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.

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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 κ-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.

An attractive interaction, commonly referred to as depletion interaction, is induced between aggregated whey protein colloid (AWC) particles when they are mixed with exocellular polysaccharides (EPSs) from a lactic acid bacterium. This interaction originates from a loss of conformational entropy of the EPSs near the surfaces of neighboring AWC particles and leads to a phase separation at high enough EPS and AWC concentrations. The effect of the depletion interaction on the properties of the mixtures of EPS and AWC particles is first studied in the stable, that is, one-phase, region. Small-angle neutron scattering (SANS) and dynamic light scattering (DLS) were used to characterize the strength of attractions. The SANS results can be described quantitatively by a model for depletion interaction. From Ornstein-Zernike plots, we derive the position of the spinodal. The DLS results can be described qualitatively quite well by using a recently derived expression for the wavevector (Q)-dependent diffusion coefficient as a function of the correlation length. Further, the experimental phase boundary is determined and compared with a meanfield theory, which evaluates the free energy of a mixture of colloids and large nonadsorbing polymers. The independently calculated spinodal was found to be consistent with the experimentally determined position of the phase boundary. Spinodal phase separation kinetics is investigated by small-angle light scattering (SALS). At low Q, a scattering peak was detected, which shifted to lower Qs 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.