2.4. Effect of polymer concentration

Associating polymers contain hydrophobic pendant groups, which are important contact points between the hydrophobic tails of the surfactant and the cavity of the β-CD leading to the self-association and formation of supramolecular three-dimensional (3D) networks. Therefore, the effect of polymer concentration on the properties of the SAP-AP systems at a fixed concentration of surfactant and β-CD (i.e. optimum concentration: 70 ppm surfactant; 70 ppm β-CD) was established. The concentrations of polymer evaluated were 0.25, 0.5, and 0.75 wt%.

These results guided the selection of the optimum formulation of the SAP-AP system as follows, 0.75 wt% associating polymer (AP), 0.007 wt% (70 ppm) of surfactant, and 0.007 wt % (70 ppm) of β-CD prepared in saline solution. Figure 6(a–c) presents the frequencydependent rheological performance of the optimum SAP-AP systems for polymers AP1,

Figure 6. Frequency-dependent rheological performance of the optimum SAP-AP systems for polymers AP1, AP2, and

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The frequency-dependent rheological data (Figure 6) indicates that these systems follow the typical behavior of three-dimensional network structures showing G<sup>0</sup> > G<sup>00</sup> in the range of frequency studied [22]. As explained by Mezger, "the curves of G<sup>0</sup> and G<sup>00</sup> often occur in the form of almost parallel straight lines throughout the entire frequency range showing a slight slope only"…"The shape of the curves [also indicates that these] … network structures are exhibiting a relatively constant structural strength in the whole frequency

Salts significantly affect the viscosity of polymer solutions. The screening of the negatively charged moieties (i.e. carboxyl groups) in the polymer structure in the presence of mono- and divalent cations causes viscosity loss due to polymer coiling, polymer precipitation, and phase

Figure 7(a–c) displays the results of the frequency sweeps of polymers AP1 and AP2 and their corresponding SAP-AP systems at the following brine concentrations: 1.4, 2.1, 4.2, 6.3, and 8.4 wt%. While for polymer AP3 and its corresponding SAP-AP3 system, the effect of ionic strength was evaluated at the following brine concentrations: 2.1, 4.2, 6.3, and 8.4 wt%.

In the case of polymer AP1, Figure 7(a) shows that it is strongly affected by salinity and hardness. As brine concentration increases, the tanδ-curve shifts from lower values toward medium range values, which means that the polymer flow behavior changes to a more

indicates that an increase in ionic strength weakens the hydrophobic interactions in the associating polymer resulting from the electrostatic screening of the charged segments [13] causing

, and |η\*| decrease as salinity increases. This rheological behavior

3. Effect of ionic strength on the SAP-AP systems

AP2, and AP3, respectively.

AP3 in saline solution.

separation [2, 3, 5, 6, 23–25].

viscoelastic liquid. G00, G<sup>0</sup>

the coiling/folding of the polymer backbone.

range" [22].

Figure 5(a–c) displays the results of the oscillatory tests for the optimum SAP-AP systems at different concentrations of the respective polymers AP1, AP2, and AP3. As polymer concentration increases, G<sup>0</sup> and G<sup>00</sup> increase. A higher concentration of associating polymers increases the number of hydrophobic contact points, which promotes more intermolecular hydrophobic interactions, host-guest complexations, and other noncovalent associations (i.e. H- and Ca2+ bridging). Additionally, higher polymer concentration enhances chain overlapping, which also contributes to the formation of a network of higher structural strength [13].

Figure 5. Oscillatory tests for the optimum SAP-AP systems at different concentrations of polymers: AP1, AP2, and AP3.

Figure 6. Frequency-dependent rheological performance of the optimum SAP-AP systems for polymers AP1, AP2, and AP3 in saline solution.

These results guided the selection of the optimum formulation of the SAP-AP system as follows, 0.75 wt% associating polymer (AP), 0.007 wt% (70 ppm) of surfactant, and 0.007 wt % (70 ppm) of β-CD prepared in saline solution. Figure 6(a–c) presents the frequencydependent rheological performance of the optimum SAP-AP systems for polymers AP1, AP2, and AP3, respectively.

The frequency-dependent rheological data (Figure 6) indicates that these systems follow the typical behavior of three-dimensional network structures showing G<sup>0</sup> > G<sup>00</sup> in the range of frequency studied [22]. As explained by Mezger, "the curves of G<sup>0</sup> and G<sup>00</sup> often occur in the form of almost parallel straight lines throughout the entire frequency range showing a slight slope only"…"The shape of the curves [also indicates that these] … network structures are exhibiting a relatively constant structural strength in the whole frequency range" [22].
