Phase Behavior of Sodium Dodecyl Sulfate-n-Butanol-Kerosene-Water Microemulsion System*

LIU Huie (劉會(huì)娥)**, ZHANG Xiaokun (張孝坤), DING Chuanqin (丁傳芹), CHEN Shuang (陳爽) and QI Xuanliang (齊選良)State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266555, China

Phase Behavior of Sodium Dodecyl Sulfate-n-Butanol-Kerosene-Water Microemulsion System*

LIU Huie (劉會(huì)娥)**, ZHANG Xiaokun (張孝坤), DING Chuanqin (丁傳芹), CHEN Shuang (陳爽) and QI Xuanliang (齊選良)
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266555, China

Experiments were carried out to investigate the influences of cation from electrolytes and acidity/alkalinity on the phase behavior of sodium dodecyl sulfate-n-butanol-organics-water (with electrolytes) microemulsion system. The organics used is commercial kerosene. The volume ratio of water to organics is 1︰1. The results show that the type and valence of electrolyte cations are important factors influencing the microemulsion behavior. Bivalent Ca2+ is more effective than monovalent K+ and Na+ for the formation of Winsor type III and II microemulsion. For electrolytes with the same monovalent cation Na+, i.e. NaCl and Na2CO3, anions in the electrolyte have some effect. Bivalent anion 2 CO3? leads to a lower activity of cation Na+ than monovalent anion Cl?. NaOH (or KOH) behaves similar with NaCl (or KCl). When HCl is used as electrolyte, its acidity plays an important role. Phase inversion of microemulsion from type III (or II) to type I is observed through precipitation of Ca2+ using Na2CO3, neutralization of HCl by NaOH, and addition of water to the system, which releases the oil from the microemulsion.

microemulsion, sodium dodecyl sulfate, kerosene, phase inversion

1 INTRODUCTION

Microemulsions are thermodynamically stable, optically transparent, isotropic dispersions of aqueous and organic liquids stabilized by an interfacial film of surfactant molecules [1]. Three types of microemulsion systems are found with the change of hydrophile-lipophile balance (HLB), i.e., Winsor type I, II and III microemulsion systems. Winsor type I system is an O/W microemulsion in equilibrium with excess oil, which, in the form of oil-swollen micelles in aqueous phase, is water soluble. Winsor type II is a W/O microemulsion in equilibrium with excess water, which is oil soluble and exists in the form of water-swollen micelles in the organic phase. Winsor type III is a middle phase microemulsion coexisting with excess water and organic phases, which has a bicontinuous structure and contains large quantities of organics and water. It can be considered as an accumulation of swollen micelles, which are so numerous that they touch one another, forming dispersion or a perfectly bicontinuous structure with all water domains connected and all oil domains connected likewise [2]. The microemulsion systems have the advantages of high interfacial area and ultra-low organics/water interfacial tension.

Microemulsion is an efficient tool in the enhanced oil recovery (EOR) [3-6], because it can provide high levels of extraction. For example, Santana et al. [6] observed that with the commercial surfactant-based MCS microemulsion, a recovery factor as high as 87.5% was obtained. A modification and extension of the EOR concept is the environmental applications, such as the remediation of organic-polluted soil [7-13] or groundwater aquifers contaminated by non-aqueous phase liquids [14-18]. Different research groups [e.g. 7, 12, 14] have given similar results that for those surfactant-based washing agents, micro-emulsification (forming Winsor type III microemulsion) of organic contaminants in the processes presents higher de-polluting efficiency than others.

However, the economics of surfactant-based remediation technologies benefits from material separation and recycling of surfactant [19, 20]. For processes with high organics content of Winsor type III or II microemulsions in contaminant removal, shifting of microemulsion from Winsor type II or III to type I is an attractive method for organics separation and surfactant reuse. During the phase shifting from Winsor type II or III to type I, most organics is separated as free phase, and through reverse phase shifting, that is, Winsor type I to type III or II, the reuse of surfactants can be realized. What factors influence the phase behavior of a microemulsion system is a crucial point for this problem.

Chai et al. [21] found that there existed differences in the state with different electrolytes in sodium dodecyl sulfate (SDS) microemulsion systems. Both aliphatic acid and short chain alcohol were used as co-surfactants in their work. For the salts with the same anion but different cations (MgCl2, CaCl2, SrCl2), the solubility of alcohol (Sa) and the solubilization ability (SP) are in the same order of MgCl2>CaCl2～SrCl2, no matter in the aliphatic acid-based or in the alcoholbased microemulsion systems. For salts with the same cation but different anions [CaCl2, CaBr2, Ca(NO3)2], Saand SP are also in the same order of Ca(NO3)2>CaBr2>CaCl2for the two kinds of microemulsion systems. Anton and Salvager [22] investigated the anionic surfactant-oil-water-alcohol microemulsion systems by using sodium salts with different anions from monovalence through tetravalence. Oil phases with different equivalent alkane carbon number were used, with petroleum sulfonate sodium salts as the surfactant and sec-butanol as the cosurfactant. For the sodium salts, the correspondent anion valence showed important influence on the electrolyte activity and a correlation was given for the optimum formulation of anionic surfactant-oil-water systems. Puerto and Reed [23] found that for monovalents Li+, Na+and K+, whose hydration radii are in the sequence of Li+>Na+>K+, the optimal salinities are in the sequence of K+>Na+>Li+. It was concluded that at constant valence, the greater the hydration radius, the higher the optimal salinity.

Rudolph et al. [24] found that varying oil/water ratio changed the extension of the three-phase region for the oil/water/nonionic surfactant (2-butoxyethanol) system. The experimental results of Raijb and Bidyut indicated that increasing oil/water ratio reduced the solubilization capacity for the Brij-56/1-butanol/ n-heptane/water system, while with negligible influence on the phase behavior of Brij/SDBS mixed system [25]. Chai et al. [26] investigated the influence of oil/water ratio on the phase behavior of SDS/alcohol/oil/water microemulsion system. As the oil/water mass ratio increases, the solubility of alcohol increases while both the mass fraction of the alcohol in the interfacial layer and the solubilization ability decrease.

Kunieda and Shinoda [27] showed a HLB temperature for the aerosol OT-isooctane-brine system with the increase of temperature. A phase inversion from W/O microemulsion to three-phase microemulsion and then O/W microemulsion was observed. The influence of temperature on phase behavior of nonionic surfactant microemulsion system was also reported in [25, 28, 29].

In summary, several variables (e.g., temperature, electrolyte, surfactant and cosurfactant) are important factors influencing the property of a microemulsion system. The phase state of a microemulsion may be changed by changing one of the several variables. Cheng and Sabatini [30] shifted the contaminant-rich Winsor type III microemulsion to type I system through precipitation-based exchange of polyvalent cations (Al3+and Ca2+) with equivalent amount of monovalent cation (Na+). The contaminants used were decane and tetradecane and the surfactant was Alfoterra 145-4 PO sulfate. In this work, the phase behavior of kerosene-water-SDS-n-butanol microemulsion systems is investigated. The effects of electrolytes, including NaCl, KCl, CaCl2, Na2CO3, NaOH, KOH and HCl, are investigated to find an effective way for phase inversion of microemulsions. The influence of acidity and alkalinity and water/oil ratio are also studied. Phase inversion of Winsor type II→III→I→III→II is to be realized through manipulating electrolytes and the acidity/alkalinity. Water is added into the Winsor type II microemulsion system for the shift from Winsor type II→III→I.

2 EXPERIMENTAL

2.1 Materials

The surfactant used in this work was an anionic type, chemically pure sodium dodecyl sulfate (SDS). Other materials used were analytically pure n-butanol, NaCl, NaOH, KOH, KCl, CaCl2, Na2CO3, HCl, deionized water, and simulated contaminant-industrial kerosene [density 840 kg·m?3, distillation range 170-240 °C, total alkane content 50.5% (by mass), total naphthene content 29.9% (by mass), and total arene content 19.6%]. All chemicals were used as received without further purification.

2.2 Preparation of microemulsions

Microemulsions were prepared using SDS, 1︰1 (by volume) deionized water and kerosene, n-butanol and one cation donor (electrolyte) from the above reagents. Both salinity scan and alcohol scan were used to observe the phase behavior of microemulsions. For convenient, the concentrations of reagents were based on the total volume of water and kerosene.

For determined organics, with equal volume of organics and water at fixed surfactant and electrolyte concentration, the microemulsion will change from Winsor type I→III→II with the increase of alcohol concentration. This is called alcohol scanning method.

On the other hand, for determined organics, with equal volume of organics and water at fixed surfactant and cosurfactant concentration, if the concentration of electrolyte in the system increases, the system will also change from Winsor type I→III→II. This is called salinity scanning method.

Taking the phase volume data during the alcohol or salinity scan, simple phase diagram can be made to show the phase state and the oil-solubilization capacity of each state. The schematic phase diagram is shown in Fig. 1. To show the phase state, a pair of curves is needed. The distance between the upper curve and the 100% line is the volume fraction of oil phase, that between the two curves is the volume fraction of microemulsion phase and between the lower one and abscissa axis is that of water phase. It is Winsor type I when the lower curve coincides with the abscissa, Winsor type II when the upper one coincides with the 100% volume fraction line and Winsor type III in between.

3 RESULTS AND DISCUSSION

3.1 Influence of electrolyte cations on phase behavior

To observe the influence of cation types on the microemulsion phase behavior, CaCl2, KCl, NaCl andNa2CO3are used as the electrolyte separately. 10 ml kerosene and 10 ml water were used, with the concentration of SDS and n-butanol being 0.14 and 1.73 mol·L?1, respectively. Fig. 2 shows the salinity scanning results for different types of electrolytes for the SDS-n-butanol-kerosene-water microemulsion system. The system changes from Winsor type I→III→II with increasing electrolyte concentration, no matter what kind of electrolyte is used. With the addition of electrolyte, the critical micelle concentration of anionic surfactant SDS greatly decreases, while the aggregation number of micelle increases and micelles get bigger, solubilizing more oil.

Figure 1 Schematic phase diagram for microemulsion during salinity or alcohol scan

Figure 2 Phase diagram for SDS-n-butanol-kerosene-water microemulsion system using different electrolytes

The results also show that each type of electrolyte has its typical length of salinity for the existence of Winsor type III microemulsion. Under the conditions of Vkerosene︰Vwater=1︰1, c(SDS)=0.14 mol·L?1and c(n-butanol)=1.73 mol·L?1, for electrolytes CaCl2, KCl, NaCl, and Na2CO3, their concentrations for forming Winsor type III microemulsion are 0.041-0.099, 0.15-0.30, 0.19-0.43 and 0.21-0.47 mol·L?1, and the salinity length is 0.058, 0.15, 0.24 and 0.26 mol·L?1, respectively. High surface charge density of Ca2+makes it more effective than K+and Na+in decreasing the HLB of surfactant system, and much easier for the formation of Winsor type III and II microemulsion. At the same time, the effect of K+is stronger than Na+, similar to the results obtained by Aarra et al [31]. As to the effect of NaCl and Na2CO3, it is found that NaCl is more effective than Na2CO3although they have the same monovalent cation, Na+. Anton and Salager [22] gave similar results. They put forward a concept of “valence activity factor (VAF)”to indicate the active fraction of sodium cations, COleads to a lower activity of the sodium salt than monovalent Cl?.

It is attractive that when Winsor type II or III microemulsion is obtained using CaCl2, it may be Winsor type I for NaCl under the same valence number (see Fig. 2). Therefore, if Ca2+is replaced by Na+, Winsor type II or III microemulsion may convert into type I, and most of the oil solubilized in the microemulsion is released. Tests were carried out to approve this supposition. CaCl2was first used to prepare Winsor III microemulsion, with the volume fraction of microemulsified kerosene being 35% and then Na2CO3added into the system, causing Ca2+to precipitate as CaCO3. Thus Ca2+in the system was substituted by Na+. Fig. 3 (a) shows the phase behavior during this substituting process. Phase inversion from Winsor type III→I can be observed with the substitution of where Z is the valence of the anion. Eq. (1) indicates that the higher the anion valence, the lower its VAF, i.e. the less active the sodium salt. Thus bivalentCa2+by Na+. The volume fraction of solubilized kerosene in the Winsor type I microemulsion is only 7.6%. After exchanging Ca2+with Na+through precipitation, 74% of the oil in the microemulsion phase is released to the free oil phase. With further addition of Na2CO3, the microemulsion system changes from Winsor type I→III→II. The volume fraction of kerosene in the microemulsion phase gradually changes from 7.6% to 65%.

Winsor type II microemulsion is also formed initially using CaCl2, and then Na2CO3is added gradually. The phase diagram is shown in Fig. 3 (b). For the Winsor type II microemulsion, the volume fraction of kerosene is 67%. When proper amount of Ca2+is substituted by Na+, Winsor type I microemulsion forms, in which the volume fraction of kerosene is only 11%. It means that 85% of the oil is released from the microemulsion. Similarly, with further addition of Na2CO3, the microemulsion system changes from Winsor type I→III→II, and more oil goes into the microemulsion again.

In summary, after the replacement of Ca2+by Na+, Winsor type III (or II) goes to type I microemulsion, whose oil solubilization capacity is relatively small, releasing most of the oil initially contained. With the addition of cation Na+, Winsor type I microemulsion converts into type III (or II) again and more oil goes into the microemulsion gradually. Thus, the system will repeatedly convert between Winsor type III (or II) and I with the precipitation and re-dissolution of Ca2+(just as the addition of cations). This is a promising way for the recovery of organic contaminants and reuse of the surfactant system repeatedly. However, the content of Na+increases monotonously during the Ca2+precipitation and re-dissolution process, with no replacement of Na+taking place, which will lead to the end of the recycle because the cation concentration is too high eventually. The replacement of Ca2+with Na+and that of Na+with Ca2+are the keys for the repeated inversion between Winsor type III (or II) and I microemulsions, which is still under investigation in our laboratory.

Figure 3 Phase inversion through cation substitution [Vkerosene︰Vwater=1︰1, c(SDS)=0.14 mol·L?1, c(n-butanol)=1.75 mol·L?1] O—oil phase; M—microemulsion phase; W—water phase

3.2 Influence of acidity and alkalinity on phase behavior

The results of Section 3.1 show that different types of cations have different effects on the phase behavior of microemulsion. The effects of monovalents H+, Na+and K+are compared and analyzed further in this section. The electrolytes used include HCl, NaCl, NaOH, KCl, and KOH, to find any special information about the H+cation or about acidity and alkalinity.

The phase diagram obtained through salinity scan and alcohol scan are shown in Fig. 4. With the increase of salinity or alcohol concentration, the phase inversion from Winsor type I→II→III can be observed. Fig. 4 (a) shows that each type of monovalent cation has its typical length of salinity for forming Winsor type III microemulsion. Under the conditions of Vkerosene︰Vwater=1︰1 (10 ml︰10 ml), c(SDS)= 0.14 mol·L?1and c(n-butanol)=1.42 mol·L?1, for electrolytes HCl, NaCl, NaOH, KCl and KOH, the concentration for forming Winsor type III microemulsion are 0.15-0.23, 0.22-0.42, 0.22-0.42, 0.17-0.29, 0.17-0.29 mol·L?1, and the length of salinity is 0.54, 0.20, 0.20, 0.12 and 0.12 mol·L?1, respectively. Fig. 4 (b) is the phase diagram obtained from alcohol scan, the alcohol concentration for forming Winsor type III microemulsion is 0.88-1.42, 1.53-2.19, 1.53-2.19, 1.15-1.81 and 1.15-1.81 mol·L?1for HCl, NaCl, NaOH, KCl and KOH, respectively.

It is interesting that the microemulsion phase diagram using KCl (or NaCl) is almost identical tothat using KOH (or NaOH). That is, for the same monovalent cation (K+or Na+), the type of monovalent anion (Cl?or OH?) has little influence on the state of SDS-n-butanol-kerosene-water microemulsion under the conditions in this work, even though KOH (or NaOH) is alkalis. With the addition of electrolytes into the microemulsion system, the counterion concentration increases, compressing the electrical double layer and depressing the electrostatic repulsion between the polar heads of surfactant. SDS is an anionic surfactant. The cation, Na+or K+, is the conterion that influences the electrical double layer, while the type of monovalent anion (no matter Cl?or OH?) of the electrolyte shows little influence. Extended conditions are still under investigation in our laboratory to justify this observation.

The results in Fig. 4 show that the effect of the monovalent cations on microemulsion phase behavior decreases in the order of H+>K+>Na+. Puerto and Reed [23] considered that the greater the hydration radius, the higher the optimal salinity at constant cation valence. According to the analysis, the effect of monovalent cations in this work should be K+>Na+>H+, because the hydration radius is in the sequence of K+

Obvious difference in state exists between the microemulsion systems using HCl and NaCl as electrolyte, as shown in Fig. 4. For the kerosene-water microemulsion system [Vwater︰Vkerosene=1︰1, 10 ml for each, c(SDS)=0.14 mol·L?1and c(n-butanol)= 1.42 mol·L?1], when Winsor type III or II microemulsion is formed using HCl as electrolyte, it may be type I for NaCl. Thus if Winsor type III or II microemulsion is formed using HCl, it will convert into type I with the substitution of H+by Na+.

A test for phase inversion through acid-base neutralization was carried out. Under the conditions of c(SDS)= 0.14 mol·L?1, c(n-butanol)=1.42 mol·L?1and c(HCl)=0.23 mol·L?1, a Winsor type II microemulsion was formed initially. Then, NaOH was added into the system gradually. The microemulsion phase diagram is given in Fig. 5.

Figure 4 Effect of acidic and alkaline electrolytes on microemulsion phase state [Vkerosene︰Vwater=1︰1 (10 ml for each), c(SDS)=0.14 mol·L?1]▼ HCl; △ NaCl; ▲ NaOH; ○ KCl; ● KOH

Figure 5 Phase diagram through acid-base neutralization [Winsor II initially, c(SDS)=0.14 mol·L?1, c(n-butanol)=1.42 mol·L?1, c(HCl)=0.23 mol·L?1] O—oil phase; M—microemulsion phase; W—water phase

The inversion of Winsor type II→III→I→III→II is observed. With the addition of NaOH, HCl is neutralized and H+is substituted by Na+gradually. With the NaOH added into the system and 0.23 mol·L?1reached, all of H+should combine with OH?. All the effective cations in the system should be Na+at this point and Winsor type I microemulsion is observed, which is consistent with the results in Fig. 4 (a). Duringthis process, the content of kerosene decreases from the initial 71% (by volume) in the Winsor type II microemulsion to 12% (by volume) in the type I microemulsion, with 82% oil releases from the microemulsion phase. With further addition of NaOH, the concentration of Na+cation increases and more oil is solubilized into the microemulsion again. Winsor type II microemulsion is formed in the end.

Just as the substitution of Ca2+by Na+in Section 3.1, H+is replaced by Na+during the acid-base neutralization process. Similarly, if effective way of Na+substitution by H+can be provided, the solubilization and release of organic contaminants and thus the reuse of surfactant system can be repeated ideally, which is our aim in the future work.

3.3 Influence of water/oil ratio on phase behavior

Under the conditions of Vkerosene︰Vwater=1︰1 (10 ml︰10 ml) and the concentrations of n-butanol, SDS and NaCl being 2.07, 0.14 and 0.32 mol·L?1, respectively, Winsor type I microemulsion was formed, as shown in Fig. 6. Water was then added gradually into the system so as to change the water/oil ratio in the system. It is attractive that the change from microemulsion Winsor type II→III→I occurs and more and more oil is released gradually. When the volume of water increases to 18 ml, 9 ml or 90% (by volume) oil is released from the microemulsion.

Figure 6 Influence of water volume on phase behavior of SDS-n-butanol-kerosene-water microemulsion system [Vkerosene︰Vwater=1︰1 (10 ml︰10 ml), c(n-butanol)=2.07 mol·L?1, c(SDS)=0.14 mol·L?1, c( NaCl)=0.32mol·L?1]

Tongcumpoua et al. [8] also found that the interfacial tension between oil and water changed with the ratio of oil to water, so a phase inversion may take place. According to the description of Aarra et al. [31], for Winsor type III microemulsion, electrolyte cations are partitioned in the excess water phase and microemulsion phase, while Na+shows a strong tendency to partition in the excess water phase for a SDS-heptanewater-1-butanol-NaCl system. Bellocq et al. [32] gave similar results for SDS-toluene-water butanol-NaCl system. The results of Aarra et al. [31] were consistent with the calculation results from Robertson’s model [33]. According to Robertson’s model, the water in the surfactant/water pseudocomponent does not contain electrolyte. The remaining bulk water in the microemulsion has the same salinity as the excess water. It is the equilibrium excess-water-phase salinity that controls the phase behavior. The release of oil from Winsor type II or III microemulsion with water addition in this work can be also explained by Robertson’s model [33]. The addition of water reduces the excess-waterphase salinity, and a salinity low enough leads to the phase inversion to Winsor type I.

These phenomena mean that when organic contaminants are transferred into Winsor type II or III microemulsion for disposal of organic contaminants, water may be added to release the organics from the microemulsion and the surfactants may be reused, although a great amount of water is needed.

4 CONCLUSIONS

The experimental results show that the type and valence of electrolyte cations are important factors influencing the phase behavior of the SDS-n-butanolkerosenewater microemulsion system. High surface charge density of bivalent cation (Ca2+) makes it more effective in adjusting the HLB of the SDS surfactant system, and much easier for the formation of Winsor type III and II microemulsion than monovalent cations (K+ and Na+). For the same monovalent cation (Na+), the valence of correspondent anions in the electrolyte show some influence on its effect. Bivalent anion ( 2 CO3? ) leads to a lower activity of the cation (Na+) than monovalent anion (Cl?). For the same type of cation (K+ or Na+), the type of monovalent anion (Cl?or OH?) in the electrolyte has little influence on the microemulsion state under the operation conditions in this work, even though KOH (or NaOH) is alkalis. The cation H+ in the electrolyte HCl has strong effect on the formation of Winsor type III microemulsion. The acidity plays an important role in this process.

Winsor type III (or II) microemulsion changes to type I with the precipitation of Ca2+ using Na2CO3, neutralization of HCl by NaOH and addition of water into the system. Most of the organics enters into the free phase during this process. These may give us some inspiration for the release of organics from type III (or II) microemulsion and the recycling of surfactants.

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2013-01-13, accepted 2013-06-08.

* Supported by the National Natural Science Foundation of China (21106187), Promotive Research Funds for Excellent Young and Middle-aged Scientists of Shandong Province (BS2011NJ021), the Fundamental Research Funds for the Central Universities (11CX05016A), and the Graduate Innovation Project of CUP 2012 (CX-1214).

** To whom correspondence should be addressed. E-mail: liuhuie@upc.edu.cn