Published: October 31, 2011

r 2011 American Chemical Society 14005 dx.doi.org/10.1021/la203125x | Langmuir 2011, 27, 14005–14013

ARTICLE

pubs.acs.org/Langmuir

Tuning High Aqueous Phase Uptake in Nonionic Water-in-OilMicroemulsions for the Synthesis of Mn�Zn Ferrite Nanoparticles:Phase Behavior, Characterization, and Nanoparticle SynthesisCarolina Aubery,† Conxita Solans,† and Margarita Sanchez-Dominguez*,†,‡

†Instituto de Química Avanzada de Catalu~na, Consejo Superior de Investigaciones Científicas (IQAC-CSIC), CIBER en Biotecnología,Biomateriales y Nanomedicina (CIBER BBN), Jordi Girona 18-26, 08034 Barcelona, Spain‡Centro de Investigaci�on enMateriales Avanzados, S. C. (CIMAV), UnidadMonterrey, GENES-Group of Embedded Nanomaterials forEnergy Scavenging, Alianza Norte 202, Parque de Investigaci�on e Innovaci�on Tecnol�ogica, 66600 Apodaca, Nuevo Le�on, Mexico

bS Supporting Information

1. INTRODUCTION

Inorganic nanoparticles have been intensively investigated inrecent years for their distinctive electrical, optic, and magneticproperties. Their functionality is closely related to their structuralcharacteristics, such as crystalline structure, particle size distribu-tion, and shape, which allow their application in several fieldsincluding ferrofluid technology, chemical catalysis, contrast agentsfor magnetic resonance imaging (MRI), transistors, and magne-tically guided site-specific drug delivery.1,2 Several wet chemistrymethods can be listed for the synthesis of inorganic nanoparticles.3

Conventional methods include coprecipitation, sol�gel, andimpregnation processes; however, the very small and controlledparticle sizes required in certain applications are not achievedwith these techniques, despite modern refining that is nowadayspresent in such methods.3 A method with a growing interest isthe microemulsion reaction method (MRM),4 which has beenfound to be a suitable option to deal with those drawbacks. Amicro-emulsion is a system of water, oil, and an amphiphile, which is asingle optically transparent and thermodynamically stable liquiddispersion.5 Depending on the system components, composition

variables, temperature, electrolyte concentration, etc., water-in-oil (w/o), oil-in-water (o/w), and bicontinuous microemulsionscan be formed. Numerous works have been carried out for theformation of inorganic nanoparticles in w/o microemulsions.6�8

In most of the studies reporting the synthesis of inorganic nano-particles by theMRM, the surfactant systems used are of the ionictype, which may have some drawbacks as, for example, functionalspecies could be absorbed at the particle surface and interferewith its growth; possible contamination of ceramic nanoparticleswith the counterions; and reduction in aqueous phase uptake ofionic microemulsions in the presence of precursor salts due toscreening effects.9�14 Nonionic surfactants of the nonylphenolethoxylated type (e.g., Triton X-100) have been used as well,15�17

but these are not biodegradable. Fewer studies employ nonionicaliphatic poly(oxyethylene) alkyl ether surfactants (ethoxylatedfatty alcohols), despite the fact that these are more environmentally

Received: February 17, 2011Revised: October 29, 2011

ABSTRACT: In this work, the formation of water-in-oil (w/o)microemulsions with high aqueous phase uptake in a nonionicsurfactant system is investigated as potential media for thesynthesis ofMn�Zn ferrite nanoparticles. A comprehensive studybased on the phase behavior of systems containing precursor salts,on one hand, and precipitating agent, on the other hand, wascarried out to identify key regions on (a) pseudoternary phasediagrams at constant temperature (50 �C), and (b) pseudobinaryphase diagrams at constant surfactant (S):oil(O) weight ratio (S:O)as a function of temperature. The internal structure and dynamics ofmicroemulsions were studied systematically by conductivity and self-diffusion coefficient determinations (FT PGSE 1H NMR). It wasfound that nonpercolated w/o microemulsions could be obtained by appropriate tuning of composition variables and temperature, withaqueous phase concentrations as high as 36 wt% for precursor salts and 25 wt% for precipitating agent systems. Three compositions withthree different dynamic behaviors (nonpercolated and percolated w/o, as well as bicontinuous microemulsions) were selected for thesynthesis of Mn�Zn ferrites, resulting in nanoparticles with different characteristics. Spinel structure and superparamagnetic behaviorwere obtained. This study sets firm basis for a systematic study of Mn�Zn ferrite nanoparticle synthesis via different scenarios ofmicroemulsion dynamics, which will contribute to a better understanding on the relationship of the characteristics of the obtainedmaterials with the properties of the reaction media.

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friendly and relatively common at commercial level. Nonionicsurfactants have a great capacity of hydration by their ethoxylated(EO) units;18 hence, an appropriate selection of surfactant, oil,and precursor salts/precipitating agent concentration, in combi-nationwith the rich structural behavior that such a systemdisplays asa function of temperature, can lead to a highly optimized system interms of aqueous phase uptake and hence reactants loading.

The dynamic behavior of microemulsions depends on thesurfactant composition as well as on the concentration of thedispersed phase; that is, when the aqueous phase is increased in aw/o microemulsion, the discrete phase domains can increase insize, and the flexibility and percolation of the droplets are alsoprogressively changing;19�21 this can directly influence the re-action kinetics.22 Although some conclusions have been reportedabout several variables affecting the synthesis of nanoparticles bythe MRM, that is, reagent and aqueous phase concentrations, thetype of surfactant, oil, addition of cosurfactant, and mainlymicroemulsion droplet size,23�26 the dynamic aspects of micro-emulsions have not been sufficiently taken into account.

The present investigation is focused on the phase behavior andcharacterization of a novel microemulsion system, based on acommercial aliphatic poly (oxyethylene) alkyl ether nonionicsurfactant (Synperonic 13/6.5). The system is extensively stu-died in terms of microemulsion type and dynamic behavior, bymeans of phase diagrams, conductivity, FT PGSE 1H NMR, anddynamic light scattering (DLS). The interest in the system understudy lies on the fact that it has been designed to solubilize a highconcentration of aqueous phase containing the necessary re-agents for the synthesis of Mn�Zn ferrite nanoparticles. A highaqueous phase solubilization allows for a broad range of compo-sitions for the synthetic studies. This gives rise to ecological andeconomical advantages, due to the concomitant increase insynthesis capacity by microemulsion volume and the decreasein oil content. Furthermore, conditions have been sought so thatw/o microemulsions of both precursor salts and precipitatingagent can be obtained around 50 �C, which has been determinedas an optimal reaction temperature for the direct formation ofMn�Zn ferrite nanoparticles with a crystalline spinel struc-ture,13,27 without the need for calcination steps. Finally, a deepknowledge in the dynamics of the microemulsions formed by thissystem will allow the study of different scenarios for the nano-particle synthesis, contributing to a better understanding on therelationship of nanoparticle characteristics with the properties ofthe system used as reaction media.

2. EXPERIMENTAL SECTION

Materials. Synperonic 13/6.5 (ethoxylated isodecanol with anaverage of 6.5 EO units, cloud point ∼65 �C, HLB ∼12.5) was a giftfrom CRODA. Isooctane (Suprasolv, for gas chromatography) and sulfuricacid (95�97%, p.a.) were purchased from Merck. Tetramethylammo-nium hydroxide pentahydrate (TMAH, 98%) was purchased fromAlfa Aesar. Iron(II) sulfate heptahydrate (puriss. p.a. ACS 99.5% min),manganese(II) sulfate monohydrate (puriss. p.a. ACS 99% min), zincsulfate heptahydrate (puriss. p.a. ACS 99.5% min), and hydrogenperoxide (purum p.a. 30% w/w min) were purchased from Fluka.Deuterium oxide (99.9 atom %D) was purchased from Aldrich. Allmaterials were used as received. Deionized water (Milli-Q) had a re-sistivity of 18.2 MΩ cm�1.Phase Behavior. The samples for phase behavior were prepared

by weighing the appropriate amount of Synperonic 13/6.5, isooctane,and aqueous phase in glass vials with a long neck. The samples were

homogenized (vortex), and the vials were flame-sealed to avoid evapora-tion. The samples were left to equilibrate at 50 �C, for phase diagrams atconstant temperature. For phase diagrams as a function of temperature,the samples were left to equilibrate at each temperature, and observa-tions were made once equilibrium was reached. The aqueous phase con-sisted of Milli-Q water, aqueous solution of the precursor salts (PS), andaqueous solution of the precipitating agent TMAH (PA) 0.5 M. The PSand their concentrations were FeSO4 3 7H2O 0.25 M, MnSO4 3 1H2O0.0625 M, and ZnSO4 3 7H2O 0.0625 M, dissolved in H2SO4 0.5 M (toavoid oxidation of Fe2+).Conductivity Measurements. Conductivity experiments were

carried out using a Crison conductimeter GLP31. The conductivity cellwas a model 52-92 (Crison) with Pt electrodes and a cell constant of1 cm�1. The temperature was controlled to (0.05 �C by a CAT tem-perature sensor, model 55-31 (Crison). Different aqueous solutionconcentrations at several surfactant:oil (S:O) wt ratios were measured.The aqueous solutions were the PS and PA, with the same concentra-tions as described above. Measurements as a function of temperature(typically from 10 to 60 �C) were carried out.

Self-Diffusion Coefficients. The diffusion coefficients of the micro-emulsion components (for PA microemulsions only) were measuredwith the Fourier transform pulsed-gradient spin�echo (FT PGSE)technique monitoring the 1H NMR spectrum. The self-diffusion coeffi-cient is obtained from the attenuation of the spin echo under the in-fluence of pulsed magnetic field gradients. Most of the measurementswere developed on a Varian Inova 500 MHz spectrometer, while thediffusion coefficient of pure isooctane was measured on a Bruker DMX500 MHz spectrometer. The gradient compensated stimulated echospin lock (DgcsteSL) was the sequence selected for diffusion experi-ments, adjusting the gradient strength (G = 1�3 G/cm), the pulseinterval (Δ = 50�300 ms), and, eventually, the gradient pulse length(δ = 2�4 ms). The decay of the echo intensity is given by:

A ¼ A0 exp½ � DðγGδÞ2ðΔ� δ=3Þ�where A is the echo amplitude in the presence of the gradient pulse, A0

is the echo intensity in the absence of the gradient pulse, D is theself-diffusion coefficient, and γ is the proton gyromagnetic ratio(=2.67515255 � 108 s�1 T�1).28 Measurements were performed at50( 0.1 �C using 5 mmNMR capillary tubes, replacing the water of themicroemulsions by D2O; nevertheless, a certain amount of H2O waspresent in the aqueous phase, arising from crystallization water of TMAH.All reported values are the average of three independent measurements.

The self-diffusion coefficient D obtained by this function was relatedto the hydrodynamic radius (RH) by the Stokes�Einstein equation:

RH ¼ kBT6πμD

where kB is the Boltzmann constant, T is the absolute temperature, andμ is the viscosity of the continuous phase.

DLS. The hydrodynamic diameter of microemulsion droplets wasestimated by dynamic light scattering measurements (Malvern Instru-ments, 4700). The equipment consists of a monochromatic laser lightsource (λ = 488 nm), directed to the cuvette containing the sample,placed into a thermostatted bath. The detector amplitude, the beamintensity, and the dispersion angle were adjusted to obtain a signal of∼80 K counts s�1. According to this, the dispersion angle was set to 90�for PSmicroemulsions and 150� for PAmicroemulsions. After detection(photomultiplier), the light was analyzed by a correlator. The sampleswere measured at 50 �C. The data were analyzed by the non-negativeleast-squares (NNLS) algorithm, except for the sample corresponding tothe precipitating agent system and 10 wt % aqueous solution concentra-tion, which was analyzed with CONTIN. The viscosity of the continuousphase (isooctane) at 50 �C was μ = 0.365 cP,29 and the isooctane

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refractive index at 50 �C was experimentally determined, η = 1.3860(Optilab, rEX).Synthesis of Mn�Zn Ferrites by the Microemulsion Reac-

tion Method. Nonpercolated, percolated, and bicontinuous micro-emulsions were selected for the synthesis of nanoparticles (S:O ratio25:75, 5 wt % and 24 wt % aqueous solution concentration and S:O ratio85:15, 50 wt % aqueous solution concentration, respectively). Thesyntheses were carried out by adding the PA microemulsion to the PSmicroemulsion. Before mixing, both microemulsions were equilibratedat 50 �C, and the mixing was also carried out at this temperature. Themixing ratio was adjusted to reach pH 12.5. According to this, non-percolated and percolated microemulsions were mixed at 5:1 PA to PSmicroemulsion wt ratio, and at 6:1 microemulsion wt ratio for bicontin-uous microemulsions. The reaction mixture was kept stirring at 50 �Cduring 1 h. A small amount of concentrated H2O2 (30% w/w) was thenadded, and the reaction mixture was left stirring at 50 �C during 30 min.After the synthesis was finished, the product was separated from themicroemulsion by addition of absolute ethanol:water 1:1 v/v, followedby several cycles of centrifugation and washings. Finally, the product wasdried overnight at 60 �C. In another experiment, a one-microemulsionstrategy was used: a PSmicroemulsion with S:O ratio 25:75 and 12 wt %aqueous phase was prepared, and the PA was added directly as a solutionuntil pH 12.5 was reached. Both solutions were previously equilibratedat 50 �C. After this step, the rest of the process was the same as when thetwo-microemulsion strategy was used.Characterization of Mn�Zn Ferrites. The products were

characterized by transmission electron microscopy by using either aHRTEM 200 kV JEOL 2100 LAB6 (objective polar piece with aresolution of 2.3 Å) or a HRTEM 300 kV Philips CM30 (Super-Twinobjective polar piece, 2.0 Å resolution). The samples for TEM wereprepared by the addition of one drop of final microemulsion reactionmixture (before washings) to 1 mL of isopropanol, and dispersing byultrasound during 1 min. One drop of this solution was immediatelydeposited onto a holey-Formvar carbon TEM copper grid. The driedmaterials were also characterized by X-ray diffractometry (XRD SiemensD-500) and magnetometry (Magnetometer SQUID Quantum DesignMPMS XL). The particle size was estimated from the broadening of thespinel XRD peaks (dXRD).

3. RESULTS AND DISCUSSION

3.1. Phase Behavior. The ideal system for the synthesis ofMn�Zn ferrite nanoparticles should form microemulsionsaround 50 �C, because it is the minimum temperature that favorsdirect formation of the spinel crystalline structure, eliminatingthe need for a calcination step.13,27 On the basis of exploratoryphase behavior studies, the system selected was aqueous phase/Synperonic 13/6.5/isooctane. Three different aqueous compo-nents were used: Milli-Q water (as a reference), PS (FeSO4,MnSO4, and ZnSO4 in H2SO4 0.5 M), and PA (TMAH, whoserole is to increase the pH during the reaction) aqueous solutions.Figure 1a shows the phase behavior of the system with Milli-Qwater. The surfactant is not soluble in water, but the water isdissolved in the surfactant up to 23 wt %. The formation ofmonophasic liquid microemulsions (L region) extends along thesurfactant oil axis from a S:O ratio of 8:92 incorporating amaximum of 24 wt % water.Addition of the reactants had a great influence on the phase

behavior, as shown in Figure 1b and c for the systems with PS andPA, respectively. In both systems, the microemulsion zone isshifted toward higher water concentrations, increasing the mini-mum amount of aqueous solution needed to form one phasemicroemulsions; this was more pronounced for the PS system.

It was also observed that in both systems there are twomaxima inaqueous phase solubilization. For the PS system, one of themaxima (up to 45 wt % aqueous phase solubilized) is produced atS:O ratio 25:75, while for the PA system, the first maximum(∼26 wt % aqueous phase solubilized) is produced at S:O ratio20:80. For both systems, the second solubility maximum isproduced at around S:O ratio 85:15. From Figure 1b and c, itis observed that some overlapping is produced in the micro-emulsion regions of PS and PA systems, which allows for theselection of appropriate microemulsions to carry out synthesis(i.e., at S:O ratio 25:75; the synthesis of nanoparticles could beundertaken from 10 to 22 wt % aqueous solution).

Figure 1. Phase diagrams at 50 �C of the aqueous solution/Synperonic13/6.5/isooctane systems, showing single phase microemulsion regions(L) and multiphasic regions (M, M0). Aqueous solution: (a) water,(b) precursor salts, and (c) precipitating agent (TMAH).

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The differences in phase behavior between the system withwater and those with PS and PA solutions can be attributed todehydration of EO surfactant groups caused by the reactants inthe aqueous phase (“salting out effect”). As a consequence, adecrease in repulsion between headgroups occurs and a shift inthe hydrophilic�lipophilic balance is produced, leading to phaseseparation at low aqueous phase concentrations (M0 region). Infact, the M0 region appears to consist of a microemulsion phaseplus a very small amount of excess surfactant (not aqueous phase).Changes in the phase inversion temperature (PIT) for selectedmicroemulsions described in the Supporting Information (Figure S1)provide evidence of the influence of reactants addition on thehydrophilic�lipophilic balance properties of this system.As compared to classic w/o microemulsion systems, it may

appear at first sight that certain ionic surfactant systems, such aswater/Aerosol OT/isooctane, are superior, because for thatsystem the phase diagram indicates a maximum aqueous phasesolubilization of around 65 wt % at S:O ratio near 35:65.30

Nevertheless, when similar reagents are incorporated into thissystem, such as in the work by Yener andGiesche,9 a compositionat S:O ratio 40:60 and aqueous solution concentration of only25 wt % were outside of the microemulsion region, despitehaving a maximum water uptake of nearly 50 wt % at this S:Oratio in the absence of reactants. Although the full phase diagramwith incorporated reagents was not reported, this behavior is anindication of an important shrinkage on the microemulsionregion area upon addition of reactants. Another example is thesystem water/CTAB/hexanol; it has a maximum of 50 wt %water uptake at S:O 40:60 in the absence of reagents, which isshifted toward higher surfactant concentration upon addition ofsimilar reagents (S:O ratio 45:55 for PS and 50:50 for PA).31 Inthe present work, the maximum aqueous phase uptake was atmuch lower S:O ratios (25:75 and 20:80, incorporating 45 and26 wt % aqueous phase for PS and PA, respectively); at these S:Oratios, the water/CTAB/hexanol system incorporated less than20 wt % for PS and less than 15 wt % for PA.

To estimate the optimum water solubilization at temperaturesclose to 50 �C, and hence widen the range of microemulsioncompositions available for synthesis, a phase behavior study as afunction of temperature and aqueous solution concentration wascarried out at S:O ratio 25:75. The results showing the micro-emulsion regions are displayed in Figure 2. For the PS system(Figure 2a), the solubility maximum is located around 50 �C,while for the PA system (Figure 2b), it is located at around 46 �C,explaining why in the pseudoternary phase diagrams at 50 �C ofFigure 1, the microemulsion region for the PS system is muchwider than that of the PA system.3.2. Microemulsion Characterization by Conductivity. 3.2.1.

Conductivity at Constant Temperature (50 �C). The phase dia-grams at 50 �C hint the possible presence of different types ofmicroemulsion structures (Figure 1b and c). In particular, thesolubility maxima found along S:O ratios 25:75 and 85:15 (PSsystem), and S:O ratios 20:80 and 85:15 (PA system), areindicative of structural transitions taking place within the micro-emulsion region. Conductivity of microemulsions was studied atdifferent S:O ratios, increasing the amount of aqueous solution atconstant temperature (50 �C) to establish their dynamic beha-vior, hence the presence or absence of percolation induced byincrease in aqueous solution concentration. Figure 3a showsexample data for the PS system at a S:O ratio 25:75. It should benoted that samples below the lower solubilization limit (11 wt %)were also included; the small amount of excess surfactant wasremoved prior to measurement. The conductivity (k) remains verylow up to 9 wt % aqueous solution concentration (<4 μS cm�1).Above 9 wt %, the conductivity increased steadily up to 4 orders

Figure 3. Conductivity k (b) and d[log(k)]/dW (O) as a function ofaqueous solution concentration W for microemulsions with a S:Oratio of 25:75 at 50 �C. Aqueous phase: (a) precursor salts and(b) precipitating agent. M0 denotes the composition range in whichmicroemulsions with a slight excess of surfactant phase were obtained,while L denotes the composition range for single-phase microemulsions.

Figure 2. Phase behavior as a function of the temperature and aqueoussolution concentration, of samples of the aqueous solution/Synperonic13/6.5/isooctane systems with a S:O ratio 25:75, showing the micro-emulsion regions. Aqueous solution: (a) precursor salts and (b) pre-cipitating agent. The lower (b) and upper (O) microemulsion forma-tion limit and the percolation temperature, Tp (2), are indicated. Theregion below the Tp line denotes percolated microemulsions.

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of magnitude upon reaching the maximum water uptake for thisdilution line, which was 45 wt % (k ≈ 10 170 μS cm�1). Thisbehavior indicates that percolation is taking place. The percola-tion threshold was estimated from these data, by plotting thed[log(k)]/dW versus W, where W is the aqueous solution con-centration.21 The maximum, indicating the percolation thresh-old, was at 11 wt % aqueous phase, which equals the beginning ofone phase microemulsions for this dilution line (samples in Mregion). Figure 3b shows example data for the PA system. Theconductivity was very low (<1 μS cm�1) for all microemulsionswith S:O ratio 25:75, despite the fact that the aqueous phaseconcentration was as high as 25 wt %, which indicates that thesemicroemulsions were not percolated at 50 �C. Additional S:Oratios were also studied. The results are shown in the SupportingInformation (Figures S2 and S3). For the PS system, most of themicroemulsions presented high conductivity, except for S:O ratio50:50, which presented a very low conductivity. Regarding PAsystem, most samples had low conductivity values at 50 �C,indicative of nonpercolated microemulsions, except those withcomposition at or near the solubility maximum at high S:O ratio,which presented high conductivity.3.2.2. Conductivity as a Function of Temperature (Constant

S:O Ratio). It is well-known that poly(oxyethylene)alkyl ethernonionic surfactants increase their hydrophobicity with increas-ing temperature, which in turn induces transitions from o/wsystems to bicontinuous and finally w/o systems. Hence, a re-duction in conductivity with increasing temperature is expected.Example data of conductivity as a function of temperature at S:0ratio 25:75 and three different aqueous solution concentrations(PS system) are shown in Figure 4. For microemulsions withaqueous solution concentrations up to 32.5 wt %, conductivitydecreased steeply as temperature rose, reaching a plateau close to0μS cm�1 at high temperatures (from 52 to 57 �C), similar to thebehavior shown in Figure 4 (samples with 17, 25, and 32.5 wt %aqueous solution concentration, Figure S4). For samples withaqueous solution concentration higher than 32.5 wt % (Figure S5in the Supporting Information), phase separation was producedbefore reaching low conductivity levels.Conductivity data were also plotted as d[log(k)]/dT versusT,

to identify important changes in the slope, which indicatetemperature-induced percolation (Tp); an example graph isshown in the inset of Figure 4: a minimum was observed forsamples from 11 to 32.5 wt % aqueous phase. The minimum is

interpreted as the onset of Tp; hence percolation occurs inmicroemulsions at temperatures belowTp, whilemicroemulsionsformed at temperatures higher than Tp are not percolated.

32 Theposition of the minimum Tp varied as a function of aqueousphase concentration. The resulting Tp values are represented inthe dashed curve in Figure 2a. It is observed that Tp is well above50 �C, except for the sample with 11 wt % aqueous phase. Thearea below the Tp dashed curve is hence indicative of micro-emulsions in a percolated state. Similar studies were carried outfor the dilution lines with other dilution lines for the PS system.The graphs and full discussion are shown in the SupportingInformation (Figures S6�S12). It was confirmed that Tp of PSmicroemulsions with S:O ratio 50:50 was below 50 �C, andhence these samples were not percolated at 50 �C. In contrast,Tp

of PS microemulsions with other S:O ratios was above 50 �C;hence at this temperature these microemulsions were percolated.Conductivity of PA microemulsions with S:O ratio 25:75

decreased steeply with the increase in temperature within thewhole range of microemulsion compositions (e.g., up to 25 wt %aqueous solution), reaching a plateau close to 0 μS cm�1 (shownin the Supporting Information as Figure S13). Tp was below50 �C for all samples, which indicates that these microemulsionsare not percolated at 50 �C.Conductivity as a function of temperature for PS systemwith a

S:O ratio around 85:15 (solubility maximum at high S:O ratio)was very high (10 and 40 mS cm�1 for 40 and 70 wt % aqueousphase, respectively), and there was only a slight variation with theincrease in temperature; conversely, the d[log(k)]/dT versusT plot did not present a minimum (Figure S12, SupportingInformation). It should also be noted that the temperature range(ΔT) in which these samples were one-phase microemulsionswas only a few degrees (∼1.5�3 �C). This conductive behavior,in combination with an almost constant conductivity with theincrease in temperature, is consistent with bicontinuous struc-tures, with an absence of inversion to w/o microemulsions.Similar studies carried out with PA microemulsions with S:Oratio 85:15 are detailed in the Supporting Information (FigureS14); these results show that at high aqueous solution concen-tration these microemulsions are bicontinuous; such behavior issupported by diffusion coefficient determination by FT PGSENMR as shown below. In addition, the behavior of the 25:75 S:Oratio PS and PA microemulsions was confirmed by hydrophilicdye diffusion studies, which are detailed in the Supporting Infor-mation (Figure S15).3.3. Characterization of Microemulsion Type: Diffusion

Coefficient by FT PGSE 1H NMR. To confirm the structure ofmicroemulsions, determination of diffusion coefficients was carriedout by FT PGSE 1H NMR. The PS system could not be char-acterized by this method, due to the magnetic nature of themetallic salts, which generates disturbances on the receivedsignal. Figure 5 shows the relative diffusion coefficients of water(DW), TMAH (DTMAH), and isooctane (DO) for the PA systemas a function of the aqueous solution concentration (S:O ratio25:75). The low relative values for water and TMAH as com-pared to isooctane indicate that the aqueous phase is a confinedmedia for all microemulsions along this dilution line.33�36 Thisstudy supports conductivity results, demonstrating the water-in-oil structure of these microemulsions. On the other hand, thesample located at S:O ratio 85:15 and 50 wt % aqueous content,which corresponds to the upper maximum observed in the phasediagram (Figure 1c), gives low relative diffusion coefficients ofwater, TMAH, and isooctane (DW = 0.177, DTMAH = 0.163, and

Figure 4. Conductivity k as a function of temperature and aqueoussolution concentration for samples of the precursor salts systemwith a S:O ratio of 25:75. Aqueous solution concentration: 11 wt % (�), 12 wt %(]), 15 wt % (O). The inset show the corresponding d[log(k)]/dT as afunction of temperature. The dashed lines in the inset indicate thetemperature used for phase behavior studies (50 �C).

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DO = 0.089), which confirms the results from conductivitystudies.3.4. Microemulsion Droplet Size Determination by Dy-

namic Light Scattering Measurements. DLS measurementswere carried out as a function of the aqueous solution concen-tration for PA and PS microemulsions at a S:O ratio 25:75. Thecorrelation functions are shown in the Supporting Information(Figure S16). A decrease in the correlation function with time isobserved, and the curves tend to zero at high times. As theaqueous phase concentration is increased, the microemulsionsshow an increase in the correlation function values at short times,indicating an increase in droplet size. It should be noted that thedecay of the correlation function is faster for PAmicroemulsions,indicating smaller hydrodynamic droplet size than PS micro-emulsions, which is in good agreement with the fact that PSmicroemulsions were found to be percolated and the PA micro-emulsions were not percolated.The diffusion coefficients estimated from the intensity correla-

tion functions were related to the hydrodynamic radius (RH‑DLS) bythe Stokes�Einstein equation. Results are shown in Table 1.A general trend of hydrodynamic diameter increase with theaqueous solution concentration was observed. The polydisper-sity indexes are higher than what is typically found for micro-emulsions, a possible indication of droplet aggregation. These arehigher for the PS microemulsions as compared to the PA micro-emulsions, in agreement with the percolative behavior found byconductivity measurements. The presence of different size dis-tributions on a single measurement, as shown in the SupportingInformation (Figure S17), may be an additional indication ofdroplet aggregation. The hydrodynamic radii of the microemul-sions, calculated from the diffusion coefficients obtained by FTPGSE 1HNMR (RH‑NMR), are also shown in Table 1. The valuesobtained are almost constant as a function of the aqueous solu-tion concentration, and are generally lower than those obtained

by DLS. The limitations on the application of these techniquesfor the calculation of droplet size, for example, the neglecteddroplet�droplet interactions on the models used for calculationsand the use of either deuterated (NMR) or hydrogenated water(DLS), could be responsible of these disagreements. NMRmeasurements take into account the core of the droplets, whileDLS measurements take into account the hydrodynamic radiusof the microemulsions, which is formed by the droplet, thehydrophobic chains of the surfactant, and the solvation layer. Theincrease on hydrodynamic radius with aqueous solution concen-tration by DLS can be explained as a decrease on droplet numberwith the aqueous solution to surfactant ratio, resulting in moreseparated droplets, which can carry on more solvent concentra-tion, resulting in a higher hydrodynamic radius. The increase indroplet size can be also explained as the increase in droplet inter-action (droplets aggregation) with the increase in aqueous solu-tion concentration. The radii obtained from NMR are more con-sistent with those typically obtained from microemulsion systems.3.5. Synthesis of Mn�Zn Ferrite Nanoparticles. 3.5.1. Syn-

thesis inNonpercolated, Percolated, andBicontinuousMicroemulsionsby the Two-Microemulsion Strategy.Mn�Zn ferrite nanoparticle

Figure 6. TEM images ofMn�ZnFerriteNPs prepared using: (a) 5wt%aqueous solution, S:O ratio 25:75 (nonpercolated); (b) 24 wt % aqueoussolution, S:O ratio 25:75 (percolated); (c�f) 50 wt % aqueous solution, S:O ratio 83:17 (bicontinuous); and (g,h) one-microemulsion strategy (12wt % PSmicroemulsion plus PA solution: 48.8 wt % final aqueous solutionconcentration). Insets in (a)�(c) display HRTEM showing lattice planes.

Figure 5. Relative self-diffusion coefficientsD/D0 of water (O), TMAH(2), and isooctane (b) as a function of the aqueous solution concen-tration for PA microemulsions at S:O ratio 25:75 at T = 50 �C.

Table 1. Hydrodynamic Radius of Microemulsions Esti-mated by the Diffusion Coefficients Obtained by DLS andNMR Measurements (RH‑DLS and RH‑NMR, Respectively)

aqueous

phase

[W]/wt

%

polydispersity

index DLS

RH‑DLS/

nm

RH‑NMR/

nm

PS 10.5 0.36 10.85

12 0.21 12.05

17 0.38 15.65

PA 10 0.01 5.85 6.72

12 0.24 8.20 6.23

17 0.18 11.60 5.09

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syntheses were carried out using nonpercolated, percolated, andbicontinuous microemulsions, to compare the effect of micro-emulsion dynamic behavior on the characteristics of the nano-particles obtained. Nonpercolated microemulsions can be foundat S:O ratio 25:75 and 5 wt % aqueous solution concentration,although these samples include a small amount of surfactantexcess (which was removed prior to synthesis). On the otherhand, PS and PA percolated microemulsions do not overlap inconcentration (at 50 �C). For this reason, the concentrationselected for this reaction was at the S:O ratio 25:75 and 24 wt %aqueous solution concentration, which corresponds to a perco-lated microemulsion for the PS system, and nonpercolatedmicroemulsion for the PA system (at 50 �C). Finally, bicontin-uous microemulsions were found for both systems at S:O ratio83:17 and 50 wt % aqueous solution concentration. TEM imagesof the nanoparticles obtained are shown in Figure 6. Thesynthesis with nonpercolated microemulsions resulted in quitemonodisperse, isolated, globular-like nanoparticles (Figure 6a),with a diameter around 3 nm.When the synthesis was carried outwith a percolated PS microemulsion and a nonpercolated PAmicroemulsion (Figure 6b), nanoparticles had roughly the samesize as those obtained with nonpercolated microemulsions; how-ever, both isolated as well as agglomerated nanoparticles wereobserved. On the other hand, particles synthesized in bicontin-uous microemulsions presented a varied size distribution andparticle shapes: small globular nanoparticles (less than 5 nm,Figure 6c), elongated, needle like nanoparticles (Figure 6d and e),and faceted, prism-like nanocrystals (about 20 nm, Figure 6f).The variety in shapes may be indicative of different crystallinephases. In all cases (nonpercolated, percolated, and bicontinuous), anexample of high-resolution image is shown (insets of Figure 6a�c),revealing the presence of crystalline features for all samples. Itmust be highlighted that in the general view of the TEM grids,different behaviors were observed. The sample synthesized innonpercolated microemulsions was rather homogeneous, withan absence of agglomerates, whereas for nanoparticles synthe-sized using percolated and bicontinuous microemulsions, largeagglomerates (up to 1�2 μM) were observed (see SupportingInformation Figure S18), although these were made up bysmall particles (Supporting Information Figure S19). Taking intoaccount the differences between nanoparticles obtained in

microemulsions with different aqueous solution content anddynamics, a possibility is that the agglomerated particles wereformed within clusters of percolated microemulsion droplets(24 wt % aqueous solution) or water channels of bicontinuousmicroemulsions (50 wt % aqueous phase), which are absent inthe nonpercolated microemulsion (5 wt % aqueous solution),where only well-dispersed nanoparticles with a narrow size distribu-tion were observed.X-ray diffraction patterns are shown in Figure 7. The broad-

ness of the diffraction peaks indicates that particles have a smallcrystallite size. Spectra of samples synthesized using nonperco-lated (Figure 7a) and percolated (Figure 7b) microemulsions arealmost identical. Values of crystallite size, as estimated from thepeak broadening (Debye�Scherrer equation), are also presentedFigure 7. The diffraction peaks of these two samples can beindexed to a spinel structure. Finally, the spectrum of nanopar-ticles synthesized with bicontinuous microemulsions (Figure 7c)shows less broad peaks, indicating a crystallite size increase. Asmall amount of an additional phase was also found, which couldcorrespond to a feroxyhite phase (δ-FeOOH). The presence ofthis phase may be attributed to an insufficiently oxidant ambient,related to the lack of confinement of the aqueous phase intodiscrete domains. A similar effect was observed by Ko�sak et al,when microemulsions with high aqueous phase concentrationwere used.13 The presence of this phase may explain the presenceof elongated, needle-like particles. In all cases, it was confirmedthat spinel structure was obtained by synthesis at 50 �C, withoutthe need for calcination.

Figure 7. X-ray diffraction patterns of as-dried Mn�Zn ferrite NPsprepared at various compositions: (a) 5 wt % aqueous solution, S:O ratio25:75 (nonpercolated); (b) 24 wt % aqueous solution, S:O ratio 25:75(percolated); (c) 50 wt % aqueous solution, S:O ratio 83:17 (bicon-tinuous); and (d) one-microemulsion strategy (12 wt % PS microemul-sion plus PA solution: 48.8 wt % final aqueous solution concentration).The markers show the corresponding characteristic peaks for spinel (|)and feroxyhite (1).

Figure 8. (a) Magnetization as a function of temperature (zero fieldcooled data) and (b)magnetization as a function of the appliedmagneticfield at 300 K for as-dried Mn�Zn ferrite nanoparticles synthesized in:(O) nonpercolated microemulsions; (b) percolated microemulsions;(4) bicontinuous microemulsions; and (2) one-microemulsion strat-egy (12 wt % PS microemulsion plus PA solution: 48.8 wt % finalaqueous solution concentration).

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Magnetization studies were also carried out with nanoparticlessynthesized in nonpercolated, percolated, and bicontinuousmicroemulsions. Zero field cooled (ZFC) studies are shown inFigure 8a. Magnetization as a function of applied magnetic field(at 300 K) is presented in Figure 8b. In a ZFC measurement, thesample is cooled from room temperature to 5 K in the absence ofa magnetic field. Next, a magnetic field of 50 Oe is applied, andthe magnetization is measured while raising the temperature.ZFC measurements indicate an increase of magnetization valueswith rising temperature up to a maximum (blocking temperatureTB); afterward, a magnetization decay is observed. TB was 43 and32 K for nanoparticles synthesized using nonpercolated andpercolated microemulsions, respectively. The decrease in mag-netization for nanoparticles synthesized in percolated micro-emulsionsmay be explained by the presence of amorphousmaterial,as observed by TEM in the large agglomerates (SupportingInformation, Figure S19). To confirm this possible difference inamorphous component, detailed Rietveld analysis was attempted,without success, due to the high broadness of the peaks. Hence,with all of the analyses carried out, it can only be inferred from themagnetization data and the evidence of amorphous material byTEM, that upon comparing the samples synthesized in w/ononpercolated and percolated microemulsions, the latter appearto contain more amorphous component, despite the fact that theXRDpatterns are almost identical. This differencemay arise froma less controlled environment in the reaction carried out withpercolated PSmicroemulsion and nonpercolated PAmicroemul-sion, due to droplet clusterization and the fact that a newequilibrium must be reached upon mixture of these samples, asopposed to the reaction carried out with nonpercolated PS andPA microemulsions, which have similar equilibria, and theirmixing process is more straightforward.The maximum on ZFC measurements for nanoparticles syn-

thesized in bicontinuousmicroemulsions, which representsTB, isquite broad as compared to nanoparticles synthesized withnonpercolated and percolated microemulsions. This confirmsthe high particle size polydispersity observed by TEM images.Magnetization data as a function of the applied magnetic field areshown in Figure 8b (300 K). No coercitivity or remanence isobserved for the samples, as for paramagnetic materials. Behaviorbelow TB (5 K) is shown for all samples in the Supporting Infor-mation (Figure S20).The low TB values indicating the nanometric size of the particles

in combination with the hysteresis loops behavior (the presence ofcoercitivity and remanence at low temperature, and no hysteresisover TB) are indicative of superparamagnetic nanoparticles.

2,37�39

3.5.2. Synthesis by the One-Microemulsion Strategy. Nano-particle synthesis was also carried out by the one-microemulsionstrategy. The initial PS microemulsion contained 12 wt % aqueoussolution. To reach pH 12.5, the ratio of PS:PA solutions used was1:6; hence, at the end, the microemulsion had a content of 48.8wt % aqueous solution concentration. Figure 6g and h shows thatparticles in the order of 5�10 nm were obtained, and bothisolated nanoparticles (Figure 6h) as well as agglomerates madeup of small particles (Figure 6g) were detected. In Figure 6g,lattice fringes indicative of crystalline features can be observed.XRD results shown in Figure 7d confirm the crystalline nature ofthis sample, and only the characteristic peaks for the Mn�Znferrite spinel phase were present and these were very well-defined; the crystallite size was 10 nm, in good agreement withTEM results. Furthermore, the magnetization studies show thatthe performance of this sample was superior to all of the other

samples, as the saturation magnetization was as high as 28 emu g�1,while absence of hysteresis at 300 K and TB of 70 K were againindicative of superparamagnetic behavior. These results showthat the one-microemulsion strategy is a good option, in parti-cular if a high saturation magnetization is needed. The largernanoparticle size obtained by the one-microemulsion approachas compared to those obtained in nonpercolated and percolatedmicroemulsions (two-microemulsions approach) can be ex-plained in terms of the higher aqueous phase content of theformer. This larger particle size also favors the formation of morecrystalline particles, with the concomitant improvement of mag-netic properties. On the other hand, the bicontinuous micro-emulsion contained a similar amount of aqueous phase (50 wt %),but higher surfactant concentration; the XRD pattern showedsimilar results, except for the feroxyhite phase. It is likely that thepresence of this phase causes the lower magnetic performance ofthis sample.

4. CONCLUSIONS

The phase behavior of aqueous solution/Synperonic 13/6.5/isooctane systems where the aqueous phase consisted of water,precursor salts, or precipitating agent has been studied at con-stant temperature (50 �C) and as a function of temperature. Theresults have revealed a high aqueous phase solubilization capacity(large one-phase microemulsion regions) with great potentialtoward Mn�Zn ferrite nanoparticle synthesis studies. The peculiarmicroemulsion region shape for both PS and PA systems hintedtransitions in microemulsion structure and dynamics; compre-hensive conductivity studies confirmed it: nonpercolated w/o,percolated, and bicontinuous microemulsions (at solubility maxima)were identified. At 50 �C and S:O weight ratios suitable fornanoparticle synthesis, the microemulsions of the PS systempresented mainly percolating behavior, while for PA system thebehavior was nonpercolating. Nevertheless, phase behavior andconductivity studies at a selected S:O ratio of 25:75 as a functionof temperature and aqueous phase concentration showed that atsufficiently high temperatures (e.g., 55 �C) nonpercolatingsystems can be obtained with the PS system as well. Conversely,percolating PA microemulsions can be obtained at sufficientlylow temperatures (e.g., 45 �C). FT PGSE 1H NMR studiesconfirmed the w/o nature of PAmicroemulsions with a S:O ratioof 25:75 at 50 �C, as well as the bicontinuous nature of a PAmicroemulsion located at the upper solubility maxima (S:O ratio85:15). Droplet size determination by DLS and FT PGSE 1HNMR confirmed that the droplet size was in the typical micro-emulsion range; polydispersity supports the percolative nature ofPS microemulsions. Selected compositions, forming microemul-sions with three different dynamic behaviors (nonpercolated w/o,percolated w/o, and bicontinuous), were selected for the synth-esis of Mn�Zn ferrites. Nanoparticles with crystalline spinelstructure of varied sizes and shapes where synthesized, and themagnetization studies showed the superparamagnetic behaviorof these nanoparticles. Furthermore, the one-microemulsionapproach resulted in nanoparticles with better crystallinity andsuperior magnetization, indicating that this approach is a viableoption. Taken together, the results obtained confirm the suit-ability of the selected system as microemulsion reaction mediafor the synthesis of superparamagnetic Mn�Zn ferrite nanopar-ticles with a high aqueous phase loading. In addition, it has beendemonstrated that with this system, Mn�Zn ferrite nanoparticleswith good quality can be obtained: highly dispersed and isolated

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nanoparticles (synthesis in nonpercolated microemulsions withlow aqueous solution concentration) or high crystallinity andsaturation magnetization (synthesis in bicontinuous microemul-sions or one-microemulsion approach). The agglomeration ofthe materials synthesized in percolated and bicontinuous micro-emulsions as well as by the one-microemulsion approach may bereduced by the use of additional dispersing agents such as oleicacid and/or oleyl amine; hence this quality can be potentiallyimproved, to give rise to well-dispersed, crystalline nanoparticleswith high saturation magnetization. The synthesis studies carriedout using microemulsions with different dynamics show animportant dependence of the characteristics of the obtainedmaterials with the properties of the reaction media. A more com-prehensive synthetic study including a wider variety of micro-emulsion compositions with different structural properties anddynamics is in progress. It is hoped that this work will contributeto a better understanding of the relationship of the characteristicsof the obtained materials with the properties of the microemul-sion reaction media.

’ASSOCIATED CONTENT

bS Supporting Information. Additional microemulsion andnanoparticles characterization data. This material is available freeof charge via the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*Phone: +52 81 1156 0830. Fax: +52 81 1156 0820. E-mail:margarita.sanchez@cimav.edu.mx.

’ACKNOWLEDGMENT

We acknowledge financial support by Ministerio Ciencia eInnovaci�on (MICINN Spain, grant number CTQ2008-01979)and Generalitat de Catalunya (Agaur, grant number 2009SGR-961). M.S.-D. is grateful to CSIC for a JAE-Doc contract (May2008 to June 2010). Dr. Xavier Alcobe from the University ofBarcelona is greatly acknowledged for his assistance in XRDmeasurements and data analysis.

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