The experimental data concerning adsorption from semi-dilute and concen-trated solution allow for a general conclusion regarding the aggregative mecha-nism of adsorption.1,2,10 This mechanism may be represented in the following way. During adsorption from solutions, where the aggregation of macromolecules proceeds, the aggregates are preferentially transferred to the adsorbent surface because they are less “soluble” as compared with isolated macromolecules. After equilibrium is established, the aggregates in solution are practically absent.10They reappear after some time necessary to attain a new equilibrium state between isolated and aggregated molecules. It was found that in solution, after some time the aggregates appear again. These data confirm the transition of aggregates onto the surface and explain the main peculiarities of adsorption from semi-dilute and concentrated solutions (extensive adsorp-tion, inversion of the influence of the solvent nature, existence of maxima on iso-therms, etc.).
Decreased adsorption, after the maximum is reached, may be explained by the formation of a physical network of entanglements in solution, which hinders and, after some concentration, prevents the transition of macromolecular aggre-gates and macromolecules onto the surface. Only such a supposition allows one to explain the absence of adsorption from solution after reaching some limiting concentration. At the same time, when explaining the adsorption isotherms, it should be remembered that, with increasing concentration, the aggregation constant diminishes. At each solution concentration a definite equilibrium be-tween aggregated and isolated molecules is established. It is evident that the change of aggregation constant affects the distribution between aggregated and isolated molecules, which determines their ratio at the adsorbent surface. The preferential (but not exclusive) adsorption of aggregates is accompanied by the adsorption of isolated molecules. Their conformation is different than in dilute solution, as a result of coil overlapping.
The peculiarity of polymer adsorption, connected with its aggregative mechanism, consists in the fact that at each concentration of solution in adsor-bent presence, a new state of equilibrium between isolated and aggregated mol-ecules is established. To each point of isotherm corresponds another structure of adsorbing units (from isolated macromolecules up to aggregates of various size)
and their different distribution on the surface, depending on the solution con-centration. The appearance of a maximum on the isotherms of adsorption is caused by diminishing of the number and size of aggregates with an increase in the solution concentration, i.e., change in the ratio between aggregated and iso-lated molecules at each point of isotherm. In this case, distinct from adsorption of low molecular substances, the adsorption value and the shape of the isotherm depends not only on concentration but also on the ratio of volumes of adsorbent and solution.
Studying adsorption of polystyrene, polycarbonate, and other polymers has shown that adsorption greatly increases after reaching some definite con-centration of solution, which is accompanied by change in the shape of the iso-therm and in a fraction of bound segments.120, 121Figures 1.6 and 1.7 represent the isotherms of adsorption and fractions of bound segments at various amounts of adsorbent. Increase of the amount of adsorbent leads to diminishing adsorp-tion. Fraction of bound segments is changed non-monotonously and is higher at low values of adsorption as a result of transition on the surface of isolated macromolecules. The surface may influence the state of equilibria between ag-gregated and isolated macromolecules. Dependencies of the adsorption values and fraction of bound segments are opposite and the maximum of adsorption corresponds to the minimum of bound segments. All these data meet the concept
Figure 1.6. Isotherm of adsorption (1) and fraction of bound segments (2) for the system:
polycarbonate-fumed silica-dichloroethane (a) and oligoethylene glycoladipinate-fumed sil-ica-dichloroethane (b).
of aggregative adsorption.
Aggregative mechanism was discov-ered also for adsorption of some pro-teins onto the silica surface.122
It is evident that when this mechanism is operative, the struc-ture and thickness of adsorption lay-ers are determined by the structure and size of aggregates, and by the ra-tio between aggregated and isolated of the amount of adsorbed polymer and surface of an adsorbent (adsorp-tion from concentrated solu(adsorp-tions (up to 56.0 g/l)). The values found are of the order of 0.7 mcm, which essen-tially exceeds the thickness of a monolayer. For solutions of isotactic polystyrene and other polymers on disperse powders, the thicknesses were found in the range of 1-8 mcm,123 which may be explained by adsorption of aggregates. The thick-ness of adsorption layers of oligomeric epoxy resin on the glass powder was found to be 0.1 mcm.125This value is at least one order higher, as compared with adsorption from dilute solutions.
Molecular-aggregative mechanism of adsorption also explains a great dif-ference between the fractions of bound segments adsorbed from dilute and semi-dilute and concentrated solutions. The fraction of segments of macromolecules, forming an aggregate and directly interacting with the sur-face, should be low, even at a high degree of surface coverage. As distinct from
di-Figure 1.7. Fraction of bound segments of polybutyl-methacrylate from chloroform (a) and from mixture with polystyrene (b) at vari-ous amounts of adsorbent:1-20, 2-40 mg/ml.
lute solutions, where the fraction of bound segments monotonously decreases with increase in solution concentration, for adsorption from concentrated solu-tion, value p is changed non-monotonously, similar to the value of adsorbed polymer. This value is also dependent on the thermodynamic quality of a sol-vent, from which the adsorption layer is formed. Transition onto the surface of large aggregates, increasing adsorbed amount, leads to the diminishing of the degree of their bonding with the surface. Thus, the conclusion may be drawn that transition of macromolecular aggregates onto the adsorbent surface results in formation of adsorption layer interacting with the surface to a lesser extent, compared with dilute solutions and aggregates which are not strongly attached to the surface.
The degree of aggregate bonding depends not only on the degree of surface coverage, but on the structure of solution, depending on solvent quality and so-lution concentration. The transition of aggregates determines to a greater de-gree the layer. The continuous change in the dede-gree of aggregation with concentration has a strong influence on the structure of adsorption layers, ob-tained from solutions of different concentrations. However, the theoretical de-scription of the aggregative adsorption has yet to be done.