The role of adsorption of a natural surfactant at oil–water interface in enhanced oil recovery: interfacial rheology, and structural, and emulsifying analyses

Abstract The enhanced oil recovery (EOR) method is widely used for recovering residual crude oil after implementing conventional oil recovery techniques. Chemical additives, such as surfactants, are considered beneficial for EOR. Their efficacy is assessed based on their ability to reduce interfacial tension, modify wettability, and establish a stable emulsion system. Understanding the mechanism of surfactant adsorption at the oil–water interface is critical for effectively implementing surfactant flooding. In this work, we have studied the adsorption of a natural surfactant synthesized from Eichhornia crassipes at the oil–water interface using numerous interfacial analyses. The results show substantial adsorption at the oil–water interface, indicated by a 27% increase in zeta potential (i.e. −25.2 to −37.2 mV) and ∼27 times increase in film elasticity (i.e. 0.15–4.12 Pa at 0.01 Hz frequency and 0.01% strain). The synthesized natural surfactant demonstrates a noteworthy improvement in the stability of the oil-in-water emulsion and interfacial rheological properties, indicating its potential application in EOR. GRAPHICAL ABSTRACT


Introduction
Renewable sources of energy have received significant attention in recent years, and research around the development of these sources has been boosted considerably. However, fossil fuels (e.g. crude oil) are expected to remain the primary sources of energy for the next several decades. The commonly-practiced methods of crude oil recovery, which can deliver merely 20-40% of the oil to the surface, have become inadequate due to the exponential increase in the requirement for crude oil. The remaining 60-80% of the crude oil in the reservoir cannot be recovered because of numerous factors such as less mobility of the entrapped oil, excessive interfacial tension (IFT) between the oil and water, and electrostatic interaction between the oil and the rock (Lake and Venuto 1992;Wever et al. 2011;Sheng 2015;Machale et al. 2020). The application of enhanced oil recovery (EOR) is a useful tool for the additional recovery of trapped oil (Sheng 2010). It mainly employs thermal, gas, and chemical injection techniques. The chemical EOR method involves the injection of chemical additives (typically surfactant, polymer, and alkali) into the reservoir to improve the sweep and displacement efficiencies of the entrapped crude oil (Olajire 2014;Machale et al. 2020).
A surfactant (sometimes a mixture of surfactants) is essential in EOR because it changes the wettability of the reservoir rock and brings down the IFT between oil and water. Commercial surfactants such as sodium dodecyl sulfate, dodecyltrimethylammonium bromide, cetyltrimethylammonium bromide, alpha-olefin sulfonate, alfoterra TM , triphenyl methane, and triton X-100 are widely used in EOR (Brackman and Engberts 1991;Yuan et al. 1999;Junquera and Aicart 2002;Hai and Han 2006;Mahdavi et al. 2016;Sakthivel et al. 2017;Machale et al. 2022b). However, the majority of these surfactants are costly, environmentally harmful, and can adsorb on the surface of porous rocks. Thus, several studies have been published in recent decades on the development and utilization of plant-based natural surfactants for EOR (Saxena and Mandal 2021). The significance of the natural surfactants is increasing inasmuch as they are inexpensive, biodegradable, harmless, and stable at high pH, salinity, and temperature.
The Ziziphus spina-christi plant has gained the interest of numerous researchers for the synthesis of a novel, ecofriendly, and cost-efficient surfactant for use in EOR Shadizadeh 2012, 2018;Ahmadi et al. 2015;Emadi et al. 2019). Around 15-25% additional oil was recovered as a result of its ability to alter wettability, enhance the relative permeability of crude oil, and reduce the IFT (Ahmadi and Shadizadeh 2012;Emadi et al. 2019). A natural surfactant derived from Anabasis Setifera with good thermal stability, wettability-alteration, and IFT reduction characteristics, has achieved an effective 15.4% additional oil recovery . Dashtaki et al. (2020) derived a natural surfactant from Vitagnus and investigated its possible utilization in EOR. An additional 10% oil was recovered by injecting the synthesized surfactant at a concentration of 3 g dm À3 . This was achieved due to a noteworthy reduction in the IFT (i.e. $65.4%) and alteration in the wettability (i.e. oilwet to water-wet). A natural surfactant derived from Myrtus communis has the ability to achieve an ultra-low IFT, which aids in enhancing the oil recovery by 14.3% (Nowrouzi et al. 2022). Surfactants derived from Tribulus terrestris, Matricaria chamomilla, soapnut, mulberry, Gundelia tournefortii, Glycyrrhiza glabra, quinoa, and Zephyranthes carinata have shown similar abilities (Chhetri et al. 2009;Shadizadeh and Kharrat 2015;Moslemizadeh et al. 2016;Moradi et al. 2019;Bahraminejad et al. 2021;Yusuf et al. 2021;Norouzpour et al. 2022).
The performance of the aforesaid natural surfactants for EOR has been evaluated mainly with the help of numerous analyses such as the measurement of IFT, wettability, adsorption, rheological characteristics, phase behavior, and core flooding (Anghel 1990;Sheng 2010;Elakneswaran et al. 2021). However, the surfactant injected in the oil reservoir interacts with the crude oil and adsorbs on its surface, forming a complex interfacial film. Therefore, explicit knowledge of the film characteristics is essential for interpreting the displacement of entrapped oil from the reservoir (Sun et al. 2011). This depends on the interfacial properties of the rock-oil-water system, controlled by the composition of the aqueous phase, rock wettability, and physicochemical properties of the crude oil (Borwankar and Wasan 1986;Dong et al. 2009;Alvarado and Manrique 2010). Generally, the surfactant adsorbs spontaneously from the bulk phase at the oilwater interface, where the resultant free energy is lesser than that in the solution. The adsorbed surfactant not only changes the IFT but also develops an interfacial film with viscoelastic properties (Kr€ agel and Derkatch 2010). Interfacial rheology is a potential tool employed to examine the interfacial film structure and its stability (Roth et al. 2000;Lyu et al. 2018). It illustrates the functional association between the stress applied at the interface and its deformation. Cairns et al. (1974) examined the rheological properties of the film at the crude oil-water interface and observed that the interfacial shear viscosity increased with the aging of the film. Zhou et al. (2019a) studied the rheology of the film formed at the oil-water interface where silica nanoparticles were adsorbed, and they correlated the rheological properties with the emulsion stability. Anseth et al. (2003) studied the rheology of a film formed by a polymeric surfactant at the silicon oil-water interface. Interestingly, the chemical additives accumulated at the interface in such a way that the solid particles in the Pickering emulsion developed a solid-like network. Eventually, it enhanced the stability of the emulsion by resisting coalescence. Rane et al. (2013) examined the rheological properties of the asphaltenes adsorbed at the oil-water interface. They explained the adsorption by using the Langmuir equation of state.
The analysis of phase behavior and the electrical properties of the interface are some of the most important aspects of understanding the formation of the film at the oil-water interface by the self-assembly of the surface-active agents (Pal et al. 2017;. However, it has been observed that a significant portion of the development and stabilization of the interfacial film is not comprehended through these analyses. Therefore, along with the aforesaid analyses, small-angle X-ray scattering (SAXS) can provide more insight into the characteristics of this film (Lutz et al. 2007).
In prior studies, we delved into the development of an economical and environmentally sustainable natural surfactant derived from Eichhornia Crassipes for utilization in EOR. The synthesized surfactant was abundant in key surface-active components, including fatty acids (e.g. palmitic, oleic, and petroselenic acids), aromatic compounds, and esters (Machale et al. 2019). To assess its effectiveness, we conducted a thorough evaluation using numerous studies such as adsorption, interfacial properties under high temperature and pressure, rheology, and core flooding experiments. The results of these studies are summarized in Table 1. Our earlier works have revealed the formation of a complex interfacial Machale et al. 2022a 333 K K L ¼ 2:2 Â 10 À4 dm 3 mg À1 and q 0 ¼ 42.5 mg g À1 Berea sandstone-WH in brine (WH concentration: 1000-5000 mg dm À3 ) 298 K K L ¼ 1:3 Â 10 À4 dm 3 mg À1 and q 0 ¼ 95.7 mg g À1 Berea sandstone-WH solution (WH concentration: 1000-5000 mg dm À3 ) Where K L is the Langmuir isotherm constant (dm 3 mg À1 ), q 0 is the maximum amount of adsorbate that can be adsorbed (mg g À1 ), DG is the standard Gibbs free energy change (kJ mol À1 ), DH is the standard enthalpy change (kJ mol À1 ), and DS is the standard entropy change (kJ mol À1 K À1 ).
film as a result of the physical and chemical interactions between the synthesized surfactant and crude oil (Machale et al. 2021). Therefore, a thorough understanding of the characteristics of this film is crucial for determining the efficiency of the displacement of the trapped oil from the reservoir. The present study focuses on the impact of this natural surfactant on the properties of the oil-water interface. The adsorption of surfactant molecules at the oil-water interface was studied using interfacial tension, phase behavior, zeta potential, and SAXS analyses. Furthermore, the viscoelastic properties of the interface under shear deformation were studied extensively through interfacial shear rheology. Section 'Materials and methods' describe the materials used and systematic experimental procedures followed in the study. The results of the experiments are then analyzed and discussed in section 'Results and discussion'. Finally, section 'Summary and conclusion' presents a comprehensive summary of the study.

Adsorption of surfactant at oil-water interface
The adsorption of surfactant molecules at the oil-water interface was studied by the interfacial tension measurements. These measurements were carried out by the du No€ uy ring method using a tensiometer [model: DY300, manufacturer: Kwoya (Japan)]. The surface excess concentration ðCÞ was determined using the Gibbs adsorption equation.
where C is the surface excess concentration (mol m À2 ), R is the universal gas constant (J mol À1 K À1 ), c is the interfacial tension (N m À1 ), T is the temperature (K), and c is the surfactant concentration (wt. %).

Zeta potential at oil-water interface
The electrophoretic mobility of the paraffin oil droplets dispersed in water was measured to determine the zeta potential of the paraffin oilsurfactant solution interface. The zeta potentiometer [model: Delsa Nano C, manufacturer: Beckman Coulter (Switzerland)] was utilized to evaluate the zeta potential. The emulsion droplets were inserted into the flow cell by using the syringe. The zeta potential of the paraffin oil-surfactant solution interface was measured with the help of the Smoluchowski equation (Stachurski and MichaLek 1996) where f is the zeta potential (V), l is the viscosity (Pa s), e is the dielectric constant of the aqueous phase, e 0 is the permittivity of free space (C 2 J À1 m À1 ), and U is the electrophoretic mobility (m 2 V À1 s À1 ).

Emulsion stability
The oil-in-water emulsion was prepared by mixing the surfactant solution (i.e. 0.1-1 wt. %) in paraffin oil (at 5:1 v/v) using a homogenizer [model: T10 Basic Ultra-Turrax, manufacturer: IKA (Germany)] at 15000 rpm. The emulsion samples were transferred into flat bottom tubes and observed for 20 d. The emulsion systems thus prepared were analyzed by using a microscope [model: Axiostar plus, manufacturer: Carl Zeiss (Germany)]. The microscopic images were used to calculate the size of the oil droplets in the emulsion using the ImageJ V R software (bundled with Java 1.8.0_172).

Interfacial shear rheology
The shear rheology of the water-paraffin oil interface (at which WH was adsorbed) was investigated by using an interfacial rheometer [model: MCR 301, manufacturer: Anton Paar (Austria)].
The biconical-bob sensor (cone angle: 5 , cone diameter: 68 mm) and a measuring cell (height: 90 mm, diameter: 80 mm) were used. Initially, the aqueous WH solution was poured into the measuring cell's specified limit (i.e. 110 dm 3 ). Further, the contact of the bob with the air-water interface was confirmed by determining the normal force (see Figure S1, Supporting Information). The measuring point was calculated with the help of the in-built software (i.e. RheoPlus, version 3.62). Once the sensor reached the air-water interface, the paraffin oil was gradually transferred over the aqueous phase till the measurement mark. The steady shear test was performed to measure the interfacial shear viscosity over the shear rate range of 1À100 s À1 . On the other hand, the dynamic oscillatory test was performed to study the viscoelastic properties of the interfacial film of WH over 0.001-0.1 Hz frequency range at the constant strain of 0.01%. The schematic representation of the scope of the present work is shown in Figure 1.

Small-angle X-ray scattering
The small-angle X-ray scattering (SAXS) experiments were carried out by using a SAXS instrument [model: Nanostar, manufacturer: Bruker (USA)]. An Excillum Gallium Metaljet was used as a radiation source (wavelength 1.340 Å), which was focused with crossed G€ obel mirrors and collimated with two 150 mm pinholes. The emulsion sample was held in a 2-mm quartzglass capillary and inserted into the sample holder. The structural analysis of the emulsion was carried out by fitting the scattering data by a unified Guinier-exponential/power-law equation (Beaucage 1996) where G is the exponential Guinier prefactor, q is the scattering angle (Å À1 ), R g is the radius of gyration (Å), B is the prefactor specific to the type of power-law scattering, and P is the Porod exponent.

Results and discussion
Adsorption of surfactant at oil-water interface The IFT underwent a remarkable reduction in the presence of WH, as depicted in Figure 2  From Figure 2(c), it is clear that the concentration of the WH molecules at the oil-water interface increased with increasing WH concentration.

Zeta potential analysis
The electrostatic charge generated by the chemical additives is an essential aspect of the EOR (Dong et al. 2009). The zeta potential indicates the strength of the electrostatic repulsive force between the oil droplets in an emulsion system and hence the stability of the emulsion (Rezk and Allam 2019). The impact of surfactant concentration on the zeta potential is shown in Figure 3. The emulsion droplets were negatively charged because the surfactant adsorbed on their surfaces had the same charge (Sang et al. 2015). Thus, their rather less adsorption on the surface of sandstone is expected (Machale et al. 2020). An apparent increase in the zeta potential from À25.2 to À37.2 mV with increasing surfactant concentration signifies the surfactant's adsorption at the oil-water interface and an enhancement in the charge density (Varade and Ghosh 2017). The values of zeta potential indicate the good stability of the emulsion prepared from the oil-surfactant system (Pate and Safier 2016). Furthermore, the increase in density of the surfactant molecules at the oil-water interface and consequent increment in the surface charge density can also be inferred with the increase in the zeta potential (Varade and Ghosh 2017;Bollineni et al. 2021).

Phase behavior
The influence of surfactant concentration on the characteristics of the oil-water emulsion was observed for 20 d (Figure 4). It is evident from Figure 4    to its poor stability. Further, the emulsion's stability improved with increasing surfactant concentration due to the enhanced adsorption of the surfactant on the oil droplets (Kumar and Mandal 2018b;Machale et al. 2021).
Furthermore, we have studied the influence of surfactant concentration on the emulsion properties using microscopic analysis ( Figure 5). With increasing surfactant concentrations, the amount of oil in the emulsion increased. The number of oil droplets was rather small for the 0.1 wt. % WH system [ Figure 5(a)]. This was due to a small amount of surfactant on the surface of the oil droplets (Bollineni et al. 2021;Machale et al. 2021). Moreover, bigger oil droplets (i.e. 20-140 mm) are also evident in Figure 5(a). These droplets were prone to quick coalescence, and hence a decrease in the emulsion stability was noticed. However, it is obvious from Figure  5(b,c) that the extent of emulsification increased at 0.25 and 1 wt. % WH, which was due to the more adsorption of the surfactant molecules on the oil droplets. Furthermore, the number of small oil droplets was found to be increasing with surfactant concentration (Figure 5b,c). An increase in the number of dispersed oil droplets and more stability of the emulsion promoted a higher removal of the trapped oil, which improved the oil recovery (He et al. 2015).
The low molecular weight fatty acids (which are the main components of the synthesized natural surfactant) (Machale et al. 2019) have a strong tendency to adsorb at the oil-water interface (Roth et al. 2000). Generally, the adsorption  of surfactant on the oil droplets is a two-step process. Below the CMC, the adsorption occurs rapidly because of the presence of vacant sites at the interface. At the CMC, a monolayer of the surfactant molecules is formed on the interface. The negatively-charged headgroups of the surfactant molecules develop an electrostatic double layer around the oil droplets (Wu et al. 2016). When the oil droplets approach each other, the ions of the diffuse double layer are confined to a narrow space, which is entropically unfavorable. This leads to the repulsive disjoining pressure between the oil droplets, and the coalescence of the droplets is hindered. The force of repulsion between the droplets has been found to be proportional to their size (Mondain-Monval et al. 1996).
It is clear from Figure 4(b) that the emulsions were highly stable at the high surfactant concentrations (i.e. above 0.7 wt. % WH) and their condition unchanged even after 20 d. At the CMC, the surfactant molecules formed a monolayer on the oil droplets and covered the entire surface. As the concentration of surfactant was increased, further adsorption may lead to the development of multilayers at the interface (Penfold et al. 2007;Thomas and Penfold 2015). At such high surfactant concentrations (i.e. above the CMC), micelles are present in the film trapped between the oil droplets (Evans and Needham 1988). This phenomenon is termed as the stratification, which promotes structural repulsion (Nikolov and Wasan 1989). The strong structural repulsion inhibits the coalescence of the droplets, even at small distances [Figure 5(c)].
The formation water in the reservoir is saline in nature. The influence of salt on emulsion stability is critical in evaluating the performance of the chemical additives for EOR applications. The present study is limited by its exclusion of the effect of salt on the adsorption of the synthesized surfactant at the oil-water interface. However, our previous work (Machale et al. 2021) has studied the stability of crude oil-natural surfactant emulsion in the saline system. A minor increase in the diameter of oil droplets was observed in the saline medium due to the interaction between oil, brine, and surfactant, leading to the modification of the spontaneous curvature. Figure 6 demonstrate the interaction between the dispersed oil droplet for 1 wt. % WH emulsion system. The presence of stable multilayers of emulsion stabilized by the surfactant is well-comprehensible in Figure 6. Furthermore, it is evident that two adjacent oil-in-water bulk phases approached close to each other driven by the capillary pressure arising due to the difference in curvature of the interfaces, which can be explained by the Young-Laplace equation (Ghosh 2009). The pressure difference across the curved surfaces eventually led to the merger of the two adjacent surfaces. However, the structural repulsion between the oil droplets hindered coalescence, which led to the encapsulation of the oil droplets [ Figure 6(a-c)]. The encapsulated oil droplets (due to the adsorption of surfactant molecules at the oil-water interface) aid in reducing interfacial tension and alteration of wettability of rock. Several studies have suggested that such encapsulation improved oil recovery by 17%-43.3% (Ojo et al. 2020;Romero-Zeron et al. 2020).

Interfacial shear rheology
Interfacial viscosity is one of the crucial parameters for understanding the capillary phenomena (Lakatos and Lakatos-Szabo 2001). Capillary phenomena in oil recovery correlate with the displacement efficiency of crude oil in the reservoir. The influence of surfactant concentration on the interfacial viscosity at a constant (i.e. 10 s À1 ) shear rate is shown in Figure 7(a). With increasing surfactant concentrations, the interfacial viscosity increased significantly. This demonstrates the formation of a viscous layer at the oilwater interface due to surfactant adsorption (Zhou et al. 2019b). At 10 s À1 , the interfacial shear viscosity of the 1 wt. % WH-oil system was determined to be 1.7 mPa s m. Figure 7(b) demonstrates the influence of shear rate on the surfactant layer at the oil-water interface. It was observed that the interfacial viscosity reduced with increasing shear rate, which depicts a shear-thinning behavior. It happened mainly because of the slow weakening of the surfactant layer's microstructure at the interface. After the CMC (i.e. 0.25 wt. %) was reached, a very nominal increment in the interfacial shear viscosity was observed, which justifies the saturation of the oil-water interface. The number of surfactant molecules adsorbed at the oil-water interface is rather small at the low concentrations, resulting in a weak interfacial film. The interfacial viscosity of the film increased as the surfactant concentration increased (Xu et al. 2007). The degree of surfactant adsorption at the oil-water interface determines the interfacial film's stability, rigidity, and resistance to deform. Therefore, it has a significant effect on oil recovery (Valkovska et al. 2002;Manev and Nguyen 2005). Lin et al. (2018) have investigated the adsorption of asphaltenes (a natural surfactant in crude oil) at both air-water and oil-water interfaces using a combination of interfacial shear rheology and visualization approaches. The shear viscosity of the air-water and decane-water interfaces at  4 mg cm À2 (interfacial) concentration of the asphaltene was reported to be 10 À5 and 10 À2 Pa s m, respectively, at 1 s À1 . In another study, Vishal and Ghosh (2018) studied the interfacial shear rheological properties of hexadecyltrimethylammonium bromide (HTAB) (a cationic surfactant used for the EOR applications) and silica (SiO 2 ) nanoparticles at the air-water interface. The adsorption of the HTAB-SiO 2 composites at the air-water interface was studied by bulk rheology, interfacial rheology, and zeta potential analyses. The interfacial shear viscosity in the presence of the 0.1 mol m À3 HTAB-0.5 wt. % SiO 2 composite film was found to be 2 Â 10 À4 Pa s m at 1 s À1 , and it decreased with increasing shear rate, which implies a pseudoplastic or shear-thinning behavior.
The Boussinesq number (Bo) provides valuable information on the rate of deformation of the interfacial film. It is a ratio of the surface to bulk viscous effects (Equation 4).

BO ¼
Surface shear viscosity Bulk viscosity Â radius of the measuring cell (4) The BO was found to be greater than unity, signifying that the surfactant molecules covered most part of the oil-water interface, as seen in Figure 8 (Ojo et al. 2020). As the concentration of surfactant increased, the oil-water interface became saturated, and hence, small changes in Bo were observed in the 0.5 and 1 wt. % WH systems.
The impact of surfactant concentration on the viscoelasticity of the oil-water interface was investigated through the frequency sweep tests (Figure 9). A similar approach was adopted by Santini et al. (2007). It is clear from Figure 9 that the interface between the paraffin oil and surfactant solution depicted viscoelastic behavior. The tendency to form the viscoelastic interfacial film can also be correlated with the density of the surfactant molecules at the interface. In the event of less density, the surfactant molecules are capable of shifting without interacting with the adjacent molecules. Thus, when the oscillatory shear was applied, the interface acted like a fluid (i.e. G 00 s > G 0 s ). On the other hand, when the density of the surfactant molecules was high, the resultant interaction increased, and the interface acted more like an elastic solid (i.e. G 0 s > G 00 s ) (Anseth et al. 2003). Unlike the interfacial shear viscosity, a minor modification in the viscoelastic properties was noticed for the 0.1-0.5 wt. % WH systems. This is possibly due to the viscoelastic deformation and sensitivity of the oil-water interface. However, a substantial improvement in the viscoelasticity of the interface was observed for the 1 wt. % WH system. The increase in the interfacial film stability with increasing surfactant concentration can be correlated with the dominating elasticity (i.e. G 0 s > G 00 s ) at the high frequency. Generally, the elastic stress developed due to the viscoelastic deformation of the oil-water interface. This elastic stress helped in pulling out the residual oil from the trapped region (Zhou et al. 2019b).  The viscoelasticity of film was further observed from the interfacial complex modulus data ( Figure 10) that at low frequency, the surfactant molecules could adsorb or desorb to establish an equilibrium between the bulk and adsorbed layers. Furthermore, a dynamic equilibrium was established at a higher frequency range because of the exchange of the surfactant molecules and the change in the interface area.

Small-angle X-ray scattering (SAXS)
The SAXS analysis was used to interpret the characteristics of the film at the interface. Figure 11 shows the SAXS profile for emulsion scattering. The zeta potential for the paraffin oil-1 wt. % WH emulsion system was À37.2 mV. The scattering profile indicates the protonation state of the fatty acids, aromatic compounds, and esters within the surfactant that influence the properties of the interfacial film. It is expected that the strong electrostatic interactions and structural forces promoted the formation of a stable emulsion system (Larson-Smith et al. 2012;Rezk and Allam 2019). Based on the scattering results, we have determined the droplet size and the respective unified Guinier-exponential power-law parameters, which are shown in Table 2.
The emulsion had surface fractals since 4 > P > 3, which confirms the formation of spherical droplets (Beaucage 1996). The surface fractal scattering may possibly be restrained by the agglomeration of the surfactant molecules on the oil droplets (Cherny et al. 2019). The radius of gyration of the small droplets was found to be 28.56 Å. The film thickness (DT S ) was found to be 18.52 Å (calculated using Equation 5).
Furthermore, the surface area to volume ratio in the paraffin oil-surfactant emulsion system was 1396.3 m 2 Ácm À3 .

Summary and conclusion
This study provides insight into the multiscale (i.e. macroscopic, mesoscopic, and microscopic) characteristics of the interfacial film that forms by the adsorption of a natural surfactant (synthesized by us from Eichhornia Crassipes) at the oil-water interface. The in-depth study of the interfacial film has provided important information for effective EOR application. Based on the experimental outcomes, the conclusions are as follows: 1. The number of oil droplets, and eventually, the stability of emulsion was found to increase with increasing surfactant concentration. 2. An increase in zeta potential confirms the adsorption of surfactant molecules at the oilwater interface. 3. The spherical structure of the emulsion and the surface fractal scattering was confirmed by the SAXS analysis. 4. Surfactant adsorption at the oil-water interface resulted in an increase in the interfacial shear viscosity and the complex moduli. 5. Viscoelastic films acted as a mechanical barrier to coalescence, which aided emulsion stability.

Funding
The authors are grateful to the Indian Institute of Technology Guwahati and Curtin University for providing financial assistance and research facilities for this study as part of a collaborative PhD program. The authors also Figure 11. SAXS intensity versus scattering angle for the paraffin oil-1 wt. % WH emulsion system. Where G is the exponential Guinier prefactor, R g is the radius of gyration (Å), B is the prefactor specific to the type of power-law scattering, and P is the Porod exponent.