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Phase Behavior, Solubilization, and Phase Transition of a Microemulsion System Stabilized by a Novel Surfactant Synthesized from Castor Oil Nilanjan Pal, Neha Saxena, and Ajay Mandal* Enhanced Oil Recovery Laboratory, Department of Petroleum Engineering Indian Institute of Technology (ISM), Dhanbad, 826 004, India ABSTRACT: Phase behavior, solubilization, and phase transition of a microemulsion system stabilized by a castor oil-based novel surfactant (sodium methyl ester sulfonate) and cosurfactant (propan-2-ol) were investigated for effective application in oil recovery processes. A pseudoternary phase diagram showed the existence of different phases (S/L phase, Winsor I, Winsor II, Winsor III, and Winsor IV) by conventional titration method. With an increase in the cosurfactant-to-surfactant ratio (Kcs), the region under the Winsor III phase was found to increase. An increase in cosurfactant content in the system mixture improved interactions of the surfactant with oil and water in the microemulsion phase, thereby reducing molecular aggregations in solution. At optimal salinity, equal amounts of oil and water were solubilized in a microemulsion in the Winsor III systems and showed ultralow interfacial tension (IFT) values on the order of 10−3 to 10−4 mN/m. Phase dilution studies revealed that the microemulsion systems formed were thermodynamically stable. Salinity increased the relative phase volume of a middle-phase microemulsion, whereas an increase in water content reduced the middle phase volume fractions in the Winsor III systems. Phase transition data were analyzed and fitted using empirical relationships. An increase in salinity and brine content caused phase transformation from Winsor I to Winsor II via Winsor III. However, an increase in temperature showed reverse phase transformation from Winsor II to Winsor III.

1. INTRODUCTION The application of microemulsion systems in enhanced oil recovery (EOR) processes is gaining importance because these systems possess a high solubilizing capacity and an ability to reduce interfacial tension between crude oil and water to an ultralow value, leading to mobilization of a substantial fraction of the residual oil.1−6 The proper formulation of a microemulsion with the ability to displace reservoir oil without high surfactant losses by phase separation or adsorption is a big challenge to researchers. Microemulsions are single phase, isotropic, thermodynamically stable dispersions that are formed almost spontaneously when thermodynamic equilibrium is established between different phases of an oil/surfactant/ cosurfactant/brine system.7 Their stability is primarily due to a number of factors such as low energy requirement and good interfacial properties by virtue of their solubilizing capacity.8 The addition of cosurfactants, such as short-chain alcohols, modifies phase behavior and improves solubilization.9 Characterization of microemulsions is very important for understanding phase behavior and identifying phase transition phenomenon for achieving suitable formulation design in oil recovery applications. Different phases can coexist with microemulsions, creating different types of phase systems, namely, Winsor Types I, II, III, and IV.10−13 The Winsor Type © XXXX American Chemical Society

I phase is a microemulsion of the oil-in-water type where only a portion of oil is solubilized by a lower microemulsion solution. Inversely, Winsor Type II is a water-in-oil microemulsion where a portion of brine (or water) is solubilized by the upper phase microemulsion. Winsor Type III consists of a bicontinuous middle phase microemulsion that exists in equilibrium condition with both upper oleic and lower aqueous phases. Winsor Type IV is a single microemulsion phase, generally formed at high surfactant concentrations. Changes in salinity, water content, and oil type yield microemulsion phase transformation.10,14−16 Oil recovery is high when an optimal formulation of a three-phase state (Winsor III) is obtained. In this system, the upper layer contains oleic phase, the middle phase consists of a continuous phase of oil, surfactant, cosurfactant, and brine, and the lower phase is brine solution. Pseudoternary phase diagrams depicting different phases and outlining the importance of phase transitions in modeling experiments have been studied previously.17−20 Phase transition from oil-in-water microemulsions to water-in-oil microemulsions through a bicontinuous phase is important for phase Received: September 15, 2016 Accepted: February 10, 2017

A

DOI: 10.1021/acs.jced.6b00806 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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behavioral studies for application in EOR.21 Phase transition from a solid−liquid mixture to a two-phase region via a single phase microemulsion can be explained by use of empirical correlations.22 Also, phase transition lines in a binary phase region can be expressed in an equation by determination of fitting parameters.23 Phase behavior of microemulsion systems generally encompasses different analyses, which evaluate properties related to oil and water solubilization ratio, optimal salinity, relative phase volume fractions under conditions of varying salinity, brine content, and temperature. The solubilization phenomenon of oil and water in the middlephase microemulsion can be graphically presented as a function of salinity in terms of oil and water solubilization parameter.24 In the Winsor III system, both oil/microemulsion interface and microemulsion/aqueous interface exist. At these interfaces, interfacial tension (IFT) values are equal at the intersection point of oil and water solubilization curves. Salinity at this juncture is referred to as optimal salinity. The relationship between IFT and the solubilization parameter has been correlated25,26 and even used in simulation. However, Huh (1979)16 was the first to derive a theoretical expression for the IFT calculation at optimal salinity. The addition of salt can induce phase transformation from Winsor I to Winsor II via Winsor III and has a synergistic effect on formation of middle phase microemulsion. Increase in brine content in microemulsion systems has a reverse effect because amphiphile molecules in the oil-microemulsion and microemulsionaqueous interfaces are reduced by excess dilution.27,28 Temperature generally shows reverse transition effect, i.e. upper to middle phase transition for such surfactants.27 In the present study, the emulsifying agent used in microemulsion studies is a novel sulfonate-based surfactant (sodium methyl ester sulfonate or SMES) obtained from castor oil, as characterized and reported in our earlier papers.29,30 The IUPAC name of the anionic surfactant is “methyl (9Z)-12hydroxy-2-(sodiumylidene sulfo)octadec-9-enolate”. Its chemical structure (molecular weight = 414 g/mol) is shown in Figure 1. It consists of an aliphatic monounsaturated carbon

(EACN) value of 7, which is very close to the measured EACN of crude oil (7.47) used in our previous studies.29−31 Propan-2ol (short chain alcohol) improves oil solubilization capacity of a microemulsion and lowers IFT better as compared to medium chain and long chain alcohols owing to its ability to effectively partition in the interfaces. The size distribution profile for the bicontinuous microemulsion phase has been analyzed by dynamic light scattering technique. A dilution method is employed to determine interfacial compositions and thermodynamic spontaneity of three-phase microemulsion (Winsor III) formation. Studies on oil and water solubilization into the microemulsion phase at different salinities are performed to find out the optimal salinity. Interfacial tension at optimal salinity is calculated using Huh’s equation. Phase transition phenomenon is also observed from phase diagram data and explained by development of empirical correlations. Furthermore, the influence of a variation of NaCl content, brine volume percentage, and temperature on relative phase volume fractions of different phases is studied. The objective of the study entails the formulation of an effective n-heptane/ surfactant/propan-2-ol/water-based microemulsion system that shows good potential for application in EOR.

2. EXPERIMENTAL DESIGN AND MODELING STUDIES 2.1. Materials. A novel surfactant, sodium methyl ester sulfonate (SMES), that was manufactured from castor oil in the laboratory, was used as surfactant. The synthesis route and various petro-physical properties such as interfacial tension, wettability alteration, and flooding properties for SMES (critical micelle concentration = 6000 ppm) was reported earlier.29,30 Propan-2-ol, used as cosurfactant, was purchased in analytical grade from Rankem Ltd. n-Heptane, procured from Merck Chemicals, was used as the oil phase. Sodium chloride (99% purity, analytical grade) purchased from Sigma-Aldrich, was used to differ the salinity of aqueous solutions. 2.2. Phase Diagram Construction. Pseudoternary phase diagrams for different cosurfactant-to-surfactant ratios were constructed by the titration method.8,32−34 Initially, quasibinary mixtures of {heptane:propan-2-ol/SMES} at a fixed value of Kcs with weight ratios of 1:9, 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, 8:2, and 9:1 were prepared. The total weight of the {heptane + propan-2-ol + SMES + water} mixture was fixed at 4 g. Intermediate salinity is favorable for the formation of a stable Winsor III system, causing substantial surfactant retention and oil trapping.35 Therefore, brine solution containing 4% NaCl was used as an aqueous phase in this study. The uniform mixing stage was achieved by shaking the mixture for 1 h at 298 K. To analyze phase behavior and the transition of different phases in a pseudoternary system, the mixture was titrated with the desired brine solution (4% NaCl) followed by mixing in a Rotospin rotary mixture (Tarsons Products Ltd., Kolkata) at 50 rpm for 5 min. After the mixture was allowed to settle, the phase behavior of the mixture was visually observed. Titration was continued until there was no further change of phase behavior. The compositional data of different constituents for the S/L phase, Winsor IV, Winsor I, Winsor III, and Winsor II phases were calculated and plotted accordingly in order to construct a ternary phase diagram. The method was repeated for different cosurfactant-to-surfactant ratios (Kcs) in oil/ surfactant/cosurfactant/brine systems. 2.3. Solubilization Parameters and Optimal Salinity. Determination of solubilization parameters is very important in formulating an economical microemulsion system for applica-

Figure 1. Molecular structure of sodium methyl ester sulfonate (SMES).

chain (17 carbon atoms) with sulfonate group at α-position to the methyl ester group. HLB value of the anionic surfactant is 4.98, which indicates that its oil solubilizing capacity is greater than its ability to solubilize water/brine. The efficiency of the surfactant on the formation of a stable microemulsion for application in enhanced oil recovery has been studied by a phase behavior study. The phase behavior of heptane/SMES/ propan-2-ol/brine at different cosurfactant-to-surfactant ratios has been investigated by the construction of a pseudoternary phase diagram and different microemulsion phases were identified. N-Heptane has an equivalent alkane carbon number B



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K. In this method, a stable three-phase microemulsion consisting of 4.5 g of n-heptane, 1.0 g of surfactant/cosurfactant mixture, and 4.5 g of brine solution (with 4% NaCl) was destabilized by the addition of oil until a two-phase system is observed. The system was then restabilized by adding a requisite quantity of cosurfactant. This procedure was repeated several times to obtain corresponding values of amounts of oil and cosurfactant in each step. Concentrations of system components were expressed in terms of mole fraction scale. For evaluation of thermodynamic spontaneity, ideal behavior of the solution was considered. 2.6. Phase Transition. After construction of the phase diagram, the compositions of oil, surfactant/cosurfactant, and brine at the dividing line between different phases were examined, and the phase equilibrium data were used and analyzed to develop a phase transition model. Relations between compositions (weight) of any two of the components of microemulsion (oil/surfactant + cosurfactant/brine) were analyzed by utilization of linear, polynomial, or logarithmic models. The weight of the third component can be simply calculated by subtracting weights of the two components calculated from the model equation from the total weight of the solution. Depending on the type of model employed for each boundary condition, the values of fitting parameters for phase transition were determined. 2.7. Relative Phase Volume. Microemulsion solutions containing the desired compositions of oil, surfactant, cosurfactant, and brine were prepared by uniform mixing and allowed to settle such that the phase volumes were observed to be constant over a suitable time period and equilibrium condition was finally reached. The relative volume fractions were shown in a way such that the upper phase was at the top of the graph and the bottom of the graph represented the lower phase. The microemulsion system desired was Winsor III in which relative phase volumes were observed for different phases of the system consisting of upper oleic phase, middle microemulsion phase, and lower aqueous phase. The effects of salt concentration, brine content, and temperature on relative volumes were studied for multiphase systems by varying one factor and fixing the two others. These factors are significant when studying phase behavior and determining proper microemulsion formulations under different conditions.

tion in enhanced oil recovery. The amount of oil and water solubilized by unit weight of surfactant may be expressed in terms of solubilization parameters. To measure solubilization parameters, different solutions with 45% heptane, 10% surfactant/cosurfactant mixture, and 45% water (composition by weight) with different salinities were separately prepared. Total weight of each solution was fixed at 10 g. Test tubes containing oil/surfactant/cosurfactant/water systems with different salinities were rotated vigorously for about 30 min at 298 K in a Rotospin rotary mixer. Then, the tubes were allowed to equilibrate in a specially designed rack. After the solution settled, the amounts of oil and water (in g) solubilized in the microemulsion region were determined from eqs 1 and 2. wos = woi − wof (1) wws = wwi − wwf

(2)

where wos is the weight of oil solubilized in microemulsion, wws is the weight of brine solubilized in microemulsion, woi is the initial weight of oil in solution, wof is the weight of upper oil phase after phase transition, wwi is the initial weight of brine in solution, wof is the weight of lower aqueous phase after phase transition. Values of oil and water solubilization parameters (SPo, SPw) were obtained from eqs 3 and 4 SPo = wos/ws

(3)

SPw = wws/ws

(4)

Here ws is the weight of surfactant in the middle phase microemulsion, SPo and SPw are oil and water solubilization parameters, respectively. Since all the surfactant molecules were presumed to be in microemulsion region, the value of ws is constant. The plots showing oil and water solubilization curves may or may not intersect each other within the desired salinity range. The point of intersection of these curves, if any, was useful in determining the optimal solubilization parameter and corresponding salinity (known as optimal salinity). These data are very important for microemulsion formulation from an economic point of view. 2.4. Dynamic Light Scattering (DLS) Measurements. The droplet size distribution in the bicontinuous microemulsion phase was determined by dynamic light scattering experiments at 298 K using a Zetasizer Nano S90 (Malvern Instruments, UK) equipped with a He−Ne laser operating at a wavelength of 633 nm and scattering angle of 90°. Measurements were performed for systems with different Kcs values of 1, 2, and 3 at 298 K. In this technique, the diffusion of droplets moving under Brownian motion is measured and the volume fraction of differently sized droplets is obtained in graphical form. In this study, oil and water compositions in the microemulsion systems were fixed at 45% (by weight) each. System salinity was fixed at 4% NaCl. The values of absorbance for microemulsions with different Kcs were measured using a UV-1800 spectrophotometer (Shimadzu, Japan) set at a wavelength of 265 nm. Absorbances for all samples were found to be 2.33. The refractive index of each microemulsion was measured by a portable Refracto 30PX refractometer. Refractive indices for Kcs = 1, 2, and 3 were found to be 1.414, 1.428, and 1.435, respectively. Measurements were repeated at least three times for each sample to check reproducibility of the results. The uncertainty of measurement is ±4%. 2.5. Dilution Studies. Dilution experiments were performed to evaluate the interfacial compositions and thermodynamic stability of Winsor III systems for Kcs = 1, 2, and 3 at 298

3. RESULTS AND DISCUSSION 3.1. Characteristics of Pseudoternary Phase Diagram. Construction of phase diagrams is very important for selection of cost-effective microemulsion systems that are favorable for use in oil recovery applications. The pseudoternary phase diagrams of heptane/propan-2-ol/SMES/brine (4% NaCl) systems with Kcs = 1, 2, 3 at 298 K are shown in Figure 2 panels a, b, and c, respectively. In each figure, there are five phase areas: S/L phase, Winsor IV, Winsor I, Winsor III, and Winsor II. The solid surfactant is only slightly soluble in a solution consisting of n-heptane and brine with lower percentage of as shown by the (S/L) region of ternary phase diagram. The low solubility of the surfactant is because of its low HLB value (4.98). With the addition of more brine solution, the mixture gradually changes to a transparent singlephase microemulsion (Winsor IV). With more brine titrated into the Winsor IV phase, the phenomenon of phase transformation begins, wherein a two phase system is achieved. In the 2-phase region, a Winsor I microemulsion is formed at the start. Winsor I phase is characterized by an upper oil phase C



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Figure 2. Pseudoternary phase diagrams of {heptane/propan-2-ol/ SMES/brine (4% NaCl)} systems at 298 K with Kcs = 1, (a); 2, (b); 3, (c), where S/L = solid−liquid mixture region, W I = Winsor I phase region, W II = Winsor II phase region, W III = Winsor III phase region, W IV = Winsor IV phase region, P = Plait Point.

Figure 3. Oil and water solubilization curves showing solubilization parameter versus salinity (% NaCl) for {heptane/n-propanol/SMES/ brine} systems for (a) Kcs = 1, (b) Kcs = 2, and (c) Kcs = 3 at 298 K.

(heptane) and a lower translucent microemulsion phase. Further titration yielded a three-phase system (Winsor III) as shown by an upper oil phase, a middle phase microemulsion, and a lower aqueous phase. Finally, with excess brine titrated in, the Winsor III phase vanishes and another two-phase region (Winsor II) is observed with an upper microemulsion phase and a lower aqueous phase. It is important to note that all curves in ternary phase diagrams are binodal (coexistence) in nature; that is, the points on the curves denote conditions at which two distinct phases coexist. It is clear that Winsor I, Winsor II, and Winsor III are three coexisting phases of partially soluble components of heptane/propan-2-ol/SMES/

brine systems. These three phases approach one another and merge at a particular composition referred to as plait point (designated as P) in phase diagrams.36 Desirability of phase systems in enhanced oil recovery applications is dependent on the ability of the microemulsion phase to extract maximum amount of oil from reservoir pores. S/L phase comprises a mixture of solid and liquid (turbid state) and is highly undesirable in microemulsion systems. As Kcs is increased, the prominence of this phase decreases. From phase diagrams, it is evident that at higher weight fractions of (propan-2-ol + SMES), the Winsor IV state is generally observed. Though the Winsor IV system is basically a single D



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Figure 4. Graph showing the effect of variation of Kcs on Huh’s interfacial tension at optimal salinity. Figure 7. Plot showing experimental and fitted lines for ncs/ns versus no/ns for n-heptane/SMES/propan-2-ol/brine systems for different Kcs values at 298 K.

more water (brine) is dissolved in the oil-in-water microemulsion formed. Winsor II behavior results in surfactant retention in the upper phase due to formation of a water-in-oil microemulsion. In the Winsor III system, a middle phase microemulsion is formed between an upper oleic phase and a lower aqueous phase. The Winsor III phase region is found to shift toward the brine region and increases with Kcs. This may be due to improved microemulsion stability in the presence of higher cosurfactant concentration in the middle phase region.37 This phase is most favorable for EOR due to its cost effectiveness and ability to achieve ultralow IFT. 3.2. Determination of Solubilization Parameter and Optimal Salinity. Investigation on changes in microemulsion interfaces after mixing are relevant in determining the amount of oil and water present in Winsor III microemulsion systems. Solubilization in the Winsor III system occurs due to the equilibrium coexistence of oil and water in the presence of cosurfactant and surfactant.38 We assumed that all surfactant molecules are present in microemulsion and no surfactant is present in the upper and lower phase regions. Solubilization parameter (SP) quantifies the degree of interactions between oil and surfactant, and surfactant and brine solution. The proper formulation of microemulsion with desired properties and low surfactant concentration is of immense importance for its application in enhanced oil recovery. Graphs showing the variation of solubilization parameters with salinity for {heptane/propan-2-ol/SMES/water} systems are depicted in Figure 3a−c. Oil solubilization parameter (SPo) values are found to increase with salinity, whereas the values of the water solubilization parameter (SPw) decrease as a function of salinity. Increasing cosurfactant-to-surfactant ratio (Kcs) increases the partitioning effect of cosurfactant molecules at the interface, thereby enhancing surfactant interactions with upper oil and lower aqueous phases. This results in increase in the amounts of oil and water solubilized by the middle phase microemulsion. As a consequence, at any salinity values, the values of SPo and SPw were found to increase with Kcs. At a particular salinity, the oil solubilization parameter and water solubilization parameter (SPw) intersect mutually. This salinity value is referred to as optimum salinity.39,40 At optimum salinity, the oil and water solubilization parameters are equal in the microemulsion phase and referred to as the optimum solubilization ratio (σ*). With an increase in Kcs the values of

Figure 5. Droplet size distribution in the middle-phase bicontinuous microemulsion phase formed by systems containing 45% n-heptane, 10% surfactant/cosurfactant mixture, and 45% water at 298 K for different values of Kcs. Salinity was fixed at 4% NaCl (by weight).

Figure 6. Photographs of microemulsions with increasing Kcs values (left to right). Microemulsion is translucent for Kcs = 1, and transparent for Kcs = 2 and Kcs = 3.

phase microemulsion with high oil displacing efficiency, this phase is not generally economically ideal for oil recovery due to the requirement of high quantity of surfactant. In the Winsor I system, excess oil is still present in the reservoir as less oil and E



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Table 1. Interfacial and Oil Phase Compositions of SMES Microemulsion System with Different Kcs Values (Using Propan-2-ol as Cosurfactant) at 298 K. Uncertainities Are Included in Data Tables Kcs

intercept (I)

slope (S)

Xiw cs

Xocs

Kd

1 2 3

3.479 ± 0.152 8.848 ± 0.070 15.345 ± 0.053

1.997 ± 0.007 2.374 ± 0.004 2.362 ± 0.002

0.777 ± 0.012 0.898 ± 0.007 0.939 ± 0.005

0.666 ± 0.006 0.704 ± 0.004 0.702 ± 0.002

1.166 ± 0.007 1.277 ± 0.005 1.336 ± 0.004

Figure 8. Variation of standard Gibbs free energy change of transfer of propan-2-ol (cosurfactant) molecules with different cosurfactant-tosurfactant ratios (Kcs) at 298 K.

Figure 9. Plots showing experimental and fitting values of phase transition from S/L to Winsor IV. Solid lines represent experimental data derived from phase diagram analysis. Dotted lines represent fitted data obtained from regression analysis.

both oil SPo and SPw increase at any salinity. Optimal salinity for Kcs = 1, 2, and 3 are found to be 6.00%, 4.20%, and 3.60% (NaCl, wt %) respectively. As the Kcs ratio is increased, optimal salinity is reduced due to increased hydrophobicity and a lesser partitioning effect of NaCl in the presence of a higher amount of cosurfactant (propan-2-ol) in the interfaces. The optimal solubilization ratio (σ*) is a strong function of optimal salinity. For Kcs = 1, 2, and 3, the values of σ* are 4.95, 6.50, and 8.28, respectively. The optimal solubilization ratio (σ*) is a function of the optimal salinity parameter. For Kcs = 1, 2, and 3, the values of σ* are 9.45, 14.82, and 20.33, respectively. Increasing cosurfactant-to-surfactant ratio (Kcs) increases the partitioning

effect of cosurfactant molecules at the interface, thereby enhancing surfactant interactions with upper oil and lower aqueous phases. This results in an increase in the amounts of oil and water solubilized by the middle phase microemulsion. As a consequence, at any salinity values, the values of SPo and SPw were found to increase with Kcs. At optimal salinity (or optimal solubilization ratio) microemulsions are Winsor III, bicontinuous in nature and spongelike (oil/water continuous) in microstructure. Huh (1979)16 formulated a theoretical relation between IFT and solubilization parameter at optimal salinity. This formula gives a good

Table 2. Phase Transition (wi) from Solid−Liquid Mixture (S/L) to Single Phase Microemulsion (Winsor IV) at 298 K Showing Equilibrium Compositions of Components 1, 2, 3 and Fitting Parameters of eq 9 (± Error Values Included)a weight fractions of different components for phase transition Kcs = 1

Kcs = 2

w1

w2

w3

0.862 0.799 0.683 0.565 0.414 0.339 0.165 0.099 0.862

0.089 0.136 0.229 0.325 0.458 0.610 0.702 0.759 0.089

0.049 0.065 0.088 0.110 0.128 0.151 0.133 0.142 0.049

w1

0.858 ± 0.029 −0.906 ± 0.019 0.994

Kcs = 3 w3

0.878 0.082 0.040 0.810 0.125 0.065 0.708 0.211 0.081 0.587 0.327 0.086 0.444 0.468 0.088 0.321 0.590 0.089 0.177 0.730 0.093 0.071 0.828 0.101 0.878 0.082 0.040 fitting parameters of eq 9 for phase transition

Kcs = 1 b0 b1 R

w2

w1

w2

w3

0.871 0.817 0.775 0.631 0.497 0.381 0.241 0.145 0.871

0.082 0.113 0.211 0.306 0.439 0.548 0.701 0.799 0.082

0.047 0.070 0.014 0.063 0.064 0.071 0.058 0.056 0.047

Kcs = 2 0.892 ± 0.022 −0.944 ± 0.020 0.999

b0 b1 R

Kcs = 3 b0 b1 R

0.933 ± 0.018 −0.978 ± 0.015 0.998

a

The weight ratio Kcs was controlled to be 1, 2, and 3 for three groups of samples. wi is the weight fraction of component i. R is the correlation coefficient for the fitting equation. F



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Table 3. Phase Transition (wi) from Single Phase Microemulsion (Winsor IV) to Two Phase Region at 298 K Showing Equilibrium Compositions of Components 1, 2, 3 and Fitting Parameters of eq 10 (± Error Values Included)a weight fractions of different components for phase transition Kcs = 1

Kcs = 2

w1

w2

w3

0.860 0.761 0.681 0.598 0.525 0.322 0.279 0.194 0.098

0.092 0.159 0.208 0.263 0.300 0.339 0.398 0.448 0.483

0.048 0.080 0.111 0.139 0.175 0.319 0.323 0.358 0.419

w1

w2

0.846 0.082 0.733 0.148 0.601 0.220 0.499 0.279 0.367 0.332 0.355 0.335 0.272 0.399 0.180 0.452 0.091 0.488 fitting parameters of eq 10 for phase

Kcs = 1 b0 b1 R

0.533 ± 0.020 −0.488 ± 0.017 0.989

Kcs = 3 w3

w1

w2

w3

0.072 0.119 0.179 0.222 0.291 0.310 0.329 0.368 0.421 transition

0.823 0.712 0.620 0.535 0.418 0.353 0.251 0.159 0.092

0.067 0.139 0.207 0.265 0.331 0.408 0.479 0.519 0.538

0.100 0.149 0.173 0.200 0.251 0.239 0.270 0.322 0.435

Kcs = 2 0.543 ± 0.024 −0.540 ± 0.010 0.999

b0 b1 R

Kcs = 3 b0 b1 R

0.625 ± 0.016 −0.675 ± 0.009 0.996

a

The weight ratio Kcs was controlled to be 1, 2, and 3 for three groups of samples. wi is the weight fraction of component i. R is the correlation coefficient for the fitting equation.

3.3. Droplet Size Distribution Analysis. The dynamic light scattering technique has proven to be a convenient method for probing the internal properties of microemulsions.41 Oil/surfactant/cosurfactant/brine systems with different n-heptane/brine (4% NaCl) compositions and Kcs values (1, 2, 3) at 298 K were used in DLS studies. Figure 5 shows the volume-weighted size distribution of droplets dispersed in the bicontinuous microemulsion phase for different Kcs values at 298 K. Microemulsion samples with Kcs = 2 and 3 were found to be slightly transparent, whereas the samples with Kcs = 1 were observed to be translucent. Images showing microemulsion samples with different Kcs values are shown in the Figure 6. With increasing Kcs value, the size distribution profile shifts more to the left, showing that the volume fraction of lowdiameter aggregates increases in the microemulsion region at a higher content of propan-2-ol (cosurfactant). The addition of cosurfactant in the system mixture improves the thermodynamic stability of the microemulsion phase by increasing surfactant molecule interactions with oil and water molecules in the microemulsion phase.42 This reduces the degree of molecular aggregation in solutions. Consequently, droplet size is decreased and the ability of microemulsion phase to migrate through the rock pores increases. Dispersity measurements are an indication of the heterogeneity of sizes of droplets in the middle-phase microemulsion phase. The obtained values of polydispersity index (PDI) for the microemulsion systems with Kcs =1, 2, and 3 were 0.408, 0.322, and 0.271, respectively. With increasing Kcs values, the PDI is found to decrease with Kcs, showing that droplet size distribution narrows toward uniformity. An increase in cosurfactant content improves surfactant−oil and surfactant− aqueous interactions in the middle phase, reducing the formation of surfactant molecule aggregates in solution. 3.4. Method of Dilution. Winsor III system is favorable for EOR due to its well-established ability to lower IFT and enhanced solubilization capacities as compared to other microemulsion phase systems. A comprehensive study on the determination of interfacial composition and evidence of thermodynamic stability of middle-phase microemulsion

Figure 10. Plots showing experimental and fitting values of phase transition from Winsor IV to two-phase system. Solid lines represent experimental data derived from phase diagram analysis. Dotted lines represent fitted data obtained from regression analysis.

estimate of IFT values for a wide range of salinity, cosurfactantto-surfactant ratio, and other factors and is given in eq 5

γ = 0.3/(σ *)2

(5)

In the above equation γ is the IFT and σ* is the oil or water solubilization ratio at optimal salinity conditions. Under optimal conditions for systems with Kcs = 1, 2, and 3, the values of IFT determined by Huh’s equation are found to be 3.359 × 10−3 mN/m, 1.366 × 10−3 mN/m, and 7.258 × 10−4 mN/m, respectively. The graph showing variation of interfacial tension (obtained from Huh’s correlation) with cosurfactant-tosurfactant ratio under optimal conditions is shown in Figure 4. When cosurfactant is added into microemulsion systems, partitioning at the oil−microemulsion and microemulsion− aqueous interfaces occurs, which subsequently improves the ability of the interfaces to occupy varying curvatures to form stable microemulsions. As a result, interfacial activity is improved and IFT decreases with an increase in the Kcs ratio. G



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Table 4. Phase Transition (wi) from Winsor I to Winsor III at 298 K Showing Equilibrium Compositions of Components 1, 2, 3 and Fitting Parameters of eq 11 (± Error Values Included)a weight fractions of different components for phase transition Kcs = 1

Kcs = 2

w1

w2

w3

0.115 0.144 0.183 0.222 0.282 0.351 0.359

0.045 0.061 0.082 0.098 0.141 0.193 0.24

0.84 0.795 0.735 0.68 0.577 0.456 0.401

w1

0.101 0.033 0.161 0.044 0.213 0.061 0.27 0.08 0.341 0.119 0.398 0.188 0.41 0.269 fitting parameters of eq 11 for phase

Kcs = 1 b0 b1 b2 R

−4.013 ± 0.112 9.006 ± 0.494 −5.748 ± 0.441 0.994

Kcs = 3

w2

w3

w1

w2

w3

0.866 0.795 0.726 0.65 0.54 0.414 0.321 transition

0.098 0.161 0.202 0.268 0.353 0.411 0.434

0.015 0.028 0.041 0.068 0.101 0.159 0.253

0.887 0.811 0.757 0.664 0.546 0.43 0.313

Kcs = 2

Kcs = 3

−3.645 ± 0.094 1.780 ± 0.102 8.666 ± 0.742 0.993

b0 b1 b2 R

−4.996 ± 0.150 9.047 ± 0.587 −2.572 ± 0.299 0.992

b0 b1 b2 R

a


ncs n i + ncsw n n iw n = cs + K o = cs + K o ns ns ns ns ns

(6)

where niw cs is the total number of cosurfactant moles in the interface and aqueous phases. The linear fitting lines drawn between ncs/ns and no/ns for Kcs = 1, 2, and 3 give the value of niw cs /ns and K as intercept (I) and slope (S), respectively, as shown in Figure 7. Distribution of propan-2-ol molecules at the interface is estimated from the intercept value to obtain the value of distribution constant (Kd) in order to support the evidence of a stable three-phase system. The importance of determination of Kd in dilution experiments is already elaborated in many studies.44−47 The relation showing the partition of propan-2-ol between the interface and the upper and lower phases is expressed as eq 7

Figure 11. Plots showing experimental and fitting values of phase transition in the binary phase region from Winsor I to Winsor III. Solid lines represent experimental data obtained from pseudoternary phase diagram. Dotted lines show phase transformation data obtained from logarithmic fitting (regression).

Kd =

Xcsiw n iw /n [(ncs /no) + 1] I(S + 1) = cs s iw = S(I + 1) Xcs ncs /no[(ncs /ns) + 1]

(7)

o In the above equation, Xiw cs and Xcs are the values of mole fraction of cosurfactant in interfacial and aqueous compositions and mole fraction of cosurfactant in the bulk oil phase, respectively. Hence, knowing the value of intercept (I) and slope (S), the value of Kd can be calculated. Table 1 shows the o Xiw cs (calculated from intercept, I) and Xcs (calculated from slope, S) along with their distribution coefficient (Kd) values. It o is observed that the values of Xiw cs and Xcs generally increase with Kcs. This shows that the partitioning of propan-2-ol is easier at the interface and hence, a lesser amount of cosurfactant is needed to restabilize the system after destabilization by oil in the case of higher Kcs values. The approximately same values of Xcso for both Kcs = 2 and Kcs = 3 may be due to the inability of the oil−microemulsion interface to accommodate surplus cosurfactant molecules, thereby ceasing transfer of cosurfactant molecules into the upper oil bulk phase. This similarity is, in fact, helpful in achieving much higher values of Kd and thereby, greater partitioning effect of propan-2-ol at interfaces for Kcs = 3 as compared to Kcs = 2. A lesser amount of propan-2-ol is required

between upper oleic phase and lower aqueous phase for different Kcs values at 298 K has been made by the dilution method. Stability of any microemulsion system is dependent on the distribution of cosurfactant molecules at the oil−microemulsion and at the microemulsion−aqueous interfaces at constant temperature.24,43 The ddition of n-heptane decreases cosurfactant (propan-2-ol) content, causing system instability. Re-establishment of the three-phase equilibrium is achieved by the addition of the necessary amount of propan-2-ol into the destabilized two-phase system. The oil solubilization of surfactant is neglected throughout this experiment. The effect of variation of cosurfactant-to-surfactant ratio (Kcs) on interfacial compositions and thermodynamic parameters of the microemulsion system is investigated in this section. System composition for a stable three-phase system is a function of number of moles of cosurfactant in the interface (ncsi) and aqueous phase (ncsw), number of moles of surfactant (ns), and total number of moles of oil (no) in the system, written in eq 6 H



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Table 5. Phase Transition (wi) from Winsor I to Winsor III at 298 K Showing Equilibrium Compositions of Components 1, 2, 3 and Fitting Parameters of eq (12) (± Error Values Included)a weight fractions of different components for phase transition Kcs = 1

Kcs = 2

w1

w2

w3

0.333 0.274 0.221 0.164 0.114 0.079 0.055

0.345 0.322 0.321 0.241 0.183 0.122 0.085

0.322 0.404 0.478 0.603 0.703 0.799 0.860

w1

w2

0.371 0.331 0.307 0.349 0.215 0.370 0.155 0.345 0.103 0.308 0.065 0.260 0.052 0.181 fitting parameters of eq 12 for phase

Kcs = 1 b0 b1 b2 R

Kcs = 3 w3

w1

w2

w3

0.288 0.334 0.415 0.500 0.589 0.675 0.767 transition

0.401 0.259 0.161 0.110 0.053 0.042 0.025

0.338 0.378 0.378 0.375 0.327 0.259 0.180

0.261 0.343 0.459 0.515 0.620 0.699 0.795

Kcs = 2

−3.161 ± 0.331 15.175 ± 0.912 −27.584 ± 1.134 0.994

−1.892 ± 0.254 7.673 ± 0.600 −15.384 ± 0.999 0.916

b0 b1 b2 R

Kcs = 3 b0 b1 b2 R

−1.617 ± 0.240 6.140 ± 0.428 −12.274 ± 0.950 0.840

a


The increase in cosurfactant-to-surfactant ratio improves the flexibility of the oil−microemulsion and microemulsion− aqueous interfaces to take up different curvatures that allow for the formation of a middle-phase microemulsion that remains in equilibrium with both upper and lower phases. As a consequence, micelles in the middle phase microemulsion have a high surface-to-volume ratio, a necessary condition to achieve thermodynamic stability. This improves fluidity of the interface and thermodynamic stability of the middle phase microemulsion.50,51 3.5. Phase Transition Model. For phase transition analysis, the phase diagrams of {heptane/propan-2-ol/SMES/ brine (4% NaCl)} systems with different Kcs at 298 K are used. In each ternary phase diagram, five different phase regions are identified based on composition of the system. In the present study, phase transition behavior is analyzed in two different stages, namely, transition from S/L to Winsor IV to two-phase region and transition from Winsor I to Winsor III to Winsor II. Transition from S/L to Winsor IV, and Winsor IV to a twophase region were explained by polynomial equations of first order, whereas transitions among Winsor I, Winsor III, and Winsor II phases were modeled with the help of second order polynomial equations. This is due to varying influences of different components (n-heptane, surfactant/propan-2-ol, brine) in the microemulsion system. This information is very useful as it gives us an idea about the phase behavior of any microemulsion state at any composition and vice versa.52 For the analysis of phase transition by different empirical models, nheptane, mixture of surfactant and cosurfactant with specific values of Kcs and brine are designated as components 1, 2, and 3, respectively. The closeness of experimental data to the data obtained from the fitted regression models is measured in terms of R- statistic value, also referred to as the correlation coefficient. 3.5.1. Phase Transition from S/L to Winsor IV to TwoPhase Region. The phase equilibrium data from the solid− liquid mixture (S/L) to Winsor IV for systems with Kcs = 1, 2, 3 are shown in Table 2. This transition may be expressed as a linear relationship in eq 9

Figure 12. Plots showing experimental and fitting values of phase transition in the binary phase region from Winsor III to Winsor II. Solid lines represent experimental data obtained from pseudoternary phase diagram. Dotted lines show phase transformation data obtained from logarithmic fitting (regression).

for restabilization of the microemulsion sytem for Kcs = 3 after destabilizing it with oil. Therefore, a microemulsion formulation with Kcs = 3 is more useful from the stability point of view. The determination of interfacial compositions of microemulsion systems is useful in getting information about their spontaneity of formation of a stable microemulsion phase.48,49 The standard Gibbs free energy change of transfer from upper oil phase and lower aqueous phase to the interfaces is obtained from eq 8 ΔGt° = −RT ln Kd

(8) −3

where R is universal constant (8.314 × 10 kJ/mol/K) and T is temperature (K) at which dilution experiments are conducted. ΔGt° are found to be negative for all experimental systems, showing that microemulsion formation is spontaneous. Higher values of −ΔGt° are obtained for microemulsion systems with higher Kcs values as shown in Figure 8. This shows that transfer of propan-2-ol molecules to interfaces becomes more spontaneous with an increase in the Kcs ratio. I



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Figure 13. Volume fractions of different phases versus NaCl content for heptane/propan-2-ol/SMES/brine systems with (a) Kcs = 1; (b) Kcs = 2; and (c) Kcs = 3.

w2 = b0 + b1w1

(9)

In the above equation, w1 is the weight of n-heptane, w2 is the weight of surfactant/cosurfactant mixture. The value of w3 may be calculated by subtracting w1 and w2 from the total weight of the system. The parameters of the fits for phase transition from S/L phase to Winsor IV are also tabulated in Table 2. Figure 9 shows the experimental and fitted values of phase transition data in graphical form. Compositions of dissolution equilibrium from Winsor IV phase to two-phase region for Kcs = 1, 2, 3 are listed in Table 3. The phase equilibrium data set is fitted with a linear model as eq 10

w2 = b0 + b1w1

Figure 14. Effect of brine solution percentage on relative volume fractions of heptane/propan-2-ol/SMES/brine (4% NaCl) systems with (a) Kcs = 1; (b) Kcs = 2; and (c) Kcs = 3.

The parameters of the fits for phase transition from Winsor IV to the binary phase region are tabulated in Table 3. For phase transition from the Winsor IV to the binary phase region, plots showing experimental data and values obtained from fitting parameters are shown in Figure 10. At high surfactant concentrations (data points above the fitted curves), a Winsor IV phase system is established.53−55 The probability of

(10) J



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occurs. The compositions for phase transformation from Winsor I (two-phase region) to Winsor III (three-phase region) are listed in Table 4. The phase equilibrium data for both transitions may be fitted by empirical eq 11 ln(w2) = b0 + b1w1 + b2w12

(11)

Since two values are known from the above equation, the third value can be easily calculated. Fitting parameters for phase transition are also given in Table 4. Experimental and fitted data of phase transformation from Winsor I to Winsor III are depicted in Figure 11, respectively. The formation of reverse micelles may be physically detected by a marked increase in their ability to solubilize water.55 As a consequence of micelle hydration, droplet size increases and an independent aqueous phase is formed inside micelles.56,57 Inversion of micellar structure from swollen micelles to reverse micelles begin at this curve, resulting in the gradual appearance of Winsor III system. The phase equilibrium data from solid−liquid mixture (S/L) to Winsor IV for systems with Kcs = 1, 2, 3 are shown in Table 5. This transition may be expressed as an empirical logarithmic relation in eq 12 ln(w2) = b0 + b1w1 + b2w12

(12)

Fitting parameters for phase transition from Winsor III to Winsor II are given in Table 5. Figure 12 shows the experimental data as well as the data obtained from regression analysis of phase transformation from Winsor III to Winsor II. Points on each curve show that most or all micelles are completely reversed in the microemulsion phase, and the oil phase is either slightly or completely dispersed into the bicontinuous microemulsion. The Winsor III phase system completely disappears beyond this equilibrium data.58,59 This transition is found to occur at a slow rate and is achieved by the gradual disappearance of the upper oleic phase. 3.6. Relative Volume Fractions of Various Phases. Winsor III system is favorable for enhanced oil recovery applications since the microemulsion phase is in equilibrium with both oleic and aqueous phases in a three-phase system. The relative phase volumes of n-heptane/propan-2-ol/SMES/ brine systems were studied at different salinity and content of brine solution. Depending on the composition of the system, different phases, namely, Winsor I, Winsor II, and Winsor III were observed. Experimental studies on the effect of salinity and brine content on phase volume fractions showed interesting results. Temperature also exhibited a significant effect on the relative phase volume of different phases. 3.6.1. Effect of Salinity. Relative volume fractions are plotted in Figure 13 panels a, b, and c as a function of salinity for a system with 45 wt % n-heptane, 10 wt % {propan-2-ol + SMES} and 45 wt % water with Kcs = 1, 2, 3 at 298 K. This type of data set is known as a salinity scan. Initially the Winsor I phase is observed until salinity reaches 2% NaCl for Kcs = 1 and 1% NaCl for Kcs = 2 and 3. Thereafter, the Winsor I phase vanishes and phase behavior turns to Winsor III. With a further increase in NaCl content, relative volume percentages of the middle phase microemulsion increases at the expense of the upper oleic phase, generally near the boundary of phase shift. Because of its low HLB value (4.98), the water solubilizing capacity of SMES under initial salinity conditions in the microemulsion phase is very low. Salinity increase does not, at all, improve water solubilization into the middle phase microemulsion due to reduction in the water−microemulsion

Figure 15. Influence of temperature on volume fractions of heptane/ propan-2-ol/SMES/brine (4% NaCl) systems with (a) Kcs = 1; (b) Kcs = 2; and (c) Kcs = 3.

occurrence of the Winsor III phase system is, therefore, higher at the abscissa points below the regression curve. 3.5.2. Phase Transition from Winsor I to Winsor III to Winsor II. In the binary phase region, phase transition also K



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Analysis of a pseudo-ternary phase diagram showed the presence of five different phase systems, namely, S/L phase, Winsor I, Winsor II, Winsor III, and Winsor IV. The ternary diagram region under the most desirable phase system for enhanced oil recovery applications (Winsor III) was found to increase with increase in the Kcs ratio. With an increase in Kcs value, the droplets size distribution profile was found to shift more toward the left, showing that the volume fraction of lowdiameter aggregates increases in a microemulsion region at a higher content of propan-2-ol (cosurfactant). This has the desired effect of improving the thermodynamic stability of the microemulsion phase. Phase dilution experiments revealed that the formation of SMES microemulsion systems at Kcs = 1, 2, 3 were thermodynamically spontaneous. The stability of Winsor III systems improved with increase in Kcs ratio. Optimum solubilization of oil and water was achieved for salinity values of 6.0%, 4.2%, and 3.6% NaCl for systems with Kcs = 1, 2, and 3. IFT values at optimal salinity were found to be in the ultralow range of the order of 10−3 to 10−4 mN/m. The phase transition model was instrumental in estimating the phase behavior of any microemulsion state at any composition and vice versa. The phase transition model encompassed the concept of two coexistent phase systems at points on the fitted regression curves. An increase in NaCl concentration increased the relative volume fractions of the middle-phase microemulsion in stable three-phase systems. An increase in water content reduced the relative phase volume of Winsor III microemulsions formed by n-heptane/SMES/propan-2-ol/brine systems. A salinity increase caused a transition of phase from Winsor I to Winsor II via Winsor III. A rise in temperature exhibited reverse phase transformation, that is, from Winsor II to Winsor III. Investigations carried out on the phase behavior, solubilization, and phase transition of microemulsion systems under different conditions of salinity, cosurfactant content, water percentage, and temperature show high potential in enhanced oil processes.

interfacial activity. However, beyond 6% NaCl, Winsor III ceases to exist and the Winsor II phase appears. This phase transition behavior (Winsor I to Winsor II via Winsor III) may be explained by the prediction of variations of microemulsion structure at different salinities. Initially the Winsor I phase is observed at no and low salinities, the hydrophobic tail regions are sequestered and the polar head groups are in contact the surrounding microemulsion phase. As NaCl content is increased repulsive forces between the polar groups reduce and form clusters with hydrophilic heads sequestered at the center. This results in the gradual inversion of structure in the microemulsion phase and subsequent appearance of the Winsor III system. Beyond 6% NaCl, the Winsor II phase appears. As a result, phase transition occurs from an oil-in-water microemulsion to a middle-phase microemulsion to a water-in-oil microemulsion.60−62 3.6.2. Effect of Water Content. The effect of water content on volume fractions in the heptane/propan-2-ol/SMES/water systems with Kcs = 1, 2, 3 at 298 K are shown in Figure 14 panels a, b, and c, respectively. The weight ratio of n-heptane to (propan-2-ol + SMES) is fixed at 3.50 and salinity of the system is 4% NaCl. Winsor III phase is observed for percentage of water solution in the system in the range from 30% to 70%. The Winsor III microemulsion region initially decreases with increasing water content and, at higher brine percentages, almost remains constant at the expense of upper excess oil phase. This may be due to decrease in excess area of amphiphilic region.28,63 Addition of aqueous solution to the microemulsion solution increases lower aqueous volume and microemulsion phase is reduced by displacement of lighter oil in microemulsion phase with denser brine solution in the middle region. 3.6.3. Effect of Temperature. Influence of temperature on phase transformation and relative volume percentages of different phases for oil/surfactant/cosurfactant/water systems with composition of n-heptane (45 wt %), surfactant/ cosurfactant mixture (10 wt %) and water (45 wt %) at Kcs = 1, 2, 3 are shown in Figure 15 panels a, b, and c, respectively. System salinity is controlled at 4% NaCl. Phase behavior of the system at 293 K is Winsor II due to complete dilution of organic molecules (oil, cosurfactant) in the microemulsion phase. As the temperature approaches 298 K Winsor III behavior is observed. The relative phase volume of excess brine remains unchanged with increase in temperature. A rise in temperature enhances the forces of repulsion between the hydrophilic head groups and causes the upper oleic phase to extrude out of the microemulsion phase due to improvement in intramolecular motion. This prevents surfactant retention in the otherwise excess oil dissolved in the upper microemulsion phase in Winsor II. Inverted micelles present in the Winsor II phase change their structure to normal micelles due to extrusion of the oil phase from the microemulsion region. Thus, reverse phase transformation from Winsor II to Winsor III is beneficial for oil recovery in reservoirs.27

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ajay Mandal: 0000-0003-2947-4261 Notes

The authors declare no competing financial interest.

■

REFERENCES

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