Polyelectrolyte adsorption behavior

There are several major parameters that need to be well considered before the polyelectrolyte solutions are employed for the fabrication or modification of NF membranes. These adjustments are especially important when the polyelectrolyte solutions need to be self-adsorbed onto the membrane surfaces. One of the parameters studied by researchers was the solution pH. A detailed study on the role of polyelectrolyte solution pH was carried out using weak polyelectrolytes in the construction of polyelectrolyte multilayers (PEMs) using poly(acrylic acid) and poly(allylamine hydrochloride) (PAH) (Shiratori and Rubner 2000). Researchers postulated that through the polyelectrolyte solution pH adjustment, the thickness of an adsorbed polyanion or polycation layer can be varied from SA to 80A. Besides, researchers also found that a significant change in the polyelectrolyte adsorption behavior, and an alteration in the polyelectrolyte layer thickness can be detected over a very narrow pH range. This research work has provided some insightful ideas in the development of NF membranes with controllable adsorption behaviors and characteristics. 

The adsorption of HPAM on sand (Figure 4) is not detected below a threshold value of Ca2+ due to strong electrostatic repulsion between the polyelectrolyte and the highly charged negative surface. This threshold value, which was also observed in the case of monovalent ions (9), represents the point where the critical adsorption energy is overcome, and once this value is surpassed, adsorption increases sharply. This form of adsorption behavior is in line with predictions of theories on polyelectrolyte adsorption (20). 

Existing theories of the adsorption of polyelectrolyte allow effects of the polymer charge density, the surface charge density, and the ionic strength on the adsorption behavior to be predicted. The predicted adsorption behavior resembles that of nonionic polymers if the ionic strength is high or the polymer charge density is very low. 

The adsorption behavior of AB- or ABA-type block copolymers in which block A is polyelectrolytic and block B hydrophobic is very interesting. As expected, these polymers serve as dispersants, micelle-forming agents and surface-active agents. 

Adsorption of block copolymers onto a surface is another pathway for surface functionalization. Block copolymers in solution of selective solvent afford the possibility to both self-assemble and adsorb onto a surface. The adsorption behavior is governed mostly by the interaction between the polymers and the solvent, but also by the size and the conformation of the polymer chains and by the interfacial contact energy of the polymer chains with the substrate [115-119], Indeed, in a selective solvent, one of the blocks is in a good solvent it swells and does not adsorb to the surface while the other block, which is in a poor solvent, will adsorb strongly to the surface to minimize its contact with the solvent. There have been a considerable number of studies dedicated to the adsorption of block copolymers to flat or curved surfaces, including adsorption of poly(/cr/-butylstyrcnc)-ft/od -sodium poly(styrenesulfonate) onto silica surfaces [120], polystyrene-Woc -poly(acrylic acid) onto weak polyelectrolyte multilayer surfaces [121], polyethylene-Wocfc-poly(ethylene oxide) on alkanethiol-patterned gold surfaces [122], or poly(ethylene oxide)-Woc -poly(lactide) onto colloidal polystyrene particles [123],... 

Synthetic polymers are widely applied to modify the surface properties of materials, and their adsorption mechanism is very different from small ions or molecules discussed in previous sections. Moreover, special methods are applied to study polymer adsorption, thus, polymer adsorption became a separate branch of colloid chemistry. Polymers that carry ionizable groups are referred to as polyelectrolytes. Their adsorption behavior is more sensitive to surface charging than adsorption of neutral polymers. Polyelectrolytes are strong or weak electrolytes, and the dissociation degree of weak polyelectrolytes is a function of the pH. The small counterions form a diffuse layer similar to that formed around a micelle of ionic surfactant. 

The similarity observed between the electro-optical effect behavior of a suspension, stabilized by polyelectrolyte adsorption, and that of the same polyelectrolyte in solution [12-18] reveals an additional possibility for experimental investigation of dilute systems, since when the polyelectrolyte molecules adsorb on colloidal particles, the electro-optical effect becomes strong enough to be observed in very dilute solutions. 

Several examples can be given in order to demonstrate the similarity existing between the electro-optical behavior of a suspension stabilized by polyelectrolyte adsorption and the behavior of polyelectrolyte in solution. 

Petkanchin IB, Buleva M. Influence of polyelectrolyte adsorption on the electro-optical behavior of aerosil particles. Comm Dept Chem BAS 1991 24 570-575. 

According to the principles discussed in Section 15.4.1, the adsorption behavior of flexible polyelectrolytes may be predicted as well. Because of the charge they carry, polyelectrolytes are strongly expanded in aqueous solution in other words, water is an excellent solvent for flexible polyelectrolytes. As a consequence, the formation of... 

The aim of the present study was to immobilize PEG on metal oxide surfaces such as silicon dioxide, titanium dioxide, and niobium pentoxide. The adsorption behavior of polyelectrolytes on such metal oxide surfaces has been characterized, " and polycations, in particular, were found to form stable adsorbed layers on negatively charged oxides such as silicon dioxide and titanium dioxide. 

Fig. 3 Illustration of the adsorption behavior of a polyelectrolyte on planar, cylindrical, and spherical surfaces with the corresponding dependencies of the critical surface charge densities on the inverse Debye screening length k [48]...

Comparing the results obtained by the WKB method with the exact solutions for the planar and spherical surface, we find, within 2% error, quantitative agreement in the planar case. For a sphere, we find the same asymptotic dependence of critical adsorption behavior for a wide range of geometries. The main advantage of the WKB method is a unified approach for the various geometries based on the same level of approximations. It can be applied at the same level of complexity to virtually any shape of the polylectrolyte-surface adsorption potential. Recent advances in polyelectrolyte adsorption under confinement [49,167] and adsorption onto low-dielectric interfaces [50] have been presented. 

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