Physicochemical behaviour of partially miscible multicomponent systems with AOT: Liquid–liquid phase diagrams, density of conjugate phases, and interfacial tension

Colloids and Surfaces A: Physicochem. Eng. Aspects 368 (2010) 64–74

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Physicochemical behaviour of partially miscible multicomponent systems with AOT: Liquid–liquid phase diagrams, density of conjugate phases, and interfacial tension夽 Blanca Estela García-Flores, Arturo Trejo ∗ , Jacinto Águila-Hernández Instituto Mexicano del Petróleo, Dirección de Investigación y Posgrado, Programa de Ingeniería Molecular, Área de Investigación en Termofísica, Eje Central Lázaro Cárdenas Norte 152, 07730 México, DF, Mexico

a r t i c l e

i n f o

Article history: Received 27 April 2010 Received in revised form 14 July 2010 Accepted 15 July 2010 Available online 23 July 2010 Keywords: Density Hydrocarbons Interfacial tension Liquid–liquid equilibrium Methanol Sodium bis(2-ethylhexyl)sulfosuccinate (CAS No. 577-11-7) Solubilization enhancement Surfactant Tie-lines Water

a b s t r a c t Experimental liquid–liquid phase diagrams have been obtained for five-component systems containing 2,2,4-trimethylpentane (isooctane), benzene, methanol, water, and sodium bis(2-ethylhexyl) sulfosuccinate (Aerosol OT), at 298.15 and 308.15 K. The density and interfacial tension of conjugate phases whose equilibrium concentration is located on the isothermal binodal curve have also been experimentally determined at 298.15 K for the five-component systems. The phase diagram is of type II when the multicomponent system has 5 mass% of AOT. The overall miscibility of the system is enhanced with 15 mass% of AOT, at 298.15 K, hence the phase boundary decreases and the phase diagram changes to type I. Both the density of each conjugate phase and the interfacial tension of each tie-line are valuable indicators of the degree of miscibility of the multicomponent systems. Water solubility in the hydrocarbon-rich phases is greatly increased due to the formation of reverse micelles with AOT. The experimental tie-lines results were regressed with the NRTL and UNIQUAC solution models with satisfactory quantitative reproduction of the results. © 2010 Elsevier B.V. All rights reserved.

1. Introduction The use of oxygenated compounds in the reformulation of gasoline is becoming a common practice in several countries, in particular alcohols are considered as the best option. Methanol blends are a good selection as vehicular fuel because of methanol’s physicochemical properties [1–3]; it also enhances gasoline’s octane number and reduces CO and hydrocarbon emissions. The use of methanol as fuel has been widely discussed under environmental, technological, economical, and energy security factors [4]. Methanol is probably the most attractive alternative vehicle fuel as it can be produced economically from a variety of sources, such as syngas, natural gas, coal, petroleum, and biomass. Furthermore, there exist many other important applications of methanol in the gas and oil industry, e.g. conversion of methanol to methyl-tertbutyl ether and reforming of methanol to hydrogen [5], methanol

夽 This paper is dedicated to Professor Fernando del Río (Universidad Autónoma Metropolitana-Iztapalapa; México), to commemorate his 70th birthday. ∗ Corresponding author. Tel.: +52 55 9175 8373; fax: +52 55 9175 6312. E-mail address: [email protected] (A. Trejo). 0927-7757/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2010.07.018

can be used in several stages of natural gas processing (mainly in dehydration plants and gas sweetening processes) [6], methanol is widely used as a hydrate inhibitor in very low temperature applications such as in turbo expanders for gas refrigeration plants of LPG recovery and hydrocarbons flow assurance [7]; methanol as a fuel for fuel cells is a subject of great interest, hence, large efforts are devoted to applications in transport and portable electronics applications due to the clear advantages for the protection of the environment [8–10]. With the aim of continuing assessing methanol’s physicochemical attributes in fuels applications our Laboratory has performed several studies on the solubility limits of methanol in gasoline models with and without the presence of water, as a function of temperature and under atmospheric pressure [11–13]. A recent comprehensive bibliographic search on experimental results for binary, ternary, and multicomponent systems shows that methanol is partially miscible with linear and branched paraffins and also with naphtenic hydrocarbons [14]. Although methanol is more soluble in aromatic hydrocarbons, the simultaneous presence of the different types of hydrocarbons already mentioned in gasoline leads to the presence of two liquid phases in well defined concentration regions, when methanol is used to produce oxygenated

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Nomenclature CAOT F r, q T xi wo wijk

concentration of AOT in mass% objective function structural parameters in the UNIQUAC model temperature in K mole fraction of the i component molar water-to-surfactant ratio weighting factors

Greek letters ˛ characteristic parameter of the NRTL model  activity coefficient  interaction parameters of the solution models  density  interfacial tension x standard deviation Superscripts I, II liquid phases in equilibrium ∧ estimated value Subscripts i component j phase k tie-line

gasoline [15,16]. Methanol-gasoline blends are sensitive to the presence of water and due to its polar nature, methanol molecules strongly associate with water molecules through hydrogen bonds which also results in the separation of the oxygenated gasoline into two liquid phases. A relatively small amount of water reduces the miscibility of methanol with mixtures of hydrocarbons as has been shown through phase diagrams in previous publications [11,12]. The phase separation of methanol-gasoline blends caused by the presence of water can be prevented through the mechanism called co-solvency [17,18]. To determine quantitatively the solubility enhancement due to co-solvency, an additive has been used in different concentrations in the model system of gasoline isooctane–benzene–(80 mass% methanol–20 mass% water) [13]. Isobutyl alcohol was used as cosolvent because it acts as a pseudosurfactant in the studied multicomponent systems; hence the interfacial tension of the equilibrium liquid phases decreases as the concentration of isobutanol increases. Therefore, the present work has been focused on investigating the solubilization of the multicomponent system formed by 2,2,4-trimethylpentane (isooctane) + benzene + (80 mass% methanol + 20 mass% water) with the addition of known quantities of the ionic surfactant sodium bis(2-ethylhexyl) sulfosuccinate (Aerosol OT or AOT, with linear chemical formula C20 H37 NaO7 S) through the experimental determination of complete liquid–liquid phase diagrams at 298.15 and 308.15 K, under atmospheric pressure. The study also includes the experimental determination of density for the conjugate phases and the interfacial tension with the aim of relating the solubilization phenomenon with the physicochemical behaviour of the interfaces. The experimental tie-line data for multicomponent systems studied were successfully regressed with the NRTL [19] and UNIQUAC [20] solution models. The use of AOT in this work is justified because it is well known that relatively large amounts of water can be dissolved in reverse micelles of AOT/nonpolar systems. Water is dispersed as minute spherical droplets coated and stabilized by a monolayer of oriented surfactant molecules through hydrogen bonds with the sulfo

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group (i.e. SO3 ) of AOT, whose hydrophobic chains interact with the bulk nonpolar organic solvent [21,22]. Thermodynamic and spectroscopic behaviour of water and aqueous solutions confined in AOT reverse micelles have been reported [21,23–27]. Fewer studies have been reported on the solubilization through reverse micelles of other polar hydrogen bonded substances such as methanol in AOT [28–30]. Therefore, both water and methanol, as polar solvents, should be incorporated into the micelar core giving rise to overall greater miscibility of the multicomponent systems considered here. Liquid–liquid interfacial tension together with density of conjugate phases, at atmospheric pressure, have been reported in the literature for ternary systems whose components include either water, an alcohol or a hydrocarbon [31–39], however no data were found for the multicomponent systems studied in this work. 2. Experimental 2.1. Materials The chemicals methanol (methyl alcohol), ethanol (ethyl alcohol) -used as internal standard- and benzene were supplied by Baker (Xalostoc, Mexico), whereas 2,2,4-trimethylpentane (isooctane) was provided by Sigma–Aldrich. All the samples were HPLC grade and their purity is reported by the suppliers to be higher than 99 mol%. Sodium bis(2-ethylhexyl) sulfosuccinate, with purity greater than 99 mol%, SigmaUltra, was purchased from Sigma–Aldrich. Methanol and isooctane were twice distilled in order to eliminate impurities [11–13]. Doubly distilled and deionized water with a conductivity of 17 ␮/cm was used for all measurements. The other substances were used without any further purification. The hydrocarbon samples were stored over sodium whereas methanol and ethanol were stored over molecular sieve. No impurities were detected in the liquid samples using gas chromatography with a thermal conductivity detector (TCD). AOT was dried at high temperature and under vacuum for several days. A Karl-Fisher titration apparatus, Photovolt Aquatest 8, was used to monitor the water content in each pure sample during present study. The maximum water content in mass fraction found in methanol and ethanol was 3 × 10−3 % and 0.5% in AOT. 2.2. Methods 2.2.1. Chromatographic calibration curves In order to quantify the concentration of each component in the equilibrium liquid phases of each studied tie-line the internal standard method was used to obtain the response factors or calibration of the TCD’s response (i.e. chromatographic areas) as a function of known concentrations of the different components considered in the multicomponent system. The method has been described in detail in previous publications [11–13]. As in previous work, it is of interest to underline that each point on the chromatographic calibration curve for each compound was obtained using multicomponent mixtures of different known concentration prepared in the completely miscible region. Ethanol was chosen as internal standard because it has chromatographic properties, i.e. retention time, close to those of the compounds in the considered multicomponent systems. Since AOT as a pure compound has a high melting point (173–179 ◦ C) and consequently a very high boiling point, in the present work it was necessary to protect the chromatographic column from AOT through an experimental setup in the chromatograph. The arrangement consisted in adapting a separation column immediately after the chromatograph injector and before the analytical column to retain the AOT present in the liquid

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multicomponent samples to be analyzed for the liquid–liquid equilibrium studies. This arrangement was successful in protecting the analytical column and at the same time generating reliable quantitative results for the concentration of the different components in the studied conjugate phases. All the homogeneous mixtures of known concentration used for obtaining the calibration curves of each of the components of the quaternary system were prepared by mass. To determine the mass of each compound in the different multicomponent mixtures, including a constant amount of the internal standard, a Sartorious 1006 MP analytic balance with a precision and accuracy of ±0.0001 g was used. The mixtures of known concentration as well as the tie-line phases were analyzed in a Varian Model 3400 gas chromatograph using a Porapak-Q packed column (with bonded polystyrene-divinylbenzene for the separation of nonpolar and polar compounds). The chromatograph is controlled by a personal computer and commercial software. The chromatographic calibration results for each of the four components included in the system isooctane + benzene + (80 mass% water + 20 mass% methanol) were reported previously [12]. Each calibration curve was obtained with nine experimental points and each of them was the average of at least five injections of the quaternary system of known concentration into the chromatograph. Since in this work AOT has been added to the quaternary system we performed a test on the experimental setup mentioned above using four of the mixtures of known overall concentration used in the calibration. Each mixture was divided into two samples which were additioned with 5 and 15 mass% of AOT, respectively. The chromatographic results for the eight systems clearly showed that AOT was totally separated in the adapted pre-column, hence the averaged chromatographic areas for each of the other four components agreed with the calibration results obtained from the quaternary system free of AOT. A statistical analysis that considered all the involved variables along all the steps of the experimental work established the uncertainty of the equilibrium concentration in mole fraction of the experimental tie-lines obtained in this work to be ±0.0045 for isooctane, ±0.0030 for benzene, ±0.0063 for methanol, and ±0.0013 for water. 2.2.2. Liquid–liquid phase behaviour The experimental investigation of the liquid–liquid equilibrium (LLE) in the multicomponent systems was carried out using equilibrium cells built of Pyrex glass with a total volume of 25 cm3 . This volume was enough to take samples from the two equilibrium liquid phases corresponding to each tie-line to perform the chromatographic quantification of their concentration, to measure the density of each conjugate phase, and determining the interfacial tension between the two phases. The characteristics of the equilibrium cells and the complete experimental setup were described in previous works [13,40]. The temperature in the equilibrium cells was kept constant within ±0.003 K by circulating water as thermal fluid with a bath-circulator, Julabo F70. The temperature in the cells was measured with a Systemteknik AB digital thermometer, model S 1220, using platinum resistance probes with precision of ±0.001 K, whose readings in the range 273–315 K were compared with a calibrated Hart Scientific thermometer; model 1529 Chub-E4, with an accuracy of ±0.005 K traceable to the US National Institute of Standards and Technology (NIST). The accuracy of the temperature measurements with the Systemteknik thermometer, through comparisons in a narrow range with the reference thermometer, was ±0.02 K. In order to completely define the binodal curve of each studied quinary system with several different tie-lines, six quaternary mixtures of isooctane + benzene + (80 mass% methanol + 20 mass% water) of known concentration were prepared by mass. The final concentration of each quinary system was achieved by adding 5 or

15 mass% of AOT. Once each of the multicomponent systems was transferred into its respective equilibrium cell stirring was carried out with magnetic bars during a period of at least 8 h, then allowed to settle for a 16 h period to obtain two liquid phases in equilibrium in each cell, at 298.15 or 308.15 K. Samples from both phases were taken simultaneously through the sampling glass capillaries in the equilibrium cells [11–13,40] using glass syringes with a capacity of 5 cm3 . For the LLE measurements a constant mass of the internal standard, ethanol, was added to all the samples of the equilibrium phases, which after thorough mixing were injected with a precise syringe for analysis into the chromatograph. The specific chromatographic conditions used to carry out the quantification of the concentration of the components in the different samples were: the detector and the injector temperatures were kept at 553.15 K, the column had a temperature program of 433.15–523.15 K with flow rate of 40 ml min−1 with a initial time of 1.5 min and the last temperature of 3.8 min [41]. The reported values of equilibrium concentrations of the conjugate phases represent the average of at least five injections of the same equilibrium sample into the chromatograph. 2.2.3. Density Density values of each conjugate phase were measured with a Picker–Sodev densimeter, model 03-D, based on the technique of the mechanical oscillator developed by Kratky et al. [42,43]. The temperature of the studied samples was controlled within ±0.002 K using a Haake A80 bath-circulator and measured with a digital thermometer, Hewlett Packard 2804 A, using a quartz sensor with a precision of ±0.001 K. The readings from this thermometer in a short range were compared with those from a calibrated thermometer Hart Scientific, model 1529 Chub-E4, whose accuracy of ±0.005 K is traceable to the US NIST. As above, the accuracy of the temperature measurements with the HP thermometer, through comparisons in a narrow range with the reference thermometer, was established to be ±0.02 K. The details on the calibration of the densimeter together with the experimental procedure have been included in several previous publications [13,40,44–50]. The reported density results for each saturated liquid phase were obtained from the average period of vibration, which was in turn obtained from at least 20 stable measurements. The final uncertainty of the density results was evaluated through a complete statistical analysis on the propagation of uncertainties for all the known variables involved that considered the use of the so-called Student’s t distribution. The uncertainty reported in this work was always determined with a 95% confidence for the best values of density experimentally determined. The estimated uncertainty, which is equal to the accuracy, for the experimental density values of this work is ±0.01% in the studied range [13,50]. 2.2.4. Interfacial tension The interfacial tension values of conjugate phases were determined using the pendant drop method with a First Ten Angstrom (FTA), model 200, contact angle system; this is a highly reproducible method and has been reported in previous works from our laboratory [13,40,48,51]. The main components of the apparatus include a light source, a computer-controlled dosing system, a thermal cell, and a video system mounted on an optical platform which shows the pendant drop of the system under study live on the computer screen, so that the images are captured for analysis with a special software for fitting the Laplace–Young equation to the drop shape coordinates [51]. The commercial tensiometer includes a thermal cell with cuvette to carry out the experimental measurements, however, the cuvette has a capacity of 25 cm3 which is too large; therefore it was necessary to design and construct a new

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thermal cell with a 1 cm3 cuvette [13,40]. This cell was successfully adapted to the equipment to accomplish all measurements of interfacial tension. The cell and cuvette were constructed in glass with flat walls to obtain appropriate images of the drop. In this work a drop of the sample of the equilibrium upper phase, always rich in hydrocarbons, was suspended from a needle, which has a ground flat tip, and inserted into the sample of the equilibrium lower phase, always rich in methanol and water. The system (cuvette + liquid–liquid samples) was surrounded by a jacket through which water circulated from a Julabo F70 circulating bath. The temperature in the equilibrium cell was kept constant within ±0.003 K. The temperature was measured in the cell with a Systemteknik AB digital thermometer, model S 1220, using a platinum resistance probe with precision of ±0.001 K, whose readings in the range 273–315 K were compared with a calibrated Hart Scientific thermometer; model 1529 Chub-E4, with an accuracy of ±0.005 K traceable to the US National Institute of Standards and Technology (NIST). The accuracy of the temperature measurements with the Systemteknik thermometer was ±0.02 K. Once the cuvette with the liquid lower phase sample was introduced into the thermal bath, this was positioned in front of the video system mounted on an optical platform of the FTA 200 tensiometer to allow the use of the computer-controlled dosing system that already holds the syringe which contains the hydrocarbonsrich liquid phase sample. The experimental setup made possible to use the automated formation of the pendant drops once the working temperature of the system was constant. After thermal equilibrium was achieved, the pendant drop was formed inside the sample of the liquid lower phase and the final value for the interfacial tension was obtained. The reported interfacial tension results of this work correspond to an average value obtained with at least 20 measurements reported by the calculation procedure implemented by the commercial software of the FTA tensiometer. The accepted standard deviation for each set of 20 measurements was established to be ±0.02 mN m−1 , hence the estimated uncertainty for the reported interfacial tension values is ±0.04 mN m−1 .

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Fig. 1. Liquid–liquid phase diagram for the system isooctane + benzene + [80 mass% methanol + 20 mass% water], at 298.15 K [12]. Experimental equilibrium concentrations in mol% for conjugate phases (䊉); NRTL model (); UNIQUAC model (). Tie-lines and binodal curve are given as full lines (—) [12].

3.1. Liquid–liquid phase behaviour of multicomponent systems

composed of only isooctane and the binary (methanol + water); tielines 2–5 correspond to the quaternary system, hence the conjugate phases are constituted by isooctane + benzene + methanol + water; and tie-line 6 contains benzene and the binary (methanol + water). Fig. 1 includes the calculated equilibrium concentrations for the six tie-lines studied from the NRTL and UNIQUAC models [12]. Each of the six tie-lines of the reference pseudoternary system was studied in this work with the addition of 5 mass% of AOT to define the overall phase behaviour and the solubilization increase of the polar components, at 298.15 K and atmospheric pressure. This was done by using samples from the same stock of partially miscible systems used for the study of the six tie-lines of the quaternary system. The experimental equilibrium results for the quinary system are depicted in Fig. 2 also as a pseudoternary system because the representation of multicomponent systems is difficult to visualize on a plane. It can be observed that this system possesses a partially mis-

To establish the concentration region of total miscibility and to quantify the partition of both methanol and water into the two liquid phase that appear in the gasoline model system, i.e. isooctane + benzene, one further objective of the present work is to study the solubilization phenomenon of water and methanol in hydrocarbon blends through the addition of different amounts of the surfactant AOT. For this purpose complete liquid–liquid phase diagrams for quinary systems were experimentally determined through the determination of the concentration of the components in each conjugate phase of six different tie-lines, at constant temperature and under atmospheric pressure (i.e. 0.1 MPa). Therefore, to clearly observe and discuss the effect of the concentration of the surfactant on the phase diagrams it is appropriate having as reference the liquid–liquid phase diagram for the quaternary or pseudoternary system isooctane + benzene + (80 mass% methanol + 20 mass% water), which was previously studied by García-Flores et al. at 298.15 and 308.15 K [12]. Fig. 1 shows the experimental liquid–liquid phase diagram for the aforementioned reference system, plotted as pseudoternary system in the triangular diagram, at 298.15 K. It is observed that the system exhibits a large partially miscible region originating a phase diagram of type II. The phase diagram was defined with six equilibrium tie-lines, which were numbered 1–6 from bottom to top. Tie-line 1 corresponds to the base of the phase diagram, which is

Fig. 2. Liquid–liquid equilibrium results for the system isooctane + benzene + [80 mass% methanol + 20 mass% water] + 5 mass% AOT, at 298.15 K. Experimental equilibrium concentrations in mol% for conjugate phases (䊉); NRTL model (); UNIQUAC model (). Tie-lines and binodal curve are given as full lines (—).

3. Results and discussion

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Table 1 Experimental values of concentration on AOT-free basis mole fraction, x, for the liquid–liquid equilibrium of the system: x1 isooctane–x2 benzene–(x3 80 mass% methanol + x4 20 mass% water) + 5 mass% AOT, at 298.15 K. Tie-line

1 2 3 4 5 6

Methanol + water-rich (lower) phase

Hydrocarbon-rich (upper) phase

x1

x2

x3

x4

x3 + x4

x1

x2

x3

x4

x3 + x4

0.0084 0.0100 0.0097 0.0110 0.0077 0.0000

0.0000 0.0377 0.0206 0.0220 0.0638 0.0980

0.6018 0.5825 0.7447 0.6481 0.5778 0.5719

0.3898 0.3697 0.2220 0.3185 0.3507 0.3301

0.9916 0.9522 0.9697 0.9666 0.9285 0.9020

0.9530 0.4744 0.7458 0.7026 0.2339 0.0000

0.0000 0.4193 0.1839 0.2435 0.6202 0.7981

0.0287 0.0872 0.0478 0.0382 0.1213 0.1613

0.0183 0.0192 0.0225 0.0157 0.0246 0.0406

0.0470 0.1064 0.0703 0.0539 0.1459 0.2019

cible region which is similar in amplitude to that corresponding to the quaternary system (Fig. 1). The experimental concentrations of isooctane, benzene, methanol, and water in each conjugate phase for the six tie-lines are presented in Table 1, as mole fraction on AOT-free basis. It is to be noted that more than necessary mole fraction results are given with the only aim of allowing a straightforward view of phase partitioning of the different compounds. Comparing the phase diagrams in Figs. 1 and 2 it is clear that adding 5 mass% of AOT to the reference system did not modify its main characteristics, hence the phase diagram of the quinary system is also of type II. Although the effect of AOT seems to be subtle, some important changes are noted. It is observed that the partially miscible region of the quinary system in Fig. 2 is smaller than that for the reference system (Fig. 1). This is quantitatively verified by comparing the values of the concentrations in the hydrocarbonrich phase, which corresponds to the left-hand side of the binodal curve in the phase diagrams, given in Table 1 with the corresponding values reported in Table 4 of reference [12]. The effect of adding 5 mass% of AOT is also manifested by the increase of water concentration in the hydrocarbon-rich phase of the quinary system with respect to the concentration obtained in the same phase for the quaternary system. For example, for the latter system [12] the water mole fraction in the mentioned conjugate phase of tie-line 1 is 0.0072, thus there exists an increase of 254% in the concentration of water for the same phase of the same tie-line by adding AOT (Table 1); for tie-line 2 the water mole fraction in the hydrocarbon-rich phase for the quaternary system is 0.0076, which means that there is a 253% increase in the water concentration when adding AOT to the system; for tie-line 3 water mole fraction in the hydrocarbon-rich phase of the quaternary system is 0.0102, therefore, from the results in Table 1 there is an increase in water solubility of 221% in the quinary system. Overall, there exists an average increase of water concentration in the six studied tie-lines of 184% with respect to the results for the reference quaternary system, i.e. AOT free. These results clearly demonstrate that AOT is a very good solubilizer of water in the hydrocarbonrich phase through the formation of reverse micelles or water-in-oil (w/o) micelles. Since pure methanol is also encapsulated or confined into the core of AOT micelles [28–30,52,53] it is of interest to analyze its concentration in the hydrocarbon-rich phase in the systems with 5 mass% AOT and carry out a comparison as above. Methanol mole

fraction in tie-line 1 in the system AOT free was reported to be 0.0222. Looking at the results given in Table 1 one can see that the addition of AOT leads to an increase of 129%, for tie-line 2 the concentration in the system without AOT was 0.0366, hence there is a 238% increase in the system with AOT. The comparison for tie-lines 3–6 shows that there is a slight decrease of methanol concentration in the system with AOT respect to the system without AOT, so that the molar ratio of methanol concentration between the tielines with AOT and those AOT free is 0.941, 0.687, 0.721, and 0.907, respectively. Theses results indicate that the behaviour of methanol in the presence of micelles is different from that observed for water, so that its concentration does not increase significantly in the hydrocarbon-rich phase. This behaviour can be explained as being the result of the relatively high partial miscibility of methanol in isooctane [14,54], i.e. partitioning of methanol molecules between the bulk hydrocarbon-rich phase and micelles is preferred towards the former. This behaviour is at variance with the almost total immiscibility of water in isooctane, which greatly favors migration of water into the cavities of the micelles. Another factor that intervenes against methanol’s solubility is that methanol molecules within micelles do not interact with as many and as strongly hydrogen bonds with the polar heads of AOT as water molecules do [28,29,52,53]. With the aim of establishing the effect of increasing the concentration of AOT on the overall solubility of the reference system the concentration of AOT was augmented to 15 mass% in each of the same six tie-lines of the quaternary system, at 298.15 K. The experimental concentration results obtained for the different components in the conjugate liquid phases are given in Table 2. The corresponding LLE phase diagram is plotted in Fig. 3. The results show that the overall solubility increases to a great extent; therefore, the partially miscible region is now appreciably much smaller than that for the reference quaternary system (Fig. 1) and also than that for the system with 5 mass% AOT (Fig. 2). As a consequence of greater overall mutual solubility of the components of the system the phase diagram has changed into type I, i.e. benzene is now completely miscible with the binary mixture of (80 mass% methanol + 20 mass% water). The fact that the miscible region includes a very wide range of concentrations for isooctane + benzene + methanol + water clearly indicates that gasoline reformulation with methanol as oxygenate is highly viable from the physicochemical point of view.

Table 2 Experimental values of concentration on AOT-free basis mole fraction, x, for the liquid–liquid equilibrium of the system: x1 isooctane–x2 benzene–(x3 80 mass% methanol + x4 20 mass% water) + 15 mass% AOT, at 298.15 K. Tie-line

1 2 3 4 5 6

Methanol + water-rich (lower) phase

Hydrocarbon-rich (upper) phase

x1

x2

x3

x4

x3 + x4

x1

x2

x3

x4

x3 + x4

0.0367 0.0204 0.0243 0.0277 0.0365 0.0409

0.0000 0.0125 0.0316 0.0608 0.1118 0.2001

0.7485 0.6357 0.6219 0.6595 0.6486 0.5849

0.2148 0.3140 0.3222 0.2520 0.2032 0.1741

0.9633 0.9671 0.9431 0.9115 0.8518 0.7590

0.8577 0.7287 0.5766 0.4430 0.2851 0.1271

0.0000 0.1018 0.2166 0.3402 0.4321 0.4760

0.0857 0.0789 0.1085 0.1414 0.1918 0.2767

0.0567 0.0905 0.0983 0.0754 0.0910 0.1202

0.1424 0.1694 0.2068 0.2168 0.2828 0.3969

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Fig. 3. Liquid–liquid equilibrium results for the system isooctane + benzene + [80 mass% methanol + 20 mass% water] + 15 mass% AOT, at 298.15 K. Experimental equilibrium concentrations in mol% for conjugate phases (䊉); NRTL model (); UNIQUAC model (). Tie-lines and binodal curve are given as full lines (—).

A quantitative comparison will help to better visualize the effect of adding 15 mass% AOT to the quaternary system. To clearly observe the increase in the solubility of water in the hydrocarbonrich phase then the comparison is carried out as above. For tie-line 1 the solubility of water in the hydrocarbon-rich phase with 15 mass% of AOT increased by a factor of 7.88 (i.e. 788%) with respect to the same equilibrium line of the reference quaternary system; in tieline 2 the increase is by a factor of 11.91; in tie-line 3 the increase is by a factor of 9.64; the increase in tie-line 4 is by a factor of 6.67. The mean value of the increase concentration factor for the six tie-lines is 7.29 (i.e. 729%). When such comparison is performed between the concentration results for the system with 15 mass% AOT and those for the system with 5 mass% AOT the average increase of the water mole fraction in the hydrocarbon-rich phase of the six tie-lines is given by a factor of 3.94, which shows that the increase in water solubility is proportional to the increase of AOT concentration. The increase of AOT concentration up to 15 mass% increases the concentration of methanol in the hydrocarbon-rich phase. The concentration of methanol for tie-line 1 in the system without AOT was reported [12] to be 0.0222, whereas methanol concentration included in Table 2 for tie-line 1 is 0.0857 which gives a molar ratio of 3.86 equivalent to an increase of 386%. The increase of methanol concentration for tie-lines 2–6 is 216, 214, 254, 114, and 156%, respectively. It should be noted that the increase of water concentration is much larger than these values. The solubility behaviour of water should necessarily be correlated with methanol concentration in the six tie-lines in the hydrocarbon-rich phase of the three systems included in the dis-

69

Fig. 4. Solubility increase of water in the hydrocarbon-rich phase due to micellization with Aerosol OT in multicomponent systems, at 298.15 K. () average molar ratio of water for six tie-lines in multicomponent systems with AOT respect to reference system without AOT; () average molar ratio of methanol to water for six tie-lines of three different multicomponent systems. Lines are drawn to show tendencies.

cussion. It is concluded from the above discussion that the increase of water concentration is higher than that for methanol for a given tie-line in the systems with 5 and 15 mass% AOT. Fig. 4 shows in a concise way the variations in concentration discussed above. It contains the average water concentration increase factor (molar ratio) and the average values for the ratio of methanolto-water mole fraction for the six tie-lines of the three different systems discussed above, i.e. 0, 5, and 15 mass% AOT. It is observed that the average increase of water concentration varies linearly with AOT concentration whereas the mole ratio methanol/water decreases as AOT concentration increases, i.e. the solubility of water increases dramatically as compared with methanol solubility in the hydrocarbon-rich phase. These results confirm that AOT micelles solubilize preferentially water over methanol in their hydrophilic core. It is known that the phase behaviour, and particularly water solubility in the hydrocarbon-rich phase, of multicomponent systems studied in this work can be further modified either by increasing the amount of surfactant or increasing the temperature. Since experimental LLE results were reported [12] for the reference quaternary system isooctane + benzene + (80 mass% methanol + 20 mass% water), at 308.15 K, we found interesting to study this system with 15 mass% AOT, also at 308.15 K, and carry out a comparison for the overall miscibility and equilibrium concentrations. The experimental results are reported in Table 3 and the corresponding phase diagram with six tie-lines is depicted in Fig. 5. This diagram is of type I, whereas the phase diagram for the quaternary system without AOT, at 308.15 K (Fig. 6 of reference [12]) is of type II. This result was expected since it was shown above that at 298.15 K the same transition from type II to type I was observed when adding 15 mass% AOT.

Table 3 Experimental values of concentration on AOT-free basis mole fraction x, for the liquid–liquid equilibrium of the system: x1 isooctane–x2 benzene–(x3 80 mass% methanol + x4 20 mass% water) + 15 mass% AOT, at 308.15 K. Tie-line

1 2 3 4 5 6

Methanol + water-rich (lower) phase

Hydrocarbon-rich (upper) phase

x1

x2

x3

x4

x3 + x4

x1

x2

x3

x4

x3 + x4

0.0224 0.0244 0.0258 0.0299 0.0336 0.0422

0.0000 0.0127 0.0321 0.0587 0.0997 0.2042

0.6265 0.6281 0.6023 0.5961 0.5838 0.5082

0.3511 0.3348 0.3399 0.3153 0.2829 0.2455

0.9776 0.9629 0.9422 0.9114 0.8667 0.7537

0.8736 0.7907 0.6109 0.4469 0.2955 0.0672

0.0000 0.0989 0.2309 0.3350 0.4247 0.2729

0.0574 0.0451 0.0738 0.1295 0.1819 0.4044

0.0690 0.0652 0.0845 0.0886 0.0980 0.2555

0.1264 0.1103 0.1583 0.2181 0.2799 0.6599

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15 mass% of AOT respect to the reference system without AOT, 1.84 and 7.29, respectively, and for the system with 15 mass% AOT respect to the system with 5 mass% AOT, 3.94, all at 298.15 K. We obtained a straight line with a correlation coefficient of 0.983. The extrapolation of the linear equation to the molar ratio of 1.2 gave a concentration of 4.2 mass% of AOT, which means that the increase in water concentration in the hydrocarbon-rich phase due to a 10 K increase would also be obtained having a multicomponent system with 19.2 mass% of AOT (15 mass% AOT in the system at 298.15 K + 4.2 mass% AOT to achieve higher water solubility). 3.2. Density

Fig. 5. Liquid–liquid equilibrium results for the system isooctane + benzene + [80 mass% methanol + 20 mass% water] + 15 mass% AOT, at 308.15 K. Experimental equilibrium concentrations in mol% for conjugate phases, (䊉); NRTL model (); UNIQUAC model (). Tie-lines and binodal curve are given as full lines (—).

It is then useful to carry out a comparison between the equilibrium concentration results of this work for the multicomponent systems with 15 mass% AOT at 298.15 K and those for the same system at 308.15 K. It is observed, from Figs. 3 and 5, that the overall solubility of the system was enhanced as a consequence of the temperature increase; hence, the partially miscible region has evidently decreased. By obtaining the molar ratio for the concentration of water in the hydrocarbon-rich phase for the system at 308.15 K with respect to the system at 298.15 K an average water molar ratio of 1.2 is obtained for the six tie-lines, which indicates that water solubility increased as the temperature increased 10 K. Also, in the same fashion the average methanol molar ratio for the two systems is 0.88, indicating that methanol concentration decreases in the hydrocarbon-rich phase as water concentration increases as a consequence of the temperature increase. These results are congruent with the well characterized micelar aggregation process that allows taking more water molecules within the spherical core of the stable reverse micelles, so that, the so-called water pool increases in size with increasing temperature, whereas methanol molecules do not interact so strongly with the polar heads of the surfactant, hence its solubility does not increase as more micelles are formed [53,55–57]. Another important competing behaviour against methanol’s solubility in the core of micelles is its partial miscibility in the bulk phase rich in isooctane. In order to establish an empirical equivalence between the 10 K increase and the AOT concentration necessary to obtain the same average enhancement in water solubility we regressed the average water molar ratios given above for the systems with 5 and

The experimental method used here for the determination of the interfacial tension of the conjugate phases for each tie-line requires as input information the density of each equilibrium phase (i.e. hydrocarbon-rich and methanol-rich phases, respectively). Hence in this work we have obtained experimental density results for such saturated phases, which will also be useful to understand the phase behaviour of the studied systems, particularly the solubility of water in the hydrophobic phase. The experimental results for the density of each conjugate phase for the quinary system isooctane + benzene + (80 mass% methanol + 20 mass% water) + 5 mass% AOT, at 298.15 K, are reported in Table 4. As expected, the results show that the density of the (methanol + water)-rich phases is always higher than that of the corresponding conjugate hydrocarbon-rich phases in all tie-lines. It is also observed that the density difference between both equilibrium phases gets smaller (i.e.180.28, 151.01, 129.91, 110.91, and 26.55 kg m−3 , respectively), as the tie-lines approach the critical point or plait point because the properties of the equilibrium phases tend to become equal. It is also worth to note that the density of the (methanol + water)-rich phases is almost constant along the different tie-lines whereas the density of the hydrocarbon-rich phases clearly increases from tie-line 1–5. These results are in agreement with the fact that the hydrocarbon-rich phase of each tie-line incorporates more water, through the formation of reverse micelles, as the top of the binodal curve is approached. This behaviour is congruent with the observation that the radius of the spherical reverse micelles, hence the size of the “water pool”, increases linearly with the water-to-AOT molar ratio, wo [58]. It is then clear that each tie-line can be considered as an independent system whose overall value of wo or the specific wo value of the hydrocarbon-rich phase increases as the top of the binodal curve is approached. With the aim of establishing the effect of adding different amounts of AOT on the physicochemical behaviour of the multicomponent system it was decided to do it through the study of a single tie-line. Table 5 shows the experimental results of density of the conjugate phases for tie-line 2 of the system isooctane + benzene + (80 mass% methanol + 20 mass% water) as a function of AOT concentration, at 298.15 K. It is observed that as the concentration of the surfactant increases the density of the (methanol + water)-rich phase also increases whereas the density

Table 4 Experimental values of density, , for conjugate phases and interfacial tension, , for multicomponent systems. Tie-line x1 isooctane + x2 benzene + (x3 1 2 3 4 5 x1 isooctane + x2 benzene + (x3 1 a

Methanol + water-rich phase  (kg m−3 )

Hydrocarbon-rich phase  (kg m−3 )

80 mass% methanol + x4 20 mass% water) + 5 mass% AOT, at 298.15 K. 868.04 687.76 865.02 714.01 863.29 733.84 862.73 751.82 864.86 838.31 80 mass% methanol + x4 20 mass% water) + 15 mass% AOT, at 298.15 K. 855.41 742.52

It was not possible to measure a reliable value.

 (mN m−1 ) 2.29 2.29 1.92 1.45 0.45 a

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71

Table 5 Experimental values of density, , and interfacial tension, , for tie-line No. 2 of the system x1 isooctane + x2 benzene + (x3 80 mass% methanol + x4 20 mass% water), as a function of AOT concentration, at 298.15 K. CAOT (mass%)

Methanol + water-rich phase  (kg m−3 )

Hydrocarbon-rich phase  (kg m−3 )

 (mN m−1 )

0 0.11 0.40 0.67 3 5 10

841.75 841.20 841.84 844.26 857.13 865.02 880.72

722.06 715.90 716.10 716.08 714.52 714.01 719.14

5.26 5.36 5.09 4.83 3.59 2.29 0.97

of the hydrocarbon-rich phase decreases. This means that the density difference between both saturated phases increases with AOT concentration. This behaviour is at variance with that observed in Table 4 as discussed in the precedent paragraph. The explanation of that behaviour is found in the fact that both the overall value of wo in the chosen tie-line and the specific value of wo in the hydrocarbonrich phase decrease as the AOT concentration increases, hence the amount of water encapsulated in the polar core of the micelles decreases; therefore, so does the density of the hydrocarbon-rich phase. 3.3. Interfacial tension The interfacial tension between the liquid equilibrium phases was measured for the system isooctane + benzene + (80 mass% methanol + 20 mass% water) + 5 mass% AOT, at 298.15 K. For the system isooctane + benzene + (80 mass% methanol + 20 mass% water) + 15 mass% AOT, at 298.15 and 308.15 K, it was not possible to determine a reliable value of the interfacial tension because the drops were spherical and very small, indicative of a very low interfacial tension which was not possible to measure with our device. The experimental values of interfacial tension, , for the conjugate phases of the multicomponent system with 5 mass% AOT are incorporated in Table 4. The values are characteristic of oilwater systems with microemulsions [59]. It is observed that the values decrease from 2.29 mN m−1 to 0.45 mN m−1 , i.e. a five-fold decrease, as the top of the binodal curve is approached. This property should be zero at the plait point of the system. The behaviour of the interfacial tension is consistent with the fact that the miscibility of the conjugate phases increases from bottom to top of the phase diagram. The dependence of the interfacial tension of the conjugate phases on the density difference of the equilibrium phases for the different tie-lines of the multicomponent system is shown in Fig. 6. It is observed that the lower the interfacial tension of a given tieline the lower the density difference between the equilibrium liquid phases for that line. This behaviour is closely related to the increase in solubilization of the components in the studied system. Hence, the addition of the surfactant to the system produces low interfacial tension and high solubilization [17]. As mentioned in the previous section tie-line 2 was chosen to study the effect of AOT concentration on the physicochemical properties of the multicomponent system. The results for the interfacial tension are given in Table 5. It is observed that the values vary from 5.26 mN m−1 for the system without AOT down to 0.97 mN m−1 with 10 mass% AOT, which indicates that the mutual solubility of the phases increases with increasing AOT concentration, as expected. From Table 5 it is also observed that as the interfacial tension decreases the density difference of the conjugate phases increases, which was explained above. Fig. 7 shows the dependence described for the interfacial tension on density differences.

Fig. 6. Experimental interfacial tension as a function of density difference of the conjugate phases of the system isooctane + benzene + [80 mass% methanol + 20 mass% water] + 5 mass% AOT, at 298.15 K. Symbols represent experimental values. Full line is drawn to show tendencies.

Fig. 7. Variation of the experimental interfacial tension and density difference of the conjugate phases of tie-line 2 of the system isooctane + benzene + [80 mass% methanol + 20 mass% water] as a result of changes in AOT concentration, at 298.15 K. Symbols represent experimental values. Full line is drawn to show tendencies.

4. Correlation of the liquid–liquid equilibrium concentrations The experimental liquid–liquid equilibrium results of the multicomponent systems studied here (Tables 1–3) were correlated with the well known activity coefficient models NRTL [19] and UNIQUAC [20]. The algorithms used in this work have been described in detail [60,61] and tested in previous works from our Laboratory [11–13]. For the NRTL model the parameter ˛ was set equal to 0.2 in all the systems. The thermodynamic stability test considered as part of the regression of our results has been based on the tangent plane criterion following the works by Barker [62] and Michelsen [63]. This stability test allows verifying whether the system is stable or not. In the latter case, it provides an estimation of the composition of an additional phase; the number of phases is then increased by one,

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and equilibrium is achieved by minimizing the Gibbs energy. This approach, adapted as a stage wise procedure, [63,64] is continued until a stable solution is found. In this technique, the stability analysis of a homogeneous system of composition z, based on the minimization of the distance separating the Gibbs energy surface from the tangent plane at z, is considered [62,63]. In terms of activity coefficients,  i , this criterion for stability can be written, for all concentrations x, as

F (x) =

N 

xi [ln xi + ln i (x) − hi ] ≥ 0 (ϕ)

where hi = ln xi + ln i (x(ϕ) ) i = 1, . . . , N and x(ϕ) is the concentration of the system when it is homogeneous. The regression of the interaction parameters for the NRTL and UNIQUAC models to correlate the experimental data reported in this work was performed through a procedure that involves the minimization of the sum of the squared differences between the calculated and experimental mole fractions with the following objective function: Neq Nph N   k=1 j=1 i=1



∧

wijk xijk − xijk

2

+Q

Component

Parameters [65]

1 2 3 4

r

q

5.8463 3.1878 1.4311 3.4535

5.0080 2.4000 1.4320 3.0480

(1)

I=1

Fx =

Table 6 Parameters r and q for (1) isooctane, (2) benzene, (3) methanol, and (4) water, for the UNIQUAC model.

Npar 

p2m ,

(2)

m=1

where wijk , are the weighting factors that replace the experimental uncertainties, xijk represents the experimental concentration in mole fraction, and xˆ ijk represents the calculated concentrations in mole fraction of component i in phase j corresponding to the tie-line k. The second term of the right-hand side in Eq. (2) is added in order to ensure that relatively small parameters are obtained without increasing the minimum of the objective function and at the same time it helps to avoid the risk of multiple solutions. Hence, this term promotes convergence during the fitting procedure and is activated only if one or more of the model parameters are greater than a fixed value [62].

Figs. 2, 3 and 5 show the mole fraction results obtained with the NRTL and UNIQUAC solution models compared with the experimental values of the liquid–liquid equilibrium for each one of the studied systems: x1 isooctane–x2 benzene–[x3 80 mass% methanol + x4 20 mass% water]–5 mass% AOT at 298.15 K and x1 isooctane–x2 benzene–[x3 80 mass% methanol + x4 20 mass% water]–15 mass% AOT, at 298.15 and 308.15 K. It is observed that the two solution models reproduce the experimental results with adequate accuracy. The structural parameters r and q used in the UNIQUAC model, included in Table 6, were taken from the literature [65] and are the same as those used previously [11–13]. The values of the adjustable interaction parameters for the two models applied in this work for each of six binary interactions ( ij and  ji ) of each of the multicomponent systems studied are reported in Table 7 together with the corresponding absolute standard deviation for the equilibrium concentration in mole fraction. The values of the absolute standard deviation indicate that both models reproduce the experimental liquid–liquid equilibrium results satisfactorily. The highest value for the standard deviation was obtained with the NRTL model for the system with 15 mass% of AOT at 308.15 K, 0.69 mol%, whereas the lowest standard deviation was obtained with the UNIQUAC model, 0.45 mol%, for the system with 15 mass% of AOT at 298.15 K. Overall, for the systems studied the standard deviations obtained from both models are essentially the same and very close to the experimental uncertainty of the equilibrium concentration results.

Table 7 Values of the binary interaction parameters for the NRTL and UNIQUAC solution models fitted to the experimental tie-line results of studied multicomponent systems. The standard deviation in mole percent,  x , of the fit for each system is also included. NRTL i,j

UNIQUAC  i,j

 j,i

 i,j

 j,i

T = 298.15 K

x1 isooctane + x2 benzene + [x3 80 mass% methanol + x4 20 mass% water] + 5 mass% AOT

1,2 1,3 1,4 2,3 2,4 3,4

−0.0031 881.89 1400.7 951.14 258.71 0.1596

207.16 621.91 708.64 170.95 885.45 758.27

−7.4781 −0.7576 419.65 99.680 7.0703 1.2950

52.740 979.36 667.25 436.50 628.17 453.14  x = 0.49%

 x = 0.51% T = 298.15 K

x1 isooctane + x2 benzene + [x3 80 mass% methanol + x4 20 mass% water] + 15 mass% AOT

1,2 1,3 1,4 2,3 2,4 3,4

−667.91 376.26 −63.581 188.71 −238.34 −315.40

949.37 694.35 1913.3 446.35 809.59 360.13

−224.43 738.65 45.774 445.71 −92.324 −334.79

278.44 −12.266 349.12 68.172 −117.28 134.35  x = 0.45%

−79.336 880.69 1006.5 495.82 1139.5 −68.608

246.32 897.72 88.741 398.83 −132.76 −280.99

−143.31 −13.451 268.84 −30.859 605.30 −199.35  x = 0.64%

 x = 0.56% T = 308.15 K

x1 isooctane + x2 benzene + [x3 80 mass% methanol + x4 20 mass% water] + 15 mass% AOT

1,2 1,3 1,4 2,3 2,4 3,4

102.37 461.76 −145.02 −3.2140 −198.92 −351.15  x = 0.69%

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5. Conclusions

References

We have presented experimental information on the effect of an anionic surfactant on the liquid–liquid phase behaviour of multicomponent systems, which are of interest for the reformulation of oxygenated gasoline. The overall miscibility of the system isooctane + benzene + methanol + water was increased with the addition of different concentrations of the surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT), at constant temperature, with respect to the same system without AOT. This result allows inferring that under extreme conditions of water concentration in a given oxygenated gasoline the presence of the right amount of surfactant will efficiently mitigate partial miscibility. Furthermore, the overall miscibility increase observed in the multicomponent systems is explained mainly through the enhancement of the solubility of water in the hydrocarbon-rich phase of all the studied tie-lines, at constant temperature, as a consequence of the addition of AOT. This very large increase in water solubility is congruent with the formation of reverse micelles in the nonpolar phase, which preferentially incorporate in their core more water than methanol. This observation is also in agreement with the well known result that as the molar water-to-surfactant ratio increases the micelles core encapsulates more water, hence increasing the size of the micelles. This behaviour was observed through the study of different tie-lines for a given multicomponent system, at constant temperature. It is of interest to underline that although the microscopic mechanism of encapsulating pure methanol into the core of AOT micelles has been reported to be essentially the same as that for pure water, in the systems studied here AOT micelles definitively favor the solubilization of water over methanol. Temperature and surfactant concentration are important parameters in the present study. Temperature was increased and the overall miscibility of the multicomponent system increased as expected. The effect of increasing the surfactant concentration on water solubility in the hydrocarbon-rich phase showed once again that the molar water-to-surfactant ratio is important for interpreting the experimental results. It was also shown that both density and interfacial tension are important thermophysical properties that are useful to gain a complete understanding of the miscibility of the conjugate phases and also of the solubility behaviour of water in the hydrocarbon-rich phases. The values of these two properties are intimately related to the presence of AOT in the studied systems. The results and analysis presented in this work are of scientific interest and should also be of practical interest for gasoline reformulation under extreme conditions that involve the undesirable, although sometimes unavoidable, presence of water. If the concentration of water is low as expected in a commercial oxygenated gasoline, the surfactant AOT is an adequate option to prevent phase separation. Furthermore, the present results are also relevant for other scientific and technological fields such as the study of formation-inhibition of different types of clathrates hydrates of hydrocarbons. We found that the experimental liquid–liquid equilibrium data for the multicomponent systems studied in this work can be accurately reproduced with either NRTL or UNIQUAC models.

[1] T.E. Daubert, R.P. Danner, Data Compilation Tables of Properties of Pure Compounds, American Institute of Chemical Engineers, New York, 1985. [2] J.A. Riddick, W.B. Bunger, T.K. Sakano, Organic Solvents, Physical Properties and Methods of Purification, 4th ed., John Wiley, New York, 1986. [3] B. Elvers, S. Hawkins, G. Schulz (Eds.), Ullman’s Encyclopedia of Industrial Chemistry, vol. A16, 15th ed., VCH Verlagsgesellschaft mbH, Weinheim, 1990. [4] A.T. Peaslee Jr., G.R. Thayer, Report LA-9068-MS, Los Alamos National Laboratory, TX, 1981. [5] W.-H. Cheng, H.H. Kung (Eds.), Methanol Production and Use, Marcel Dekker Inc., New York, 1994. [6] A.L. Kohl, R.B. Nielsen, Gas Purification, 5th ed., Gulf Publishing Company, Houston, 1997. [7] E.D. Sloan Jr., Clathrate Hydrates of Natural Gases, 2nd ed., Marcel Dekker Inc., New York, 1998. [8] D.J.L. Brett, A. Atkinson, D. Cumming, E. Ramírez-Cabrera, R. Rudkin, N.P. Brandon, Chem. Eng. Sci. 60 (2005) 5649–5662. [9] L.M. Gómez-Sainero, R.T. Baker, I.S. Metcalfe, M. Sahibzada, P. Concepción, J.M. López-Nieto, Appl. Catal. A 294 (2005) 177–187. [10] S. Ren, G. Sun, C. Li, Z. Wu, W. Jin, W. Chen, Q. Xin, X. Yang, Mater. Lett. 60 (2006) 44–47. [11] B.E. García-Flores, G. Galicia-Aguilar, R. Eustaquio-Rincón, A. Trejo, Fluid Phase Equilibr. 185 (2001) 275–293. [12] B.E. García-Flores, M. Gramajo de Doz, A. Trejo, Fluid Phase Equilibr. 230 (2005) 121–130. [13] B.E. García-Flores, A. Trejo, J. Águila-Hernández, Fluid Phase Equilibr. 255 (2007) 147–159. ˜ [14] A. Trejo, P. Yánez, R. Eustaquio-Rincón, J. Chem. Eng. Data 51 (2006) 1070–1075. [15] V. Ruzicka, R. Frydova, J. Novak, Fluid Phase Equilibr. 32 (1986) 27–47. [16] F. Ancillotti, E. Pescarollo, Hydroc. Process. November (1977) 295–299. [17] K. Shinoda, P. Becher, Principles of Solution and Solubility, Marcel Dekker Inc., New York, 1978. [18] A. Li, S.H. Yalkowsky, Ind. Eng. Chem. Res. 37 (1998) 4476–4480. [19] H. Renon, J.M. Prausnitz, AIChE J. 14 (1968) 135–144. [20] D.S. Abrams, J.M. Prausnitz, AIChE J. 21 (1975) 116–127. [21] R. Day, B.H. Robinson, J. Clarke, J. Doherty, J. Chem. Soc. Faraday Trans. 1 (75) (1979) 132. [22] P.D.I. Fletcher, J. Chem. Soc. Faraday Trans. 1 (82) (1986) 2651. [23] H. MacDonald, B. Bedwell, E. Gulari, Langmuir 2 (1986) 704. [24] A. D’Aprano, A. Lizzio, V. Turco Liveri, J. Phys. Chem. 91 (1987) 4749. [25] T.K. Jain, M. Varshney, A. Maitra, J. Phys. Chem. 93 (1989) 7409. [26] A. Maitra, T.K. Jain, Z. Shervani, Colloids Surf. 47 (1990) 255. [27] G. Onori, A. Santucci, J. Phys. Chem. 97 (1993) 5430. [28] R.E. Riter, E.P. Undiks, J.R. Kimmel, N.E. Levinger, J. Phys. Chem. B 102 (1998) 7931. [29] R.E. Riter, J.R. Kimmel, E.P. Undiks, N.E. Levinger, J. Phys. Chem. B 101 (1997) 8292. [30] H. Shirota, K. Horie, J. Phys. Chem. B 103 (1999) 1437. [31] I. Pliskin, R. Treybal, J. Chem. Eng. Data 11 (1966) 49–52. [32] E. Sada, S. Kito, M. Yamashita, J. Chem. Eng. Data 20 (1975) 376–377. [33] S. Ross, R.E. Patterson, J. Chem. Eng. Data 24 (1979) 111–115. [34] B. Li, J. Fu, J. Chem. Eng. Data 37 (1992) 172–174. [35] L. Bartovska, M. Cechova, J. Matous, J.P. Novak, Collect. Czech. Chem. 65 (2000) 1487–1496. [36] M.L. Kijevcanin, I.S.A. Ribeiro, A.G.M. Ferreira, I.M.A. Fonseca, J. Chem. Eng. Data 48 (2003) 1266–1270. [37] B.M.S. Santos, A.G.M. Ferreira, I.M.A. Fonseca, Fluid Phase Equilibr. 218 (2003) 1–21. [38] M.L. Kijevcanin, I.S.A. Ribeiro, A.G.M. Ferreira, I.M.A. Fonseca, Fluid Phase Equilibr. 218 (2004) 141–148. [39] H. Kahl, T. Wadewitz, J. Winkelmann, J. Chem. Eng. Data 49 (2004) 997–1002. [40] C.G. Aranda-Bravo, A. Romero-Martínez, A. Trejo, J. Águila-Hernández, Ind. Eng. Chem. Res. 48 (2009) 1476–1483. [41] B.E. García-Flores, Ph.D. Thesis, Estudio fisicoquímico del equilibrio líquidolíquido de sistemas ternarios de interés para la reformulación de gasolina oxigenada, Universidad Nacional Autónoma de México, 2004. [42] O. Kratky, H. Leopold, H. Stabinger, Z. Angew. Phys. 27 (1969) 273–277. [43] P. Picker, P.A. Leduc, P.R. Philip, J.E. Desnoyers, J. Chem. Thermodyn. 3 (1971) 631–642. [44] F. Murrieta-Guevara, A. Trejo, J. Chem. Eng. Data 29 (1984) 204–206. [45] R. González, F. Murrieta-Guevara, A. Trejo, J. Solution Chem. 15 (1986) 791–802. [46] D. Apam-Martínez, A. Trejo, J. Chem. Soc., Faraday Trans. 1 (84) (1988) 4073–4086. [47] R. Eustaquio-Rincón, A. Trejo, J. Chem. Soc., Faraday Trans. 90 (1994) 113–120. [48] J. Águila-Hernández, A. Trejo, J. Gracia-Fadrique, Fluid Phase Equilibr. 185 (2001) 165–175. [49] J. Águila-Hernández, R. Gómez-Quintana, F. Murrieta-Guevara, A. RomeroMartínez, A. Trejo, J. Chem. Eng. Data 46 (2001) 861–867. [50] M.E. Rebolledo-Libreros, A. Trejo, J. Chem. Eng. Data 51 (2006) 702–707. [51] J. Aguila-Hernández, A. Trejo, B.E. García-Flores, Colloids Surf. A: Physicochem. Eng. Aspects 308 (2007) 33–46. [52] D.S. Venables, K. Huang, C.A. Schmuttenmaer, J. Phys. Chem. B 105 (2001) 9132–9138. [53] P. Hazra, D. Chakrabarty, N. Sarkar, Langmuir 18 (2002) 7872–7879. [54] R.W. Kiser, G.D. Johnson, M.D. Shetlar, J. Chem. Eng. Data 6 (1961) 338–341.

Acknowledgements The authors acknowledge the financial support to carry out this research under Research Projects D.00338, D.00406, I.00348, and I.00432 of the Instituto Mexicano del Petróleo. We thank Dr. F. García-Sánchez of the Instituto Mexicano del Petróleo for the gift of the executable computer program and for advice.

74 [55] [56] [57] [58] [59] [60]

B.E. García-Flores et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 368 (2010) 64–74 J. Zhang, F.V. Bright, J. Phys. Chem. 95 (1991) 7900–7907. P. Hazra, N. Sarkar, Chem. Phys. Lett. 342 (2001) 303–311. B. Strus, A. Sobczynska, M. Wisniewski, Fuel 87 (2008) 957–963. G. Calvaruso, A. Minore, V. Turco Liveri, J. Coll. Interface Sci. 243 (2001) 227–232. D.K. Rout, S. Chauhan, A. Agarwal, Ind. Eng. Chem. Res. 48 (2009) 8842–8847. F. García-Sánchez, G. Eliosa-Jiménez, A. Salas-Padrón, O. Hernández-Garduza, D. Ápam-Martínez, Chem. Eng. J. 84 (2001) 257–274.

[61] F. García-Sánchez, J. Schwartzentruber, M.N. Ammar, H. Renon, Rev. Mex. Física 43 (1997) 59–92. [62] L.E. Barker, A.C. Pierce, K.D. Lucks, Soc. Pet. Eng. J. 22 (1982) 731. [63] M.L. Michelsen, Fluid Phase Equilibr. 9 (1987) 1–19. [64] L.X. Nghiem, Y.-K. Li, Fluid Phase Equilibr. 17 (1984) 77–95. [65] J.M. Sorensen, T. Magnussen, P. Rasmussen, A. Fredeslund, Fluid Phase Equilibr. 3 (1979) 47–82.