Colloidal particles, polymer-induced

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]. 

In Section 3.4a we examine a model for the second virial coefficient that is based on the concept of the excluded volume of the solute particles. A solute-solute interaction arising from the spatial extension of particles is the premise of this model. Therefore the potential exists for learning something about this extension (i.e., particle dimension) for systems for which the model is applicable. In Section 3.4b we consider a model that considers the second virial coefficient in terms of solute-solvent interaction. This approach offers a quantitative measure of such interactions through B. In both instances we only outline the pertinent statistical thermodynamics a somewhat fuller development of these ideas is given in Flory (1953). Finally, we should note that some of the ideas of this section are going to reappear in Chapter 13 in our discussions of polymer-induced forces in colloidal dispersions and of coagulation or steric stabilization (Sections 13.6 and 13.7). 

Stability in mixtures of colloidal particles and polymer molecules, dispersed in a solvent, has been the subject of experimental and theoretical investigations for a long time and it has applications in diverse fields such as paint technology, wastewater treatment, emulsion polymerization, biology etc. It has now been well recognized that polymer molecules can be used to induce either stabilization or flocculation (phase separation) in colloidal dispersions. It is important to distinguish between polymers which are adsorbed on the particle surface and those that are free in solution because the two situations usually lead to qualitatively different effects. Stability imparted by adsorbed polymers is known as steric stabilization and the flocculation or phase separation caused by the free polymer is due... 

The steric stabilization, which is imparted by polymer molecules grafted onto the colloidal particles, is extensively employed.3 Amphiphilic block copolymers are widely used as steric stabilizers. The solvent-incompatible moieties of the block copolymer provide anchors for the polymer molecules that are adsorbed onto the surface of the colloidal particles, and the solvent-compatible (buoy) moieties extend into the solvent phase. When two particles with block copolymers on their surface approach each other, a steric repulsion is generated bet ween the two particles as soon as the tips of the buoy moieties begin to contact, and this repulsion increases the stability of the colloidal system.4-6 Polymers can also induce aggregation due to either depletion 7-11 or bridging interactions.12 15... 

There are three chapters in this volume, two of which address the microscale. Ploehn and Russel address the Interactions Between Colloidal Particles and Soluble Polymers, which is motivated by advances in statistical mechanics and scaling theories, as well as by the importance of numerous polymeric flocculants, dispersants, surfactants, and thickeners. How do polymers thicken ketchup Adler, Nadim, and Brenner address Rheological Models of Suspensions, a closely related subject through fluid mechanics, statistical physics, and continuum theory. Their work is also inspired by industrial processes such as paint, pulp and paper, and concrete and by natural systems such as blood flow and the transportation of sediment in oceans and rivers. Why did doctors in the Middle Ages induce bleeding in their patients in order to thin their blood ... 

Fl-FFF is the most universally applicable FFF technique as the separation only relies on differences in the diffusion coefficients. Thus, it nicely complements S-FFF or Th-FFF with respect to size distribution analysis [225]. Fl-FFF is capable of separating almost all particles (up to 50 pm [226] or even much larger) and colloids and polymers down to -2 nm [17] or 103 g/mol [227]. The lower limit is determined by the pore size of the membrane material. The need for such membrane covering the accumulation wall is the principle disadvantage of Fl-FFF due to possible interactions with the solute and the danger of a membrane-induced non-uniformity in the channel thickness, especially since thinner channels are highly favored for faster separations. However, the advantages of Fl-FFF usually more than balance the potential pitfalls and sources of error. Consequently, Fl-FFF is the FFF technique which has been developed the most in recent years in instrumentation [48] and has found the most widespread distribution. 

The second part is devoted to adsorption of polyelectrolytes at interfaces and to flocculation and stabilization of particles in adsorbing polymer solutions. A recent theory of the electrostatic adsorption barrier, some typical experimental results, and new approaches for studying the kinetics of polyelectrolyte adsorption are presented in the first chapter of this part. In the following chapters, results are collected on the electrical and hydrodynamic properties of colloid-polyelectrolyte surface layers, giving information on the structure of adsorbed layers and their influence on the interactions between colloidal particles examples and mechanisms are analyzed of polyelectrolyte-induced stabilization and fragmentation of colloidal aggregates ... 

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... 

For completeness we note that two other FFF subtechniques can be applied to certain polymeric materials, although applications are so far limited. Sedimentation FFF is the most notable example. For this system the driving force (centrifugally induced sedimentation) is directly proportional to molecular mass in a form that is calculable from first principles (see eqn 8.7). Accordingly, molecular mass distributions can in theory be obtained by calculation without empirical calibration. This principle has been successfully applied to the determination of the molecular mass and particle size distribution of numerous colloidal particles including viruses, latices, emulsions, liposomes, protein aggregates, and water-borne colloids [5,7,9]. However, as noted earlier, sedimentation FFF is not applicable to many polymers of interest because sedimentation forces (even in a powerful centrifuge) are not adequate to drive the components to the accumulation wall of the FFF channel. Thus molecular masses of less than 10 cannot be well... 

Polymers are often applied to promote either stabilization or aggregation. Examples of polymer-induced aggregation are found in water purification, clarification of wine and fruit juices, and in papermaking. Stabilization of colloidal particles by polymers is applied in foodstuffs, paints, and dyes, in the dispersion of particles on magnetic tapes and photographic paper, as well as in many cosmetic and medical salves and lotions. A rather recent development is the use of polymers to reduce or prevent the deposition of protein molecules and biological cells on surfaces in order to avoid biofouling and, hence, improve the biocompatibility of these surfaces. 

For a polymer with M = 1 O, (r = 6 nm and at M= 10 the value reaches 20 nm. Consequently, macromolecules with M> 10 are of the sizes needed to stabihze colloidal particles (macromolecules are, of comse, to induce repulsion of particles). 

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