2.3.2. Binding Isotherm

The binding isotherm of the anionic surfactant to the polycation PDADMAC was determined by potentiometric titration using a surfactant selective electrode model 6.0507.120 from Metrohm (Herisau, Switzerland). The binding degree of surfactant β was estimated from the potentiometric measurements, as was proposed by Mezei and Meszaros [22]

$$\beta = \frac{c\_s^{frcc}}{c\_{momancr}} \tag{1}$$

where *c f ree <sup>s</sup>* and *cmonomer* are the concentrations of free surfactant in solution and charged monomers of the polyelectrolyte chains, respectively. This method of determining the binding isotherm provides information about the amount of free surfactant remaining in the solution.

#### 2.3.3. Surface Tension Measurements

Surface tension measurements as functions of the surfactant concentration (SLMT or SLES) for pure surfactant and polyelectrolyte-surfactant solutions were performed using different tensiometers. In all the cases, the adsorption was measured until the steady state conditions were reached. Special care was taken to limit the evaporation effects. Each value was obtained as an average of three independent measurements. All experiments were performed at 25.0 ± 0.1 ◦C. From the results of the experiments, it is possible to define the surface pressure as Π(*c*s)= γ<sup>0</sup> – γ(*c*s), where γ<sup>0</sup> is the surface

tension of the bare water–vapor interface and γ(*c*s) is the surface tension of the solution–vapor interface. Further details on surface tension experiments can be obtained from a previous study [23].

