Surfactant binding isotherms

The clusters consisting of surfactant molecules only are indicated by // = 0, so that we have 1 = Zi,0 in the distribution function (7.95). There are junctions of multiplicity k = li only in such pure surfactant micelles. We then have jk= for fc = /1, otherwise jk = 0. The distribution function becomes 

3 Isotherm of surfactant molecules bound by bifunctional (/ = 2) polymers. The fraction 0 of the adsorbed surfactant molecules is plotted against their total concentration. The minimum multiplicity of the junctions is changed from curve to curve with fixed /km = 8. The curve starts from a finite value and shows a peak at the SMG concentration. (Reprinted with 

The fraction u x )/u z) out of the total volume fraction 4 of surfactants remains unassociated to the polymers. The rest 

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One of the fundamental and necessary tasks in the study of polyelectrolyte-surfactant interactions is the determination of surfactant binding isotherms. 

Although the electrostatic force plays the main role in the interactions of oppositely charged surfactants and polymers, hydrophobicities of polymer and surfactant also play a role. Zana and Benrraou (2000) studied the interaction of two polyelectrolytes PSl and PS4 (copolymers of disodium maleate and methyl or butyl vinyl ether, respectively), and quaternary ammonium bromide surfactants (3-dodecyldimethyl(alkyl)ammonium bromides and two dimeric surfactants of the polymethylene-a,fi)-bis(dodecyldimethylammonium bromide) type). They used the surfactant-binding isotherms method and spectrofluorometry using pyrene as a fluorescent probe to detect the onset of binding. Their results showed that surfactant binding to the polymer is more pronounced when the polymer is more hydrophilic for a given surfactant. 

FIGURE 13.26 Surfactant binding isotherms for SDS with nonionic polymers at 20°C for (—) CST-103 ... 

When DsPheno1 Dsmlcelle, significant binding of phenol to micelles occurs, )Pheno1 is largely reduced. From the binding isotherm, an estimation of phenol molecules per surfactant molecule can be obtained. 

Fig. 9.3. Cu(ll)-binding isotherms for the resins (a) SDS as a co-surfactant, 0.5 g. (b) SDS, 1.0 g. Open circles, non-imprinted resin solid circles, Cu(Il)-imprinted resin solid triangles, Zn(II)-imprinted resin solid squares, Cd(II)-imprinted resin. Conditions 0.5 g of resin and Ckno.i = 0.1 mM, at 25°C for a potentiometric titration.

The effect of simple salt on the surfactant binding confirms the electrostatic nature of the surfactant ion-polyion interactions (step 1, Scheme 1). Any increase in the ionic strength of solution shifts the onset of binding toward higher free surfactant concentrations (compare isotherms in water, 0.01 M, and 0.1 M NaCl, Figure 6) and decreases the amount of bound surfactant. These observations can be related to the screening influence of the simple salt, which acts to diminish the electrostatic interactions between surfactant cations and polyanions. This is also well documented in the literature for a variety of polyelectrolyte-surfactant pairs [26-29],... 

Hansson P, Almgren M. Interaction of C TAB with sodium(carboxymethyl) cellulose effect of polyion linear charge density on binding isotherms and surfactant aggregation number. J Phys Chem 1996 100 9038-9046. 

Figure 20.2. The binding isotherm of a surfactant to a polymer without distinct hydrophobic moieties, giving the concentration of bound surfactant as a function of the free surfactant concentration, can be interpreted as a lowering of the surfactant CMC by the polymer, or a strongly co-operative binding...

Since the mixed aggregates dominate, we have a low concentration of free surfactant and essentially no free micelles until the concentration of micelles exceeds the concentration of polymer hydrophobic groups. From this, we can understand the binding isotherm of a surfactant to an HM-polymer, as schematized in Figure 20.12. The binding isotherm of an ionic surfactant to a nonionic HM-polymer will be very similar to its binding... 

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