Surface charge polyelectrolyte adsorption

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. 

When a polyelectrolyte adsorbs on an oppositely charged surface the adsorbed amount compensates, or slightly over-compensates, the surface charge [6], Adsorption of cationic polymers continues even after the zeta potential is reversed [7], This is the case for cationic starch. 

Fleer GJ, Cohen Stuart MA, Scheutjens JMHM et al (1993) Electrostatic effects charged surfaces and polyelectrolyte adsorption. In Polymers at interfaces. Chapman Hall, London... 

The process of adsorption of polyelectrolytes on solid surfaces has been intensively studied because of its importance in technology, including steric stabilization of colloid particles [3,4]. This process has attracted increasing attention because of the recently developed, sophisticated use of polyelectrolyte adsorption alternate layer-by-layer adsorption [7] and stabilization of surfactant monolayers at the air-water interface [26], Surface forces measurement has been performed to study the adsorption process of a negatively charged polymer, poly(styrene sulfonate) (PSS), on a cationic monolayer of fluorocarbon ammonium amphiphilic 1 (Fig. 7) [27],... 

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). 

Hesselink attempted to calculate theoretical adsorption isotherms for flexible polyelectrolyte chains using one train and one tail conformation (1) and loop-train conformation (2) as functions of the surface charge, polyion charge density, ionic strength, as well as molecular weight. His theoretical treatment led to extensive conclusions, which can be compared with the relevant experimental data. 

The effect which polyelectrolyte adsorption has upon the surface charge (zeta potential) of fibres and fines is also important—particularly for retention—and both molecular weight and charge density of the adsorbed polyelectrolyte are known to affect the particle surface charge, although not always in an intuitively predictable way. 

The ionic strength of the solution also significantly influences polyelectrolyte adsorption. In general, the higher the ionic strength of the medium, the less extended and the more coiled the polymer conformation becomes (due to preferential interaction with counter ions in solution rather than with other segments of the polymer chain). The coiled polymer becomes more accessible to the internal porous structure and adsorption is increased. However, for the same reason, it is less influential on the surface charge. 

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. 

As a further surface modification, concept PEMs may be deposited using negatively and positively charged PECs. Recently, the consecutive adsorption of PEC+ (n—/n+=0.6) of PDADMAC/PMA-MS with the respective PEC (n—/ n+=1.6) was reported [94]. In contrast to oppositely charged polyelectrolytes, a lateral growth of the PECs was obtained, which was evident from ATR-FTIR spectral data. Obviously, the oppositely charged PEC nanoparti-... 

Borukhov et al. [11] investigated the effect of polyelectrolyte adsorption on the intercolloidal forces. They assume that the adsorption of polyelectrolyte molecules on the surface of the plates was solely due to the electrostatic interactions between the negatively charged plates and the positively charged polyelectrolyte segments. However, there are additional interactions between the surfaces of the plates and the segments, such as the van der Waals interactions. 

The formation of multilayer structure can be carried out by several ways 1) adsorption on liquid/liquid interface - Langmuir-Blodgett films [5,6] 2) adsorption on solid/liquid interface - alternate adsorption of oppositely charged polyelectrolytes (PE) and surfactants on flat surfaces or spherical particles [7,8], To control the process of multilayer systems formation, it is necessary to under-... 

It is important to concentrate special attention on the first step of the process, viz on the study of polyelectrolyte adsorption on charged surface. The time dependence of layer growing and structural reconstruction can give the information of formation mechanism of the multilayer. 

Since the negative charged surface of fused quartz was used as a substrate, the first adsorbed layer was the layer of cationic polyelectrolyte. In our previous works [18-20] the adsorption kinetics of cationic polyelectrolyte CS-DAPM was studied in detail. The estimation of adsorption was carried out by changing of f potential of charged quartz surface during the cationic polyelectrolyte adsorption. 

Schwarz S, Buchhammer H-M, Lunkwitz K, Jacobasch HJ (1998) Polyelectrolyte adsorption on charged surfaces study by electrokinetic measurements. Colloids Surf A Physicochem Eng Aspects... 

In the latter case the total interaction, which is what can be measured, is affected by the net charge of the surface and the adsorbed layer, ion-ion correlations, bridging interactions and steric confinement of the polymer chain [116]. We note that polyelectrolytes are often present as additives in colloidal dispersions and the character of the forces generated by the polyelectrolyte adsorption layers has a paramount influence on stability of these colloidal systems. With the aim to illustrate what can be learnt about polyelectrolyte adsorption layers using the SFA, we will look at the influence of the polyelectrolyte charge density on the forces acting between surfaces coated with polyelectroytes. We will consider an example where the polyelectrolyte charge density is varied by a systematic... 

The problem is to relate v (z) to the surface potential - v (0) or the surface charge density a° = a(O)) and the volume fraction profiles of the components. Early versions t-2) of a polyelectrolyte adsorption model neglected the volume of the small Ions and solved (numerically) the Poisson-Boltzmann equation 13.5.6). A more sophisticated, yet simpler, approach was proposed by Bflhmer et al. who accounted for the Ion volume by adopting a multilayer Stem model, see fig. 5.17. This Is a straightforward extension of the monolayer Stern model discussed in sec. 3.6c. The charges of the ions and the segments are assumed to be located on planes in the centres of the lattice layers. The lattice is thus con-... 

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