Charge density, polyelectrolyte adsorption

Rojas OJ, Claesson PM, Muller D, Neuman RD. The effect of salt concentration on adsorption of low charge density polyelectrolytes and interactions between polyelectrolyte coated surfaces. J Colloid Interface Sci 1998 205 77— 88. 

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. 

Polyelectrolytes are required to be adsorbed by fibres and fillers in order to perform their function, and the adsorption process is dependent upon both charge density and molecular weight. In... 

The effect of charge density is, however, the same as for polyelectrolyte adsorption by non-porous substrates, i.e. the lower the charge density the higher the level of adsorption. For example, the... 

This is because their charge densities arise as a result of protonation of the amino nitrogen atom (Figure 6.12) and this increases at low pH. Since high charge densities give rise to lower levels of adsorption these polyelectrolytes are adsorbed less effectively at low pH. 

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. 

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. 

Recently, Bartels and Arends113 studied the adsorption of poly(4-vinylpyridinium fluoride) with different hexadecyl group content on hydroxyapatite. Adsorbance decreased as the hexadecyl content, i.e. the charge density, was increased. Desorption experiments showed that the adsorption of this polyelectrolyte in water is essentially irreversible. However, the polymer partially desorbed when excess calcium ions were added. Bartels and Arends concluded that adsorption of poly(4-vinylpyridinum fluoride) occurs as a result of the uptake of fluoride ions by hydroxyapatite which releases phosphate ions into water. They also suggested that this adsorption phenomenon can be interpreted in terms of an ion-exchange mechanism. 

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-... 

Figure 5.32. Schematic overview of polyelectrolyte adsorption behaviour. The adsorbed amount is plotted as a function of the salt concentration c. for various combinations of the segment charge the surface charge density a°. and the chemical peu-ameters x rid The curves are numbered to facilitate the discussion (see text).

While the covalent bonding of polymers on oxide surfaces requires certain reaction conditions adapted for the particular reaction system, for example highly purified non-aqueous solvents or thermally activated surface groups, surface modification with polyelectrolytes can be carried out in aqueous environment [31-46]. Numerous parameters can be adjusted to control the conformation of the polyelectrolyte molecules, e.g. their charge density and the charge density of the metal oxide surface. In other words, the adsorption process and the surface properties of the final product can be influenced in many different ways. This diversity is a challenge for academic research to develop novel hybrid materials for technical applications. 

Control of the ionic properties of the polyelectrolyte, e.g. charge density and acid-base strength, is possible by simple proton adsorption because the charge carriers, commonly -NH3+X groups, are directly located along the polymer backbone. For numerous inorganic oxides, for example the silica or titania used, the adsorption of the PVAm can be carried out under condi-... 

FIG. 11 Attachment barrier for a polyelectrolyte (charge density zp = —1), approaching an (initially) uncharged surface. The curves are calculated for several degrees of polymer coverage ffq, i.e., for different stages in the adsorption process. (Calculations by Hoogendam [14].)... 

With new data for polyelectrolytes obtained with the techniques described above it should become possible to determine carefully the effects of ionic strength and externally controlled surface potential on the rate of adsorption of poly electrolytes. We hope that the effects of molar mass and charge density of the poly electrolyte, as well as the nature of that charge (annealed or quenched) can be established. This should stimulate further theoretical research aimed at constructing an adequate equivalent of the Von Smolu-chowsky-Fuchs theory for the rate of flocculation. At present, it would seem that an analysis of the adsorption process taking all complications into account necessitates a simulation-oriented approach. 

Figure 3 illustrates the electro-optical effect dependence on the polymer charge density for a suspension of /3-FeOOH particles, stabilized by the adsorption of polyacrylamides with degrees of hydrolysis 3.4, 9.8, and 19.1%, respectively. The calculated electric polarizabilities y at plateau value of the corresponding adsorbed polyacrylamide T are presented in Table 1. They are of an order that is typical for all other systems described in this review—10 2S-10 32 Fm2. Values of the electric polarizability between 10 28 and 10 32 Fm2 have also been obtained for most of the polyelectrolytes investigated in solution [1,49,50]. 

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