Effect of electrolytes on solubilization of PAHs with CTAB

The solubilization activity ofnaphthaleneand anthracene in the presence of different electrolyteshas been studied to check the augmented effectiveness over contamination concerns, commercially as well as environmentally.Hence, to determine the efficacy of electrolyte for extended applicability,solubilization in pure CTAB has beencarried out initiallybefore studying the solubilization behavior of naphthalene and anthracene with different electrolytes.

The influence of three different electrolytes NaCl (1:1), Na₂SO₄ (1:2) and CaCl₂ (2:1) on solubilization of PAHs was studied in the presence of cationic surfactant (CTAB).The cationic surfactant shows enhanced solubilization proficiency for PAHs than other ionic surfactants ^[2]may be due to adsorption at micellar-water periphery in addition to solubilization at inner core of micelle.In the presence of electrolytes, CMC was reached at concentration far below that of the original CMC, depending on the concentration of the

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electrolytes. Since surfactant molecules adsorbed at the water PAH interface, after adding electrolytes lateral interactions between tail group (hydrophobic bonding) increases whereas electrical repulsion between similar charged head groups is weakened due to the presence of counterions.According to the Schulze-Hardy rule, the charge screening efficiency or the ability to reduce the Debye length of a multivalence ion is comparatively greater than that of monovalence ion, hence,multivalence salt requirement is significantly less than monovalence to acquire similar value of surface tension at reduced CMC and enhanced solubilization.

When we add electrolytes in small concentrations to the surfactant solution, the solution becomes more thermodynamically stable and hence further decrease in interfacial tension can be observed as the energy required to increase the surface area is lowered, hence, solubilization of PAHs becomes more significant ^[33].

4.4.3.1 Effect of NaCl (1:1) on solubilization of PAHs

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Fig. 4.4:Solubilization of (a) naphthalene and (b) anthracene using CTAB with different NaCl concentrations.

The effect of constant NaCl concentration has been studied over the PAH solubilization efficiency with varying surfactant concentration. Fig. 4.4 and Table 4.1 illustrates that, with increasing concentration of NaCl, the slope of solubilization curve (MSR) & Log (Km) values increases, which implies that with increasing concentration of NaCl the solubilization capacity of CTAB increases. In the present study, for 50mM NaCl solution, the CMC of CTAB reduces from 0.9mM to 0.5mM and naphthalene solubilization enhances to 32.28% as compared to solubilization in pure CTAB solution. Similar event observed for anthracene as

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the solubilization increases from 0.005421mM/L to 0.010409mM/L at 2 CMC with the addition of 50mM NaCl. When we add NaCl salt in cationic surfactant solution, Cl^- mitigates the head group repulsion as well as the hydrophilic solubilization, thus amplify the lipophilic or hydrocarbon-hydrocarbon interaction instigating enhancement in solubilization ^[31].This can be attributed to the fact that at a very low surfactant concentration with electrolyte, the surfactant cation and counteranion are fully dissociated and reaches to a plateau level upto the CMC and, above CMC, the solubilization of PAHs initiated. CMC was reduced far from the original setting the fact that we can achieve the desired reduced surface tension at a very low surfactant concentration with the addition of electrolyte.

Table 4.1: Values of CMC, MSR and log Km for different systems.

Napthalene Anthracene

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The depression of CMC with increasing electrolyte concentration may be due to the decrease in thickness of ionic atmosphere surrounding the ionic headgroups and the consequent decreased repulsion between them in the micelle.

4.4.3.2 Effect of Na₂SO₄ (1:2) on solubilization of PAHs Bivalent counterion (SO4

2-) requires very less electrolyte concentration to attain the same ionic strength as compared to monovalent (Cl^-) counterion as well as reduces the surfactant concentration. Fig. 4.5 illustrates the effect of constantNa₂SO₄ over naphthalene and anthracene solubilization efficiency with varying surfactant concentration. With the addition of 3.5mM Na₂SO₄to 2mM of CTAB, solubilization of naphthalene reaches to 0.248732mM/L from 0.22023mM/L with pure CTAB which is nearly equal to the solubilization with 10mM NaCl.

Fig. 4.5: Solubilization of (a) naphthalene and (b) anthracene using CTAB with different Na2SO4 concentrations.

The effective counter ion charge density is high for the higher valence ions, henceforth;

electrolytes with higher valency are more effective to screen the charge of the surfactant head groups. Due to the presence of more number of counter ions the micellar size becomes denser with a bigger size; hence micellar core is able to accommodate more hydrophobic compounds.Ionic strength plays a crucial role to enhance the solubility of PAHs; meanwhile, the valency of the counter-ion is another important factor to enhance the solubilization as well as partition coefficient between micellar and aqueous phase. With increasing valency of electrolytes the surfactant consumption can be reduced to achieve the desired solubilization

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with very less concentration of electrolytes. But with high concentration of bivalent electrolyte the solubilization starts to decline as with much high concentration of Na2SO4 entire micelle may covered with only electrolyte and possibly there is no room for PAH to get inside the micellar core for solubilization. Another possible reason for decrease in solubilization could be stated as, with much higher concentration of bivalent electrolyte the anionic PAH maybe repelled strongly by the electrolyte subsequently there is less or no diffusion of PAHs from aqueous phase to micellar phase, resulting in decrease in partition coefficient as number of molecules of PAH is more in aqueous phase than in micellar phase.

From Fig. 4.5: the solubilization of anthracene is also declining after certain increase in Na2SO4concentration.

4.4.3.3 Effect of CaCl2 (2:1) on solubilization of PAH

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

Fig. 4.6: Solubilization of anthracene using CTAB with different CaCl2 concentrations.

Solubilization experiment of anthracene by CTAB along with CaCl2has been carried out (Fig.

4.6)to study the effect of co-ions on PAH solubilization. With increasing concentration of CaCl2 electrolyte, there is no enhancement of solubilization happens as compared to pristine cationic surfactant CTAB, NaCl and Na2SO4, whereas it has been observed thatdue to thepresence of higher valence co-ions in CaCl₂, solubilization diminishes.MSR and Log (K_m) values are less than pure CTAB and gradually reduces with increasing concentration of CaCl_2. It has to notify that, though the CMC usingCaCl₂ is reducing to nearly same value of Na₂SO₄ but solubilization of organic contaminants is not significant comparatively. This can be attributed to the fact that co-ion is less significant to enhance the solubilization as it increases the head group repulsion for cationic surfactant. On the other hand, counterion reduces the electrical double layer thickness around the surfactant heads, which reduces the

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repulsion between two adsorbed surfactant molecules. One possible explanation may be that cationic surfactant favors the electrostatic attraction with counterions to enhance the solubilization.