Depletion-induced biaxial nematic states of boardlike particles

With the aim of investigating the stability conditions of biaxial nematic liquid crystals, we study the effect of adding a non-adsorbing ideal depletant on the phase behavior of colloidal hard boardlike particles. We take into account the presence of the depletant by introducing an effective depletion attraction between a pair of boardlike particles. At fixed depletant fugacity, the stable liquid crystal phase is determined through a mean-field theory with restricted orientations. Interestingly, we predict that for slightly elongated boardlike particles a critical depletant density exists, where the system undergoes a direct transition from an isotropic liquid to a biaxial nematic phase. As a consequence, by tuning the depletant density, an easy experimental control parameter, one can stabilize states of high biaxial nematic order even when these states are unstable for pure systems of boardlike particles

Controlled Nanoparticle Formation by Diffusion Limited Coalescence

Polymeric nanoparticles (NPs) have a 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 nanoparticles with a priori determined size.Comment: 4 pages, 3 figure

Phase diagram for a mixture of colloids and polymers with equal size

We present the phase diagram of a colloid-polymer mixture in which the radius a of the colloidal spheres is approximately the same as the radius R of a polymer coil (q=R/a1). A three-phase coexistence region is experimentally observed, previously only reported for colloid-polymer mixtures with smaller polymer chains (q0.6). A recently developed generalized free-volume theory (GFVT) for mixtures of hard spheres and non-adsorbing excluded-volume polymer chains gives a quantitative description of the phase diagram. Monte Carlo simulations also agree well with experimen

Design principles for metamorphic block copolymer assemblies

Certain block copolymer assemblies in selective solvents undergo dynamic morphology transitions (metamorphism) on varying the solution temperature. Despite the great application potential, there is a lack of fundamental understanding of the relationship between copolymer composition and the thermally-induced metamorphic behavior. Herein this relationship is studied by applying Scheutjens-Fleer Self-Consistent Field (SF-SCF) theory to develop fundamental design principles for thermoresponsive diblock copolymers exhibiting metamorphic behavior. It is found that metamorphism is caused by variation in the degree of stretching of the lyophobic blocks in response to changes in solvency. An optimal lyophobic/lyophilic block length ratio interval 3.5 ≲ fB ≲ 5.5 is identified. Such a fB window allows switching between spheres, cylinders and vesicles as preferred morphologies, with relatively small changes in the lyophobic block solvency. The transition from spheres to cylinders and from cylinders to bilayers can be controlled by varying fB, the overall degree of polymerization of the diblock copolymer, and by choosing an appropriate lyophilic block. Empirical relationships are provided to establish a connection between the SCF–SCF predictions and experimental observations

Depletion induced isotropic-isotropic phase separation in suspensions of rod-like colloids

When non-adsorbing polymers are added to an isotropic suspension of rod-like colloids, the colloids effectively attract each other via depletion forces. We performed Monte Carlo simulations to study the phase diagram of such rod-polymer mixture. The colloidal rods were modelled as hard spherocylinders; the polymers were described as spheres of the same diameter as the rods. The polymers may overlap with no energy cost, while overlap of polymers and rods is forbidden. Large amounts of depletant cause phase separation of the mixture. We estimated the phase boundaries of isotropic-isotropic coexistence both, in the bulk and in confinement. To determine the phase boundaries we applied the grand canonical ensemble using successive umbrella sampling [J. Chem. Phys. 120, 10925 (2004)], and we performed a finite-size scaling analysis to estimate the location of the critical point. The results are compared with predictions of the free volume theory developed by Lekkerkerker and Stroobants [Nuovo Cimento D 16, 949 (1994)]. We also give estimates for the interfacial tension between the coexisting isotropic phases and analyse its power-law behaviour on approach of the critical point

Phase Behaviour of Binary Hard-Sphere Mixtures: Free Volume Theory Including Reservoir Hard-Core Interactions

Comprehensive calculations were performed to predict the phase behaviour of large spherical colloids mixed with small spherical colloids that act as depletant. To this end, the free volume theory (FVT) of Lekkerkerker et al. [Europhys. Lett. 20 (1992) 559] is used as a basis and is extended to explicitly include the hard-sphere character of colloidal depletants into the expression for the free volume fraction. Taking the excluded volume of the depletants into account in both the system and the reservoir provides a relation between the depletant concentration in the reservoir and in the system that accurately matches with computer simulation results of Dijkstra et al. [Phys. Rev. E 59 (1999) 5744]. Moreover, the phase diagrams for highly asymmetric mixtures with size ratios q . 0:2 obtained by using this new approach corroborates simulation results significantly better than earlier FVT applications to binary hard-sphere mixtures. The phase diagram of a binary hard-sphere mixture with a size ratio of q = 0:4, where a binary interstitial solid solution is formed at high densities, is investigated using a numerical free volume approach. At this size ratio, the obtained phase diagram is qualitatively different from previous FVT approaches for hard-sphere and penetrable depletants, but again compares well with simulation predictions.Comment: The following article has been accepted by The Journal of Chemical Physics. After it is published, it will be found at https://doi.org/10.1063/5.003796

An exocellular polysaccharide and its interactions with proteins

In the food industry polysaccharides are used as thickening or gelling agents. Polysaccharides are usually extracted from plants. Micro-organisms are also capable of excreting polysaccharides: exocellular polysaccharides (EPSs). In some cases EPSs are produced in-situ in food products, notably in acidified milk products. These EPSs function effectively as food thickeners but do not need to be declared in the food label.Systematic physical analysis of an exocellular polysaccharide produced by a lactic acid bacterium has hardly been performed until now. In order to obtain a better understanding of the role of EPS in (acidified) milk products the physical properties of an EPS from the lactic acid bacterium strain Lactococcus lactis subsp. cremoris B40 were studied (Chapters 2-4) as well as its interactions with milk proteins (Chapters 5-8). The ionic strength of the EPS solutions was always set at 0.10 M, about the ionic strength in milk.In Chapter 2 the isolation, purification and analysis of the molecular properties of EPS from L. lactis B40, our 'model' EPS, are investigated. The polysaccharide was separated from most low molar mass compounds in the culture broth by filtration processes. Gel permeation chromatography (GPC) was used to size-fractionate the polysaccharide. Fractions were analyzed by multi-angle static light scattering in aqueous solutions from which a number- (M n ) and weight-averaged (M w ) molar mass of (1.47 ± 0.06)·10 3and (1.62 ± 0.07)·10 3kg/mol, respectively, were calculated so that M w /M n1.13. The number-averaged radius of gyration was found to be 86 ± 2 nm. The hydrodynamic radius as determined from dynamic light scattering was consistent with the radius of gyration.The viscosity of the EPS solutions was studied in simple shear flow as described in Chapter 3. Firstly, the zero-shear viscosity was determined as a function of the concentration. The intrinsic viscosity was determined from the data in the low concentration range. The intrinsic viscosity and the concentration dependence of the (zero-shear) viscosity of the B40 EPS could be predicted from the molar mass and the hydrodynamic radius. In addition the shear-thinning behavior was measured at several concentrations. The shear rate at which the viscosity starts to decrease scales with polymer concentration in accordance with the Rouse theory. By combining existing theories (Rouse and Bueche) it is possible to predict the intrinsic viscosity, concentration dependence of the viscosity, and shear-thinning behavior in terms of the molar mass and the hydrodynamic radius.The measurements and theoretical description of the dynamic rheological properties of the EPS are presented in Chapter 4. Dynamic rheological measurements were performed as a function of frequency and EPS concentration. The dynamic properties could be described by the bead-spring model of Rouse. Concentrated EPS solutions have a significant elasticity at high concentrations and high frequencies, which is indicative of the presence of significant normal stress differences. It is suggested that these normal stresses may explain the contribution of the EPSs to the ropy behavior of yogurts.Having characterized the EPS in aqueous solution, its interaction with the most relevant colloidal (protein) particles present in milk products was studied. As the polysaccharide studied in this thesis occurs in dairy products our focus was on the interactions and phase behavior of EPS with the colloidal components in milk. There are three distinctly different types of particles in the colloidal size range in milk: fat globules, casein micelles and whey proteins. Smaller molecular species (over 100,000 in milk) are considered as part of the continuous phase.In Chapter 5 the interactions with whey proteins are described. Native whey proteins and EPS were co-soluble; they could be mixed in all proportions. However, an effective attraction (a depletion interaction) is induced between aggregated-whey-protein colloid (AWC) particles when they are mixed with the EPS. This depletion interaction originates from a loss of conformational entropy of the EPSs near the surface of neighboring AWC particles and leads to a phase separation at high enough EPS and/or AWC concentrations. The effect of the depletion interaction on the properties of the mixtures of EPS and AWC particles was first studied in the stable, i.e. one-phase region. The strength of attractions was characterized by small-angle neutron scattering (SANS) and dynamic light scattering (DLS). The SANS results could be described quantitatively by the Vrij theory and integral theory (Ornstein-Zernike with HNC closure) in combination with the Schaink-Smit theory and allowed a determination of the position of the spinodal. The DLS results could be described reasonably well by using a theory of Dhont and Kawasaki.Furthermore, the experimental phase boundary was determined and compared with the Schaink-Smit theory, a mean-field theory which evaluates the free energy of a mixture of colloids and large non-adsorbing polymers. The spinodal so calculated was found to be consistent with the experimentally determined position of the phase boundary.Spinodal phase separation kinetics was investigated by small-angle light scattering (SALS). At low Q a scattering peak was detected which shifted to lower Q's with time, in agreement with other experimental data and theoretical predictions for spinodal decomposition. Both the scaling of the scattered intensity with Q and the scaling of the Q-position of the peak with time agree with theoretical predictions of Furukawa and Siggia.The interactions between EPS B40 and casein micelles are treated in in Chapter 6. Casein micelles become mutually attractive when the EPS is added to skim milk. The attraction can be explained as a depletion interaction between the casein micelles induced by the non-adsorbing EPS. We used three scattering techniques (SANS, turbidity measurements and DLS) to measure the attraction. The Vrij theory in combination with integral theory and all the experiments showed that casein micelles became more attractive upon increasing the EPS concentration.The phase separation arising from depletion interaction in mixtures of casein micelles and EPS is described in Chapter 7. We have determined a phase diagram that describes the separation of skim milk with EPS into a casein-micelle-rich phase and an EPS-rich phase. We compared the phase diagrams with those calculated from theories developed by Vrij, and by Lekkerkerker and co-workers, showing that the experimental phase boundary can be predicted quite well. From measurements of the self-diffusion of the casein micelles in the presence of EPS the spinodal was calculated, which corresponds to the visual observations.The effect of adding the EPS to an oil-in-water emulsion, stabilized with whey proteins, is reported in Chapter 8. Even at low EPS concentrations the emulsion phase separates. The phase line could be described by depletion interaction theory of Vrij. At high EPS concentrations and dispersed phase volume fractions above 10% we found a stable 'gel'-like region in the phase diagram. In that region the oil droplets attract one another so strongly that a space-filling network is formed at sufficient oil volume fractions.A kinetic study showed that the rate of creaming/demixing decreases with volume fraction of oil of the system (hydrodynamics) and strongly depends on the concentration of EPS (strength of depletion interaction and continuous-phase viscosity). At low EPS concentration the creaming rate strongly increased with EPS concentration since attractions enhance creaming. At higher EPS concentrations creaming was slowed down by the viscosity increase of the continuous phase and the particle network which was created. This network became so strong at high EPS concentrations that creaming was absent in the 'gel' region. The rheological behavior of the 'gel' was studied by measuring flow curves which could be interpreted by the Potanin model, which describes the rheology of a dispersion of weakly aggregating particles.In Chapter 9 the practical implications of this work are described. In order to understand the thickening effect of EPSs the molar mass, radius of gyration, and their interrelation are very important. It is indicated how the effectivity of a polysaccharide can be analyzed on the basis of the molar mass and the radius of gyration. The relation between the radius of gyration and the molar mass depends on the kind of monosaccharide residues, the linkage type, and the solvent. Further it is addressed how a fundamental understanding of the interactions between polysaccharides and proteins leads to predictions of the phase line and interpretation of the measured phase behavior. The unwanted effect of phase separation can then be suppressed by using only biopolymer concentrations at which the system is still stable. An understanding of the biopolymer interactions may thus make it possible to adjust the properties of food dispersions. Finally, some suggestions for further research are given.</p