Hard sphere colloidal dispersions: Mechanical relaxation pertaining to thermodynamic forces

The complex viscosity of sterically stabilized (hard) silica spheres in cyclohexane has been measured between 80 Hz and 170 kHz with torsion pendulums and a nickel tube resonator. The observed relaxation behaviour can be attributed to the interplay of hydrodynamic and thermodynamic forces. The validity of the Cox-Merz rule is checked

Adhesive Hard-Sphere Colloidal Dispersions. A Small-Angle Neutron-Scattering Study of Stickiness and the Structure Factor

Small-angle neutron-scattering structure factor measurements were made on sterically stabilized silica spheres dispersed in benzene up to volume fractions of 0.30. Benzene is only a marginal solvent for the stabilizing layer on the surface of the particles. The particles are made attractive by lowering temperature. This attraction is modeled by a square well potential, the depth of which varies with temperature. At the highest temperature studied, our experimental system behaved effectively as an assembly of hard spheres, whereas at the lowest temperature the system approaches a spinodal. Using Baxter's theory we were able to evaluate the interaction parameters and to calculate the structure factor. Experimental structure factors were satisfactorily reproduced over the entire temperature range studied

Thixotropy in macroscopic suspensions of spheres

An experimental study of the viscosity of a macroscopic suspension, i.e. a suspension for which Brownian motion can be neglected, under steady shear is presented. The suspension is prepared with a high packing fraction and is density-matched in a Newtonian carrier fluid. The viscosity of the suspension depends on the shear rate and the time of shearing. It is shown for the first time that a macroscopic suspension shows thixotropic viscosity, i.e. shear-thinning with a long relaxation time as a unique function of shear. The relaxation times show a systematic decrease with increasing shear rate. These relaxation times are larger when decreasing the shear rates, compared to those observed after increasing the shear. The time scales involved are about 10000 times larger than the viscous time scale and about 1000 times smaller than the thermodynamic time scale. The structure of the suspension at the outer cylinder of a viscometer is monitored with a camera, showing the formation of a hexagonal structure. The temporal decrease of the viscosity under shear coincides with the formation of this hexagonal pattern

Casein Micelles: Size Distribution in Milks from Individual Cows

The size distribution and protein composition of casein micelles in the milk of Holstein-Friesian cows was determined as a function of stage and number of lactations. Protein composition did not vary significantly between the milks of different cows or as a function of lactation stage. Differences in the size and polydispersity of the casein micelles were observed between the milks of different cows, but not as a function of stage of milking or stage of lactation and not even over successive lactations periods. Modal radii varied from 55 to 70 nm, whereas hydrodynamic radii at a scattering angle of 73° (Q2 = 350 μm−2) varied from 77 to 115 nm and polydispersity varied from 0.27 to 0.41, in a log-normal distribution. Casein micelle size in the milks of individual cows was not correlated with age, milk production, or lactation stage of the cows or fat or protein content of the milk

Lactoferrin binding to transglutaminase cross-linked casein micelles

Casein micelles in skim milk were either untreated (untreated milk) or were cross-linked using transglutaminase (TGA-milk). Added lactoferrin (LF) bound to the casein micelles and followed Langmuir adsorption isotherms. The adsorption level was the same in both milks and decreased the micellar zeta potential, indicating charge neutralization and the formation of complex coacervate-type interactions. For the untreated milks, the adsorption of LF was initially accompanied by an increase in turbidity of the milk and in the size of the casein micelles; however, after several hours, the turbidity and the casein micelle size of these milks decreased markedly. For the TGA-milks, no change in casein micelle size was observed on adsorption of LF, but the turbidity increased due to the increased mass of the casein micelles, and remained constant on holding. These results indicate that the cross-linking of the casein micelles prior to adding LF prevents disintegration of the casein micelles

Co-acervates of lactoferrin and caseins

On mixing positively charged lactoferrin (LF) with negatively charged caseins (*CN) it is observed that complexes are formed. The * stands for α, β, κ or Na. The size of the complex co-acervates appears to grow indefinitely and asymptotically near the point of charge equivalency. Away from the charge equivalent ratio it seems that build-up of (surface) charges limits complex size. We proposed a simple scaling law so as to predict the size of the complex. By assuming that surface charge density is constant or can reach only a maximum value, it follows that scattering intensity is proportional to |(1 − x/xcrit)|−3 where x is the mole (mass) fraction of the cationic protein and xcrit the value of the mole (mass) fraction at the charge equivalent ratio. Both scattering intensity and particle size obey this simple assumption. We investigated three different caseins, all of which formed co-acervate complexes with LF, but at different molar ratios. Critical composition varied inversely with pH, showing that charge neutrality is the determining factor. Sodium caseinate formed complexes as well but the growth was limited, presumably due to the intrinsic surfactant properties of whole casein. Adding NaCl diminishes the interaction and above 0.4 mol L−1 of NaCl no β-CN–LF complexes are formed. The charge neutral composition shifts to the LF side on adding NaCl, probably because the casein can wrap around the LF more effectively

Coacervates of lysozyme and β-casein

Complexes are formed when positively charged lysozyme (LYZ) is mixed with negatively charged caseins. Adding b-casein (BCN) to LYZ leads to flocculation even at low addition levels. Titrating LYZ into BCN shows that complexes are formed up to a critical composition (x = [LYZ]/([LYZ] + [BCN]). The formation of these complex coacervates increases asymptotically toward the molar charge equivalent ratio (xcrit), where the size of the complexes also seems to grow asymptotically. At xcrit, insoluble precipitates of charge-neutral complexes are formed. The precipitates can be re-dispersed by adding NaCl. The value of xcrit shifts to higher values on the LYZ side with increasing salt concentration and pH. Increasing the pH, de-protonates the BCN and protonates the LYZ, and therefore, charge neutrality will shift toward the LYZ side. xcrit increases linearly from 0.2 at no salt to 0.5 at 0.5 M NaCl. It ends abruptly at a salt concentration of 0.5 M after which a clear mixed solution remains. Away from the charge equivalent ratio, it seems that the buildup of charges limits the complex size. A simple scaling law to predict the size of the complex is proposed. By assuming that surface charge density is constant or can reach only a maximum value, it follows that scattering intensity is proportional to |(1 x/xcrit)| 3 where x is the mole fraction of one protein and xcrit the value of the mole fraction at the charge equivalent ratio. Both scattering intensity and particle size seem to obey this simple assumption. For BCN–LYZ, the buildup occurs only at the LYZside in contrast to lactoferrin which forms stable complexes on either side of xcrit. The reason that the complexes are formed at the BCN side only may be due to the small size of LYZ, which induces a bending energy in the BCN on adsorption

Coacervates of lysozyme and β-casein

Complexes are formed when positively charged lysozyme (LYZ) is mixed with negatively charged caseins. Adding b-casein (BCN) to LYZ leads to flocculation even at low addition levels. Titrating LYZ into BCN shows that complexes are formed up to a critical composition (x = [LYZ]/([LYZ] + [BCN]). The formation of these complex coacervates increases asymptotically toward the molar charge equivalent ratio (xcrit), where the size of the complexes also seems to grow asymptotically. At xcrit, insoluble precipitates of charge-neutral complexes are formed. The precipitates can be re-dispersed by adding NaCl. The value of xcrit shifts to higher values on the LYZ side with increasing salt concentration and pH. Increasing the pH, de-protonates the BCN and protonates the LYZ, and therefore, charge neutrality will shift toward the LYZ side. xcrit increases linearly from 0.2 at no salt to 0.5 at 0.5 M NaCl. It ends abruptly at a salt concentration of 0.5 M after which a clear mixed solution remains. Away from the charge equivalent ratio, it seems that the buildup of charges limits the complex size. A simple scaling law to predict the size of the complex is proposed. By assuming that surface charge density is constant or can reach only a maximum value, it follows that scattering intensity is proportional to |(1 x/xcrit)| 3 where x is the mole fraction of one protein and xcrit the value of the mole fraction at the charge equivalent ratio. Both scattering intensity and particle size seem to obey this simple assumption. For BCN–LYZ, the buildup occurs only at the LYZside in contrast to lactoferrin which forms stable complexes on either side of xcrit. The reason that the complexes are formed at the BCN side only may be due to the small size of LYZ, which induces a bending energy in the BCN on adsorption

Phase separation of sterically stabilized colloids as a function of temperature

Colloidal dispersions of sterically stabilized silica spheres in toluene are found to separate into two layers when the temperature drops below a certain threshold. The phase diagram shows a dependence on the overall concentration. The behaviour of the dilute phase compares with theoretically calculated binodals using polydispersity in the polymer-like attraction. For the concentrated phase no agreement is obtained