Polyelectrolyte adsorbed layers

Adsorption of polyelectrolyte on interfaces is concerned with various applications such as flocculation and steric-stabilization of colloidal particles in an aqueous phase, oil recovery, and soil conditioning. In these cases, both the adsorbance of polyelectrolytes and the conformation of the adsorbed polymer, which is connected with the thickness of the adsorbed layer, are very important. 

For homopolyelectrolyte, we first studied the ellipsometric measurement of the adsorption of sodium poly(acrylate) onto a platinum plate as a function of added sodium bromide concentration (5). We measured the effect of electrolyte on the thickness of the adsorbed layer and the adsorbances of the polyelectrolyte. It was assumed that the Donnan equilibrium existed between the adsorbed layer and the bulk phase. The thickness was larger and the adsorbance of the polyelectrolyte was lower for the lower salt concentration. However, the data on the molecular weight dependence of both the adsorbance and the thickness of the adsorbed polyelectrolyte have been lacking compared with the studies of adsorption of nonionic polymers onto metal surfaces (6-9). 

In recent years, the measurement of thick adlayers has received an increasing interest in the field of so-called polyelectrolyte multilayer films. These films sometimes have a thickness of several micrometers. Such large thicknesses are obviously not easily monitored using conventional waveguide sensors because of the limited penetration depth into the adlayer. Simply, waveguide sensors loses their sensitivity when the adsorbed layer thickness exceeds 2 3 times the penetration depth of the evanescent field and cannot be used to monitor films thicker than 350 nm12. 

Polyelectrolytes. The most striking feature of polyelectrolytes is that due to the electrostatic repulsion between the segments, the formation of thick adsorbed layers is prevented. Polyelectrolytes tend to adsorb in rather flat conformations. If adsorbent and polyelectrolyte bear opposite charges, this attraction can be of an electric (coulombic) nature if the charges have the same sign, adsorption takes place only if the non-electrostatic attraction outweighs the electrostatic repulsion (Lyklema, 1985). 

In both cases the top layer of these layered polyelectrolyte films contains many ion sites that can bind redox ions by ion exchange vdth the electrolyte solution. Homo polypeptides such as poly(L-lysine) and poly(L-glutamic add) have been employed to form layered polyelectrolyte films with Fe(CN)6 " electrostatically adsorbed onto ammonium sites in poly(lysine) [45]. Modified electrodes with polyelectrolytes mono-layers have also been deposited using the Langmuir-Blodgett technique [46-48]. 

While the structure of nonredox polymer and polyelectrolytes thin layers has received much attention in the past [116, 117], only recently has a molecular theory able to treat, from a molecular point of view, redox polyelectrolytes adsorbed on electrodes, been presented [118-120]. The formulation of the theory, its scope, advantages and limitations will be discussed in detail in Section 2.5.2, and therefore we will limit ourselves to show here some predictions that are relevant for the understanding of the structure of polyelectrolyte-modified electrodes. The theory was applied to study the particular system depicted in Figure 2.5, which consists of a single layer of PAH-Os adsorbed on a gold surface thiolated with negatively charged mercapto... 

In many practical cases, stabilisation by polymers involves a combination of steric and charge interactions. Unlike simple electrolytes, multiple adsorption effects permit polyelectrolytes to continue to adsorb well beyond the point where the adsorbed layer charge exceeds that of the particle surface. In this way, the effective charge on particles can be increased substantially at relatively low surface coverage by the polymer. 

Dissolved polymer molecules can be adsorbed by polymer particles via electrostatic attractive force or hydrophobic interaction. When polyelectrolyte is adsorbed on an opposite-charge particle, the polymer molecules usually have a loop-and-tail conformation and, as a result, inversion of charge occurs. For example, sulfatecarrying particles behave as cationic ones after they adsorb poly(lysine). Then poly(-styrene sulfonate) can be adsorbed on such cationic particles and reinvert the charge of particles to anionic (14). Okubo et al. pointed out that the alternate adsorption of cationic and anionic polymers formed a piled layer of polyelectrolytes on the particle, but the increment of adsorbed layer thickness was much less than expected. This was attributed to synchronized piling of two oppositely charged polyelectrolytes (15). 

The first ellipsometric measurement of the thickness of the adsorbed layer and the adsorbance of a polyelectrolyte and a negative adsorbance of salt onto a solid surface was reported by Takahashi et al.U4) They measured the adsorption of sodium poly(acrylate) (M = 950 x 103) onto a platinum plate as a function of the concentration of added sodium bromide. In an aqueous polyelectrolyte solution with an added simple salt, the bulk phase is a three-component system which consists of a polyelectrolyte, a simple salt, and water. The adsorbed layer on the solid surface is a three-component phase as well. The adsorbance of polyelectrolytes thus cannot easily be determined from measurements of the refractive index nf of the adsorbed phase. Hence, it was assumed that the adsorbed layer is a homogeneous layer of thickness t and further that nf is represented by the Lorenz-Lorentz equation as follows ... 

Fig. 26. Plots of thickness of the adsorbed layer v. polyelectrolyte concentration at various ionic strengths114). Symbols are the same as in Fig. 24...

Takahashi et al.67) prepared ionene-tetrahydrofuran-ionene (ITI) triblock copolymers and investigated their surface activities. Surface tension-concentration curves for salt-free aqueous solutions of ITI showed that the critical micelle concentration (CMC) decreased with increasing mole fraction of tetrahydrofuran units in the copolymer. This behavior is due to an increase in hydrophobicity. The adsorbance and the thickness of the adsorbed layer for various ITI at the air-water interface were measured by ellipsometry. The adsorbance was also estimated from the Gibbs adsorption equation extended to aqueous polyelectrolyte solutions. The measured and calculated adsorbances were of the same order of magnitude. The thickness of the adsorbed layer was almost equal to the contour length of the ionene blocks. The intramolecular electrostatic repulsion between charged groups in the ionene blocks is probably responsible for the full extension of the... 

The most important factor influencing the degree of steric stabilization is the thickness of the adsorbed layer in comparison with the size of the particles [292], The term protection has also been used because the steric stabilizing effect can cause significant salt tolerance on the part of a colloidal dispersion. Some suspensions have been prepared, using high concentrations of polyelectrolytes, that are quite stable in concentrated salt solutions [49]. 

Shiratori SS, Rubner MF (2000) pH-Dependent thickness behavior of sequentially adsorbed layers of weak polyelectrolytes. Macromolecules 33 4213—4219... 

Electrokinetic Study of Layer-by-Layer Polyelectrolyte and Surfactant Adsorbed Layers... 

Abstract Investigations of alternate adsorption regularities of cationic polyelectrolytes a) copolymer of styrene and dimethylaminopropyl-maleimide (CSDAPM) and b) poly(diallyldimethylammonium chloride) (PDADMAC) and anionic surfactant - sodium dodecyl sulfate (SDS) on fused quartz surface were carried out by capillary electrokinetic method. The adsorption/desorption kinetics, structure and properties of adsorbed layers for both polyelectrolytes and also for the second adsorbed layer were studied in dependence on different conditions molecular weight of polyelectrolyte, surfactant and polyelectrolyte concentration, the solution flow rate through the capillary during the adsorption, adsorbed layer formation... 

Keywords Adsorbed layer deformation Adsorption Aging Polyelectrolytes Streaming potential method... 

In this paper we investigate the process of alternate adsorption of cationic polyelectrolyte and anionic surfactant, structure and properties of adsorbed layers depending on different factors (molecular weight of PE, concentration of polyelectrolyte and surfactant, adsorbed layer formation time, the flow rate of the solution) by measuring potential and streaming current using the capillary electrokinetic method. 

Experimental procedure. The experiment was initiated by pumping of background electrolyte solution KC1 10 4M through the capillary. The initial surface potential was measured. Than a polyelectrolyte solution, as the first adsorbed layer, and anionic surfactant solution, as the second one, were pumped through the same capillary. Between these two stages the capillary was rinsed with KC1... 

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. 

The observed chaige reversal can prove the presence of two types of the PE adsorption sites on the capillary surface. At low concentration, the electrostatic adsorption of positively charged PE molecules predominantly occurs on the negatively charged sites of quartz surface. Thereafter (or simultaneously), on the surface of a capillary covered with a polymer adsorbed layer, the adsorption of the PE molecules can occur due to the forces of molecular attraction and attraction between hydrophobic sites of polyelectrolyte and surface (e.g. siloxane groups). Their competition with the electrostatic repulsion forces that increase in the course of further adsorption of PE molecules determines the completion of the adsorption and the formation of equilibrium (with the solution) adsorbed layer. 

The second adsorbed layer was formed due to the pumping through the capillary with preadsorbed polyelectrolyte layer of anionic surfactant solution of different concentrations below erne. To clear out the influence of the first adsorbed PE layer on the formation of the second anionic surfactant layer, we studied the adsorption of SDS on PE layers of different structures (see Deformation of Adsorbed Layers) when the PE molecules adsorbed in flat conformation (CSDAPM at C = 10—4 g/1) and when the extended layer with loops and tails was formed (CSDAPM at C = KT2 g/1 and PDADMAC at C = KT2, KT3 and 10-4g/l). 

CSDAPM 10—4 g/1 f potential values reach the equilibrium in about 50-60 minutes while at the same SDS concentration on PDADMAC 10-4 g/1 - in 120 minutes. Such difference can be connected with the nature of polyelectrolytes, molecular weight and different deformation of adsorbed layers on which the SDS adsorption takes place. 

According to these data, one may draw a conclusion that at low concentrations PE molecules adsorb in flat conformation and at high concentrations more extended layer with loops and tails is formed. These data about conformation of polyelectrolyte molecules are in a good agreement with other experimental and theoretical works [21-23], Note that the curves are reversible and f potential values establish immediately at each pressure value after pressure rising and decreasing. This is the argument that the deformation of adsorbed layers but not desorption of macromolecules takes place on experimental time scale, since our measurements are carried out in polyelectrolyte-free solution. 

The absence of CSDAPM adsorbed layer aging can be the sequence of this PE nature this polyelectrolyte is weak and has low molecular weight in comparison with PDADMAC. Hence, the reconstruction inside the adsorbed layer may occur during less time, in our case during the adsorption time scale. 

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