Investigation on Physical Properties and Morphologies ofMicroemulsions formed with Sodium Dodecyl Benzenesulfonate,Isobutanol, Brine, and Decane, Using Several ExperimentalTechniquesAyako Fukumoto, Christine Dalmazzone,* Didier Frot, Loïc Barre, and Christine Noïk

IFP Energies nouvelles, 1-4 avenue de Bois Preau, 92852 Rueil-Malmaison, France

ABSTRACT: In the context of chemical enhanced oil recovery (EOR), there is no well-established way to characterize andunderstand the physical properties and structures of microemulsions composed of crude oil and industrial surfactants due to theircomplexity. Making a comparison to a well-studied model system is a simple and effective way to investigate the complexmicroemulsions. The purpose of this study is to provide and complete experimental characteristic data of model-microemulsionsas a basis of analysis of the complex system. Types of microemulsions studied in the present work were oil in water (O/W),bicontinuous, and water in oil (W/O). The system was composed of water, sodium dodecyl benzenesulfonate (SDBS),isobutanol, sodium chloride (NaCl), and decane. We performed several experiments at room temperature such as spinning dropmethod, differential scanning calorimetry (DSC), Karl Fischer titration, Hyamine titration, dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), and electron cryomicroscopy (cryo-SEM). The measurements provided a deeper insight into acorrelation between physical properties and morphologies of water/oil in the microemulsions.

■ INTRODUCTION

Water and oil can be dispersed with each other by addingsurfactants in a system. Microemulsions are thermodynamicallystable, composed of water, oil, surfactants, and sometimescosurfactant such as alcohol.1 One can observe the structuraltransition of microemulsions from water in oil (W/O) to oil inwater (O/W) through bicontinuous by changing physicalparameters of a system like temperature, salinity, surfactantconcentration, and water to oil ratio (WOR). A bicontinuousphase can be pictured as a structure that has aqueous and oildomains separated with a surfactant layer with periodicity. It issimilar to that of a lamellar phase, but each phase is connectedto itself and the domains are chaotically intertwined.2

Commonly, a microemulsion’s system is classified into WinsorI−III types.3 A Winsor I consists of O/W microemulsions as alower phase and excess oil as an upper phase. A Winsor IIconsists of excess water phase as a lower phase and W/Omicroemulsions as an upper phase. A Winsor III has threephases; bicontinuous microemulsions in between excess water(lower) and oil (upper) phases. Types of microemulsions aredetermined by the behavior of surfactants that inhabit thewater/oil interface.4,5 When the surfactant has an equal affinityfor oil and water, the interfacial tensions (IFT) between oil andwater can be ultralow, and this leads to bicontinuousstructure.6−8 Microemulsions have been studied for chemicalenhanced oil recovery (EOR) and pharmaceutical applications,among others.9−12 Physical properties and characterizations ofthe microemulsions are usually investigated by differentexperimental measurements such as, conductivity, IFT,remained surfactant concentration, water content, and so on.In the context of chemical EOR, there is no well-establishedway to characterize and understand the physical properties andthe structures of microemulsions composed of crude oil and

industrial surfactants due to their complexity. Making acomparison to a well-studied model system is a simple andeffective way to investigate the complex microemulsions.Numerous works have been done with different model

systems in the past. Saito and Shinoda4 were one of the first toreport the phase transition of the microemulsions (W/O to O/W) by studying a phase stability of W/O microemulsion as afunction of temperature. Lindman et al.13 demonstrated thatboth oil and water are continuous in a middle phase with anuclear magnetic resonance (NMR) experiment. Aveyard etal.14 investigated on phase diagrams and physical properties ofthe microemulsions. They measured IFT and concentration ofthe surfactant in aqueous and oil phases as a function oftemperature and salinity. Viscosity, conductivity, and hydro-dynamic radius were studied as well on W/O microemulsions.The structure of bicontinuous phase was studied with a small-angle X-ray scattering (SAXS),15 and a model16 was proposedto understand the characteristic length of the structure. Imagesof the structures were presented by Jahn and Strey17 using afreeze fracture electron microscopy. Garti et al.18 studieddistributions of water in W/O microemulsions with adifferential scanning calorimetry (DSC). Dynamical behaviorof the bicontinuous phase was studied with a quasielastic lightscattering.19 Among the many model systems studied, fewstudies share the same component. Therefore, it is needed toprovide characteristics of experimental results of a certainmodel-microemulsion for further investigations on the complexmicroemulsions. With this aim, we have developed a completemethodology based on the application of several probes. This

Received: March 11, 2016Revised: May 4, 2016

Article

pubs.acs.org/EF

© XXXX American Chemical Society A DOI: 10.1021/acs.energyfuels.6b00595Energy Fuels XXXX, XXX, XXX−XXX

methodology will be relevant to more complex systems such asthe one including real oils and will become of prime importanceto the reemerging field of chemical EOR, especially surfactantflooding.The purpose of this study is to show experimental

characteristics of model-microemulsions as a basis of analysisof the complex system. In this study, we performed severalexperiments at room temperature such as spinning dropmethod, DSC, Karl Fischer titration, Hyamine titration,dynamic light scattering (DLS), SAXS, and electron cryomicro-scopy (cryo-SEM). The system studied in this work iscomposed of water, sodium dodecyl benzenesulfonate(SDBS), isobutanol, sodium chloride (NaCl), and decane.The anionic surfactant SDBS was chosen because it is a well-defined molecule used in chemical, biochemical, and industrialworks.This paper is organized as follows. First, experimental

techniques are described in detail. Results from the measure-ments are discussed in two parts. One is from aphysicochemical, and the other is from a morphological pointof view. The characteristics of the bicontinuous microemulsionat a salinity of 52 g/L are the focus of the conclusions.

■ MATERIALS AND METHODSMaterials. SDBS (technical grade) was purchased from Sigma-

Aldrich. Isobutanol (purity >99%) and decane (99% purity) werepurchased from Alfa Aesar. NaCl (99.9% purity) was purchased fromVWR chemicals. All compounds were used without furtherpurification. Aqueous solutions were prepared with Milli-Q watermade with a resistivity of 18.2 MΩ/cm.Preparation of Samples. A microemulsion was prepared at room

temperature in a glass tube by following procedures. First, solutions ofSDBS (140 g/L) and NaCl (200 g/L) were prepared separately inflasks. The mass of SDBS and NaCl was measured with an electronicmass balance (AE200, Mettler toledo) with a precision of ±0.1 mg.Second, 1 mL of the SDBS solution was inserted in a glass tube with apipet. After, water and NaCl solutions were inserted in the glass tubewith pipettes. Here, the volume of water and the solution wascalculated from the set salinity (from 30 to 80 g/L) and set volume ofaqueous phase (6.79 mL). Also, 0.42 mL of isobutanol was added inthe aqueous phase and the solution was carefully mixed. Finally, 6.79mL of decane was inserted in the glass tube. Samples were mixedgently by turning the tubes upside down for approximately 20 times,and left to be equilibrated for 1 month at room temperature. Tosummarize the composition of the samples, the concentration of SDBSin aqueous phase was 20.6 g/L, WOR was set to 1, and isobutanol was3% (ratio in total volume).Spinning Drop Method. IFT of the microemulsion and aqueous/

oil phase was measured at 20 °C using the spinning drop videotensiometer (SVT 20N, Dataphysics). Equilibrated solutions weresampled for this method. First, density of the sample was measuredwith a densimeter (DMA 4500 M, Anton Paar) at 20 °C. After, about1 mL of the aqueous phase was inserted in a glass capillary, and a dropof the phase, contacting with the aqueous phase, was injected. Thecapillary was rotated at high speed (5000 rpm).DSC. Crystallization and dissociation temperatures/enthalpies of

microemulsions were measured using a DSC apparatus (DSC 1,Mettler toledo). The calibration of the setup was carried out with highpurity indium. Water was used to determine the uncertainty of thephase change temperature and the enthalpy, which were found to be0.40 °C and 20%, respectively. Between 10 to 20 mg of micro-emulsions were inserted in a cell. Temperature of the furnace wasdecreased from 20 to −80 °C at a cooling rate of 5 °C·min−1 and keptconstant for 5 min. After, temperature was increased to 20 °C at aheating rate of 5 °C·min−1. Temperature of crystallization andenthalpy of fusion were measured to investigate the water morphology

and salinity of the microemulsions. This scanning was also conductedwith SDBS + NaCl + isobutanol + water solutions as a reference.

Karl Fischer Titration. Mass fraction of water in oil phase ormicroemulsion phase was measured by the Karl Fischer titration. Thetitration was conducted with 787-KF Titrino from Metrohm with areagent of HYDRANAL-Composite 5 purchased from Fluka. Aweighted sample (about 10 mg) was injected into a measuring cell, andthe amount of water in the sample was calculated according to thevolume of reagent consumed. All the samples were analyzed at leastthree times.

Hyamine Titration. The concentration of SDBS in the aqueousphase was estimated by Hyamine titration (Hyamine 1622-solution,Merck Millipore) using a titrator (862-compact titrosampler,Metrohm). For this, 1 mL of the sample was inserted in a measuringcell. The titrant (Hyamine) was added in the sample with anincrement of 0.03 mL. An electrode was used for the potentiometrictitration to determine the amount of the anionic surfactants in thesample.

DLS. Dynamics of microemulsions were studied using a DLS setup(Vasco-flex, Cordouan technologies) equipped with a diode laser (λ =656 nm) and a single photon counting. Measurements were conductedat a scattering angle (θ) of 90°. A detailed description of the setup canbe found elsewhere.20 It is noteworthy that this measurement did notrequire any sampling. The equipment was specially modified in orderto mount a tube on the setup so that the laser beam can directly gothrough the phase of interest in order to observe its dynamics.

In this measurements, intensity fluctuations of the scattered lightwere collected as a function of time. From the measured fluctuation,the autocorrelation function of the intensity G2(q, t) can be derivedusing the Siegert relation and can be expressed as

α β= +G q t g q t( , ) ( , )2 12

(1)

where α and β are a baseline and the coherence factor, respectively. q(= 4πn sin(θ/2)/λ0) and t represent scattering vector and time,respectively. n is the refractive index of the medium, and λ0 is theradiation wavelength in vacuum. The normalized field autocorrelationfunction g1(q, t) is related to the dynamic behavior of the sample. Formonodispersed particles, it is appropriate to describe g1(q, t) as a singleexponential decay:

= −Γg q t t( , ) exp( )1 (2)

where Γ is the decay rate. The diffusion coefficient of the particles canbe given as D = Γ/q2. In the case of hard spheres well dispersed in asolution with no interactions and only diffuse as Brownian motion, thehydrodynamic radius RH of the sphere is given by the Stokes−Einsteinequation as

πν=D kT R/6 0 H (3)

where k, T, and ν0 represent the Boltzmann constant, temperature, andviscosity of solvent, respectively.

SAXS. An in-house experimental setup was used to investigate thestructural characteristics of microemulsions. The sample was injectedinto a capillary with a diameter of 1.5 mm. Based on measurements ofan external standard (Ag behenate), the position of each pixel, relativeto the position of the direct beam, was converted into the magnitudeof the scattering vector q (= 4πn sin(θ/2)/λ0). The range of thescattering angle (θ) enables a range of q from 7 × 10−3 to 3 × 10−1 Å−1

to be covered. After normalization with respect to thickness,transmission, and measuring time, the solvent signal was subtractedfrom the sample signal, and the raw intensities were converted to thescattering cross section I(q) in absolute scale (cm−1). Further details ofthe description of the setup can be found elsewhere.20−22

Cryo-SEM. This technique was used to visualize the structure ofbicontinuous microemulsions. In this study, a small piece of the middlephase solutions at a salinity of 52 g/L was shock frozen in nitrogenslush at −210 °C, fractured, and transferred in a vacuum to thescanning electron microscope (Supra 40, Zeiss) fitted with a cold stageunit.

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Rheometer. The viscosities of O/W and W/O microemulsionswere measured with AR2000 rheometer (TA Instruments) using adouble cylinder geometry. After carefully loading the sample in thegeometry, 1 min of equilibration time was imposed. Shear ratemeasurements were performed between 0.1 and 700 s−1. Thismeasurement was conducted in order to estimate the hydrodynamicradius of the microemulsions from the DLS measurements asexplained later in the Results and Discussion.

■ RESULTS AND DISCUSSIONPhysicochemical Analysis. Phase Behavior. Phase

transitions of the samples were investigated with a salinityscan in the range from 24 to 96 g/L with a minimum interval of2 g/L. The samples were kept at room temperature for 1month in order to have equilibrated phases. It was found thatthe phases reached equilibrium 4 days after the preparation,because heights of the phases remained constant since. Middlephases were observed at salinities from 36 to 74 g/L. The phasetransitions in terms of the microemulsions can be described asfollows: Winsor I (O/W) ranged from 24 to 34 g/L, Winsor III(bicontinuous) ranged from 36 to 74 g/L, and Winsor II (W/O) ranged from 76 to 96 g/L. The photographic and schematicimages of some of the formulated samples are shown in Figure1. The aqueous phase was white-translucent at a salinity of 30

g/L because of dispersed oil droplets. At salinities of 40, 52, and64 g/L, three phases were observed. The middle phase at 40 g/L was slightly yellow-colored. The color is due to SDBSmolecules because the similar color was also seen with aconcentrated SDBS solution. As for samples at 52, 64, and 80g/L, all phases were transparent. Note that the dark region atthe top of the lower phase is seen in the photographic imagebecause of an optical effect at the interface.IFT. Figure 2 shows the IFTs between equilibrated

microemulsion and oil/water phases measured with thespinning drop method. At salinities of 30, 52, and 80 g/L,the measurements were conducted three times, and meanvalues are plotted. The error of the measurements were foundto be less than 7%, which is smaller than the symbol (filled

circle) in the figure. As can be seen from the figure, the order ofmagnitude of IFT was −2 when the system is Winsor III. Forcomparison, the IFT measured by Moire 23 with the sametechnique is plotted in the figure. The IFT reported from theliterature is substantially smaller than ours at a salinity of 52 g/L. When a system is Winsor III, the interfaces can be easilydisordered because of the ultralow IFT, especially on theoccasion of samplings. It can cause a transfer of SDBSmolecules between phases, which can result in the deviation ofIFTs between the measurements. At salinities of 30 and 80 g/L,the results agree well with the literature. This is reasonablebecause when the system is Winsor I/II, sampling is easier. Onecan say that optimum salinity of the system is in the vicinity of50 g/L, meaning that the amphiphilicity is high in this region.

Water Content of Oil Phase and Surfactant Concen-tration in Aqueous Phase. Compositions of the aqueous andoil phases were investigated by the Karl Fischer and theHyamine titration. The results are summarized in Figure 3. It

shows that about 0.4−1 wt % of water was in oil phase whensystems are Winsor I and III. The water content increasedrapidly at a salinity of 80 g/L, which is in the range of Winsor II(W/O microemulsions). As for the SDBS concentration inaqueous phase, while about 95% of the surfactant remained inthe aqueous phase when the system is Winsor I, less than 1%remained when it is Winsor II/III. The results refer that thesecompositions of the oil and aqueous phases are goodindications for characterizing the state of a system; low watercontent of oil phase and high concentration of surfactants in

Figure 1. Photographic and schematic images of formulatedmicroemulsions with increase of salinity. Samples were kept at roomtemperature for 1 month. System was composed of water, NaCl,SDBS, isobutanol, and decane. WOR is unity. WI, WII, and WIIIrepresent Winsor I, II, and III, respectively. A and O denote aqueousand oil phases, respectively.

Figure 2. IFT between equilibrated microemulsion and water/oilphases as a function of salinity. Data from literature23 is shown as acomparison.

Figure 3. Water content of oil phase measured by Karl Fischertitration and concentration of SDBS in aqueous phase measured byHyamine titration.

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aqueous phase indicate a system is Winsor I, when theproperties are contrary, a system is Winsor II. Both parametersare low when Winsor III, because surfactants are concentratedin the middle phase, thus few oil can transfer to the aqueousphase.Salinity and Water Content of Microemulsion. From the

heat profiles of the melting process obtained from DSCmeasurements, one can estimate the concentration of NaCl inthe microemulsions by comparing the onset temperature of theprogressive melting of ice with that of the aqueous solutions.Figure 4 shows heat flow profiles of the aqueous solutions and

decane. During the melting process, one peak was detectedwhen no NaCl was present. Two negative peaks were observedwith the solutions with NaCl; first one detected around −20 °Ccorresponded to eutectic melting of NaCl and ice, and thesecond broad peak was detected soon after the first one, whichwas due to the progressive melting of ice. The meltingtemperature of decane was about −30 °C, which is in a goodagreement with the literature.24 From Figure 4, a relationbetween temperature of the progressive melting of ice andsalinity was found as T = 4.99 × 10−4x2 − 1.11 × 10−1x withthe average absolute deviations (AAD) of 0.13 °C, where T isthe melting temperature in degrees Celcius and x is theconcentration of NaCl in grams per liter. It is noted that thisrelation is only valid within the studied salinities. Here, thetemperature of progressive melting of ice was extrapolated froman intersection point of a baseline and a line which goesthrough the peak point of the progressive melting. For aninclination of the line, the same value of the tangent at aninflection point of the eutectic melting was used. The heat flow

profiles of the microemulsions at different salinities are shownin Figure 5. At a salinity of 80 g/L, a negative peak was

observed around −30 °C, which corresponds to a dissociationof decane. It is suspected that the heat absorbed by melting ofdispersed water droplets was too small to be detected (watercontent was 3 wt %, see Figure 3). As for salinities at 30, 40, 52,and 64 g/L, three peaks were detected. First peak observedaround −30 °C was induced by decane melting. Second andthird peaks were due to eutectic and progressive melting of ice,respectively. The temperature of the progressive melting of icein microemulsion was determined with the same procedure asthe aqueous solutions, and the salinity was estimated using theequation shown above. The comparison between the initialconcentration of NaCl and the value obtained from the DSCmeasurements are shown in Figure 6a. Error bars of the y axiswere estimated from the uncertainty of the DSC setupmeasured with water. Note that the concentration of NaClwas not estimated with the microemulsion at a salinity of 80 g/L since a significant peak of the ice melting was not detected.The estimation from the DSC measurements slightly under-estimates the salinity. The deviations were 7, 10, 14, and 9 g/Lat 30, 40, 52, and 64 g/L, respectively.Water content of microemulsion can also be estimated by

comparing the total enthalpies (surface area of the peaks) of theeutectic and progressive melting of ice in microemulsions withthat in aqueous solutions. A relation between the meltingenthalpy and salinity (within the studied range of salinities),when water content was 100%, was found as ΔH = 5.00 ×10−1x − 3.25× 10−2 with the ADD of 2.4%. Here, ΔH

Figure 4. Heat flow profiles of aqueous solutions composed of SDBS+ water + isobutanol with different salinities and decane.

Figure 5. Heat flow profiles of microemulsions with different salinities.Types of microemulsions are O/W at 30 g/L, bicontinuous at 40, 52,and 64 g/L, and W/O at 80 g/L.

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represents the melting enthalpy in joules per gram. Likewise, oilcontent of microemulsion can be estimated by comparing theenthalpy of decane melting. Melting enthalpy of decane wasfound as 240 J/g, which is in the error range of the literature.25

This value was used as reference of oil content of 100%. Thecomparison of water content measured with the Karl Fischermethod and the DSC measurements are shown in Figure 6b.Squares were calculated using the melting enthalpies of theaqueous solutions as reference. While circles were calculatedwith the decane melting, assuming wwater = 1 − wdecane. Here wrepresents mass fraction of each component. The estimation ofDSC using the aqueous melting roughly equals the valuemeasured with the Karl Fischer titration. The estimation fromthe decane melting, however, deviated about 20 wt % from theKarl Fischer titration. The possible reason can be anassumption we made in the estimation, which ignored themass contribution of surfactant, NaCl, and alcohol. Due to thisassumption, water content was overestimated. Despite the timecost, it is recommended to use melting profiles of aqueoussolutions when estimating water contents from DSC measure-ments. From Figure 6b, one can say that a salinity of 52 g/L isclose to the optimum salinity because the volume of water inmicroemulsion was estimated to be 48 wt % with the DSCresult.Morphological Analysis. Images of Bicontinuous Micro-

emulsions. Cryo-SEM allowed us to have the images of thestructure of the bicontinuous microemulsion. Obtained imagesof the sample at a salinity of 52 g/L are shown in Figure 7.From Figure 7a, it was observed that the bicontinuous phase isa homogeneous structure at this scale. Figure 7b shows an

image at a higher resolution. From the color contrast, which isdue to the difference of chemical composition and topology, itis confirmed that water and oil domain are intertwined at thisscale. The reader is directed to the literature,17,26 which showssimilar images of the bicontinuous phase taken with an electronmicroscopy.

Crystallization of Microemulsions. Dalmazzone et al.27

demonstrated the characterization of opaque-emulsions using aDSC. Temperature of crystallization is related to morphologyof water in microemulsions since a probability of nucleation ofice is dependent on the volume of water. It has been shown inthe literature that the smaller the volume is, the lower thefreezing temperature is.As shown in Figure 5, during the cooling process, sharp

positive peaks were observed around −20 °C at salinities of 30and 40 g/L, which are in the same range as measured in theaqueous solutions (Figure 4). It indicates the existence of waterin a volume scale of mm3. This is understandable for the sampleat a salinity of 30 g/L, since the microemulsion was O/W,meaning oil droplets dispersed in a continuous water phase. Asfor 40 g/L, one could conclude that water channels in themicroemulsion were so large that the peak did not shifteddespite the formation of the bicontinuous structure. However,it is not the case for this sample. The heat profile similar to 30g/L was obtained because water and oil domains in thebicontinuous microemulsion were separated during the coolingprocess. The phase separation was visually confirmed by asimple experiment; about 5 mL of the bicontinuous micro-emulsion was frozen in a household freezer, and defrosted in aroom temperature. The phase separation was observed at asalinity of 40 g/L, but not at salinities of 52 and 64 g/L. Webelieve that the microemulsion at 40 g/L destabilized because itis near the verge of Winsor III region and the structure canchange more easily than that at 52 and 64 g/L. For the samplesat 30 and 40 g/L, second peaks were observed around −35 °C,which correspond to decane crystallizations. Following the

Figure 6. (a) Comparison of concentration of NaCl in microemulsioncalculated from DSC measurements and that at initial state. (b)Comparison of water content of microemulsion calculated from DSCmeasurements and measured with the Karl Fischer titration. Numbersshown next to symbols represent the salinities. Square is calculatedusing results of aqueous solution melting. Circle is calculated usingresults of decane melting. The lines correspond to f(x) = x.

Figure 7. Images of bicontinuous microemulsions (middle phase) at asalinity of 52 g/L obtained with cryo-SEM. Images a and b were takenat different resolutions (see scales shown in each image).

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second peaks, third peaks were detected between −45 and −40°C. These small peaks were attributed to NaCl crystallizations.It must be noted that the bicontinuous microemulsions at asalinity of 52 g/L exhibited only one peak of crystallization.This supports the fact that for the bicontinuous phase at aroundthe optimum salinity, water and oil phases are neighboring eachother laying the surfactant molecules in between the layers.Thus, the crystallizations of water and oil were inducedtogether. At a salinity of 64 g/L, the peak of decanecrystallization was following a small sharp peak observed at−32 °C. This sharp peak suggests the presence of water in avolume scale of cubic micrometers in the bicontinuous phase. Itcan be interpreted as a bicontinuous phase with a poorconnection of the water channel. As for W/O microemulsionsat a salinity of 80 g/L, one peak was obtained at −35 °C, whichis mainly due to the decane crystallization. It is suspected thatthe crystallization of water droplets were induced by thesurrounding decane, hence the one peak corresponds to bothdecane and water freezing.Dynamical Behavior. The dynamics of O/W and W/O

microemulsions can be evaluated using the hydrodynamicradius from the Stokes−Einstein equation as mentioned beforesince they are composed of dispersed particles. However, forbicontinuous microemulsions, it is not suitable for theevaluation. To compare the dynamics of all phases, thediffusion coefficient D was used for the sake of simplicity.The cumulant method was applied to have a representativedecay rate Γ of the auto correlation data. The diffusioncoefficient D, which describes the averaged fluctuation of thescattered intensity, was obtained (D = Γ/q2). It is noted that Dobtained from a middle phase does not represent the diffusivityof components in the phase, but it only corresponds to thecharacteristic time of the dynamics, which is defined as τc = 1/Γ. The higher D, the smaller the characteristic time.Figure 8 shows the measured autocorrelation functions of

microemulsions at 30, 52, and 80 g/L. The solid lines represent

the exponential decays using Γ. It is noteworthy that all thesamples were quite transparent and there was no multi-scattering of the light. Also, there was no dust in the samples,which was confirmed by measuring the total scattered intensityversus long time. In Figure 9, D of aqueous, middle, and oilphases are plotted at different salinities. The error bars in thefigure represent the reproducibility of the measurements, whichwas found to be around 10%. D of the middle phase increased,but it decreased beyond this concentration. The order ofmagnitude of D of middle phases measured in this study are

consistent with the literature,19,28 which reports the diffusioncoefficient of microemulsions formed with anionic surfactants,brine, and simple alkane oil. In bicontinuous microemulsions,the fluctuation is dependent on the size of water and oildomains because it is mainly due to a topology change in thephase; breaking and mending of water/oil channels. As can beseen in Figure 9, D of the middle phase maximized near theoptimum salinity because the domain size is smaller and hencemore frequent topology change was observed. As for theaqueous phase, the fluctuation decreased along with the salinityincrement when system was Winsor I. This can be due toincrements of size of dispersed oil droplets or interactionsbetween the particles. When system was Winsor II or III, nofluctuation of the intensity was observed in the aqueous phase(plotted as D = 0 in the figure). Although the D of oil phase inWinsor I and III was 1 order of magnitude smaller than that inWinsor II, the fluctuation of the phase was confirmed at allsalinities studied in this work. This is consistent with ourobservation with the Karl Fischer titration (Figure 3), whichconfirmed the presence of water in the oil phase in all Winsorsystems.

Characteristic Lengths of Structures. Structures of themicroemulsions were investigated, and the characteristiclengths were extracted from the SAXS measurements. Figure10 shows the SAXS spectra of microemulsions. Note that the

signals of the solvents (water and decane) were subtracted withrespect to volume fractions. The volume fractions of water (φw)and decane (φo) were estimated by the water fraction in mass(ww) obtained from the Karl Fischer titration and the density(ρME) as φw = ww × ρME/ρw, where ρw represents the density ofwater. φo is calculated as 1 − φw. From the figure, the

Figure 8. Normalized autocorrelation against time measured atdifferent salinities. Types of microemulsions are O/W at 30 g/L,bicontinuous at 52 g/L, and W/O at 80 g/L. Symbols are from DLSmeasurements and solid lines correspond to the exponential decay.

Figure 9. Diffusion coefficient (D) of relaxation process of aqueous,middle, and oil phases as a function of salinity.

Figure 10. SAXS spectra of microemulsions. Types of microemulsionsare O/W at 30 g/L, bicontinuous at 40, 52, and 64 g/L, and W/O at80 g/L. Solid lines are the best fits by T-S model or Guinier equation.A dashed line is the power law of q−4.

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dispersion of the measured intensity is clearly visible at large qvalues for 80 g/L. For other spectra, especially for bicontinuousmicroemulsions, uncertainties are smaller than the symbol sizes.The scattering curve of microemulsion at salinities of 40, 52,

and 64 g/L exhibited a single broad peak, which is a typicalpattern from bicontinuous microemulsions.16,29−31 As for 30and 80 g/L, no characteristic peak was observed. In the small qregime, the scattered intensity is a plateau for the O/Wmicroemulsion at 30 g/L. On the other hand, the intensityexhibits a more pronounced q dependence for the W/Omicroemulsion at 80 g/L. One can conclude that themicroemulsion at 80 g/L had significant interactions ofdispersed water droplets such as aggregations. In the large qregime, the scattering intensities of all the tested samplesdecreased with the same scale as q−3.2, meaning that thestructure on a small length scale exhibits no differencedepending on the salinity. Theoretically speaking, one shouldobserve the decay of the curve scaling as q−4 (Porod’s law) inthe large q regime, which is a characteristic of a sharp interface(i.e., a step function of scattering length density). Thedifference between the law and our measurements is attributedto a SDBS molecule at the interface. This makes the scatteringlength density more complex than the simple approximationusing the solvents such as φwφo(ρw − ρo)

2. It is suggested touse a profile of the scattering length density at the interface thatis closer to the actual one. The scattering length densities at theinterface can be calculated by combining the bound coherentscattering length and the molar volume32 of the atom/group.The values related to the interface are 7.12 × 1010 and 9.4 ×1010 cm−2 for decane and water, respectively. As for SDBS, asulfonate ion is 23 × 1010, a phenylene is 9.5 × 1010, a straightalkyl chain is 8.4 × 1010, and a methyl is 4.5 × 1010 cm−2.The scattered intensity I(q) of the O/W microemulsion at 30

g/L was fitted with the Guinier approximation, which is givenby using radius of gyration Rg as

= −I q I q R( ) (0)exp( /3)2g

2(4)

Assuming that droplets are spherical particles homoge-neously dispersed in the solution, the statistical radius of theparticles R0 can be estimated by Rg

2 = 3/5R02.33

The scattered intensity I(q) of the bicontinuous micro-emulsions was fitted with the model proposed by Teubner andStrey16 (T-S model). In the T-S model, a real space correlationfunction ϒ(r) is described as

ξϒ = · −r Kr Kr r( ) sin( )/ exp( / ) (5)

where K is given as K = 2π/d. The scattered intensity I(q) isdescribed with d and ξ as

π η ξ=

⟨ ⟩+ +

I qc

a c q c q( )

8 /22

2 12

24

(6)

where ⟨η2⟩ is the mean square fluctuation of the scatteringlength density (ρ) given by φwφo(ρw − ρo)

2. Physicalparameters d and ξ can be written using a2, c1, and c2 in eq 6 as

ξ

π

= +

= −

−

−

⎡⎣⎢⎢

⎛⎝⎜

⎞⎠⎟

⎤⎦⎥⎥

⎡⎣⎢⎢

⎛⎝⎜

⎞⎠⎟

⎤⎦⎥⎥

ac

cc

dac

cc

12 4

,

212 4

2

2

1/21

2

1/2

2

2

1/21

2

1/2

(7)

By substituting eq 7 into eq 6, I(q) is described with threefitting parameters d, ξ, and ⟨η2⟩. Here, the parameters d and ξrepresent periodicity of the oil and water domain and the decaylength of the periodic order, respectively.Solid lines in Figure 10 were drawn by T-S model and the

Guinier approximation using the parameters listed in Table 1for the microemulsions at 30, 40, 52, and 64 g/L. As for themicroemulsion at 80 g/L, no line is drawn because the Guinierfitting does not agree well with the measurement since there isno plateau in the spectra. One can observe the good agreementin the SAXS measurements and the fitted lines in the q rangebefore 0.07 Å−1. For the bicontinuous microemulsions, it isfound that the domain size (d) takes minimum and thecorrelation length (ξ) takes maximum at 52 g/L. Uncertaintiesof d and ξ were estimated to be within 0.2 nm. A radius ofdispersed O/W microemulsions (R0) was found to be 6 nm.Although no structural length was determined for W/Omicroemulsion at 80 g/L, one can presume from the curve ofthe spectra that the size of droplet is bigger but in the sameorder of magnitude as compared to that of O/W micro-emulsion at 30 g/L.Chen et al.34 reported the physical meaning of ξ by using the

ratio of the two parameters ξ/d; the ratio is related to apolydispersity of the domain size, the smaller the ratio is, thebigger the polydispersity is. We found 0.31, 0.39, and 0.32 forthe bicontinuous microemulsions at salinities of 40, 52, and 64g/L, respectively. The ratio maximized at the salinity of 52 g/L,indicating the most ordered structure among the testedsamples. ξ/d is reported in literature for different systems;0.41−0.65 for D2O + decane + sodium bis (2-ethylhexyl)sulfosuccinate (AOT) system studied by Teubner and Strey,16

0.35−0.41 for D2O + toluene + butanol + NaCl + SDS(sodium dodecyl sulfate) studied also by Teubner and Strey,16

0.29−0.40 for water + NaCl + decane + AOT system reportedby Chen et al.29 Note that ξ/d obtained in this study is in thesame range as the literature; however, when comparing thestructural order with another system using the parameter, one

Table 1. Structural Parameters of Microemulsionsa

NaCl, g/L φw ν0, mPa·s d, nm ξ, nm η × 1010, cm−2 I(0), cm−1 R0b, nm RH

c, nm

30 87.4 1.29 12.4 5.9 9.840 57.4 23.7 7.4 1.1952 40.1 20.0 7.7 1.3664 26.8 20.6 6.6 1.4080 2.2 1.14 14.6

aφw is volume fraction of water in microemulsions. ν0 is viscosity of solvent in microemulsions. d, ξ, and η are fitting parameters of T-S model. I(0)and R0 were determined by the Guinier approximation. RH is hydrodynamic radius given by eq 3. bCalculated using SAXS results. cCalculated usingDLS results.

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should be careful with the magnitude of hydrophilicity/hydrophobility of the system, which depends on thecomposition of the system. For example, ξ/d at an optimumsalinity should be a good comparison for the polydispersitywhen comparing different systems. The observation of d and ξ/d supports the results of the DLS measurements, which showedthe more frequent topology change around the optimumsalinity. The correlation between D, d, and ξ/d is summarizedin Figure 11.

The hydrodynamic radius of O/W and W/O microemulsionscan be estimated from the DLS measurements with theStokes−Einstein equation. Here, the viscosity of the solvent ν0was estimated from the Einstein’s equation as ν = ν0(1 + 2.5φ),where ν represents the viscosity of microemulsions measuredwith the rheometer and φ is the volume fraction of a dispersedphase. Only one characteristic decay was found in bothmicroemulsions. The calculated RH is listed in Table 1. For themicroemulsion at 30 g/L, RH was found to be bigger than R0.The difference can be caused from polydispersity of thedispersed phase or a contribution of counterions to thehydrodynamic radius. RH of the microemulsion at 80 g/L wasobserved to be bigger than that of the microemulsion at 30 g/L.This is consistent with the assumption we made from the curveof SAXS spectra.

■ CONCLUSIONSIn this work, O/W, bicontinuous, and W/O microemulsionscomposed of water, NaCl, SDBS, isobutanol, and decane werestudied at room temperature using several experimentaltechniques such as spinning drop method, DSC, Karl Fischertitration, Hyamine titration, DLS, SAXS, and cryo-SEM. Themeasurements provided us a deeper insight into a correlationbetween physical properties and morphologies of water/oil inthe microemulsions.The optimum salinity was found to be around 50 g/L from

the IFT measurement. The bicontinuous microemulsion at asalinity of 52 g/L showed following characteristics. From theDSC measurement, it was found that the concentration ofNaCl slightly decreased compared to the initial salinity ofaqueous solution. Also, water content of the bicontinuousmicroemulsion was estimated to be 48 wt % from the DSCmeasurement, which agrees well with the value measured withthe Karl Fischer titration. The value supports that a salinity of52 g/L is close to the optimum salinity, at which the volume of

water and oil solubilized in microemulsion is equal. The DSCmeasurement also revealed that water and oil crystallized at thesame time in the bicontinuous microemulsion. This leads to themorphological observation that water and oil domains wereneighboring each other and well intertwined, which can also beseen in the images from cryo-SEM. Physical properties ofexcess water and oil phases were studied with the Karl Fischertitration and the Hyamine titration. It was confirmed that veryfew surfactants (0.4%) remained in the aqueous phase and asmall amount of water (0.5 wt %) existed in oil phase.Correspondingly, the time fluctuation of the excess oil phasemeasured with the DLS corroborated the presence of waterdroplets in the phase. Dynamics of the middle phase wereinvestigated with the DLS in terms of the diffusion coefficient,which maximized near the optimum salinity. SAXS measure-ments revealed that the domain size and its polydispersity tookminimum at a salinity of 52 g/L. By combining the observationof DLS and SAXS, one can conclude that the fluctuation of themiddle phase is mainly due to topology changes (breaking andmending of water/oil channels).We conclude that characteristics of the model micro-

emulsions were well studied and revealed in this study. Themethodology that has been developed is currently being appliedto complex oil systems which are more representative of realcrude oils, that means solutions of asphaltenes. This will be thesubject of a future paper. In a second step, we intend toinvestigate systems representative of microemulsions thatshould be used in chemical EOR operations (surfactantflooding), including real crude oils and complex mixtures ofsurfactants. The results shown here will be a great help wheninterpreting that of more complex systems, composed ofindustrial surfactant or crude oil, which are of our next interests.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: christine.dalmazzone@ifpen.fr.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

The authors thank M. Moire (IFPEN) for cryo-SEMmeasurements and A. Knorst-Fouran (IFPEN) for helpfuldiscussions.

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Figure 11. Characteristic lengths of bicontinuous microemulsionsversus diffusion coefficient. d and ξ/d represent domain size andpolydispersity of bicontinuous microemulsions obtained from SAXSmeasurements. D is the diffusion coefficient derived from the averagedecay rate. Numbers shown on the upper base of the figure representthe salinities.

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