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

because the molecular locations containing the β-CD complexations become more hydrophilic in

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%. 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

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

contributes to the formation of a network of higher structural strength [13].

nature [7].

208 Cyclodextrin - A Versatile Ingredient

2.4. Effect of polymer concentration

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 separation [2, 3, 5, 6, 23–25].

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 viscoelastic liquid. G00, G<sup>0</sup> , and |η\*| decrease as salinity increases. This rheological behavior 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 the coiling/folding of the polymer backbone.

throughout the entire range of angular frequency as salinity and hardness concentrations increase. These results suggest that a higher content of hydrophobic groups in the AP2 polymer makes it less susceptible to electrostatic effects. Figure 7(b) makes evident that the SAP-AP2 system displays a better rheological performance than the baseline AP2. Again, an increase in the ionic strength causes a decrease in the tanδ-values. The SAP-AP2 prepared in

hardness concentration. The SAP-AP2 formulated in 6.3 and 8.4 wt% brine displays the highest values of complex viscosity, |η\*|. These observations demonstrate that an increase in the ionic strength reinforces the inter- and intramolecular forces building up the supramolec-

Figure 7(c) presents the effect of salinity and hardness concentration on the rheological behavior of polymer AP3 and the SAP-AP3 system. Figure 7(c) demonstrates a negligible effect of brine salinity in the range from 2.1 to 6.3 wt% on the rheological properties of polymer AP3. In contrast, above this salinity range (i.e. brine 8.4 wt%), the rheological behavior of AP3 is significantly affected. For instance, the tanδ-curve shifts toward very low values and G00/G<sup>0</sup> < 1 in the entire range of angular frequency, which suggests the reinforcement of the intermolecular hydrophobic interactions generating a stronger physical network structure. These observations seem to indicate that the lower anionicity and

The viscoelasticity of the SAP-AP3 system is greater than the viscoelasticity of polymer AP3 at all salinity concentrations (see Figure 7(c)). At the lowest salinity concentration of 2.1 wt%, the SAP-AP3 system shows far superior rheological properties (i.e. enhanced

are significantly higher compared to the baseline. It seems that this salinity concentration (i.e. 2.1 wt%) greatly reinforces the strength of the inter- and intramolecular interactions

of a highly stable structural network [22]. However, increasing the ionic strength beyond

Overall, the SAP-AP systems are less sensitive to electrostatic effects compared to the AP polymers. The enhanced salt tolerance may result from the bulky size of the β-CD hostguest complexations that "sterically [hinders] the polymer chain so that the hydrodynamic

These experimental observations also indicate that the hydrophobic content in the associating polymers plays a vital role in the strength, rheological behavior, and ionic strength sensitivity of the SAP-AP systems. For instance, the SAP-AP system derived from the baseline polymer AP2, which has a medium content of hydrophobic groups, shows the formation of a stable supramolecular network highly functional in brines with high salinity and hardness concentration (i.e. brine 8.4 wt%). For this SAP-AP2 system, an increase in the ionic strength increases its elasticity and viscosifying power. This functionality is important for applications in

radius does not fully collapse to a random coil configuration at high salinity" [6].

, G00, and |η\*| also increase with salinity and

Advanced Polymer-Surfactant Systems via Self-Assembling

http://dx.doi.org/10.5772/intechopen.74618

211


, G00, and |η\*|

8.4 wt% brine shows the lowest tanδ-value. G<sup>0</sup>

taking place in this system. The G<sup>0</sup>

enhanced oil recovery.

ular SAP-AP2 system, which improves its viscosifying power [13].

larger hydrophobic content of the AP3 polymer improve its salt tolerance.

viscosifying power and elasticity) in the entire angular frequency range. G<sup>0</sup>

2.1 wt% negatively affects the viscoelasticity of this system.

Figure 7. Frequency sweeps of polymers AP1, AP2, and AP3 and the corresponding SAP-AP systems at several brine concentrations at T = 25�C.

On the contrary, for the SAP-AP1 system, Figure 7(a) demonstrates that as salinity and hardness concentration increase, G<sup>0</sup> > G<sup>00</sup> (tanδ < 1) throughout the entire frequency range; thus, in these systems, the elastic behavior dominates, which is characteristic of supramolecular aggregates [22]. Although as salinity concentration increases, G00, G<sup>0</sup> , and |η\*| decreases, the SAP-AP1 system displays higher structural strength relative to the AP1 polymer. Self-association seems to prevent electrostatic screening of the charged sections of the polymer molecule. Therefore, the SAP-AP1 system is less affected by electrostatic effects, which enhances its structural strength and functionality. Furthermore, the bulky size and shape of the β-CD molecules increase the steric hindrance that might diminish the electrostatic effects in these network structures [3, 13].

Figure 7(a) also shows that the SAP-AP1 prepared in the highest brine concentration of 8.4 wt% seems to collapse showing the same rheological behavior of the SAP-AP1 prepared in 6.3 wt% brine. These observations indicate that for the experimental conditions of this work, a brine concentration of 6.3 wt% seems to be the threshold before electrostatic effects become significant for this system.

The effect of salinity on the rheological behavior of polymer AP2 presented in Figure 7(b) is noticeably different than that for polymer AP1. In this case, it seems that increasing brine salinity and hardness strengthens the intermolecular hydrophobic interactions. The tanδvalues are shifted from tanδ > 1 (1.4 wt% brine) to tanδ < 1 (2.1 – 8.4 wt% brine) and the polymer attained a flow behavior of a physical network. G<sup>0</sup> -, G00-, and |η\*|-values increase throughout the entire range of angular frequency as salinity and hardness concentrations increase. These results suggest that a higher content of hydrophobic groups in the AP2 polymer makes it less susceptible to electrostatic effects. Figure 7(b) makes evident that the SAP-AP2 system displays a better rheological performance than the baseline AP2. Again, an increase in the ionic strength causes a decrease in the tanδ-values. The SAP-AP2 prepared in 8.4 wt% brine shows the lowest tanδ-value. G<sup>0</sup> , G00, and |η\*| also increase with salinity and hardness concentration. The SAP-AP2 formulated in 6.3 and 8.4 wt% brine displays the highest values of complex viscosity, |η\*|. These observations demonstrate that an increase in the ionic strength reinforces the inter- and intramolecular forces building up the supramolecular SAP-AP2 system, which improves its viscosifying power [13].

Figure 7(c) presents the effect of salinity and hardness concentration on the rheological behavior of polymer AP3 and the SAP-AP3 system. Figure 7(c) demonstrates a negligible effect of brine salinity in the range from 2.1 to 6.3 wt% on the rheological properties of polymer AP3. In contrast, above this salinity range (i.e. brine 8.4 wt%), the rheological behavior of AP3 is significantly affected. For instance, the tanδ-curve shifts toward very low values and G00/G<sup>0</sup> < 1 in the entire range of angular frequency, which suggests the reinforcement of the intermolecular hydrophobic interactions generating a stronger physical network structure. These observations seem to indicate that the lower anionicity and larger hydrophobic content of the AP3 polymer improve its salt tolerance.

The viscoelasticity of the SAP-AP3 system is greater than the viscoelasticity of polymer AP3 at all salinity concentrations (see Figure 7(c)). At the lowest salinity concentration of 2.1 wt%, the SAP-AP3 system shows far superior rheological properties (i.e. enhanced viscosifying power and elasticity) in the entire angular frequency range. G<sup>0</sup> , G00, and |η\*| are significantly higher compared to the baseline. It seems that this salinity concentration (i.e. 2.1 wt%) greatly reinforces the strength of the inter- and intramolecular interactions taking place in this system. The G<sup>0</sup> -curve is almost parallel to the x-axis, which is typical of a highly stable structural network [22]. However, increasing the ionic strength beyond 2.1 wt% negatively affects the viscoelasticity of this system.

On the contrary, for the SAP-AP1 system, Figure 7(a) demonstrates that as salinity and hardness concentration increase, G<sup>0</sup> > G<sup>00</sup> (tanδ < 1) throughout the entire frequency range; thus, in these systems, the elastic behavior dominates, which is characteristic of supramolecular aggre-

Figure 7. Frequency sweeps of polymers AP1, AP2, and AP3 and the corresponding SAP-AP systems at several brine

AP1 system displays higher structural strength relative to the AP1 polymer. Self-association seems to prevent electrostatic screening of the charged sections of the polymer molecule. Therefore, the SAP-AP1 system is less affected by electrostatic effects, which enhances its structural strength and functionality. Furthermore, the bulky size and shape of the β-CD molecules increase the steric hindrance that might diminish the electrostatic effects in these

Figure 7(a) also shows that the SAP-AP1 prepared in the highest brine concentration of 8.4 wt% seems to collapse showing the same rheological behavior of the SAP-AP1 prepared in 6.3 wt% brine. These observations indicate that for the experimental conditions of this work, a brine concentration of 6.3 wt% seems to be the threshold before electrostatic effects become significant

The effect of salinity on the rheological behavior of polymer AP2 presented in Figure 7(b) is noticeably different than that for polymer AP1. In this case, it seems that increasing brine salinity and hardness strengthens the intermolecular hydrophobic interactions. The tanδvalues are shifted from tanδ > 1 (1.4 wt% brine) to tanδ < 1 (2.1 – 8.4 wt% brine) and the

, and |η\*| decreases, the SAP-


gates [22]. Although as salinity concentration increases, G00, G<sup>0</sup>

polymer attained a flow behavior of a physical network. G<sup>0</sup>

network structures [3, 13].

concentrations at T = 25�C.

210 Cyclodextrin - A Versatile Ingredient

for this system.

Overall, the SAP-AP systems are less sensitive to electrostatic effects compared to the AP polymers. The enhanced salt tolerance may result from the bulky size of the β-CD hostguest complexations that "sterically [hinders] the polymer chain so that the hydrodynamic radius does not fully collapse to a random coil configuration at high salinity" [6].

These experimental observations also indicate that the hydrophobic content in the associating polymers plays a vital role in the strength, rheological behavior, and ionic strength sensitivity of the SAP-AP systems. For instance, the SAP-AP system derived from the baseline polymer AP2, which has a medium content of hydrophobic groups, shows the formation of a stable supramolecular network highly functional in brines with high salinity and hardness concentration (i.e. brine 8.4 wt%). For this SAP-AP2 system, an increase in the ionic strength increases its elasticity and viscosifying power. This functionality is important for applications in enhanced oil recovery.
