Polymer adsorption irreversible

There are numerous references in the literature to irreversible adsorption from solution. Irreversible adsorption is defined as the lack of desotption from an adsoibed layer equilibrated with pure solvent. Often there is no evidence of strong surface-adsorbate bond formation, either in terms of the chemistry of the system or from direct calorimetric measurements of the heat of adsorption. It is also typical that if a better solvent is used, or a strongly competitive adsorbate, then desorption is rapid and complete. Adsorption irreversibility occurs quite frequently in polymers [4] and proteins [121-123] but has also been observed in small molecules and surfactants [124-128]. Each of these cases has a different explanation and discussion. 

In order to generate information on the mechanism of flocculation by polymers it is, however, necessary to correlate flocculation with various system properties, particularly adsorption. Thus, if particle/polymer-polymer/particle contact is the aggregation mechanism, the flocculation responses should be expected to continuously increase with surface coverage. On the other hand, if particle/polymer-particle contact is predominant and if the polymer adsorption is essentially irreversible, maximum flocculation might be expected under submonolayer conditions. In order to determine the nature of this relationship for the present systems, selected flocculation responses are plotted in Figures 8 and 9 as a function of surface coverage for the nonionic and the anionic polymer respectively. The assumptions involved in the computation of the surface coverage are to be noted at this point ... 

Because polymer adsorption is effectively irreversible, and because adsorption and floe growth occur simultaneously, flocculation is a non-equilibrium process. As a result, performance is largely determined by the kinetics of adsorption and aggregation. Both of these can be regarded as collision processes involving solid particles and polymer molecules. In each case, collisions can arise due to either Brownian motion or agitation of the suspension. The collision frequency v between particles and polymer molecules can be estimated from °... 

Because polymer adsorption tends to be irreversible, inadequate mixing, with non-uniform distribution of polymer in the particle suspension, cannot be rectified by subsequent rearrangement. Locally high concentrations can lead to rapid floe growth, but with corresponding regions of starvation elsewhere in the... 

The last Issue to be dealt with Is the apparent irreversibility of the adsorption. One quite often encounters the opinion, especially In the older literature, that polymer adsorption would be an Irreversible phenomenon. These ideas are based on the hysteresis found In the adsorption isotherms desorption Isotherms (obtained by dilution with solvent) do not coincide with adsorption Isotherms (obtained by adding more polymer at given amount of solvent). Qualitatively, this was already discussed in sec. 5.3d. An experimental example Is given in fig. 5.31, for the adsorption of a polydisperse rubber from heptane on two types of carbon black (differing In specific surface area) ). The desorption Isotherms are found to He considerably above the adsorption Isotherms, the extent of desorption being very small. 

Although these hysteresis effects are real, there is no reason to conclude that the adsorption of polymers Is Irreversible. The situations for a point on the desorption Isotherm and on the adsorption isotherm are different in one Important aspect is higher In the former case because the solution has been diluted with solvent. This solvent addition lowers the concentration In the solution (which contains only the "losers") but does not affect the "winners" on the surface. The latter would desorb only by excessive dilution (below see fig. 5.27). which cannot be done under the usual experimental conditions (unless the chains are short and the adsorption weak), As a matter of fact, a "desorption isotherm" as given In fig. 5.31 (whereby is continuously Increased) corresponds to going from some point on the plateau of an adsorption isotherms at low to a similar point (with nearly the same D on an adsorption Isotherm at higher one crosses a set of adsorption Isotherms to the left at essentially constant r. This does not reflect irreversibility, but simply a change in the mass beilance. 

Note that the Langmuir model is an equilibrium relationship, and its application assumes adsorption is instantaneous and reversible in terms of polymer concentration. When polymer adsorption is considered to be irreversible, the Langmuir model cannot be used directly when the polymer concentration is declining. An additional parameter, Cp p x, must be used to track the adsorption history so that Eq. 5.33 applies. 

In most cases, polymer adsorption is considered irreversible that is, it does not decrease as polymer concentration decreases (Szabo, 1979 Lakatos et al., 1979 Gramain and Myard, 1981). The irreversible effect is caused by polymer adsorption on rock. However, this is not exactly true because small amounts of polymer can be removed from porous rock using prolonged exposure to water or brine injection. Usually, however, the rate of release is so small that it is not possible to measure the concentrations accurately. It is thus more accurate to state that the rate of polymer retention is much greater than the rate of polymer removal. Retention also may occur when flow rates are suddenly increased. This process is called hydrodynamic retention, which is reversible (Green and Willhite, 1998). 

Bondor et al. (1972) assumed that the permeability reduction is caused by polymer adsorption, and the adsorption process is irreversible. They further assumed the maximum permeability reduction corresponds to the polymer adsorptive capacity on the rock, AdC. The permeability reduction factor is linearly interpolated based on the ratio of the amount of polymer adsorbed to the adsorptive capacity ... 

As discussed earlier, however, polymer adsorption is not a fully irreversible process. Prolonged water injection will reduce the polymer adsorption. Then the rock permeability to the water after polymer flood will not be the same as that to the polymer solution. It will gradually come back to the initial water permeability. In general, Ek < Ek but the process may take many pore volumes of water flush (Gogarty, 1967). 

State (h) represents the case whereby the particles are not completely covered by the polymer chains. In this situation, the simultaneous adsorption of one polymer chain onto more than one particle occurs, leading to bridging flocculation. If the polymer adsorption is weak (low adsorption energy per polymer segment), the flocculation may be weak and reversible, but if the adsorption of the polymer is strong then tough floes will be produced and the flocculation will be irreversible. The latter phenomenon is used for soUd/Uquid separation, for example in water and effluent treatment... 

As a last remark on polymer adsorption, let s consider Fig. 6.3. If a polymer with a hydrodynamic radius Rg is present at low concentration, its configuration at the interface will be relatively flat with trains on the interface. After more polymer is added, the irreversible adsorption on the surface will produce an adsorbed layer thickness on the... 

The whole discussion of polymer adsorption so far makes the fundamental assumption that the layer is at thermodynamic equilibrium. The relaxation times measured experimentally for polymer adsorption are very long and this equilibrium hypothesis is in many cases not satisfied [29]. The most striking example is the study of desorption if an adsorbed polymer layer is placed in contact with pure solvent, even after very long times (days) only a small fraction of the chains desorb (roughly 10%) polymer adsorption is thus mostly irreversible. A kinetic theory of polymer adsorption would thus be necessary. A few attempts have been made in this direction but the existing models remain rather rough [30,31]. 

The presentation in Figure 16.4 answers the question of irreversibility versus reversibility of polymer adsorption. When r > 50, 0 reaches such a low value that, experimentally when using low polymer concentrations, one cannot detect any polymer in solution. This means that on dilution of the system after complete adsorption, desorption is practically impossible, since one has to reach such low polymer concentrations in the bulk... 

However this description is based on composition only, and it assumes that for one given composition the system will always reach a unique equilibrium state. This may not be the case, because polymer adsorption on a particle may be irreversible if sufficiently many monomers are bound. Thus the path followed to prepare samples may be important, and depending on this path many different non equilibrium states could be. This question of approach to equilibrium is discussed next... 

Figure 1 Simplistic depiction showing why polymer adsorption tends to be irreversible segment a trying to escape is restrained by adsorbed segments b, c, d, and e (shown without loops — see Final Comments). 

In the case of polymer adsorption, all three of the above items have potential relevance items (b) and (c) would assume special importance in the case of hydrophobic or hydrophobically modified polymers. In addition, a factor of unique significance in the case of polymers is that even small energies of interfacial attraction, when expressed per monomer group, can become appreciable when summed up for the polymer molecule as a whole smaller forces, such as dipole/dipole, which may be only of the order of 1 kT, become a substantial nkT when expressed for an n-mer. This is one of the reasons that the adsorption of polymer tends to be irreversible, as indicated in Chapter 2, and desorption is kinetically unfavorable. Also, put simplistically, a segment such as a endeavoring to desorb is restrained by segments b, c, d, etc., already adsorbed (Fig. 1). 

Several theoretical models describing surface interactions between irreversibly adsorbed flexible polymers have been developed. The most common approaches are based on scaling arguments (181) or on self-consistent mean field calculations (180). The irreversibility criterion implies that the polymer adsorption/desorption rate has to be slow when compared to the approach rate of the surfaces. Under such circumstances, the total amount of polymers on the surfaces is independent of the surface separation and the system is not in true equilibrium with the bulk solution. However, it is often the case that the speed of approach is sufficiently slow for the irreversibly adsorbed polymers to adopt the most favourable conformation for each surface separation. Hence, there is equilibrium within the layer. This situation is referred to as quasi-equilibrium, or restricted equilibrium. 

---