Journal of Colloid and Interface Science 386 (2012) 218–227

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Journal of Colloid and Interface Science

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Mixed O/W emulsions stabilized by solid particles: A model systemfor controlled mass transfer triggered by surfactant addition

Audrey Drelich a, Jean-Louis Grossiord b, François Gomez a, Danièle Clausse a, Isabelle Pezron a,⇑a EA 4297 Transformations Intégrées de la Matière Renouvelable UTC/ESCOM, Université de Technologie de Compiègne, rue Personne de Roberval, 60200 Compiègne Cedex, Franceb Physicochimie-Pharmacotechnie-Biopharmacie, UMR CNRS 8612, Université Paris XI, Faculté de Pharmacie, 5 rue Jean-Baptiste Clément, 92290 Châtenay-Malabry Cedex, France

a r t i c l e i n f o a b s t r a c t

Article history:Received 8 April 2012Accepted 23 July 2012Available online 2 August 2012

Keywords:O/W emulsionsPickering emulsionsMicelle-facilitated transportDifferential scanning calorimetryMass transfer

0021-9797/$ - see front matter � 2012 Elsevier Inc. Ahttp://dx.doi.org/10.1016/j.jcis.2012.07.072

⇑ Corresponding author.E-mail address: isabelle.pezron@utc.fr (I. Pezron).

This article deals with a model mixed oil-in-water (O/W) emulsion system developed to study the effectof surfactants on mass transfer between dispersed oil droplets of different composition. In this purpose,our goal was to formulate O/W emulsions without any surface active agents as stabilizer, which wasachieved by replacing surfactants by a mixture of hydrophilic/hydrophobic silica particles. Then, to studythe specific role of surfactants in the oil transfer process, different types and concentrations of surfactantswere added to the mixed emulsion after its preparation. In such a way, the same original emulsion can beused for all experiments and the influence of various surface active molecules on the oil transfer mech-anism can be directly studied. The model mixed emulsion used consists of a mixture of hexadecane-in-water and tetradecane-in-water emulsions. The transfer between tetradecane and hexadecane dropletswas monitored by using differential scanning calorimetry, which allows the detection of freezing andmelting signals characteristic of the composition of the dispersed oil droplets. The results obtainedshowed that it is possible to trigger the transfer of tetradecane towards hexadecane droplets by addingsurfactants at concentrations above their critical micellar concentration, measured in presence of solidparticles, through micellar transport mechanism.

� 2012 Elsevier Inc. All rights reserved.

1. Introduction

Mixed emulsions, which are obtained by gently stirring twosimple emulsions containing droplets of different nature, arewell-suited model systems to study transfer mechanisms across li-quid membranes [1–3]. For instance in the case of oil-in-water (O/W) emulsions, these systems allow to model transport processesbetween two oil phases separated by an aqueous membrane [4–14]. Other types of emulsions, as water-in-oil emulsions [13,15–18] or oil–water–oil (O/W/O) and water–oil–water (W/O/W) mul-tiple emulsions have also been used in this purpose [13,19–25].Different transfer mechanisms have been reported, includingsolution-diffusion [15] or micelle-facilitated transport [4–7,9–11,13,14,25]. In these systems, surfactant molecules are involvedin the different steps of the experiment: first in the emulsion for-mation, the decrease in interfacial tension induced by the surfaceactive molecules affecting the droplets formation and theirbreak-up process, secondly in the stabilization of the emulsionby the formation of an adsorbed film at the oil droplet/water inter-face, and then in the oil transfer process by modifying the solubil-ity of oil in the aqueous phase, which may involve (or not) the

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solubilization of oil in micelles. A slight change in surfactant con-centration may then not only affect the oil droplet size distributionof the emulsion, but also the emulsion stability and the solubilityof oil in the aqueous phase, which makes more complex the mod-elling of oil transfer mechanisms in these systems.

The objective of our study is to investigate more specifically therole of the surface-active molecules in the mass transfer process. Inthis purpose, our goal was first to identify a way to formulate theemulsions without any surface active agents. Then, to study theirrole in the oil transfer process, different types and concentrationsof surfactants were added to the mixed emulsion after its prepara-tion. In such a way, the same original emulsion can be used for allexperiments and the influence of surface active molecules on theoil transfer can be directly compared.

We chose a mixed O/W emulsion system based on previouswork performed in our laboratory. These previous studies con-cerned O1/W/O2 multiple emulsions [13,24,25], made of n-tetrade-cane (O1) in water emulsion dispersed in n-hexadecane (O2)stabilized by Tween�20 surfactant, and O1 + O2/W mixed emul-sions [12,13], obtained by mixing n-tetradecane (O1) in wateremulsion and n-hexadecane (O2) in water emulsion, both stabi-lized by Tween�20 surfactant. These studies revealed a mass trans-fer between the oil phases, facilitated by Tween�20 micelles, andoccurring preferably from n-tetradecane to n-hexadecane droplets,n-tetradecane and n-hexadecane being miscible at all ratio. In

A. Drelich et al. / Journal of Colloid and Interface Science 386 (2012) 218–227 219

order to prepare similar simple and mixed emulsion systems, con-taining n-tetradecane and n-hexadecane droplets dispersed inwater, without Tween�20 molecules as stabilizers, emulsions wereformulated in the presence of solid particles dispersed in the aque-ous phase. Indeed, recent work has shown that various types ofemulsions can be obtained by replacing surface-active emulsifiersby solid particles, often providing enhanced stability [26–31]. Itleads to the well-known Pickering emulsions [32,33], which at-tracted renewed attention during the last decade. The droplet sizedistribution of the simple and mixed emulsions obtained was char-acterized by laser light scattering. Then, to study the transfermechanism, different types and concentrations of surfactant weresubsequently added to the continuous water phase of the originalmixed emulsion after its preparation. Critical micellar concentra-tions of surfactants were measured in presence of solid particlesin order to investigate the role of micelles in the transfer mecha-nism. The transfer between n-tetradecane and n-hexadecane oildroplets was monitored by using the Differential Scanning Calo-rimetry (DSC) technique, which allows the detection of freezingand melting signals characteristic of the dispersed oil droplets.The dependence between the crystallization temperature and thecomposition of the oil droplets allows to follow the evolution ofthe droplet composition and therefore to monitor the oil exchangebetween the droplets [12,13,34]. The advantage of this procedurewould be to be able, through the post-addition and the choice ofthe nature and concentration of surfactant, to trigger and controlthe rate of oil transfer in liquid membrane systems.

2. Materials and methods

2.1. Materials

Commercial n-hexadecane oil CH3(CH2)14CH3 (manufacturerspecifications: min. 99% pure, molecular weight 226.45 g/mol,density 0.773 g/cm3 at 20 �C and melting point 17–18 �C) and n-tetradecane oil CH3(CH2)12CH3 (manufacturer specifications: min.99% pure, molecular weight 198.4 g/mol, density 0.763 g/cm3 at20 �C and melting point 5–7 �C) were purchased from Sigma–Al-drich (Steinheim, Germany). Ultra-pure water (resistivity18 MX cm), used for all experiments, was produced by a purifica-tion chain provided by Aquadem/Veolia water STI (Wissous,France). The particles used are fumed silica from the Aerosil�

range, hydrophilic Aerosil�A200 particles (water contact an-gle � 14� [35]) and moderately hydrophobic Aerosil� R711 parti-cles (water contact angle � 55� [35]), supplied by EvonikIndustries AG (Rheinfelden, Germany). They consist in sinteredbranched network, called aggregates, obtained by fusion of pri-mary spheres of 12 nm of silicone dioxide in a combustion process.Hydrophobic character comes from the substitution of silanolgroups by methacrylsilane moieties. The same type of particleswas used in previous studies of transfers in mixed water-in-paraf-fin oil emulsions [17], and to stabilize simple water-in-paraffin oil[31] or water-in-toluene emulsions [36] or silicon oil-in-wateremulsions [37].

Hydrophilic surfactants studied are: non-ionic surfactant Poly-oxyethylene (20) sorbitan monolaurate C58H114O26 (manufacturerspecifications: molecular weight � 1250 g mol�1, critical micelleconcentration � 4 � 10�5 M at 25 �C and HLB number = 16.7),commercially also known as Tween�20, supplied by VWRInternational (Strasbourg, France); ionic surfactant sodiumdodecyl sulfate C12H25NaO4S (manufacturer specifications:molecular weight � 288.4 g mol�1, critical micelle concentra-tion � 8 � 10�3 M at 25 �C and HLB number = 40), commonlycalled SDS, supplied by Sigma–Aldrich (Steinheim, Germany);non-ionic polymeric surfactant Polyoxyethylene (PEO)–polyoxy-

propylene (PPO)–polyoxyethylene (PEO) tri-block copolymers(PEO)101–(PPO)56–(PEO)101 (molecular weight � 12,000 g mol�1),commercially named Synperonic�F127, produced by ICI (Wilton,UK).

2.2. Emulsion preparation

Simple oil-in-water emulsions containing 40 wt.% of n-hexadec-ane droplets or 40 wt.% of n-tetradecane droplets and solely stabi-lized with silica particles were prepared. The external aqueousphase was prepared by dispersing variable amounts of hydropho-bic and hydrophilic silica particles in ultra-pure water, by usingan Ultra Turrax T25 high speed homogenizer Janke&Kuntel (Stau-fen, Germany) at 25,000 rpm for 3 min at room temperature. About10 g of each kind of simple oil-in-water emulsion were prepared byslowly adding the required amount of oil (40% in weight) to theaqueous phase while mixing the system using the same homoge-nizer at 25,000 rpm for 10 min at room temperature. Mixed emul-sions containing 20 wt.% of n-hexadecane droplets and 20 wt.% ofn-tetradecane droplets were obtained by slowly mixing equalmasses of the two primary solid-stabilized oil-in-water emulsionsusing a low speed mixer with three helixes at 3 rpm. To study theeffect of surfactant on the rate of mass transfer in mixed emulsions,surfactant was added in such mixed emulsion after its preparation.A small amount (0.5 mL) of concentrated aqueous surfactant solu-tion was slowly added to the mixed O/W emulsion (10 g) in orderto obtain an adjustable surfactant concentration ranging from8.8 � 10�4 mol L�1 up to 5.4 � 10�2 mol L�1 of the aqueous phase.Mixed emulsions with added surfactant were kept at 20 �C in athermostated chamber and continuously homogenized using thelow speed mixer at 3 rpm during all the mass transfer experiments.

2.3. Droplet size distribution measurements

Droplet size distributions of emulsions were determined by la-ser light scattering using a Mastersizer X Malvern Laser Diffraction(Worcestershire, England) with a lens of 300 nm, which is adaptedto measure sizes ranging between 0.2 and 600 lm. Measurementswere performed at room temperature (20 ± 2 �C). About 100 mg ofemulsion samples were diluted in a sample dispersion unit of100 mL containing pure water phase, and put into circulation witha rate about 1 L/min. For each kind of emulsion, the reproducibilitywas verified from several measurements performed on three emul-sions having the same composition and prepared in the same con-ditions and typical droplet size distributions were reported.Emulsion samples were taken in different points of the emulsion.In addition, emulsions were observed with a Labophot-2 Nikonoptical microscope (Japan) equipped with an ExwaveHAD Sonycolor video camera (Japan), which allowed an estimation of theirdroplet size. Emulsion samples without previous dilution werecovered with a thin glass lamella to optimize the opticalobservation.

2.4. Surface tension measurements

As surfactants are added to an aqueous phase in which solidparticles are dispersed, surface tension measurements were per-formed in the presence of solid particles to evaluate their effecton the critical micellar concentration of the surfactant. Liquid/airsurface tensions were measured with the Wilhelmy plate methodwith a K100 Krüss tensiometer (Hamburg, Germany). Measure-ments were performed at 20 ± 0.5 �C by using an external thermo-stat. Aqueous dispersions were prepared with a constant mixtureof hydrophobic and hydrophilic silica particles by using a magneticstirrer at 1500 rpm at room temperature. Various concentrations ofTween�20 surfactant were added in aqueous dispersions after 24 h

220 A. Drelich et al. / Journal of Colloid and Interface Science 386 (2012) 218–227

of homogenization. Surfactant concentrations in aqueous phaseswere chosen up to 6 � 10�2 mol L�1 of the aqueous phase. Beforesurface tension measurement, silica particles were separated fromthe aqueous dispersions by using a MR1812 Jouan centrifuge at10,000 rpm during 10 min. The reported values are the averagesof at least three measurements with corresponding standarddeviations.

2.5. Differential scanning calorimetry measurements

The oil transfer between the two populations of droplets wasmonitored by following the freezing and melting transitions ofthe dispersed phase by using Differential scanning calorimetrytechnique with a DSC821e Mettler calorimeter (Greifensee, Swit-zerland). Emulsion samples of about 20 mg were taken from themixed emulsion at successive time intervals and introduced in acalorimeter cell at ambient temperature. Then, the calorimetric cellwas placed in the head of the calorimeter, equilibrated for 300 s at20 �C, followed by a cooling experiment from 20 �C to �8 �C and aheating experiment back to 30 �C, at a regularly rate of 2 �C/min.The cooling experiment was stopped at �8 �C to avoid the freezingof the continuous aqueous phase, which was found between�14 �C and �20 �C for the volumes concerned. It also allows avoid-ing the overlap between the melting signals of the water phase(spread out from �2 �C to about 8 �C) and the n-tetradecane oil(spread out from 3 �C to about 12 �C), as far as the scanning ratedoes not exceed 2 �C/min and for the sample volume and percent-age of droplets concerned. DSC experiments corresponding to timezero were performed on mixed emulsions solely stabilized by silicaparticles right after their preparation or right after the added sur-factant was incorporated in the solid-stabilized mixed emulsion. Inthese conditions, the aging time of mixed emulsified systems didnot exceed 5 min before the first DSC analysis. For each type ofemulsion, the reproducibility was verified by studying at leastthree identical emulsions prepared in the same conditions.

The DSC technique allows detecting and measuring the heat re-leased or absorbed by time unit during the cooling and heatingexperiments [38,39]. These exchanges of energy are well repre-sented by a curve of heat flow rate signal recorded as a functionof time and temperature. A linear baseline is obtained when nochange occurs in the sample, whereas an exothermic peak is devel-oped in the case of crystallization, and an endothermic peak is ob-tained in the case of melting transition [40–43]. The total energyreleased DHc or absorbed DHm during the crystallization or themelting transition of a sample is proportional to the amount ofcrystallized or melted mass and is measured by the surface areaof the corresponding peak [40–43]. Due to undercooling phenom-enon, solidification occurs with a delay and solidification signalsare observed at lower temperatures than the melting signal[44,45]. For a pure material, the undercooling degree DT is definedby the difference between the melting temperature Tm of the mate-rial and, either the crystallization temperature Tc in the case of asingle bulk system, or the mean temperature of crystallization T�

in the case of dispersed droplets that freeze at different tempera-tures [46]. From nucleation theory, this phenomenon depends onboth the nature and the size of the sample and the undercoolingdegree is amplified in the case of droplets compared to bulk system[34,41,43,46–48].

In the case of bulk material, the crystallization temperature Tc isdetermined as the onset of the exothermic peak. In the case of dis-persed material, the solidification temperature is defined at the topof the broad exothermic peak and is referred to as the mean tem-perature of crystallization T� corresponding to the maximum num-ber of droplets expected to freeze around this temperature [46]. Onthe contrary, whatever the state of dispersion of the material, the

melting temperature Tm is determined as the temperature of theonset of the endothermic signal [34,38–48].

In the case of O/W mixed emulsion constituted, at time zero, oftwo populations of oil droplets of different nature, two exothermicpeaks are observed, corresponding to the crystallization of eachpopulation of oil droplets. Consequently, as the most probablefreezing temperature of oil droplets in mixed emulsion dependson their composition, the shift in the position of the crystallizationpeaks with time allows monitoring the oil exchange between thetwo populations of droplets. However, this principle can only beapplied to a mixed emulsion showing noticeable freezing signalsand containing a mixture of oils droplets with two very differentcrystallization temperatures, which is the case for hexadecaneand tetradecane. This principle needs to establish a calibrationcurve giving the relationship between the most probable tempera-ture of crystallization T� of oils droplets and their composition. Inthat purpose, simple O/W emulsions were prepared with a dis-persed oil phase made of a mixture of hexadecane and tetradecaneof known composition, and the values of T� as a function of the oilcomposition were determined by DSC in the range of compositionnecessary for this study from 0% to 50% of n-tetradecane in mass.

2.6. Stability measurements

Emulsion stability was assessed by the bottle test method bymonitoring the phase separation of oil-in-water emulsion withtime. Simple and mixed emulsions were stored in graduated glasstest tubes of 10 mL kept in a refrigerated chamber at 20 �C ± 0.5 �C.The volume of separated phase was measured at regular timeintervals. The percentage of each phase (emulsion, oil phase,water) was monitored as a function of time for up to 15 days.

3. Results and discussion

3.1. Characterization of the solid-stabilized simple emulsion

Solid-stabilized simple emulsions using different fumed silicaparticles from the Aerosil� range were studied. Bottle test observa-tions showed that hydrophilic A200 silica particles alone are notable to produce stable n-tetradecane-in-water or n-hexadecane-in-water emulsions and that only a mixture of hydrophilic Aerosil�

A200 particles and hydrophobic Aerosil� R711 particles allow togenerate oil-in-water emulsions with n-tetradecane or n-hexadec-ane droplets. By varying the ratio of hydrophilic/hydrophobic silicaparticles, it was found that stable oil-in-water emulsions were ob-tained, with very little creaming of the emulsion for several days,as assessed by the bottle tests, by using a mixture of 2 wt.% ofhydrophilic Aerosil� A200 silica particles and 2 wt.% of hydropho-bic Aerosil� R711 silica particles (% of particles are expressed as amass fraction of the aqueous phase) [49]. No creaming was ob-served when emulsions were maintained continuously homoge-nized using a slow speed mixer, after 15 days of storage.

Droplet size distribution measurements showed that both so-lid-stabilized simple emulsions are characterized by a broad distri-bution peak centered on around 15 lm, which is reproducible andstable with time (the data will be shown together with the resultsobtained for the mixed emulsions in the next section).

DSC results for each simple emulsion stabilized by silicaparticles are reported on Fig. 1. The analysis was performed onemulsions right after their preparation. DSC curves show that so-lid-stabilized simple emulsions are both characterized by a solidi-fication signal with a most probable crystallisation temperature T�

of around +12 �C ± 0.3 �C for the crystallisation of pure n-hexadec-ane droplets and �2 �C ± 0.3 �C for pure n-tetradecane droplets.Melting signals can be observed at higher temperature for each

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Fig. 1. Typical DSC curves showing the solidification and the melting of simplesolid-stabilized emulsions right after their preparation (at t = 0): (a) Emulsion of n-hexadecane droplets, (b) emulsion of n-tetradecane droplets.

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A. Drelich et al. / Journal of Colloid and Interface Science 386 (2012) 218–227 221

oil, with respectively Tm = 17.3 �C ± 0.2 �C for n-hexadecane(Fig. 1a) and Tm = 5.8 �C ± 0.2 �C for n-tetradecane (Fig. 1b), inagreement with the suppliers’ specifications for the pure chemicals(see Section 2.1). It is interesting to notice that the DSC curves re-veal an undercooling degree DT = Tm–T� for each pure dispersed oilphase, which is found to be DT � 5.3 �C in the case of n-hexadecanedroplets and DT � 7.8 �C in the case of n-tetradecane droplets.

In the case of simple emulsions in which the oil phase is a mix-ture of n-hexadecane and n-tetradecane, the main freezing signalhas been attributed to the formation of a solid solution in the drop-lets in agreement with previous works found in the literature onbulk samples [12,50,51]. This temperature has been retained to fol-low the mass transfer between the droplets. To establish thedependence between the temperature T� of oil droplets and theircomposition, a series of solid-stabilized simple oil-in-water emul-sions containing 40 wt% of oil droplets of variable n-tetradecane/n-hexadecane ratio were prepared. The experimental crystallization

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Fig. 2. Calibration curve giving the most probable crystallization temperature T� asa function of the mass fraction of n-tetradecane in the oil droplets dispersed insimple solid-stabilized emulsions.

temperatures T� obtained are plotted on Fig. 2 as a function ofthe mass fraction of n-tetradecane in oil droplets.

As it has been stressed, freezing is a kinetic phenomenondepending essentially of the volume sample and the scanning rate;therefore, the calibration curve is not universal, and slightly differ-ent values can be found in the literature for different experimentalconditions [12,50,51]. Additional experiments showed nosignificant influence of the different added surfactant used at aconcentration of 1.6 � 10�2 mol/L and no significant influence ofthe Tween�20 surfactant with different concentrations up to5.4 � 10�2 mol/L on the calibration curve (within experimental er-rors). As a consequence, this calibration curve can be used for allthe emulsified systems studied in this work.

3.2. Characterization of the solid-stabilized mixed emulsion

As observed for the simple emulsions, slow creaming of the oildroplets in the solid-stabilized mixed emulsion, and a limited sep-aration of the water phase were observed at the bottom of the glasstest tubes after 15 days of storage. Creaming was prevented whenthe solid-stabilized mixed emulsion was maintained continuouslyhomogenized using a low speed mixer at 3 rpm, after 15 days ofstorage.

Droplet size distribution in volume of simple and mixed emul-sion stabilized by silica particles alone are reported on Fig. 3. Theanalysis was performed on simple emulsions right after their prep-aration and on mixed emulsion at regular time intervals. Dropletsize distribution measurements showed that the solid-stabilizedmixed emulsion is characterized by a broad distribution peak cen-tered on around 15 lm, with a population of tiny droplets from0.5 lm and a population of large droplets up to 200 lm, which isreproducible and similar to the droplet size distribution of solid-stabilized simple emulsions (Fig. 3a). Measurements performedat regular time intervals during 14 days of storage under homoge-nizer system indicate no evolution of the morphology of the mixed

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222 A. Drelich et al. / Journal of Colloid and Interface Science 386 (2012) 218–227

emulsion (Fig. 3b). These results suggest the stability of the solid-stabilized mixed emulsions under homogenizer system during theexperimental time scale.

DSC results for mixed emulsion stabilized by silica particles arereported on Fig. 4. The analysis was performed on emulsions rightafter their preparation. DSC curves show that the crystallisationand melting signals of solid-stabilized mixed emulsion, at t = 0,appear as the superposition of the DSC signals obtained for thetwo primary emulsions (Fig. 1a and b).

Two distinct peaks are observed: a first broad solidification sig-nal characterized by T� = +12 �C ± 0.2 �C, corresponding to themean temperature of crystallization of pure n-hexadecanedroplets, and a second solidification signal observed withT� � �2 �C ± 0.2 �C characteristic of the mean temperature of crys-tallisation of the pure n-tetradecane droplets. Melting signals canbe observed for oils at higher temperature, at 5.5 �C ± 0.3 �C forn-tetradecane and at 17.6 �C ± 0.3 �C for n-hexadecane, in agree-ment with the experimental results obtained by DSC measurementperformed on each solid-stabilized simple emulsion (Fig. 1).

DSC experiments were then performed at successive time inter-vals on mixed emulsion only stabilized by solid particles. Typicalresults of oil droplets crystallization signals at successive timeintervals for this system are reported on Fig. 5. DSC cooling curveswere arbitrarily shifted on the vertical scale to facilitate the pagesetting and the readability of the figure. DSC cooling curves showno significant evolution of the solidification signals of oil dropletsin the solid-stabilized mixed emulsion observed for up to 13 days.These results suggest that no modification of the droplets oil com-position occurred during this time scale.

These results demonstrate that the mixed emulsion stabilizedby silica particles still contains dispersed droplets of pure tetrade-cane and pure hexadecane, and that the droplet composition didnot evolve during this time scale.

3.3. Effect of the nature of the surfactant on mass transfer within solid-stabilized mixed emulsions

DSC experiments were performed at successive time intervalson solid-stabilized mixed emulsion after the addition of differentsurfactant molecules at a fixed concentration (1.6 � 10�2 mol/L).Typical results of oil droplets crystallization signals for each sys-tem are reported on Fig. 6.

In the case of the solid-stabilized mixed emulsion with addednon-ionic and ionic surfactants, (Fig. 6a and b), an evolution ofthe two oil solidification peaks was observed with time: a progres-sive displacement of the solidification signal of n-hexadecane from+12 �C towards lower temperatures and a progressive reduction ofthe area of the solidification signal of dispersed n-tetradecane. Theeffect is more pronounced with the non-ionic surfactant than with

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the ionic surfactant. The area reduction observed can be related tothe decrease of the total amount of dispersed pure n-tetradecanewith time.

The displacement of the solidification signal of n-hexadecanetowards lower temperature can be related to a progressive modifi-cation of the composition of the n-hexadecane droplets due to thetransfer of n-tetradecane, as shown by the calibration curve. Theseevolutions of the solidification peaks are characteristic of a prefer-ential and global oil exchange from the n-tetradecane dropletstowards the n-hexadecane droplets, during which all n-hexadec-ane droplets are equally affected by the n-tetradecane oilincorporation.

For the system containing Tween�20, which is a non-ionic sur-factant, a unique solidification peak was detected around5 �C ± 0.2 �C after 12 days of evolution (Fig. 6a). According to thecalibration curve, it corresponds to a mass fraction of n-tetrade-cane equal to 50%. This was verified by comparing this solidifica-tion signal to the DSC curve obtained for a solid-stabilized simpleemulsion directly prepared with 50/50 wt.% of n-tetradecane andn-hexadecane as the oil phase as shown on Fig. 7. The DSC curvesshow that the temperatures T� are the same for the two emulsionsbut the freezing signal is slightly broader for the mixed emulsionkept for 12 days. Nevertheless, the DSC curves indicate that the un-ique solidification signal obtained after 12 days of evolution ischaracteristic of a unique composition of oil droplets with 50/50 wt.% of n-tetradecane and n-hexadecane. This result evidencesa complete mass transfer, in agreement with the initial mixedemulsion formulation containing an equal amount of dispersedpure n-hexadecane and pure n-tetradecane.

For systems with SDS, which is a charged surfactant, the twocrystallization signals were still observed after 7 days of evolution.However, the position of the n-hexadecane solidification peakshowed an evolution towards lower temperatures (Fig. 6b) up toT� = 7.6 �C ± 0.2 �C, and the area of the n-tetradecane solidificationpeak decreased with time. As a comparison, for systems containingTween�20, the position of the n-hexadecane solidification peakafter 7 days of evolution is observed at T� = 5.7 �C ± 0.2 �C. These re-sults show that, with SDS, the mass transfer is incomplete withinthe time scale of the experiment performed (7 days) and slowerthan with the Tween�20 within a comparable time scale (for thesame surfactant concentration equal to 1.6 � 10�2 mol/L).

In the case of the solid-stabilized mixed emulsion with addednon-ionic copolymer surfactant (Fig. 6c), no evolution of the solid-ification signals of oil droplets was observed for up to 7 days. Theresult indicates that this kind of surfactant does not allow any

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t = 24h

t = 0

(c) Synperonic®127 surfactant

Heat

Flo

w (

W/g

)

Fig. 6. Evolution with time of cooling curves of solid-stabilized mixed emulsion with different nature of added surfactant: (a) Tween�20 surfactant, (b) SDS surfactant, (c)Synperonic�127copolymer surfactant.

0.0

0.2

0.4

0.6

0.8

1.0

-10 -5 0 5 10 15 20 25

Hea

t Flo

w (W

/g)

Temperature (°C)

Solidification signals simple emulsion (a)

mixed emulsion (b)

Fig. 7. Comparison of typical DSC cooling curves of solid-stabilized emulsions: (a)Simple emulsion with an equal mass mixture of n-tetradecane and n-hexadecane att = 0, (b) mixed emulsion with 1.6 � 10�2 mol/L of added Tween�20 surfactant att = 12 days.

A. Drelich et al. / Journal of Colloid and Interface Science 386 (2012) 218–227 223

mass transfer between the oils droplets at the chosen concentra-tion, at least during 7 days of experimental time scale.

The results obtained therefore directly evidence how the rate ofn-tetradecane transfer depends on the nature of the addedsurfactant.

It is interesting to notice that the results obtained by DSC alsounderline that there is no coalescence of the oil droplets withinthis system. The coalescence of n-tetradecane droplets withn-hexadecane droplets at an early stage of the experiment would

imply the simultaneous presence of droplets containing, pure n-hexadecane, pure n-tetradecane, and a hexadecane/tetradecanemixture. As a consequence, three solidification peaks would be ob-served in the DSC experiment, with the apparition of solidificationsignal at an intermediate temperature between �2 �C and +12 �C,corresponding to the crystallization of droplets with a compositionof n-tetradecane/n-hexadecane according to the calibration curve(Fig. 2). In this work, the absence of intermediate solidification sig-nal and the gradual evolution of the two solidification peaks evi-dence the mass transfer of n-tetradecane towards n-hexadecaneand reject the hypothesis of coalescence phenomenon.

The asymmetric character of the transport phenomena, charac-terized by a preferential transfer from tetradecane to hexadecanedroplets, was already reported in previous studies of our laboratorywith the same oil phases incorporated in O1/W/O2 multiple emul-sions, and in which the hypothesis of a steady hexadecane phaseallowed a good fitting between the mass transfer kinetic modeland the experiments [25]. In addition, the rate of solubilizationof alkanes of different chain lengths from alkane-in-water simpleemulsions into micelles has been investigated by S.R. Dunganand collaborators, who evidenced a much faster rate of solubiliza-tion (around 15 times faster) of tetradecane into Tween�20 andSDS micellar solutions as compared with hexadecane [52]. If the ef-fect, which is in agreement with our results, was clearly shown, thereasons underlying the phenomenon are still not obvious [52]. Themechanism of transfer of oil into micelles is a complex phenome-non which has been the object of debate for several decades. The

224 A. Drelich et al. / Journal of Colloid and Interface Science 386 (2012) 218–227

main transfer pathways principally involve either a direct transferof oil from the emulsion droplets to the micelles at the interface, ora first step of solubilization of oil into water followed by the uptakeof oil by the micelles, or even the contribution of both mechanisms[53]. Whereas the solubility of tetradecane in water was reportedto be at least an order of magnitude higher than the solubility ofhexadecane in water (estimated at around respectively1.9 � 10�9 and 4.4 � 10�11 mol/L at 25 �C [54]), the solubility ofthe two oils in micellar solutions of Tween�20 or SDS appears tobe close to each other (estimated at around 2.4 � 10�3 mol/L fortetradecane and 1.8 � 10�3 mol/L for hexadecane in 2 wt.%Tween�20 solutions [11,52,55]). It can be noticed that the equilib-rium concentrations in pure water are not easy to measure and dis-crepancies can be found in the literature, due to the low solubilityvalues and the need for very long equilibration times [54]. As sol-ubility of tetradecane in water is much higher than the solubility ofhexadecane in water, the uptake of oil in micelles from the aque-ous phase, if it is fast enough, would displace the solubilizationequilibrium from the oil phase to the aqueous phase faster fortetradecane than hexadecane. Therefore, there will be a largernumber of micelles carrying tetradecane molecules than hexadec-ane molecules. In this case, the difference in aqueous solubility ofthe two oils could be the main reason for the preferential transportof tetradecane towards hexadecane droplets, although other mech-anisms may not be excluded.

To investigate the role of micelles in the transport of tetrade-cane towards hexadecane droplets in solid-stabilized emulsions,the experiments were repeated with varying amounts of addedTween�20 molecules.

3.4. Effect of Tween�20 concentration on the transfer of n-tetradecanein solid-stabilized mixed emulsion

DSC experiments were also performed at successive intervals oftime on solid-stabilized mixed emulsions with different concentra-tions of added Tween�20 surfactant. The evolution with time of oildroplets solidification peaks obtained for several concentrations ofTween�20 surfactant are displayed on Fig. 8. Concentrations usedvaried from 8.8 � 10�4 mol/L to 5.2 � 10�2 mol/L, which arerespectively above and below the concentration used in the previ-ous experiment (Fig. 6a).

In the case of solid-stabilized mixed emulsion with8.8 � 10�4 mol/L of added Tween�20 surfactant (Fig. 8a), nochange in the solidification signal of oil droplets was observed

0

0.5

-10 -5 0 5 10 15 20

Heat

Flo

w (W

/g)

Temperature (°C)

(a) 8.8 10-4 mol/L Tween®®20

t = 72h

t = 0

t = 24h

t = 7 days

t = 14 days

Fig. 8. Evolution with time of cooling curves of solid-stabilized mixed emulsion with3.3 � 10�2 mol/L.

for up to 14 days. This result indicates that low concentrations ofadded Tween�20 surfactant do not allow mass transfer betweenoil droplets during the experimental time studied.

On the contrary, for solid-stabilized mixed systems with3.3 � 10�2 mol/L of added Tween�20 surfactant (Fig. 8b), thereduction of the solidification signal area of pure n-tetradecanedroplets and the shift of crystallization temperature of initiallypure n-hexadecane droplets were observed. Finally, a unique solid-ification peak characteristic of a complete mass transfer was de-tected around 5 �C after 120 h of evolution. Similar results wereobserved in the case of solid-stabilized mixed emulsion containing5.2 � 10�2 mol/L of added Tween�20 surfactant (not represented),but with a faster evolution and a unique solidification peak ob-tained after 53 h of evolution, and in the case of solid-stabilizedmixed emulsion with 1.6 � 10�2 mol/L of added Tween�20surfactant (Fig. 6a), but with a slower evolution and a unique solid-ification peak obtained after 12 days of evolution.

These results show that high concentrations of addedTween�20 surfactant facilitate the oil exchange and indicate thatn-tetradecane transfer is faster when the surfactant concentrationincreases, in agreement with the literature [7,9,10]. On the con-trary, oil exchange did not occur when a low surfactant concentra-tion was used. These results suggest the implication of surfactantmicelles in the oil transfer process, in agreement with the mi-celle-facilitated transport mechanism reported in the literature inthe case of emulsions in which the surfactant played both rolesof emulsifier and transporter [11,13,25]. Surprisingly, our resultsdid not reveal any n-tetradecane transfer for a concentration of8.8 � 10�4 mol/L of added Tween�20 surfactant (Fig. 8a), althoughthis concentration is well above its CMC value in pure water(4 � 10�5 mol/L). However, the CMC value can be affected by thepresence of solid particles, mainly through adsorption.

Therefore, to confirm this hypothesis, surface tension measure-ments were performed on aqueous dispersions containing variousconcentration of Tween�20 surfactant up to 6 � 10�2 mol/L and afixed mixture quantity of 2 wt.% of hydrophobic and 2 wt.% ofhydrophilic silica particles. The evolution of the water/air surfacetension versus Tween�20 surfactant concentration is reported onFig. 9.

Before surface tension measurement, silica particles were sepa-rated from the aqueous dispersions by centrifugation in order toavoid their attachment to the Wilhelmy plate, which was observedand found to disturb the surface tension measurement. First, thesurface tension of a pure water phase in equilibrium with the

0

0.5

-10 -5 0 5 10 15 20

Heat

Flo

w (W

/g)

Temperature (°C)

t = 24h

t = 0

t = 9h

t = 120h

t = 50h

(b) 3.3 10-2mol/L Tween®20

different concentrations of added Tween�20 surfactant: (a) 8.8 � 10�4 mol/L, (b)

0

0.1

0.2

0.3

0.4

0.5

0 50 100 150 200 250 300

M

ass

frac

tion

of

n-te

trad

ecan

e

Time / h

Fig. 10. Evolution with time of mass fraction of n-tetradecane incorporated in n-hexadecane oil droplets in solid-stabilized mixed emulsion containing differentconcentrations of added Tween�20 surfactant: (e) 5.4 � 10�2 mol/L; (N)3.3 � 10�2 mol/L; (s) 1.6 � 10�2 mol/L; (j) 8.8 � 10�4 mol/L; and (�) without addedsurfactant. Dotted lines represent the fit to first-rate order kinetics.

30

35

40

45

50

55

60

65

70

75

1E-07 1E-06 1E-05 1E-04 1E-03 1E-02 1E-01

Surf

ace

Tens

ion

/ mN

/m

Tween20® Concentration / mol/L

air / pure water

with silica particles

without silica particles

Fig. 9. Evolution of water/air surface tension with concentration of Tween�20surfactant: (N) without silica particles, (s) after equilibration with the mixture ofsilica particles followed by centrifugation, (---) guide line.

A. Drelich et al. / Journal of Colloid and Interface Science 386 (2012) 218–227 225

mixture of silica particles was measured to ensure the absence ofsurface-activity originating from the silica particles used. Valueof 71.9 ± 0.7 mN/m was obtained, close to the value of72.4 ± 0.1 mN/m measured for pure water/air surface.

In the case of systems containing Tween�20 surfactant withoutsilica particles, the water/air surface tension progressively de-creases with the increase of Tween�20 surfactant concentration,down to 35.4 ± 0.9 mN/m at a concentration around 4 � 10�5 mol/L. Above this concentration, the surface tension remains constant,which suggests that the CMC for the formation of micelle has beenreached. This experimental value is in agreement with the valuefound in the literature and specified by the supplier.

For systems containing Tween�20 solutions equilibrated withsilica particles, the water/air surface tension evidenced a similarbehaviour with a significant shift towards higher concentrations.The water/air surface tension progressively decreases whileincreasing Tween�20 concentration down to 35.4 ± 0.6 mN/m ata concentration around 1.5 � 10�2 mol/L, corresponding to theCMC of the system with silica particles, and then remains constantfor higher concentrations.

This result suggests a strong interaction between surfactantmolecules and the silica particles, most likely through theiradsorption on the fumed silica particles, which are characterizedby a very large specific area [56]. Consequently, in mixed emul-sions stabilized with silica particles, less surfactant molecules willbe available to form micelles and to participate in the mass trans-fer, in comparison to similar system without silica particles. Theconcentration of 8.8 � 10�4 mol/L of Tween�20 is now significantlybelow the CMC measured in presence of silica particles.

3.5. Determination of the kinetics of n-tetradecane transfer in thesolid-stabilized mixed emulsions

DSC results presented in the previous section show a progres-sive reduction of the area of the solidification signal of pure n-tetradecane droplets and a progressive displacement of the solidi-fication signal of n-hexadecane towards lower temperatures dur-ing experimental time scale. These evolutions of the solidificationpeaks are characteristic of a global oil exchange. Considering thishypothesis, to model the n-tetradecane transfer, we supposed thatall n-hexadecane droplets are simultaneously and similarly af-fected by the n-tetradecane oil incorporation. So, the entire popu-lation of n-hexadecane diluted by the n-tetradecane is supposed tobe identical in mass composition at a given time t. Consequently,only two populations of droplets coexist into the mixed emulsionat time t: a population of pure n-tetradecane droplets and a popu-lation of n-hexadecane/n-tetradecane mixture droplets with a

mass fraction xT(t) of incorporated n-tetradecane. The expected fi-nal mass fraction of n-tetradecane in the oil droplets when themass transfer phenomenon is achieved is xe = 0.5, whereas the ini-tial mass fraction is x0 = 0, corresponding to pure n-hexadecanedroplets. Consequently, the mass fraction xT(t) of n-tetradecaneincorporated in the n-hexadecane droplets varies from x0 = 0 toxe = 0.5 during the evolution of the emulsions.

Using the calibration curve (Fig. 2), the amount of n-tetradecaneincorporated in the n-hexadecane droplets was directly deter-mined from the solidification temperature T� obtained for solid-stabilized mixed emulsions containing different concentrations ofadded Tween�20 surfactant. For example, a first solidification sig-nal detected with a temperature of crystallization T� = 9.7 �C corre-sponds to an n-tetradecane composition xT = 0.20 ± 0.03 accordingto the calibration curve, meaning that 20 wt.% of n-tetradecanewas incorporated in the n-hexadecane droplets. Therefore, fromthe evolution of the crystallisation temperature T� with time, themass fraction xT of n-tetradecane incorporated in the n-hexadec-ane oil droplets can be determined. The results obtained arereported on Fig. 10 for each solid-stabilized mixed emulsion con-taining different concentrations of added Tween�20 surfactant.

In the case of mixed emulsion without added surfactant andwith 8.8 � 10�4 mol/L of added Tween�20 surfactant, which is be-low the CMC of the system with added particles, results indicatethat the mass fraction of n-tetradecane in hexadecane droplets re-mains equal to xT = 0 during the experimental time studied.

On the contrary, for solid-stabilized mixed systems with1.6 � 10�2 mol/L, 3.3 � 10�2 mol/L, and 5.4 � 10�2 mol/L of addedTween�20 surfactant used above the CMC of the system with solidparticles, results show that the final mass fraction of n-tetradecaneevolves from xT = x0 = 0 and reaches xT � xe = 0.5, corresponding toan equal amount of n-tetradecane and n-hexadecane in the oildroplets, as it is expected when the mass transfer is achieved.

Globally the mass transfer observed in the solid-stabilizedmixed emulsions appears to follow an exponential first-order evo-lution with time, t, as it is modelled by the dotted lines on Fig. 10following the equation:

xT ¼ xe � xe � expð�k� tÞ ð1Þ

Values of the first-order rate coefficient k have been determinedfrom the best fit to the model and reported on Fig. 11 as a functionof the concentration of added Tween�20 surfactant in linear scale(in the insert) and in logarithmic scale, superimposed to surfacetension variations in presence of particles.

The insert reveals that the first-order rate coefficient k scaleslinearly with the concentration of Tween�20 surfactant from the

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0

10

20

30

40

50

60

70

80

1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00

k (h

-1)

surf

ace

tens

ion

(mN

/m)

Tween20 concentration (mol/L)

Fig. 11. Superimposed variations of the surface tension of Tween20 solutions withadded particles (�), and of the first-order rate coefficient k (e), as a function of theconcentration of added Tween�20 surfactant in solid-stabilized mixed emulsion.The insert shows the variations of k with the surfactant concentration in a linearscale (the dotted lines above the CMC correspond to the linear fit).

226 A. Drelich et al. / Journal of Colloid and Interface Science 386 (2012) 218–227

x-intercept of around 1 � 10�2 mol/L, which is of the same order ofmagnitude than the CMC value of surfactant solution equilibratedwith solid particles (experimentally determined at around1.5 � 10�2 mol/L). In addition, plotting the variations of k and thesurface tension on the same graph clearly shows that the valueof k, which is very low and close to zero at low surfactant concen-tration, abruptly increases with surfactant concentrations from aconcentration close to the CMC measured in presence of addedparticles, which is therefore the relevant CMC value in the transferprocess.

These results suggest a determining role of micelles in the sol-ubilization and transport of tetradecane from tetradecane dropletsto hexadecane droplets, by a mechanism of micelle-facilitatedtransfer in which the composition ripening rate is limited by theflux across the aqueous phase.

We should notice that the mass transfer determined by the DSCtechnique is more accurate in the first stage of the experiment thantowards the end of the transfer, when the peaks are partiallysuperimposed. Effects which have been reported in the literature,of delayed mass transfer due to drop surface shrinkage leading tothe compression of adsorbed particles layer at the end of masstransfer phenomenon have not been investigated here but cannotbe excluded [57].

As the emulsions used for all experiments are the same, the sur-face available for oil transfer is the same in all cases, so that thekinetics obtained for the various concentrations of surfactant usedcan be directly compared. In such a way, a control of the rate of oiltransfer in this type of systems could be achieved, and triggered bythe addition of surfactant at the proper concentration.

4. Conclusion

A well-suited model of oil-in-water mixed emulsion stabilizedonly with silica particles was prepared to study oil exchange facil-itated by surfactant micelles. The study demonstrates that the sys-tem developed does not allow any mass transfer and provides anappropriate way to rigorously and directly compare surfactant ef-fects on the rate of oil transfer in mixed oil-in-water emulsion,by decoupling their dual role as emulsifier and as transporter.The DSC technique is well-suited to detect droplet compositionchange by analysing the droplet solidification peaks and to deter-mine the mass transfer kinetics.

The result shows that mass transfer process depends both on thenature and the concentration of the added surfactant. Non-ionic

and charged surfactants, which are able to form micelles in the con-tinuous water phase, seem to facilitate mass transfer, contrary tocopolymer surfactant. Slower mass transfer rate was observed inthe case of charged surfactant compared to non-ionic surfactantat the studied concentration. Surfactant concentrations below theCMC, measured for the non-ionic Tween�20 surfactant in presenceof solid particles, did not allow any oil exchange without micelles inthe continuous aqueous phase, and the rate of oil exchange in-creased when higher concentrations were used. The results are inagreement with the mechanism of micelle-facilitated transport ofoil in the continuous phase proposed previous studies in our labo-ratory on the same system formulated as multiple emulsions [25]and in the literature for various systems [4–7,9–11,13,14]. To betterunderstand and control mass transfer in such mixed systems, fur-ther work will be performed on additional concentrations of addedsurfactant, and a kinetic model of mass transfer will be developed.Moreover, future work will be focused on the specific interactionsbetween surfactant molecules and solid particles, and their com-petitive adsorption at the n-tetradecane oil/water interface in orderto define more precisely the amount of surfactant micelles availableto participate in the mass transfer. This calculation will permit torigorously model and control the kinetic of transfer in such systems.

Acknowledgements

The work benefited of financial contribution from the RégionPicardie. The European Space Agency is gratefully acknowledgedfor the financial support from the MAP-project FASES (Fundamen-tal and Applied Studies of Emulsion Stability).

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