Colloidal particles, polymer-induced attraction

Charge neutralization, polymeric flocculation effect, 4-5 Colloid-polymer surface layers, electrical and hydrodynamic properties investigated with electro-optics, 121-135 Colloid stability, 161 Colloidal particles, polymer-induced attraction, 97... 

In section C2.6.4.3 it was shown how tlie addition of non-adsorbing polymer chains induces a depletion attraction between colloidal particles. If sufficient polymer is added, tliese attractions can be strong enough to induce a phase separation of tire colloidal particles. An early application of tliis was tire creaming of mbber latex [93]. 

Since the beginning of colloids science, however it is also known that the agglomeration of colloids and dispersed particles can be prevented or controlled by stabilization [8]. The attractive interactions between the colloidal particles, caused by van-der-Waals forces, need to be compensated by repulsive interactions. The latter can be based either on electrostatic repulsion due to same-sign surface charges (electrostatic stabilization), or on repulsion via a polymer shell formed through adsorption of polymers to the particle surface (steric stabilization, in presence of polyelectrolytes termed electrosteric stabilization due to additional charged-induced repulsion) [9, 10]. The stabilization by control of the interaction forces between colloidal particles has been in the focus of extensive research efforts. Already... 

The effect of ionic strength on the adsorption of protein onto poly[NIPAM] is more complex than was expected. In fact, salinity affects not only electrostatic interactions but also the colloidal properties of such thermally sensitive particles (1) the increase in ionic strength leads to a reduction in particle size induced by lowering the volume phase transition temperature (i.e., the LCST of linear thermally sensitive polymer decreases as the salinity of the medium increases) and (2) salinity affects the degree of attractive and repulsive electrostatic interactions. As a result, the adsorption of proteins onto thermally sensitive microgel particles is generally and dramatically reduced as salinity increases, irrespective of temperature (as illustrated for P24 [Figure 9.26] adsorption onto poly(NIPAM) particles). 

The introduction of a second colloidal component, whether it is a particle, micelle, or dissolved polymer, can induce depletion-induced attraction forces. - These forces can lead to the manipulation of the percolation network structure. The role of colloidal structures that can contribute to the final conducting percolation network has previously been modeled. - It was shown in these works that... 

These results demonstrate that nonadsorbing polymer can induce phase separations in colloidal systems with the nature of the phases depending primarily on the ratio of the particle and polymer sizes. Since the strength of the attraction is not necessarily a monotonic function of the polymer concentration, e.g., because of penetration of the free polymer into a grafted layer, both destabilization and restabilization are possible. 

Most food systems are of a colloidal as well as a polymeric nature. The presence of a nonadsorbing polymer in a skim milk dispersion induces an effective attraction between the casein particles, called depletion interaction, resulting in phase separation at sufficiently high polymer concentration. Tuinier et al. (2003) discussed the influence of colloid-polymer size ratio, polymer concentration regime, size, poly-dispersity and charges in colloid/biopolymer mixtures, demonstrating the challenging complexity of the subject. 

The existence of short-range attractive interactions between particles leads to a much richer phase behavior, as illustrated in Fig. 3. This situation can be achieved by adding a nonadsorbing polymer to the suspensions, which induces an effective depletion attraction between the particles [105]. Such polymer-colloid mixtures can be viewed as model systems of complex fluids and are involved in many practical... 

There are three main modes of interaction between a polymer solution and a solid surface. The first interaction mode is depletion [2,3]. If the monomers are repelled by the surface (or in other words if the attractive interaction between the solvent molecules and the surface is larger than the interaction between the monomers and the surface), the polymer concentration in solution decreases as the surface is approached and a region depleted in polymer exists in the vicinity of the surface. The size of this region is the size of the polymer chain if the solution is dilute and the size of the correlation length of the solution if the solution is semidilute (if the polymer chains overlap). When two surfaces are brought in close contact, the density in the gap between the surfaces is smaller than the bulk concentration and the osmotic pressure in the gap is smaller than the bulk osmotic pressure. This osmotic pressure difference induces an attraction between the surfaces. The depletion interaction is not specific to polymers and exists with any particle with a size in the colloidal range [4]. It has sometimes been used to induce adhesion between particles of mesoscopic size such as red blood cells. The only limitation to this qualitative description of the depletion force is that at equilibrium the polymer chains (or any other particles) must leave the gap as the surfaces get closer. There is no attractive depletion force if they remain trapped in the gap. We will not consider further the depletion interaction. 

---