Scheutjens-Fleer theory, effects

Scheutjens-Fleer (SF) Theory. A conceptual model for the effects of NOM on colloidal stability can be developed by using existing theoretical and experimental investigations of polymer and polyelectrolyte adsorption on solid surfaces and of the effects of macromolecules on colloidal stability. The modeling approach begins with the work of Scheutjens and Fleer for uncharged macromolecules, termed here the SF theory (3-5). This approach has been extended to the adsorption of linear flexible strong polyelectrolytes by van der Schee and Lyldema (6), adapted to weak polyelectrolytes (7-9), and applied to particle-particle interactions (8, 10). 

A very similar effect of the surface concentration on the conformation of adsorbed macromolecules was observed by Cohen Stuart et al. [25] who studied the diffusion of the polystyrene latex particles in aqueous solutions of PEO by photon-correlation spectroscopy. The thickness of the hydrodynamic layer 8 (nm) calculated from the loss of the particle diffusivity was low at low coverage but showed a steep increase as the adsorbed amount exceeded a certain threshold. Concretely, 8 increased from 40 to 170 nm when the surface concentration of PEO rose from 1.0 to 1.5 mg/m2. This character of the dependence is consistent with the calculations made by the authors [25] according to the theory developed by Scheutjens and Fleer [10,12] which predicts a similar variation of the hydrodynamic layer thickness of adsorbed polymer with coverage. The dominant contribution to this thickness comes from long tails which extend far into the solution. 

Figure 3. The effect of degree of polymerization on surface coverage (fractional site occupancy) at various polymer concentrations. The solid lines represent the present model and the symbols correspond to the theory of Scheutjens and Fleer. The parameter values are the same as in Figure 2.

Several theories have been put forward to account for the distributicm of polymer segments in the depletion zone. The theories of Feigin and Napper [48] and Scheutjens and Fleer [49] are qualitatively different from the theory of Asakura and Oosawa and de Cannes and coworkers [50,51] in that they predict not only depletion flocculation but also depletion stabilization. Depletion stabilization has not to date been verified experimentally although depletion fiocculation has been verified experimentally for several systems [52,53]. The effect of an adsorbed poljnner layer [54] and ordered solvent layers [55] on depletion flocculation is also under theoretical attack. The depletion stabilization interaction energy cannot simply be added to the other interaction energy terms to give the total interaction energy. 

Feigin and Napper (1980b) predicted quite different behaviour with respect to the solvency of the system the better is the solvent for the free polymer, the larger are the free energy changes involved and thus the smaller are the values of V2 and V2. These predictions, however, only apply to naked surfaces. The situation is much more complicated when steric barriers are present, as they would be in the theory of Scheutjens and Fleer and in many practical situations. In that case, the steric repulsion is greater in the absence of free polymer in a better solvent. It is the subtle interplay of the effects of solvency change on both steric stabilization and depletion stabilization that determines the overall trend observed. At present there does not appear to be any data in the literature that permit a clear cut statement to be made of the effects of solvency on depletion stabilization in the presence of steric layers. 

Scheutjens JMHM, Fleer GJ. Some implications of recent polymer adsorption theory. In Tadros TF, ed. The Effect of Polymers on Dispersion Properties. London Academic Press, 1982 145-168. 

---