Synergistic performance of a Gemini nano ionic liquid and sodium dodecyl sulfate surfactants at the crude oil–water interface

3.1.1 Individual surfactants

The variation of the system IFT against the corresponding concentration are presented in Fig. 2. Evidently, either of the surfactants, brings about drastic decrease in the IFT up to CMC, from an initial value of 31.8 to 8.79 and 1.55 mNˑm⁻¹ respectively. Under this condition, the intermolecular forces at the interface will diminish, leading to a large IFT reduction. Upon reaching CMC for GSAIL and SDS (0.68 and 0.06 molˑdm⁻³), the adsorbed particles completely saturate the interface with no free sites; thus, forcing the particles to self-assemble spontaneously. The corresponding obtained parameters are listed in Table 3. Comparison between the GSAIL and SDS in IFT reduction as well as the micelle formation, reveals that the longer alkyl chain SDS with 12 carbon chain outperforms the short chain GSAIL with 4 carbon atoms.

Close

Fig. 2

The crude oil–water IFT variations with individual GSAIL and SDS concentration.

Export to PPT

3.1.2 Theoretical consideration of individual surfactants

The Frumkin adsorption isotherm, which considers the non-ideal interactions (attraction or repulsion) among adsorbed particles at the interface, acceptably covers the IFT data with individual surfactants (Birdi, 2009). Considering the two positively charged imidazolium rings in the [C₄im-C₆-C₄im][Br]₂ structure as well as anionic head group in SDS, giving strong interactions, the use of this isotherm is consistent. The equation of state and the isotherm are described as (Stubenrauch et al., 2005):

Here, $\mathrm{\Pi }={\gamma }_{°}-\gamma$ is known as interfacial pressure which represents difference between the pure system IFT, ${\gamma }_{°}$ , and the achieved value, $\gamma$ . Besides, $\theta =\mathrm{\Gamma }/{\mathrm{\Gamma }}_{\text{m,F}}$ shows the interface coverage. Other parameters are the maximum interface excess concentration, ${\mathrm{\Gamma }}_{\text{m,F}}$ , the Frumkin adsorption constant, $b_{\text{F}}$ , the van der Waals molecular interaction parameter, $\beta$ , the activity coefficient of ions, f_±, and the number of cations and anions of the ionic surface-active substance, n. The correctness of fittings was established upon achieving a low value of the objective function (OF), which is described in details in our previous studies (Kharazi et al., 2019, Saien et al., 2019). The achieved fitting parameters and the objective function, OF, are listed in Table 4.

As presented in Figs. S5 and S6, the adsorption isotherm of Frumkin fits the data very well. The low OF values confirm good fittings. The ${\mathrm{\Gamma }}_{m,F}$ of the SDS with smaller volume and longer chain of 12 carbon atoms is higher than bulkier GSAIL with shorter chains of 4 carbon atoms, which is consistent with the above results relevant to greater hydrophobicity and more adsorption at the interface for longer hydrocarbons. Further, the negative value of the molecular interaction parameter,β, confirm the existence of electrostatic repulsion among GSAIL and SDS molecules. However, this parameter is higher for GSAIL because it has two positively charged rings per molecule, creating more electrostatic repulsion. In the same manner, the Frumkin adsorption constant, b_F, is obviously higher for SDS, which is due to greater hydrophobicity leading to higher adsorption tendency. Relevantly, the minimum occupied interface area by each adsorbed molecule, A_m, can be obtained from $A_m=1/{\mathrm{\Gamma }}_{m,F}{N}_{\mathit{Av}}$ , where $N_{\mathit{Av}}$ displays the Avogadro's number. As is expected, the compact orientation of SDS at the interface decreases the occupied area by each molecule, A_m, compared to GSAIL.

Meanwhile, the thermodynamic parameters of $\mathrm{\Delta }{G}_{\text{ads}}^{°}$ and $\mathrm{\Delta }{G}_{\text{mic}}^{°}$ , related to adsorption and micellization Gibbs free energies, reflecting the adsorption and aggregation tendencies, can be determined as (Liu et al., 2012, Möbius et al., 2001):

while ${\rho }^{\prime }=\rho /18$ is the molar concentration of water. As listed in Table 5, negative values of Gibbs free energies approve the GSAIL and SDS adsorption at crude oil–water interface and that the micellization was spontaneous. However, superior hydrophobicity with less electrostatic repulsion provides greater forces for SDS. As another point, for the both used surfactants, the absolute values of $\mathrm{\Delta }{G}_{\text{ads}}^{°}$ are meaningfully greater than $\mathrm{\Delta }{G}_{\text{mic}}^{°}$ , implying that the surfactants prefer to adsorb instead of stay in the bulk to create aggregates.

3.1.3 Mixture of surfactants

The variation of the system IFT against the concentration of the surfactants mixture, at different GSAIL mole fractions ( ${\alpha }_1$ ), are presented in Fig. 3. It can be seen that IFT decreases with concentration for all the mole fractions. Reasonably, easier interfacial adsorption at low concentrations gives a steeper slope in IFT variation (Kharazi and Saien, 2022b).

Close

Fig. 3

The IFT variations with the concentration of the mixture of surfactants.

Export to PPT

In another view, IFT changes versus ${\alpha }_1$ , at different concentrations, are illustrated in Fig. 4. It can be seen that IFT significantly decreases with ${\alpha }_1$ and then increases toward the IFT of just the GSAIL. The lowest IFT with the highest synergy is corresponding to the GSAIL mole fraction of about 0.4. If there was no synergy, the performance of the mixture was expected to lie along the straight lines between the IFTs of the individual components. To better clarify, the color synergistic regions are distinguished in Fig. 4.

Close

Fig. 4

Synergism regions for different mole fraction of the GSAIL and concentration of the mixture.

Export to PPT

To justify the synergy, the Rosen concept of dilution effect at the interface adsorption layer was employed (Rosen and Kunjappu, 2012). The level of synergism can be calculated as the percentage of difference between the achieved IFT and the linear contribution of the individual surfactant IFTs to the achieved IFT. Accordingly, a maximum 97.1 % synergism was relevant with 0.25 mol·dm⁻³ mixture of surfactants at ${\alpha }_1=0.4$ , providing a low IFT value of 0.18 mNˑm⁻¹. A low IFT is essential to have stable emulsions, required for efficient EOR processes, as it raises the capillary number in the reservoirs (Pillai et al., 2018, Zhang et al., 2009). From an economic point of view, use of low doses of surfactants is important to attain low IFT values. Another remarkable economy advantage is that experiments showed several times reusing of solutions with no sensible change in results.

In the same manner, the CMC reached to the very low value of 0.03 molˑdm⁻³ (89.6 % decrease compared to the linear contribution of individual surfactants) at ${\alpha }_1=0.4$ . It is worth noting that the decrease in CMC is desired for EOR since it favors the transport of oil droplets with micelles in the surfactant flooding process (Saien et al., 2022).

With respect to the chemical structure of surfactants (Fig. 1), due to the attraction between two positively charged imidazolium rings of the GSAIL and the negatively charged hydrophilic SDS head group, a close orientation at the interface would be established bringing about high IFT reductions. A similar mechanism can be interpreted for the micelle formation. Fig. 5 schematically presents the arrangement of the surfactant molecules in the bulk and at the interface. Considering two positively charged rings in the GSAIL and one negative charge in SDS molecules, the highest dilution effect as well as the maximum degree of synergy was found at the ratio of 4:6 for GSAIL:SDS i.e. ${\alpha }_1=0.4$ . At higher mole fractions, the percentage of synergy decreases due to disrupting the balance between electrostatic interactions. Based on the results, it can be concluded that the adsorbed layers are different for different bulk mole fractions (Saien and Asadabadi, 2011).

Close

Fig. 5

Schematic arrangement of the GSAIL and SDS molecules in the crude oil–water system.

Export to PPT

For a better comparison, variation of the maximum percentage of synergism (at ${\alpha }_1=0.4$ ), with the concentration of the mixtures is shown in Fig. 6. As can be seen, maximum percentage of synergism rises incredibly at low concentrations, tends to low variations after 0.01 molˑdm⁻³ and remains almost constant, around 97 %, for concentrations more than 0.10 molˑdm⁻³. At low concentrations, due to provided space, positive and negative charged molecules are better placed next to each other, significantly neutralize the electrostatic repulsion and thus, synergism rises. However, at high concentrations, the closer orientation of the adsorbed molecules gives greater electrostatic repulsion.

Close

Fig. 6

Maximum percentage of synergism, at ${\alpha }_1=0.4$ , versus concentration of the mixture of surfactants.

Export to PPT

3.1.4 Theoretical investigation for the mixture of surfactants

The Rosen non-ideal interactions in binary mixtures (NIBM) theory (Rosen and Kunjappu, 2012), was used to determine the mole fraction of adsorbed compounds ( $X_1$ for the GSAIL as surfactant 1) and the parameter of molecular interaction between adsorbed components (β) using the following equations (Rosen and Kunjappu, 2012):

while ${\alpha }_1$ is the mole fraction of surfactant 1 (GSAIL) in the bulk solution and $C_{\text{1}}^0$ , $C_{\text{2}}^0$ , and $C_{\text{12}}$ are respectively, the bulk concentration of surfactant 1 (GSAIL), surfactant 2 (SDS), and their mixture. The $C_{\text{1}}^0$ , $C_{\text{2}}^0$ and $C_{\text{12}}$ values for a specified IFT, were determined from the graphs of IFT against the concentration of individual and the mixture of surfactants at a certain ${\alpha }_1$ (Fig. S7). The corresponding values are listed in Table S1. After that, the exact values of $X_1$ and β were calculated from above equations by iteration method (Saien and Asadabadi, 2011). Based on NIBM theory, the negative β values roughly indicate attractive interactions and positive values repulsive interactions at the interface. The determined values of the parameters at selected IFTs of 1, 2.5, 5, 10 and 15 mN.m⁻¹ are presented in Table 6. As illustrated in Fig. 7, the maximum $X_1$ is achieved at ${\alpha }_1=0.4$ , corresponding to the highest degree of synergism. These results confirm that the dilution effect causes more interfacial adsorption of the surfactants, which is the main reason of synergy.

Close

Fig. 7

The mole fraction of GSAIL at the interface versus its bulk mole fraction for different IFTs.

Export to PPT

The interfacial molecular interaction parameter, β, variation versus ${\alpha }_1$ is shown in Fig. 8. Negative β values indicate that the surfactants exhibit attractive interaction despite the dominant self-repulsive interaction between just either of surfactants. Therefore, the larger absolute value of β, the stronger synergistic effect would be (Olea and Gamboa, 2003). It can also be seen that the highest absolute interaction between the compounds corresponds to ${\alpha }_1=0.4$ , consistent with the above results.

Close

Fig. 8

The interfacial molecular interaction parameter versus the GSAIL bulk mole fraction for different IFTs.

Export to PPT