreased with increasing the length of the end group. At high concentrations a discontinuous reversible liquid-solid transition was observed below a critical temperature (T(c)) that increased with increasing length of the end group. Mixing end-capped PEO with different alkyl lengths led first to formation of the micelles by polymers with the lowest CAC into which the other polymers were incorporated when the concentration was increased. The viscoelastic properties at high concentrations are the same for pure systems and mixtures with the same average length of the alkyl end group.

• Structure and dynamical mechanical properties of suspensions of sodium caseinate.

  Authors: Anne Pitkowski, Dominique Durand, Taco Nicolai

  Journal of colloid and interface science. 11/2008; 326(1):96-102.

  Sodium caseinate is derived from casein which is the major milk protein and forms small star-like aggregates in aqueous solution. The dynamic mechanical properties of dense sodium caseinateSodium caseinate is derived from casein which is the major milk protein and forms small star-like aggregates in aqueous solution. The dynamic mechanical properties of dense sodium caseinate suspensions were studied by measuring the shear viscosity and the frequency dependent loss and storage shear modulus. The viscosity is shown to increase strongly above a concentration, C(c), due to jamming of the aggregates. C(c) depends little on the pH or on the ionic strength. The repulsion between the aggregates increases with decreasing temperature leading to jamming at lower concentrations. As a consequence the viscosity decreases strongly with increasing temperature for concentrated suspensions. The frequency dependent shear modulus of concentrated suspensions shows a solid like behaviour at high frequencies. Shear thinning is observed at Peclet numbers larger than unity.

• The influence of bond rigidity and cluster diffusion on the self-diffusion of hard spheres with square well interaction.

  Authors: Sujin Babu, Jean Christophe Gimel, Taco Nicolai, Cristiano De Michele

  The Journal of chemical physics. 06/2008; 128(20):204504.

  Hard spheres interacting through a square well potential were simulated by using two different methods: Brownian cluster dynamics (BCD) and event driven Brownian dynamics (EDBD). The structure of theHard spheres interacting through a square well potential were simulated by using two different methods: Brownian cluster dynamics (BCD) and event driven Brownian dynamics (EDBD). The structure of the equilibrium states obtained by both methods was compared and found to be almost identical. Self-diffusion coefficients (D) were determined as a function of the interaction strength. The same values were found by using BCD or EDBD. Contrary to EDBD, BCD allows one to study the effect of bond rigidity and hydrodynamic interaction within the clusters. When the bonds are flexible, the effect of attraction on D is relatively weak compared to systems with rigid bonds. D increases first with increasing attraction strength, and then decreases for stronger interaction. Introducing intracluster hydrodynamic interaction weakly increases D for a given interaction strength. Introducing bond rigidity causes a strong decrease in D which no longer shows a maximum as function of the attraction strength.

• Light scattering study of heat-denatured globular protein aggregates.

  Authors: Soraya Mehalebi, Taco Nicolai, Dominique Durand

  International journal of biological macromolecules. 04/2008;

  The structure of aggregates formed by the globular protein beta-lactoglobulin (beta-lg) after heat induced denaturation was studied using light scattering and size exclusion chromatography. TheThe structure of aggregates formed by the globular protein beta-lactoglobulin (beta-lg) after heat induced denaturation was studied using light scattering and size exclusion chromatography. The influence of varying the pH above the iso-electric point (pH 5.2) was investigated in the absence of added salt. Stable aggregates could be formed and characterized between pH 5.8 and pH 9. The large-scale structure of the aggregates was self-similar and remarkably insensitive to the pH. Below a critical association concentration, which decreased with decreasing pH from 10 to 1g/L, denatured monomers and small oligomers were formed. At higher concentrations larger so-called preaggregates formed containing roughly 100 monomers. With increasing concentration the size of the aggregates varied little until it rose sharply close to the gelation concentration that decreased with decreasing pH (50g/L<C(g)<90g/L). The effect of the pH on the interaction in solutions of native beta-lactoglobulin was also investigated.

• Diffusion limited cluster aggregation with irreversible flexible bonds

  Authors: Sujin Babu, Jean Christophe Gimel, Taco Nicolai

  02/2008;

  Irreversible diffusion limited cluster aggregation (DLCA) of hard spheres was simulated using Brownian cluster dynamics. Bound spheres were allowed to move freely within a specified range, but noIrreversible diffusion limited cluster aggregation (DLCA) of hard spheres was simulated using Brownian cluster dynamics. Bound spheres were allowed to move freely within a specified range, but no bond breaking was allowed. The structure and size distribution of the clusters was investigated before gelation. The pair correlation function and the static structure factor of the gels were determined as a function of the volume fraction and time. Bond flexibility led to local densification of the clusters and the gels, with a certain degree of order. At low volume fractions densification of the clusters occurred during their growth, but at higher volume fractions it occurred mainly after gelation. At very low volume fractions, the large scale structure (fractal dimension), size distribution and growth kinetics of the clusters was found to be close to that known for DLCA with rigid bonds. Restructuring of the gels continued for long times, indicating that aging processes in systems with strong attraction do not necessarily involve bond breaking. The mean square displacement of particles in the gels was determined. It is shown to be highly heterogeneous and to increase with decreasing volume fraction.

• Scattering and turbidity study of the dissociation of casein by calcium chelation.

  Authors: Anne Pitkowski, Taco Nicolai, Dominique Durand

  Biomacromolecules. 02/2008; 9(1):369-75.

  The dissociation of casein was studied after addition of polyphosphate that leads to calcium chelation, using light and X-ray scattering and turbidimetry. It is shown that the dissociation is aThe dissociation of casein was studied after addition of polyphosphate that leads to calcium chelation, using light and X-ray scattering and turbidimetry. It is shown that the dissociation is a cooperative process; that is, a casein complex is either completely dissociated or remains largely intact. A systematic study was done of the dependence of the rate and extent of dissociation on the polyphosphate concentration and was found to be determined by the ratio between casein and polyphosphate. The structures of the casein complex and the small micellar particles formed after dissociation were compared. Additional experiments with a different chelatant (EDTA) gave similar results.

• Tracer diffusion in colloidal gels.

  Authors: Sujin Babu, Jean Christophe Gimel, Taco Nicolai

  The journal of physical chemistry. B. 02/2008; 112(3):743-8.

  Computer simulations were done of the mean square displacement (MSD) of tracer particles in colloidal gels formed by diffusion or reaction limited aggregation of hard spheres. The diffusionComputer simulations were done of the mean square displacement (MSD) of tracer particles in colloidal gels formed by diffusion or reaction limited aggregation of hard spheres. The diffusion coefficient was found to be determined by the volume fraction accessible to the spherical tracers (phi a) independent of the gel structure or the tracer size. In all cases, critical slowing down was observed at phi a approximately 0.03 and was characterized by the same scaling laws reported earlier for tracer diffusion in a Lorentz gas. Strong heterogeneity of the MSD was observed at small phi a and was related to the size distribution of pores.

• Self-diffusion of reversibly aggregating spheres.

  Authors: Sujin Babu, Jean Christophe Gimel, Taco Nicolai

  The Journal of chemical physics. 09/2007; 127(5):054503.

  Reversible diffusion limited cluster aggregation of hard spheres with rigid bonds was simulated and the self-diffusion coefficient was determined for equilibrated systems. The effect of increasingReversible diffusion limited cluster aggregation of hard spheres with rigid bonds was simulated and the self-diffusion coefficient was determined for equilibrated systems. The effect of increasing attraction strength was determined for systems at different volume fractions and different interaction ranges. It was found that the slowing down of the diffusion coefficient due to crowding is decoupled from that due to cluster formation. The diffusion coefficient could be calculated from the cluster size distribution and became zero only at infinite attraction strength when permanent gels are formed. It is concluded that so-called attractive glasses are not formed at finite interaction strength.

• Light-scattering study of the structure of aggregates and gels formed by heat-denatured whey protein isolate and beta-lactoglobulin at neutral pH.

  Authors: Najet Mahmoudi, Soraya Mehalebi, Taco Nicolai, Dominique Durand, Alain Riaublanc

  Journal of agricultural and food chemistry. 04/2007; 55(8):3104-11.

  The structure of aggregates and gels formed by heat-denatured whey protein isolate (WPI) has been studied at pH 7 and different ionic strengths using light scattering and turbidimetry. The resultsThe structure of aggregates and gels formed by heat-denatured whey protein isolate (WPI) has been studied at pH 7 and different ionic strengths using light scattering and turbidimetry. The results were compared with those obtained for pure beta-lactoglobulin (beta-Lg). WPI aggregates were found to have the same self-similar structure as pure beta-Lg aggregates. WPI formed gels above a critical concentration that varied from close to 100 g/L in the absence of added salt to about 10 g/L at 0.2 M NaCl. At low ionic strength (<0.05 M NaCl) homogeneous transparent gels were formed, while at higher ionic strength the gels became turbid but had the same self-similar structure as reported earlier for pure beta-Lg. The length scale characterizing the heterogeneity of the gels increased exponentially with increasing NaCl concentration for both WPI and pure beta-Lg, but the increase was steeper for the former.

• Coupling between polysaccharide gelation and micro-phase separation of globular protein clusters.

  Authors: Karine Baussay, Dominique Durand, Taco Nicolai

  Journal of colloid and interface science. 01/2007; 304(2):335-41.

  The effect of gelation of the polysaccharide phase on the phase separation was investigated for mixtures of anionic polysaccharide (kappa-carrageenan) and globular protein (beta-lactoglobulin)The effect of gelation of the polysaccharide phase on the phase separation was investigated for mixtures of anionic polysaccharide (kappa-carrageenan) and globular protein (beta-lactoglobulin) clusters at pH 7 well above the iso-electric point. Gelation of kappa-carrageenan was induced by cooling in the presence of KCl. In the liquid state the protein clusters phase-separate into relatively dense micro-domains. When the polysaccharide phase gelled during cooling, the turbidity of the systems decreased dramatically. Light scattering experiments showed that the density of the micro-domains decreased, while microscopy showed that the number and size was not strongly modified. It is concluded that smaller protein clusters leave the micro-domains when kappa-carrageenan gels. The effect could be reversed by reheating the samples and thus melting the gel and was observed for repeated heating and cooling cycles. The effect of gelation on phase separation decreases with increasing polysaccharide concentration and with ageing of the liquid mixture. The latter is caused by the formation of bonds between the protein clusters in the micro-domains that slowly reinforce with time.

• Phase separation and percolation of reversibly aggregating spheres with a square-well attraction potential.

  Authors: Sujin Babu, Jean Christophe Gimel, Taco Nicolai

  The Journal of chemical physics. 12/2006; 125(18):184512.

  Reversible aggregation of spheres is simulated using a novel method in which clusters of bound spheres diffuse collectively with a diffusion coefficient proportional to their radius. It is shown thatReversible aggregation of spheres is simulated using a novel method in which clusters of bound spheres diffuse collectively with a diffusion coefficient proportional to their radius. It is shown that the equilibrium state is the same as with other simulation techniques, but with the present method more realistic kinetics are obtained. The behavior as a function of volume fraction and interaction strength was tested for two different attraction ranges. The binodal and the percolation threshold were determined. The cluster structure and size distribution close to the percolation threshold were found to be consistent with the percolation model. Close to the binodal phase separation occurred through the growth of spherical dense domains, while for deep quenches a system spanning network is formed that coarsens with a rate that decreases with increasing attraction. We found no indication for arrest of the coarsening.

• Effect of the cluster size on the micro phase separation in mixtures of beta-lactoglobulin clusters and kappa-carrageenan.

  Authors: Karine Baussay, Taco Nicolai, Dominique Durand

  Biomacromolecules. 02/2006; 7(1):304-9.

  The phase separation of globular protein clusters formed by heat-denatured beta-lactoglobulin (beta-lg) in mixtures with the polysaccharide kappa-carrageenan (kappa-car) has been studied at pH 7 andThe phase separation of globular protein clusters formed by heat-denatured beta-lactoglobulin (beta-lg) in mixtures with the polysaccharide kappa-carrageenan (kappa-car) has been studied at pH 7 and 20 degrees C. The effect of the protein cluster size on the phase separation was investigated by preparing clusters with radii between 20 nm and 1 mum. The formation of protein rich microdomains led to an increase of the turbidity starting at a minimum kappa-car concentration that decreased with increasing cluster size, but was only weakly dependent on the protein concentration. The size and number of microdomains do not depend much on the cluster size, but their density decreases with increasing cluster size leading to a lower turbidity.

• Strain hardening and fracture of heat-set fractal globular protein gels.

  Authors: Matthieu Pouzot, Taco Nicolai, Lazhar Benyahia, Dominique Durand

  Journal of colloid and interface science. 02/2006; 293(2):376-83.

  Non-linear mechanical behavior at large shear deformation was been investigated for heat-set beta-lactoglobulin gels at pH 7 and 0.1 M NaCl using both oscillatory shear and shear flow. These gelsNon-linear mechanical behavior at large shear deformation was been investigated for heat-set beta-lactoglobulin gels at pH 7 and 0.1 M NaCl using both oscillatory shear and shear flow. These gels have a self-similar structure at length scales smaller than the correlation length of the gel with fractal dimension d(f)=2. Strain hardening is observed that can be well described using the model proposed by Gisler et al. [T.C. Gisler, R.C. Ball, D.A. Weitz, Phys. Rev. Let. 82 (1999) 1064] for fractal colloidal gels. The increase of the shear modulus normalized by the low strain value (G(0)) is independent of G(0). For weak gels the elasticity increases up to a factor of ten, while for strong gels the increase is very small. At higher deformation irreversible fracture occurs, which leads eventually to macroscopic failure of the gel. For weak gels formed at low concentrations the deformation at failure is about 2, independent of the shear modulus. For strong gels fracture occurs at approximately constant stress (2 x 10(3) Pa).