Temperature-Dependent Phase Behavior, Particle Size, and

Article pubs.acs.org/jced

Temperature-Dependent Phase Behavior, Particle Size, and Conductivity of Middle-Phase Microemulsions Stabilized by Ethoxylated Nonionic Surfactants Achinta Bera, Shashikant Kumar, and Ajay Mandal* Department of Petroleum Engineering, Indian School of Mines, Dhanbad 826004, India ABSTRACT: The present paper deals with the influence of salinity and temperature on the phase behavior and conductivity of middle-phase microemulsions stabilized by ethoxylated nonionic surfactants. Optimal salinity of microemulsion increases with increases in temperature. Middle phase microemulsion shows stability for temperatures up to 330 K, and then the relative phase volume of middle phase decreases slightly. Particle size distribution of the microemulsion formulations with different ethoxylated nonionic surfactants have been studied by dynamic light scattering. Percolation behavior of the middle-phase microemulsion systems have been studied as a function of mass fraction of water for different nonionic surfactants at 0.02 mass fractions of NaCl concentration by electrical conductivity measurement. The temperature effect on the electrical conductivity of middle-phase microemulsions is also studied. With an increase in temperature, the conductivity increases and the activation energy of the system also increases with an increase in ethylene oxide number of the nonionic surfactants.

1. INTRODUCTION In microemulsion flooding for enhanced oil recovery (EOR) in deep reservoir, the influence of temperature on the microemulsion phase behavior must be considered because of drastic phase transition from Winsor type I (WI) to Winsor type II (WII) through Winsor type III (WIII) phases with temperature. The temperature variation phase behavior of microemulsion systems demand further study due to changing of phase volumes in reservoir temperature. Complete studies on phase behavior, interfacial phenomena, and physicochemical properties with variation of temperature and salinity are the primary determinants of the potential success of a microemulsion flooding for EOR technique. The fundamental importance of particle size distribution, their interaction, and dynamics are due to their controlling power of different generalized properties of microemulsions. The particle size distribution of the microemulsion is the main key factor to better understand the mechanism leading to both the penetrating power into porous media and the stability of the microemulsion. The factors that affect the phase transition between different types of systems include the salinity, the temperature, the molecular structure and nature of the surfactant and cosurfactant, the type of oil, and the water−oil ratio (WOR).1 Among the cited factors, temperature and pressure have significant roles in the phase behavior of microemulsion systems in the EOR technique to maintain reservoir conditions. Under adequate conditions, the microemulsion system is miscible with both oil and water. Optimum salinity and the amounts of solubilized oil and water contained in a microemulsion have been shown to play important roles in © 2012 American Chemical Society

obtaining low interfacial tensions (IFTs) and high oil recoveries in EOR. Since IFTs are minimal at optimal salinity and solubilization parameters are related to IFT2 estimation of both properties, it is of great help in designing economical microemulsion flooding. The optimal salinity depends upon many parameters like pressure, temperature and microemulsion compositions (viz. nature of oil, nature of surfactant, nature of cosurfactant, etc.).3,4 The influence of temperature on optimal salinity is quite a complex study. For the anionic microemulsion system, it was observed that optimal salinity increases with increase in temperature.2,5 The electrical conductivity measurement of the microemulsions has become a standard process in obtaining information on percolation.6−8 The percolation phenomenon in the microemulsion system is characterized by a sudden increase in electrical conductivity when either the temperature or the mass fraction of the dispersed phase reaches a certain threshold value. In percolation due to ion transfer, the conductance value can be enhanced by 100 to 1000 fold. A number of research groups have investigated the nature and the basic understanding of the percolation process.9−12 The compositions of the microemulsion systems and other environmental factors such as the presence of additives control the percolation threshold values.10,13−15 In the present paper, the effect of temperature on the phase behavior of the middle-phase microemulsion system has been discussed. Laser light scattering was used to monitor the Received: July 19, 2012 Accepted: October 17, 2012 Published: October 31, 2012 3617

dx.doi.org/10.1021/je300845q | J. Chem. Eng. Data 2012, 57, 3617−3623

Journal of Chemical & Engineering Data

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Table 1. Detailed Descritpions of the Surfactants Used for the Present Work Including Trade Name (TN), Molar Mass (M.M./ kg·mol−1), Hydroxyl Number (HN), HLB Value (HLB), and Category (Cat.)

The size distribution of the dispersed particles can be obtained by the in build software of the instrument. Drops of microemulsion were introduced into the sample presentation until the concentration reached the optimum one, indicated by the instrument. All of the experiments were conducted at 298 K. 2.2.2. Phase Behavior and Solubilization Parameter Study. The microemulsion phase behavior was determined by equilibrating the surfactant solution with equal masses of oil and brine and measuring the volume of the microemulsion phase and excess oil and/or excess brine phase at constant pressure and temperature by visual observation and studied as mass ratio. All of the samples were prepared with mass fractions of 0.47 brine, 0.47 oil, and 0.06 dilute surfactant solution (0.04 mass fractions). For all systems, the water−oil ratio (WOR) was maintained as 1. The details of the procedure of the salinity scan experiment were stated in our previous paper.16 To investigate the effect of temperature on relative phase volumes, the samples were kept in a thermo-stated water bath and readings were taken at different temperatures. Solubilization parameters were calculated for all samples at different temperatures assuming that all of the surfactant solutions were present in the microemulsion phase. For different temperatures, optimal salinities were calculated for all the nonionic surfactants. 2.2.3. Electrical Conductivity Measurement. The conductivity behavior of middle-phase microemulsion was measured with gradual dropwise addition of brine by using a digital conductometer (Biocraft Scientific Systems (P) Ltd.; model No. CEI-13) with cell constant of 1.074 × 102 m−1 for all of the nonionic surfactant systems. The temperature was kept constant at (302 ± 1) K throughout the experiment. In all cases, measurements were made after homogeneous mixing by a magnetic stirrer. The uncertainty (u) of the conductivity (k) measurement is u(k) = 0.01·k.

particle size of several microemulsion formulations with different ethoxylated nonionic surfactants. To understand the microstructure of the microemulsion, the conductivity study has also been carried out at constant salinity. To justify the percolation process of the microemulsion systems, a NaCl solution at a fixed concentration was added dropwise to the microemulsion systems and the change of conductance values was recorded. The activation energy for percolation of the conductance of the microemulsion systems is the main essential parameter to properly interpret the percolation phenomenon. Hence, the activation energy has been estimated in nonionic surfactant based microemulsion systems. An empirical relationship between ethylene oxide number (EON) and activation energy of the microemulsion systems has been established.

2. EXPERIMENTAL SECTION 2.1. Materials. In the present study the nonionic surfactants such as Tergitol 15-S-5, Tergitol 15-S-7, Tergitol 15-S-9, and Tergitol 15-S-12 (with mole fraction purity of 0.99 each) were purchased from Sigma−Aldrich, Germany. The details physicochemical properties of the employed surfactants used in the present work have been given in Table 1. Sodium chloride (NaCl) with 0.98 mol fraction purity, procured from Qualigens Fine Chemicals, India, was used for preparation of brine solution. Double distilled water from a Millipore water system (Millipore SA, 67120 Molshein, France) was used for preparation of solutions. The synthetic oils such as heptane, benzene, and methylbenzene were procured from Otto-kemi, India. The synthetic oil used in the present work is a mixture of 0.76 mass fractions of heptane (0.98 mol fraction purity), 0.12 mass fractions of benzene (0.98 mol fraction purity), and 0.12 mass fraction of methylbenzene with mole fraction purity of 0.98. All chemicals were used without further purification. 2.2. Experimental Procedures. 2.2.1. Particle Size Measurement. The particle size distribution for the microemulsions prepared with four different nonionic surfactants was measured after 5 h equilibrium by the method of laser diffraction with the help of Zetasizer Ver. 6.00 (Malvern Instruments Ltd., Worcestershire WR14XZ, United Kingdom).

3. RESULTS AND DISCUSSION 3.1. Particle Size Distribution. Generally all nonionic surfactants are assigned with a hydrophilic−lipophilic balance 3618



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(HLB) number, which defines the hydrophile/lipophile behavior of the surfactant defining the stability of the emulsion.17 Particle size distributions in microemulsions at 0.02 mass fractions of NaCl solutions for four ethoxylated nonionic surfactants namely Tergitol 15-S-5, Tergitol 15-S-7, Tergitol 15-S-9, and Tergitol 15-S-12 are shown in Figure 1.

Table 2. Results of the Particle Size Measurements of the Microemulsions: Z-Average Diameter (d/nm), Polydispersity Index (PDI), Intercept (I), and Peak Vol. % r·nm (PV) surfactants in microemulsion Tergitol Tergitol Tergitol Tergitol

15-S-5 15-S-7 15-S-9 15-S-12

d/nm

PDI

I

PV

429 305 226 22

0.498 0.387 0.755 0.538

0.908 0.949 0.958 0.957

14.44 50.00 43.21 49.59

increases ethylene oxide number (EON) also increases. Therefore, surface activity of the surfactant solutions and particle sizes of the microemulsions are strongly influenced by HLB values of different surfactants. 3.2. Relative Phase Volume and Solubilization Parameters. The relative phase volumes of microemulsion systems are affected by temperature and salinity. Along with these thermodynamical and nonthermodunamical factors, microemulsion compositions such as oil types, nature of surfactants, and cosurfactant types are also responsible for variation of relative phase volumes. Few of the above factors are discussed here with the experimental results. 3.2.1. Effect of Temperature on Relative Phase Volumes. The dependence of the relative phase volume of microemulsion system on temperature has been shown in Figure 2 for Tergitol Figure 1. Intensity (I) wise particle size (d/nm) distribution for microemulsions composed with different nonionic surfactants. Symbols: ■, Tergitol 15-S-5; ●, Tergitol 15-S-7; ▲, Tergitol 15-S-9; ▼, Tergitol 15-S-12.

The mean particle diameter and polydispersity index (PDI) have been calculated from intensity, mass, and number bimodal distribution. However, in each case the particle size distribution curve has been shown only as a function of intensity for clarity. Usually colloidal interactions among the particles influence the size of the particles and the particle size are significantly smaller than 10 nm. The droplet size distribution depends on the particle deformability and the width of the droplet size distribution. In case of low HLB value (10.6), microemulsion shows the Z-average diameter of 429 nm which is greater than that of microemulsions with high HLB values (Z-average diameters are 305 nm, 226 nm, and 22 nm for HLB values of 12.1, 13.3, and 14.7, respectively). The Z-average diameter is defined as the mean hydrodynamic diameter and is calculated according to the International Standard on dynamic light scattering ISO13321. The Z-average diameter is intensity weighted harmonic mean size and is therefore influenced by the presence of large particles. It is a suitable parameter for predicting the nature of some processes such as particle aggregation or crystallization. The Z-average diameter of 15-S-9 is not truly reflected in Figure 1. Actually according to dynamic light scattering the Z-average diameter is calculated by eq 1 as follows: Dz =

∑ Si/∑ (Si/Di)

Figure 2. Relative phase volume (RPV) as a function of temperature (T) for (a) Tergitol 15-S-5, (b) Tergitol 15-S-7, (c) Tergitol 15-S-9, and (d) Tergitol 15-S-12 nonionic microemulsion systems.

15-S-5, Tergitol 15-S-7, Tergitol 15-S-9, and Tergitol 15-S-12 respectively. In the present study the temperature effect on relative phase volumes of the middle-phase bicontinuous microemulsion has been investigated for a specific salinity (0.02 mass fractions NaCl). At 303 K, the systems show the Winsor type III phase behavior. As the temperature of the microemulsion increases, the volume fraction of the aqueous phase almost remains unchanged and the middle phase of the microemulsion systems reduces significantly with thte release of excess oil up to a certain temperature and then remains unaffected with a further increase in temperature. For all surfactants, the effect of temperature follows the same path. The salinity of the microemulsions in this study is very close to the optimal salinity as shown in Figure 3, and hence the middle

(1)

where, Si is the scattered intensity from particle i and Di is the diameter of particle i. Dependence of particle size on HLB provides flexibility to formulation of microemulsions. The results of the particle size analysis of the nonionic surfactants have been shown in Table 2. As HLB values of nonionic surfactants increase Z-average diameter decreases. In case of the employed nonionic ethoxylated surfactants as HLB value 3619



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factors like branching of cosurfactant, category of surfactants, and oil types (microemulsion composition) also influence the optimal salinity. In Figure 4, the variations of optimal salinity

Figure 3. Solubilization parameter for oil (SPO = Mo/Ms) and water (SPW = Mw/Ms) as a function mass fraction of NaCl (S) for (a) Tergitol 15-S-5, (b) Tergitol 15-S-7, (c) Tergitol 15-S-9, and (d) Tergitol 15-S-12 at 60 °C. Symbols: ■, solubilization parameter for water (Mw/Ms); ●, solubilization parameter for oil (Mo/Ms). Figure 4. Relationship between optimal salinity (OS/mass fraction of NaCl) and temperature (T/K) for nonionic microemulsion systems. Symbols: ■, Tergitol 15-S-5; ●, Tergitol 15-S-7; ▲, Tergitol 15-S-9; ▼, Tergitol 15-S-12.

phase microemulsion shows significant stability at higher temperature. It may be concluded that the repulsion between ethylene oxide head groups is increased by increase of temperature and spontaneous curvature changes from zero to a positive value for medium salinity microemulsion systems. Microstructural transitions in microemulsion are based on the property of the interfacial surfactant film. The bending of surfactant film toward oil and water are evaluated based on the value of the spontaneous curvature (C0).18 However, some researchers19,20 have proposed that when the C0 value tends toward zero a bicontinuous structure is formed and an idea of the nature of C0 can be obtained from the geometrical tail and head areas of the surfactant, i.e., St and Sh, respectively. The C0 value depends on the interrelationship of St and Sh. When Sh > St then C0 > 0, and when Sh < St then C0 < 0. In the case of Sh = St, C0 = 0. The change of temperature can influence the C0 value. For nonionic surfactants due to hydration of the head groups, the heads are far apart and C0 is positive. With increase in temperature, hydration number decreases and hence C0.19 That is why increase in temperature affects the microstructure of microemulsion. With increase in temperature, the C0 value tends toward zero. That is, a structural change occurs by expelling oil and water as two separate phases. The HLB is very sensitive to temperature and the balance tilts toward more hydrophobicity with an increase in temperature. That is why, Winsor I is obtained with a relatively more hydrophilic system and Winsor II with a relatively more hydrophobic system. 3.2.2. Effect of Temperature on Solubilization Parameters and Optimal Salinity. The effects of temperature on solubilization parameters of microemulsion systems for different nonionic surfactants have been studied by measuring the masses of oil, water, and surfactant, i.e., Mo, Mw, and Ms, respectively, at different temperatures. The solubilization parameters for oil (Mo/Ms) and for water (Mw/Ms) have been plotted together against NaCl concentration at 333 K temperature and are shown in Figure 3 from where one can also predict the optimal salinity. The concentration of NaCl at which two curves intersect is termed as the optimal salinity. The optimal salinity is a function of temperature and generally it increases with an increase in temperature.2,5,21 Different

with temperature have been shown for all of the nonionic surfactants used in the present study. For all surfactants, the optimal salinity increases with temperature. A summarized form of optimal salinities at different temperatures has been depicted in Table 3. The effect of temperature on optimal salinity might Table 3. Optimal Salinities (OS/Mass Fraction of NaCl) of the Nonionic Microemulsion Systems at Different Temperatures (T) OS/(mass fraction of NaCl) × 10−2 T/K

Tergitol 15-S-5

298 308 318 333

2.00 2.10 2.15 2.25

Tergitol 15-S-7 Tergitol 15-S-9 1.50 1.55 1.60 1.65

1.78 1.80 1.85 1.90

Tergitol 15-S-12 2.20 2.30 2.40 2.48

be explained by the reduction in concentration of different components (NaCl salt and surfactant), weaker hydrogen bonding, and reduced hydrophobic effect with temperature.22 The reduction in concentration of different components is due to partitioning of salt and surfactant. As the temperature increases, the partitioning of surfactant and salt in oil phase increases and subsequently the concentration decreases. The optimal salinity is also a function of the HLB of the system. Generally optimal salinity is higher for the surfactant with a high HLB value. The exception is Tergitol 15-S-5 because of its dispersible nature in water. 3.3. Electrical Conductivity of Nonionic Microemulsion Systems. The electrical conductivity behavior of the nonionic microemulsion with weight fraction of brine is depicted in Figure 5. On addition of brine, initially the conductance value increases very slowly. The conductance of the tested microemulsion system (nonionic surfactant + 0.02 mass fractions of NaCl solution + synthetic oil) was found to 3620



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Figure 6. Linear plot of conductance (k) as a function of temperature (T/K) for nonionic surfactants/brine/synthetic oil systems. Symbols: ■, Tergitol 15-S-5; ●, Tergitol 15-S-7; ▲, Tergitol 15-S-9; ▼, Tergitol 15-S-12.

Figure 5. Conductivity (k/S·m−1) behavior of different nonionic microemulsion systems with variation of mass fraction of brine (MFB) in 0.02 mass fraction of NaCl concentration. Symbols: ■, Tergitol 15S-5; ●, Tergitol 15-S-7; ▲, Tergitol 15-S-9; ▼, Tergitol 15-S-12.

⎛ E* ⎞ k = A exp⎜⎜ − cond ⎟⎟ ⎝ R gT ⎠

increase with increasing brine mass fraction as shown in Figure 5. For all nonionic surfactant systems, a quick increase of conductivity with an increase in mass fraction of brine was observed at about 0.45 mass fraction of brine. This sudden increase of the conductance value is due to the structural transition in the bicontinuous phase. This phenomenon is known as the “percolation process”. The enhanced conductance has been considered to take place by the formation of infinite clusters or association of dispersed water droplets which are stabilized by surfactants in the oil continuous phase. The charges of the ions are easily moved from one droplet to another one by “hopping”23−26 or are transferred by way of “fusion mass transfer and mass exchange”.27−30 At a higher mass fraction of brine (mass fraction of brine >0.45), the conductance value remained the same due to a further transition of phase. The conductance of the middle-phase microemulsion system is mainly driven by the exchange of counterions (or charge carriers) between droplets in close proximity.31 From Figure 5 it is clear that the electrical conductance of the microemulsion formulated with Tergitol 15S-12 is greater than other nonionic microemulsions. Since it has 12 ethylene oxide groups, therefore, its conductivity behavior is more prominent than other nonionic surfactants and it shows a higher conductance value than other surfactants in any composition of microemulsions system. The increased conductivity is due to enhanced interfacial fluidity caused by the adsorption of the nonionic surfactants into the interfaces. As a result the droplet radius of the microemulsion is decreased and is influenced by the nature and size of the ethylene oxide headgroup area of the used nonionic surfactants. 3.3.1. Effect of Temperature on Microemulsion Conductivity. In Figure 6, conductance-temperature profile is depicted for all nonionic microemulsion systems. The conductivity of the middle-phase microemulsion was found to increase with an increase in temperature of the system. The temperature effects of the conductance of nonionic microemulsion systems were studied for the calculation of the energetic of conduction flow in microemulsion systems using an Arrhenius type of equation.31,32

(2)

Equation 2 can be rearranged by taking logarithm on both sides to form eq 3. * / R gT ln k = ln A − Econd

(3)

where k is the conductance, A is a preexponetial factor, and E*cond is the activation energy of conduction. Rg is the universal gas constant, and T has its usual significance. From the linear plot of ln k with 1/T, the preexponetial factor and activation energy can be calculated. From the slope of the curves Econd * and from the intercept at Y axis, A can be determined. The results have been presented in Table 4. From Table 4, it is clear that Table 4. Activation Energy (E*cond/ kJ·mol−1) and Preexponetial Factor (A) with Standard Deviation (SD) of the Nonionic Microemulsion Systems surfactants in microemulsion Tergitol Tergitol Tergitol Tergitol

15-S-5 15-S-7 15-S-9 15-S-12

Econd * ± SD/ (kJ·mol−1) 17.68 18.77 21.12 21.29

± ± ± ±

0.82 0.56 0.82 0.56

A ± SD

R2

± ± ± ±

0.974 0.988 0.991 0.984

7814 14069 41390 51210

0.005 0.007 0.005 0.007

the activation energy and EON of the nonionic surfactant are linearly related. The interaction of microemulsion droplets during the percolation28,33,34 process is sometimes very specifically described by the “transient fusion-mass transferfission” process.35−37 As the temperature increases the average kinetic energy of the droplets also increases and the collision rate among them also improved which leads to an increase in conductivity of the system.

4. CONCLUSION The effect of temperature on relative phase volumes and conductivity behavior of nonionic microemulsion systems have been studied. From the results, it can be concluded that the 3621



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(9) Lang, J.; Zana, R.; Lalem, N. The Structure, Dynamics and Equilibrium Properties of Colloidal Systems; Kluwer Academic: The Netherlands, 1990. (10) Boned, C.; Peyrelasse, J.; Saidi, Z. Dynamic percolation of spheres in a continuum: The case of microemulsions. Phys. Rev. E 1993, 47, 468−478. (11) Feldman, Y.; Korlovich, N.; Nir, L.; Garti, N. Dielectric relaxation in sodium bis(2-ethylhexyl)sulfosuccinate-water-decane microemulsions near the percolation temperature threshold. Phys. Rev. E 1995, 51, 478−491. (12) Schubel, D. A shear-induced viscoelastic system through addition of a crown-ether to AOT w/o-microemulsions. Colloid Polym. Sci. 1998, 276, 743−746. (13) Ray, S.; Bisal, S.; Moulik, S. P. Structure and dynamics of microemulsions. Part 1. Effect of additives on percolation of conductance and energetics of clustering in water−AOT−heptane microemulsions. J. Chem. Soc., Faraday Trans. 1993, 89, 3277−3282. (14) Alexandradis, P.; Holzwarth, J. F.; Hatton, T. A. Interfacial dynamics of water-in-oil microemulsion droplets: determination of the bending modulus using iodine laser temperature jump. Langmuir 1993, 9, 2045−2052. (15) Peyrelasse, J.; Boned, C. Structure and Dynamics of Strongly Interacting Colloids and Supramolecular Aggregates in Solution; Kluwer Academic: The Netherlands, 1992. (16) Bera, A.; Ojha, K.; Mandal, A.; Kumar, T. Interfacial tension and phase behavior of surfactant-brine−oil system. Colloids Surf. A 2011, 383, 114−119. (17) Griffin, W. C. Classification of surface active agents by HLB. J. Soc. Cosm. Chem. 1949, 1, 311−326. (18) Safran, S. A. Micellar Solutions and Microemulsions: Structure, Dynamics and Statistical Thermodynamics;; Spinger-Verlag: New York, 1990. (19) Langevin, D. Microemulsions - interfacial aspects. Adv. Colloid Interface Sci. 1991, 34, 583−595. (20) Aveyard, R.; Binks, B. P.; Fletcher, P. D. I. In the Structure, Dynamics and Equilibrium Properties of Colloidal Systems; Kluwer Academic Publishers: Norwell, MA, 1990. (21) John, A. C.; Rakshit, A. K. Phase behavior and properties of a microemulsion in the presence of NaCl. Langmuir 1994, 10, 2084− 2087. (22) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes; Wiley: New York, 1981. (23) Safran, S. A.; Grest, G. S.; Bug, A. L. R. Microemulsion Systems; Dekker: New York, 1987. (24) Hilfilker, R.; Eicke, H. F.; Geiger, S.; Furlur, G. Optical studies of critical phenomena in macrofluid-like three-component microemulsions. J. Colloid Interface Sci. 1985, 105, 378−387. (25) Peyrelasse, J.; Boned, C.; Saidi, Z. Percolation phenomenon in waterless microemulsions, Prog. Colloid Polym. Sci. 1989, 79, 263−269. (26) Kim, M. W.; Huang, J. S. Percolation like phenomena in oilcontinuous microemulsions. Phys. Rev. A 1986, 34, 719−722. (27) Lang, J.; Lalem, N.; Zana, R. Droplet size and dynamics in water-in-oil microemulsions. Colloids Surf. A 1992, 68, 199−206. (28) Borkovec, M.; Eicke, H. F.; Hammerich, H.; Das Gupta, B. Two percolation processes in microemulsions. J. Phys. Chem. 1988, 92, 206−211. (29) Dutkiewicz, E.; Robinson, B. H. The electrical conductivity of a water-in-oil microemulsion system containing an ionic surfactant: Part I. Temperature effect. J. Electroanal. Chem. 1988, 251, 11−20. (30) Mukhopadhyay, L.; Bhattacharya, P. K.; Moulik, S. P. Additive effects on the percolation of water/AOT/decane microemulsion with reference to the mechanism of conduction. Colloids Surf. A 1990, 50, 295−308. (31) Paul, B. K.; Moulik, S. P. Biological microemulsions: Part IIIThe formation characteristics and transport properties of saffolaaerosol OT-hexylamine-water system. Indian J. Biochem. Biophys. 1991, 28, 174−183. (32) Paul, B. K.; Das, M. L.; Mukhopadhyay, D. C.; Moulik, S. P. Phase behaviour and physicochemical properties of a microemulsion

microemulsion compositions affect not only the phase behavior and phase volumes of microemulsion systems but also the thermodynamic parameters of microemulsion systems. Therefore, like salinity, temperature can also induce the phase transition of the microemulsion systems. However, at optimal salinity, the middle phase microemulsion shows its highest stability even at higher temperature. Optimal salinity of the microemulsion system has been found to increase with an increase in temperature. The results also show that optimal salinity generally increases with HLB vales of water-soluble nonionic surfactants. The conductivity of nonionic microemulsion systems depends upon EON. It has been observed that the higher the EON, the higher the microemulsion conductivity with the same compositions. A clear water volume percolation process has been observed in the present system. The mechanism of the percolation has been discussed on the basis of the “transient-fusion-mass transfer-fission” and “hopping” methods. The temperature effect on the electrical conductivity is also investigated. With an increase in temperature, the conductivity increases and the activation energy of the system increases with increase in EON.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 91-326-2296632. Funding

This work was financially supported by the University Grant Commission [F. No. 37−203/2009(SR)]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the University Grant Commission [F. No. 37-203/2009(SR)], New Delhi to the Department of Petroleum Engineering, Indian School of Mines, Dhanbad, India. Thanks are also extended to all persons who are directly or indirectly associated with the project work.

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REFERENCES

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Temperature-Dependent Phase Behavior, Particle Size, and

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