Microemulsions as Reaction Media for the ... - ACS Publications

Article pubs.acs.org/Langmuir

Microemulsions as Reaction Media for the Synthesis of Mixed Oxide Nanoparticles: Relationships between Microemulsion Structure, Reactivity, and Nanoparticle Characteristics Carolina Aubery,† Conxita Solans,† Sylvain Prevost,‡,§ Michael Gradzielski,‡ and Margarita Sanchez-Dominguez*,†,⊥ †

Instituto de Quimica Avanzada de Cataluña, Consejo Superior de Investigaciones Cientificas (IQAC−CSIC) and CIBER en Biotecnologia, Biomateriales y Nanomedicina (CIBER-BBN), Jordi Girona 18-26, 08034 Barcelona, Spain ‡ Stranski-Laboratorium für Physikalische und Theoretische Chemie, Institut für Chemie, Strasse des 17. Juni 124, Sekr. TC7, Technische Universität Berlin, D-10623 Berlin, Germany § Soft Matter Department, Helmholtz-Zentrum Berlin, Hahn-Meitner-Platz 1, D-14109 Berlin, Germany ⊥ Centro de Investigacion en Materiales Avanzados, S. C. (CIMAV), Unidad Monterrey, GENES-Group of Embedded Nanomaterials for Energy Scavenging, Alianza Norte 202, Parque de Investigación e Innovacion Tecnologica, 66600 Apodaca, Nuevo Leon, Mexico S Supporting Information *

ABSTRACT: Phase behavior, dynamics, and structure of W/ O microemulsions of the system aqueous solution/Synperonic 13_6.5/1-hexanol/isooctane were studied, with the goal of determining their effect on Mn−Zn ferrite nanoparticle formation, kinetics and characteristics. Microemulsion structure and dynamics were studied systematically by conductivity, dynamic light scattering (DLS), differential scanning calorimetry (DSC), and small-angle neutron scattering (SANS). The main effect of cosurfactant 1-hexanol was a decrease in microemulsion regions as compared to the systems without cosurfactant; nevertheless, overlap of microemulsion regions in the systems with precursor salts (PS) and precipitating agent (PA) was achieved at lower S/O ratios, compared to the system without cosurfactant. At 50 °C, PA microemulsions are nonpercolated, while PS microemulsions are percolated. SANS indicates small prolate ellipsoidal micelles with the absence of free water up to 18 wt % PS solution; DSC studies confirm the absence of free water in this composition range. Kinetic studies show an increase in the reaction rate with increasing concentration of the aqueous solution; but the most significant effect in reaction kinetics was noted when cosurfactant was used, regardless of microemulsion dynamics and structure. On the other hand, the main difference regarding the characteristics of the obtained nanoparticles was observed when bicontinuous microemulsions were used as reaction media which resulted in 8 nm nanoparticles, versus a constant size of ∼4 nm obtained with all other microemulsions regardless of aqueous solution content, dynamics, and presence or absence of cosurfactant. The latter effect of constant size is attributed to the fact that the water present is dominantly bound to the EO units of the surfactant.

1. INTRODUCTION

emulsions), or with both aqueous and oily continuous domains as interconnected sponge-like channels (bicontinuous microemulsions). With most single-chain ionic surfactants, the addition of cosurfactant is necessary in order to form microemulsions, while microemulsion formation can be achieved with most ethoxylated nonionic surfactants with the appropriate combination of ethylene oxide and hydrocarbon chain lengths, which determine their hydrophilic−lipophilic properties. Nevertheless, the hydrophilic−lipophilic balance temperature (THLB) and the solubilization capacity of nonionic

Nanoparticle synthesis is a very active research field in chemistry; there are numerous synthetic methods, from the gas phase to the liquid phase techniques.1 A soft chemistry technique with a growing interest is the microemulsion reaction method, due to the efficient control of size, shape, and composition of nanoparticles obtained thanks to reagents confinement and particle stabilization by surfactant molecules.2−6 A microemulsion is a system of water, oil, and amphiphile, which is a single optically isotropic and thermodynamically stable liquid solution.7 Microemulsions can exist as oil-swollen direct micelles dispersed in water (O/W microemulsions), water-swollen inverse micelles dispersed in oil (W/O micro© 2013 American Chemical Society

Received: September 22, 2012 Revised: January 9, 2013 Published: January 10, 2013 1779

dx.doi.org/10.1021/la303817w | Langmuir 2013, 29, 1779−1789

Langmuir

Article

Table 1. Composition of the Samplesa surfactant

[aqueous solution]/wt %

Csurf [mol dm−3]

Φcore

ΔSLD 10−4 nm2

n(OH) /n(EO)

n(water) /n(EO)

n(ion) /n(EO)

PS PA

Synperonic 13_6.5

5.3 5.3

0.285 0.285

0.11 0.11

2.79 2.82

0.154 0.154

0.980 0.987

0.040 0.013

PS5CS

PS

5.3

0.271

0.11

2.83

0.188

1.026

0.042

PA5CS

PA

Synperonic 13_6.5/1Hexanol

5.3

0.272

0.11

2.86

0.188

1.033

0.014

PS13 PS18

PS PS

Synperonic 13_6.5

13.0 18.0

0.343 0.329

0.18 0.21

3.74 4.29

0.154 0.154

2.031 3.050

0.083 0.125

aqueous solution

PS5 PA5

sample ID

Csurf is the surfactant concentration, Φcore is the volume fraction of all species considered in the core (EO units, D2O, H2O, salts or TMAH, H2SO4). ΔSLD is the scattering length density difference between the hydrophobic and the hydrophilic domain of the microemulsion. Molar ratios of species in the core, relative to the quantity of ethoxy units, are reported. a

dynamic behavior of the microemulsions (the largest change was observed with bicontinuous microemulsions). In this work, the effect of cosurfactant in the microemulsion properties of the aqueous solution/Synperonic 13_6.5/ isooctane system and, consequently, in the formation of Mn− Zn ferrite nanoparticles have been studied. The reaction conditions used in the present study (pH 12.5, T = 50 °C) have been set so that spinel structure is directly obtained in the microemulsions, eliminating the need for calcination;22,23 this allows for a more direct correlation between the properties of the microemulsions and the characteristics of the obtained nanoparticles. Our approach was to determine the phase diagrams of the two starting microemulsion systems (containing PS and PA, respectively), and to classify the microemulsions depending on the structure and dynamic behavior. On the basis of this knowledge, the kinetics of nanoparticle formation was studied by transmittance measurements versus time with the stopped-flow method.24 Different scenarios for nanoparticle synthesis were compared, based on the microemulsion dynamic behavior and the aqueous solution content. The aim of this work was to study the effects of cosurfactant 1-hexanol on microemulsion phase behavior, reaction kinetics, and nanoparticle formation, as well as controlling nanoparticle formation by the type of microemulsion used for synthesis.

microemulsion systems can be modulated by the addition of electrolytes and cosurfactant.8−10 The effects of several variables on the characteristics of nanoparticles synthesized by the W/O microemulsion reaction method have been reported, e.g., aqueous phase content, reagent concentration, solvent, surfactant, and cosurfactant type, etc. However, the correlation between the size of microemulsion droplets and the finally formed nanoparticles is still far from clear.11−14 The relationship between the dynamic behavior of the microemulsions and the kinetics of nanoparticle formation has not been sufficiently taken into account in a systematic way, and it could be an important variable affecting the nanoparticle formation in microemulsions. The dynamic behavior of microemulsions depends on their composition, temperature, and the concentration and nature of the dispersed phase (e.g., the salts added in the aqueous phase).15−17 When the aqueous solution content is progressively increased at constant S:O ratio, the formation of clusters also increases up to a certain concentration, Cp (percolation threshold), at which the first infinite water cluster appears, and it is characterized by the increase in conductivity values.18,19 The subsequent increase in water content can lead to the appearance of bicontinuous structures. Such change in structure can also be induced by temperature (T) at constant composition; for instance, for microemulsions based on ethoxylated nonionic surfactants, transitions from nonpercolated to percolated microemulsions occur upon temperature decrease.20 The percolation temperature Tp appears as a minimum when the data are represented as d[log κ]/dT versus T. We have recently reported a study on nanoparticle synthesis using microemulsions with different structure: nonpercolated, percolated, and bicontinuous microemulsions. The system studied was aqueous solution/Synperonic 13_6.5/isooctane, and the aqueous solutions included the necessary reagents for the formation of Mn−Zn ferrite nanoparticles (Mn0.5Zn0.5Fe2O4).21 Phase behavior studies at 50 °C indicated that the precursor salts (PS) and precipitating agent (PA) systems form large microemulsion regions with maximum aqueous solution solubilization of 75 and 50 wt %, respectively. Optimum PS and PA microemulsion region overlap was obtained at a surfactant-to-oil (S:O) wt ratio of 25:75. Conductivity (κ) studies indicated that PS microemulsions were percolated at an S:O ratio of 25:75, while PA microemulsions were in a nonpercolated regime.21 Nanoparticles with different sizes were obtained depending on the

2. EXPERIMENTAL SECTION 2.1. Materials. Synperonic 13_6.5 (branched C13 alcohol ethoxylated with an average of 6.5 EO units, cloud point ∼65 °C, HLB ∼ 12.5) was a gift from CRODA. Isooctane (Suprasolv, for gas chromatography) and sulfuric acid (95−97%, p.a.) were purchased from Merck. Tetramethylammonium hydroxide pentahydrate (TMAH, 98%) was purchased from Alfa Aesar. Iron(II) sulfate heptahydrate (ACS 99.5% min.), manganese(II) sulfate monohydrate (ACS 99% min.), zinc sulfate heptahydrate (ACS 99.5% min), hydrogen peroxide (30% w/w min.), and 1-hexanol (99.0% GC) were purchased from Fluka. Deuterium Oxide (99.9 atom % D) was purchased from Aldrich. All materials were used as received, except for the samples for SANS and NMR for which the TMAH pentahydrate was vacuum-dried in order to remove any adsorbed H2O (only crystallization H2O was left). Deionized water had a resistivity of 18.2 MΩ·cm−1. 2.2. Methods. 2.2.1. Phase Behavior. The surfactant-tocosurfactant weight ratio was kept constant (21.1:1). Two aqueous solutions were used: (a) inorganic precursor salts (PS) FeSO4·7H2O 0.25 M, MnSO4·1H2O 0.0625 M, and ZnSO4·7H2O 0.0625 M, dissolved in H2SO4 0.5 M (acidic pH is used in order to prevent oxidation of Fe2+ by dissolved oxygen, since this process is hindered at 1780


Langmuir

Article

low pH)25 and (b) precipitating agent (PA) TMAH 0.5 M. More experimental details can be found elsewhere.21 2.2.2. Conductivity Measurements, Dynamic Light Scattering (DLS), Density Measurements, Differential Scanning Calorimetry (DSC), and UV−Vis Spectrophotometry. Experimental details can be found in the Supporting Information. 2.2.3. Small-Angle Neutron Scattering (SANS). SANS spectra were acquired on V4 at the Helmholtz-Zentrum-Berlin, Germany. Experimental details and data reduction can be found in the Supporting Information. Samples for SANS measurements were composed of hydrogenated oil and surfactant, and either PS or PA dissolved in D2O (core contrast). Moles of water were kept at the same values used for other studies (phase behavior, conductivity, DSC), in order to keep the same w values ([moles of water/mol of surfactant]). All cosurfactant was assumed to be located at the oil/ water interface, due to the small amount involved, as has been shown previously by SANS studies to be the case for 1-hexanol in microemulsions.26,27 Other studies have also shown that the majority of 1-hexanol resides at the oil/water interface of microemulsions and that transfer of the alcohol from the oil phase to the interface is exothermic.28 Deduced volume fractions and scattering length densities are presented in Table 1. 2.2.4. Stopped-Flow Experiments. Kinetic studies were performed with a stopped-flow device (Bio-Logic, SFM400), adapted to a monochromator spectrophotometer MOS (BioLogic). The cuvette (Suprasil) had a path length of 1.5 mm. The syringes, valves, delay lines, and the cuvette are enclosed in a water jacket to allow temperature regulation of the whole system. The reference was isooctane. The PA-to-PS volume ratio was set to 6:1, and transmittance of the reaction mixture as a function of time for different microemulsion compositions was measured. After preliminary tests, experimental conditions were fixed as follows: total volume/shot 527 μL, flow rate 7 mL·s−1, exposure time 2 ms, number of points 8000, and acquisition start 99 ms. 2.2.5. Nanoparticle Preparation and Characterization. The syntheses were carried out by mixing the reagent microemulsions with a PA-to-PS microemulsion weight ratio of 6:1, in order to reach pH 12.5. Reaction temperature was either 43 or 50 °C. The products were characterized by transmission electron microscopy (TEM, JEOL JEM 2100), X-ray diffractometry (XRD Siemens D-500), and magnetization measurements (Magnetometer SQUID Quantum Design MPMS XL). The crystallite size was estimated by using the Debye−Scherrer equation (for each sample, crystallite size was estimated from all peaks and an average was calculated). More details on nanoparticle synthesis and characterization can be found elsewhere.21

The addition of cosurfactant resulted in two main effects in phase behavior (Figure 1, filled circles): a remarkable decrease

Figure 1. Effect of cosurfactant 1-hexanol on the phase behavior of the system aqueous solution/surfactant/isooctane at 50 °C: phase diagrams showing single-phase microemulsion regions (L) and multiphase regions (M). System: (○) without cosurfactant; (●) with cosurfactant 1-hexanol (Synperonic 13_6.5:1-hexanol weight ratio of 21.1:1). Aqueous solution: (a) PS; (b) PA.

in the microemulsion region and the formation of microemulsions at lower S:O weight ratios in both PS (Figure 1a) and PA (Figure 1b) systems, as compared to the systems without cosurfactant.21 This could be attributed to interfacial effects (e.g., lowering the interfacial rigidity, modification of interfacial tension) or shifts in the hydrophilic−lipophilic properties of the systems caused by the presence of cosurfactant (the system becomes more lipophilic upon addition of cosurfactant, e.g. the HLB is decreased, and so are the THLB values of the microemulsions as shown in Table S2 in the Supporting Information). The maximum aqueous phase solubilized with cosurfactant was 32 and 20 wt % for the PS and PA systems, respectively. These values were lower than those found for the system without cosurfactant because addition of this component renders the system less hydrophilic. In contrast to the PA system, the microemulsion region of the PS system is completely detached from the S:O axis and a minimum of 10 wt % aqueous solution is required to form single-phase microemulsions at 50 °C. Samples in the M (multiphase) regions were not studied in detail; however, it was noticed that most samples in the M region with low water content between the oil−surfactant axis and the microemulsion zone consisted mainly of a microemulsion phase and a very small amount of surfactant excess. 3.2. Microemulsion Characterization. 3.2.1. Conductivity Measurements. The dynamic behavior of microemulsions

3. RESULTS AND DISCUSSIONS 3.1. Phase Behavior. The effect of cosurfactant 1-hexanol on the phase behavior of the aqueous solution/Synperonic 13_6.5/isooctane system was investigated at 50 °C, with an emphasis on microemulsion formation. The aqueous solutions comprise the reagents for Mn−Zn ferrite nanoparticle synthesis (precursor salts PS or precipitating agent PA). A surfactant:cosurfactant weight ratio of 21.1:1 was selected since it resulted in an optimum overlap of PS and PA microemulsions at a surfactant-to-oil (S:O) weight ratio of 20:80 (Figure S3, Supporting Information). This maximum overlapping was produced at a higher S:O ratio (25:75) in the system without cosurfactant. However, it should be highlighted that w values ([moles of water]/[moles of surfactant]) of microemulsions with and without cosurfactant are very close at the respective maximum overlapping. For example, w = 10.9 for samples with 10 wt % aqueous phase without cosurfactant and an S:O ratio of 25:75, while with cosurfactant, 10 wt % aqueous phase and an S:O ratio of 20:80, w = 11.6. 1781


Langmuir

Article

Figure 2. Conductivity studies of aqueous solution/Synperonic 13_6.5/1-hexanol/isooctane microemulsions with S:O ratio 20:80. Aqueous solution: (a, b) PS and (c, d) PA; (a, c) correspond to studies as a function of the aqueous solution content and (b, d) as a function of the temperature. Symbols in (a, c): (●) conductivity; (○) d[log κ]/dW. Symbols in (b, d) represent different aqueous solution content: (□) 10 wt %; (○) 12.5 wt %; (Δ) 14 wt %, and ( × ) 15 wt %. Insets in (b, d) show d[log κ]/dT vs T.

results is illustrated in Figure 3, where Tp values are plotted as a function of aqueous solution content. The reaction temperature (50 °C) is indicated by the dashed lines. Independently on the aqueous component and the presence or absence of cosurfactant, Tp values increase with the aqueous solution content and reach a plateau. This behavior is expected due to the nonionic nature of the surfactant and packing parameter transition. With the increase in temperature, a change from elongated to spherical structures is expected.29 However, as the aqueous phase content is increased, the EO hydrophilic headgroup becomes effectively larger, and correspondingly the sample requires a higher temperature for this transition (e.g., for dehydration of EO moieties). 3.2.2. Dynamic Light Scattering (DLS). Further microemulsion characterization was carried out by DLS. The correlation functions for PS and PA microemulsions are shown in the Supporting Information (Figure S4). In general, the curves show a monoexponential relaxation. For the PS system an increasingly slower relaxation with increasing concentration of the aqueous solution is observed that corresponds to an increase of the hydrodynamic radius (Rh). In contrast, no such clear tendency was observed for PA microemulsions. As the samples are in a rather concentrated regime, concentration effect should not be neglected. Since the system is oil-continuous one may safely assume that the droplets interact effectively via a hard sphere potential. However, for hard spheres the concentration-dependent diffusion coefficient has been calculated to be

containing cosurfactant, with an S:O weight ratio of 20:80 was determined by conductivity measurements as a function of the aqueous solution content (at 50 °C) and temperature (Figure 2). Figure 2a shows data for the PS system; samples in the M region with low water content were also studied (the small amount of excess surfactant was removed prior to measurement). The conductivity remains very low up to 10 wt % aqueous solution content, increasing sharply about 2 orders of magnitude above this concentration. The maximum obtained when the data were represented as d[log κ]/dW vs W confirmed that the percolation concentration Cp = 10 wt % aqueous solution. Figure 2b shows a decrease in conductivity with the increase in temperature for PS microemulsions. The data are also represented as d[log κ]/dT vs T, where the minima indicate the percolation temperature Tp, confirming that PS microemulsions over 10 wt % aqueous solution content are percolated at the reaction temperature (50 °C). For the PA system, no maximum in conductivity values as a function of aqueous solution content was obtained (Figure 2c) and the absolute conductivity values were below 1 μS cm−1, indicating that PA microemulsions between 10 and 15 wt % aqueous solution content are nonpercolated at 50 °C. Conductivity results as a function of temperature (Figure 2d) show that Tp is well below 50 °C. Comparing the dynamic behavior of microemulsions with and without cosurfactant,21 it is observed that even though in both cases PS microemulsions are percolated, with cosurfactant the conductivity values are more than 1 order of magnitude lower, suggesting that the clusterization is less pronounced in these microemulsions. In addition, a summary of conductivity

D = D0(1 + 1.454ΦHS) 1782

(1)


Langmuir

Article

3.2.3. Small-Angle Neutron Scattering (SANS). Microemulsions with an S:O ratio of 20:80 and 5.3 wt % aqueous solution content, without and with CS, for both PS and PA samples (samples PS5, PA5, PS5CS, and PA5CS, as described in Table 1) were analyzed. Microemulsions with higher aqueous solution content were also studied at an S:O ratio of 25:75, without cosurfactant, corresponding to microemulsions with 13 and 18 wt % PS (samples PS13 and PS18, described in Table 1). It should be noted that aqueous solution content of 5.3, 13, and 18 wt % of samples prepared with D2O (for SANS characterization) correspond to 5, 12, and 17 wt % aqueous solution content for samples prepared with H2O used in the rest of the studies; in this way w values were kept constant. The SANS spectra are shown in Figure 4. All samples display spectra typical for W/O microemulsions, with an intensity mostly flat at low q followed by a sudden decrease (around q−5), due to a characteristic structural size. Model-free analysis was carried out (Supporting Information, Table S3); it was consistent with the assumption of a homogeneous core of aqueous phase and PEG chains instead of a core−shell model. That means that up to the composition studied (with a maximum aqueous phase content of 18 wt %), water molecules are embedded (as bound or interphasal water) in the EO chains of the surfactant, with a negligible presence of free (unbound or bulk) water. Water molecules per EO unit were calculated for samples analyzed by SANS (Table 1), and the values were between 1 and 3 water molecules per EO unit. Assuming a homogeneous core of aqueous phase and PEG chains is in good agreement with NMR studies, which have shown that about 4−6 molecules of water are bound per EO group of a surfactant.33 In addition, DSC investigations by Garti et al. also indicated up to three water molecules per EO unit behaving as “interphasal” water in inverse nonionic microemulsions, and free water was only detected by DSC above this ratio.34 Moreover, it must be taken into account that the surfactant used in the present study, Synperonic 13_6.5, is a commercial product, with a mixture of EO chain lengths, in the order of 4−9 EO units. Therefore, there should be enough EO chains of sufficient length to be present in the core, bound to water. Between the low-q regime with an almost flat intensity and the mid-q regime with an abrupt intensity decay, a transition is observed that is smooth for PS5 and PS5CS, but presents a bump for PA5 and PA5CS (similar in composition to PS5 and PS5CS but with precipitating agent instead of precursor salts) and a peak for PS13 and PS18. The position of this bump or peak corresponds to the average spacing d = 2π/q between the centers of mass of scattering fluctuations. In a simple cubic model the q value of this bump is related to the number density of micelles with N = (1/d)3 = (q/2π)3. With Φ as the volume fraction of scattering material (polar core), an approximate droplet volume V = Φ(2π/q)3 reported in the Supporting Information (Table S3, “Vq(peak)’’) can be extracted. Values obtained range from 350 to 1400 nm3, i.e., at least 10 times larger than spherical aggregates considering a radius of 1.9 nm. Therefore, it can be assumed that the polar cores do not have a spherical shape. A strong anisotropy of the aggregates should result in an increase of intensity at low q (following q−1 for cylinders and q−2 for lamellae); such a strong increase is never seen, due to the opposite trend of the repulsive interactions. In Figure 4b, where low q is highlighted by using a linear scale, it is observed that the behavior of samples PS5 and PS5CS, and PA5 and PA5CS, is similar with only a smaller corrected

Figure 3. Percolation temperature (Tp) as a function of the aqueous solution content for the (a) precursor salts and (b) precipitating agent systems. (●) Systems with cosurfactant (S:O ratio 20:80) and (○) without cosurfactant (S:O ratio 25:75).21

where D0 is the diffusion coefficient at infinite dilution and ΦHS the hard sphere volume fraction.30 This has been shown before to be an accurate description for the situation in microemulsion droplets.26 By using this relation and accounting for the surfactant volume fraction as forming part of the hard spheres, D0 was calculated and this value was introduced in the Stokes− Einstein relation to calculate Rh. The calculated Rh and polydispersity indexes (PDI) are indicated in Table 2. The Table 2. Hydrodynamic Radius (RH) and Polydispersity Index (PDI) for PS and PA Microemulsions with Cosurfactant at an S:O Ratio of 20:80 and 50 °C As Obtained by DLS aqueous soln

[aqueous soln]/wt %

RH/nm

PDI

PS

10 12.5 15 10 12.5 15

19.1 25.8 32.0 10.7 13.2 13.4

0.21 0.20 0.28 0.14 0.16 0.22

PA

rather high PDI values of PS microemulsions can be attributed to their percolated state.31 The relatively large values of Rh are explained by the fact that they do not just account for the aggregate core but also for the surfactant that extends well into the oil phase and typically to a certain additional oil layer attached strongly to the microemulsion droplet.32 Furthermore, it has to be considered that SANS (3.2.3) proves the presence of anisometric aggregates, which is in good agreement with the values by DLS. 1783


Langmuir

Article

Figure 4. SANS data (markers, 1 of every 3 shown) and fits (lines) for the following: PS5, PS5CS, PA5, and PA5CS microemulsions (a) log−log plot and (b) linear−log plot; PS5, PS13, and PS18 microemulsions (c) log−log plot and (d) linear−log plot. Fits: biaxial prolate ellipsoids with sticky hard sphere interactions. Spectra plotted in log-scale are incrementally shifted by a factor of 4, except for PS5.

on data modeling are shown in the Supporting Information. Parameters for PS5, PS5CS, PA5, and PA5CS are consistent with stickiness increasing when metal salts are present, and with volumes in agreement with the Integral Structural Parameters outcome. The small semiaxes are around 2.6 nm, in agreement with the length of the stretched 6.5 EO chain. Comparing the results obtained by SANS and DLS, the former technique results into lower droplet dimension (R × ε, considering the major semiaxes) than the latter. Larger sizes are expected by DLS since the hydrodynamic diameter corresponds to droplets surrounded by a layer of solvent molecules diffusing with the same velocity, whereas the droplet size results obtained from SANS correspond only to the core (aqueous solution + EO chains). 3.2.4. Differential Scanning Calorimetry (DSC). Aqueous domains in W/O microemulsions can be composed of bound water (hydrating the surfactant/cosurfactant hydrophilic domains), interphasal (interfacial) water, and free water (droplet core with similar properties as bulk water).36,37 This aspect was studied by DSC. Results corresponding to PS microemulsions are displayed in Figure 5 for systems with and without cosurfactant. Endothermic signals are obtained between −30 and 20 °C, which become more pronounced as the aqueous solution content is increased. Regarding the system with cosurfactant (Figure 5a), the onset temperatures of these endothermic signals are around −15 to −18 °C with rather low enthalpy (H) values (Supporting Information, Figure S5). This

intensity for the curves; therefore the addition of cosurfactant only seems to reduce slightly the size of the micelle. The comparison of the same microemulsions but with PS or PA shows a pronounced change at low q (factor of 2) indicating either a growth or more attractive interactions with PS, or both. Data were fitted with a model of homogeneous monodisperse biaxial ellipsoids (semiaxes R, R, εR) with a Baxter S(q).35 This model describes hard spheres with surface adhesion, hence introducing the stickiness parameter (St), which characterizes the adhesive strength between droplets. The stickiness of PS5, PA5, PS5CS, and PA5CS (samples with 5.3 wt % aqueous solution, Table 3) appears appropriate due to their proximity to the microemulsions phase boundary. Details Table 3. Parameters Resulting from the Fit of Data with a Model of Biaxial Sticky Micellesa sample

T/°C

R/nm

ε

Vmic/nm3

Nagg

St

ϕs/ϕcore

PS5 PA5 PS5CS PA5CS PS13 PS18

45 45 45 45 50 50

2.6 2.7 2.6 2.6 3.5 4.2

5.9 10.3 4.9 3.6 4.8 4.5

437 834 378 264 902 1337

683 1324 576 409 1067 1276

0.53 ∞ 0.57 ∞ 0.52 0.59

1.7 1.2 1.9 1.5 1.9 1.7

R = semiaxes (nm), ε is the axial ratio, Vmic is the volume (of the core, nm3), Nagg is the aggregation number, and St is the stickiness.

a

1784


Langmuir

Article

Figure 5. DSC heating curves (endothermic peaks) for microemulsions with different aqueous solution content: (a) PS/Synperonic 13_6.5/1-hexanol/isooctane system at S/O ratio of 20:80 and (b) PS/ Synperonic 13_6.5/isooctane system at an S:O ratio of 25:75.

corresponds to the interphasal (and/or bound) nature of water and the absence of free water in these microemulsions; again, this is in good agreement with NMR and DSC studies as mentioned previously.33,34 Similar results were obtained for the PS system without cosurfactant, and up to 17 wt % aqueous solution content (Figure 5b); however, above this composition (24 wt % aqueous solution content), a second endothermic signal is obtained with onset around −7 °C, consistent with the presence of free (core) water in this range of compositions. Concerning PA microemulsions (Supporting Information, Figure S6), endothermic signals indicated a lack of free water, independently on the aqueous solution content; similar results were obtained in PA microemulsions without cosurfactant. 3.3. Reaction Kinetics: Stopped-Flow Technique. The wavelength selected for kinetic studies was 450 nm, where the absorbance values of PS and PA microemulsions are low, and the maximum transmission contrast was obtained for the reaction mixture (Figure S7, Supporting Information). It should be noted that the formation of nanoparticles cannot be expected to follow a simple law observed by an absorbance measurement. In general it has to be expected that the process of particle formation will occur by stepwise addition of material onto the growing nanoparticles, which leads to a rather wide size distribution and the sum of all particles present then would lead to a rather complex evolution of the absorbance. However, any such process could be described by a sum of exponentials and in order to have a general way of describing the process as simple as possible, we analyzed the transmission data by a single exponential (eq 2, Figure 6), which works well for times below 0.1 s (Figure 6b), and for longer times one then needs a second exponential. The apparent rate constant k (s−1) then can be obtained from the slope of ln[(T0 − T∞)/(T − T∞)] vs time and is a measure for the effective rate with which the reaction proceeds. ln[T − T∞] = −kt + ln[T0 − T∞]

Figure 6. Change of the transmittance during synthesis of Mn−Zn ferrite nanoparticles in aqueous solution/Synperonic 13_6.5/1hexanol/isooctane microemulsions at an S:O ratio of 20:80 at 50 °C. Aqueous solution content: (○) 10 wt %, (△) 11.25 wt %, and (×) 12.5 wt %. Plots a and b are different scales of the same results.

(Table S4, Supporting Information), and T is the transmittance at time t. Results indicate an increase in the reaction rate as the aqueous solution content increases, hence, the reactions are faster with higher aqueous solution content (k = 0.40, 0.87, and 1.28 s−1 corresponding to 10, 11.25, and 12.5 wt % aqueous solution content, respectively). For times longer than 2 s, the slope of the curves is about the same for all samples, which could indicate similar kinetics at that time scale. The reaction rates obtained in the absence of cosurfactant also increase with increasing aqueous solution content (Supporting Information, Figure S8), with k equal to 0.05, 0.63, and 0.83 s−1 for reactions carried out with 5, 15, and 20 wt % aqueous solution content, respectively (S:O ratio of 25:75), but these values were lower as compared to the reaction rates obtained with the cosurfactant system. These results may be attributed to an interfacial effect caused by the addition of cosurfactant, which in general has a tendency to decrease the film rigidity of nonionic surfactant monolayers,27 favoring faster exchange processes during droplet coalescence. PS and PA microemulsions overlapping on both composition and dynamics were found previously at 43 °C.21 According to this, nonpercolated, percolated, and bicontinuous microemulsions of both PS and PA systems can be produced at an S:O ratio of 25:75 and aqueous solution content of 5, 12, and 52 wt %, respectively, as shown in the Supporting Information, Figure S9. Figure 7 shows the transmittance as a function of time at 43 °C for reactions carried out with these microemulsions. A transmittance decrease as a function of time is observed for reactions carried out with nonpercolated and percolated microemulsions. This can be attributed to the nucleation and growth of the nanoparticles. In contrast, for reactions carried out in bicontinuous microemulsions, the

(2)

where T0 is the initial transmittance, as determined by stoppedflow experiments at the initial time (t0), T∞ is the transmittance at long time, as determined by UV−vis spectrophotometry 1785


Langmuir

Article

Figure 7. Transmittance as a function of time for reactions carried out in the aqueous solution/Synperonic 13_6.5/isooctane system with an S:O ratio of 25:75 at 43 °C. Aqueous solution content: (○) 5 wt % (nonpercolated microemulsions), (△) 12 wt % (percolated microemulsions), and (×) 52 wt % (bicontinuous microemulsions).

transmittance first decreases to a minimum, followed by a steady increase at relatively low times. This increase in transmittance can be attributed to a fast sedimentation of the nanoparticles (as observed by visual inspection). In any case, it can be observed that reaction kinetics are much faster when bicontinuous microemulsions are employed. 3.4. Synthesis and Characterization of Mn−Zn Ferrite Nanoparticles. Nanoparticle synthesis was carried out in microemulsions with an S/O ratio of 20/80 and 10−15 wt % aqueous solution content at 50 °C, using the system with cosurfactant. For comparative purposes, nanoparticle syntheses were also carried out in microemulsions without cosurfactant, with an S:O ratio of 25:75 and 10−24 wt % aqueous solution content. Irrespective of the presence or absence of cosurfactant and the aqueous solution content, small nanoparticles (3−5 nm) have been evidenced by TEM. Spinel structure and crystallite size around 4 nm were established (XRD), in agreement with TEM results. Examples of these results are shown in Figure 8, corresponding to nanoparticles synthesized with 12.5 wt % aqueous solution content and cosurfactant. Hence, in terms of particle size, shape, and crystallinity, no difference was observed for samples synthesized in microemulsions with and without cosurfactant, despite the differences in kinetics. Nanoparticle synthesis was carried out by using nonpercolated, percolated, and bicontinuous microemulsions in the system without cosurfactant at 43 °C (S:O ratio 25:75; 5, 12, and 52 wt % aqueous solution content, respectively). According to TEM results, nanoparticles around 3−5 nm in diameter were obtained when using nonpercolated and percolated microemulsions, while the use of bicontinuous microemulsions resulted in a wider particle size population, in the range of 2−10 nm in diameter (Figure 9a). In addition, it should be highlighted that according to TEM results, nanoparticles synthesized in nonpercolated microemulsions appear well dispersed, while as dynamic behavior increased (e.g., in percolated and bicontinuous microemulsions), nanoparticle agglomeration became more important. Synthesis in nonpercolated microemulsions resulted in wide and poorly defined XRD peaks, which become narrower and more defined for nanoparticles synthesized with percolated microemulsions (Figure 9b); this is an indication of an increase in nanoparticle crystallinity with the increase in aqueous solution content, due to the increase in the concentration of precursors. Thus, nanoparticles synthesized in bicontinuous microemulsion present a well-defined diffraction pattern, but it also presents an additional reflection, which is consistent with

Figure 8. (a) TEM image and (b) XRD pattern of nanoparticles synthesized with the aqueous solution/Synperonic 13_6.5/1-hexanol/ isooctane system at T = 50 °C. S:O ratio of 20:80, pH 12.8. Aqueous solution content: 12.5 wt %. Inset in part a shows high-resolution TEM (HRTEM).

δ-FeO(OH) (Feroxyhite), with superposed peaks at 54.3° and 63.1°; this effect was previously reported for synthesis carried out in microemulsions with high aqueous solution content.22 The particle size observed by TEM images and the crystallite size estimates obtained by XRD analysis (Debye−Scherrer equation) are in very good agreement, with crystallite size around 4 nm for nanoparticles synthesized with nonpercolated and percolated microemulsions, and around 8 nm for nanoparticles synthesized in bicontinuous microemulsions. The materials were also characterized by magnetization measurements (Figure 10). The results from zero field cooled (ZFC) measurements reveal low blocking temperatures (TB, which corresponds to the temperature at which the maximum in magnetization is obtained). TB can be related to the size of the particles, e.g., TB is proportional to the particle size. Results indicate similar TB values, 33 and 34 K, for nanoparticles synthesized with nonpercolated and percolated microemulsions, respectively, while TB for nanoparticles synthesized with bicontinuous microemulsions is higher (52 K), in agreement with TEM and XRD results. The behavior observed in the magnetization (M) versus applied magnetic field (H) plot illustrated in Figure 10b, was linear for particles synthesized with nonpercolated and percolated microemulsions, hence these samples have paramagnetic behavior; in contrast, particles synthesized in bicontinuous microemulsion presented superparamagnetic behavior.38,39 The magnetization as a function of the magnetic field at 300 K shows no magnetization hysteresis. The highest magnetization value was achieved with the sample 1786


Langmuir

Article

4. CONCLUSIONS The effect of cosurfactant 1-hexanol on the phase behavior of the aqueous solution/Synperonic 13_6.5/isooctane system was studied at 50 °C. The addition of cosurfactant caused a decrease in the microemulsion regions, but the minimum surfactant concentration needed to form microemulsions with 5−15 wt % aqueous phase was lower compared to the system without cosurfactant. Conductivity studies reveal that PA microemulsions are nonpercolated, while PS microemulsions are percolated. SANS studies indicate the presence of small prolate ellipsoidal micelles with the absence of free water in the composition range studied (up to 18 wt % aqueous solution content for PS microemulsions), which is in good agreement with DSC results. Kinetic studies suggest a decrease of interfacial film rigidity due to addition of cosurfactant, leading to a faster exchange of the aqueous solution. Comparison of reaction kinetics with nonpercolated, percolated, and bicontinuous PA and PS microemulsions for the system without cosurfactant, at an S:O ratio of 25:75 and 43 °C, revealed a substantial increase in the reaction rate as the microemulsions droplets become more connected. Synthesis of Mn−Zn ferrite nanoparticles in these microemulsions results in the formation of small globular nanoparticles (around 4 nm) with a spinel structure independently of the presence or absence of cosurfactant, the aqueous solution content, and microemulsion dynamic behavior. However, nanoparticles synthesized in bicontinuous microemulsions are about two times larger. For reactions carried out in different types of W/O microemulsions (percolated and nonpercolated), it may be implied that reactions are so fast that the nucleation step is the dominant process, with a limited growth step. This could be attributed to the large excess of precipitating agent, confinement effects, and thermodynamic stabilization by surfactant molecules. The predominantly bound nature of water in these W/O microemulsions as indicated by DSC and SANS may also explain the independence of particle size on aqueous phase content, microemulsion dynamics, and addition of cosurfactant.

Figure 9. (a) TEM images and (b) XRD patterns of nanoparticles synthesized in the aqueous solution/Synperonic 13_6.5/isooctane system at T = 43 °C. S:O ratio of 25:75, pH 12.8. Aqueous solution content: (1) 5 wt % (nonpercolated microemulsions); (2) 12 wt % (percolated microemulsions), and (3) 52 wt % (bicontinuous microemulsions).

■

ASSOCIATED CONTENT

* Supporting Information S

Additional microemulsion and nanoparticles characterization data. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +52 81 1156 0830. Fax: +52 81 1156 0820. Notes

Figure 10. Magnetization studies (a) as a function of temperature (ZFC at 100 Oe) and (b) as a function of the magnetic field (at 300 K) for nanoparticles synthesized in (○) nonpercolated, (△) percolated, and (×) bicontinuous microemulsions with the aqueous solution/Synperonic 13:6.5/isooctane system at an S:O ratio of 25:75 and 43 °C.

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We acknowledge financial support from Ministerio Ciencia e Innovacion (MICINN Spain; grant CTQ2011-29336-C03-01) and Generalitat de Catalunya (Agaur, grant 2009SGR-961). M.S.D. is grateful to CSIC for a JAE-Doc contract. Dr. Xavier Alcobe (U. Barcelona) is greatly acknowledged for assistance in XRD analysis. We thank HZB for allocation of neutron beamtime. This research was supported by the European Commission (seventh FP) through the “Research Infra-

synthesized in bicontinuous microemulsions, due to the larger particle size and improved crystallinity. 1787


Langmuir

Article

structure” action, “Capacities” Programme, NMI3-II Grant No. 283883.

■

(18) Borkovec, M.; Eicke, H.-F.; Hammerich, H.; Das Gupta, B. Two percolation processes in microemulsions. J. Phys. Chem. 1988, 92, 206−211. (19) Moulik, S. P.; Paul, B. K. Structure, dynamics and transport properties of microemulsions. Adv. Colloid Interface Sci. 1998, 78, 99− 195. (20) Schlicht, L.; Spilgies, J.-H.; Runge, F.; Lipgens, S.; Boye, S.; Schiibel, D.; Ilgenfritz, G. Temperature-, electric field- and soluteinduced percolation in water-in-oil microemulsions. Biophys. Chem. 1996, 58, 39−52. (21) Aubery, C.; Solans, C.; Sanchez-Dominguez, M. Tuning High Aqueous Phase Uptake in Nonionic Water-in-Oil Microemulsions for the Synthesis of Mn−Zn Ferrite Nanoparticles: Phase Behavior, Characterization, and Nanoparticle Synthesis. Langmuir 2011, 27, 14005−14013. (22) Košak, A.; Makovec, D.; Drofenik, M. The preparation of MnZn-ferrite nanoparticles in a water/CTAB, 1-butanol/1-hexanol reverse microemulsion. Phys. Status Solidi 2004, 1, 3521−3524. (23) Košak, A.; Makovec, D.; Drofenik, M. The Preparation of Spinel Ferrite Nanoparticles Using Precipitation in Water-in-Oil Microemulsions. J. Metastable and Nanocryst. Mater. 2005, 23, 251−254. (24) Bommarius, A. S.; Holzwarth, J. F.; Wang, D. I. C.; Hatton, T. A. Coalescence and solubilizate exchange in a cationic four-component reversed micellar system. J. Phys. Chem. 1990, 94, 7232−7239. (25) Morgan, B.; Lahav, O. The effect of pH on the kinetics of spontaneous Fe(II) oxidation by O2 in aqueous solution  basic principles and a simple heuristic description. Chemosphere 2007, 68, 2080−2084. (26) Gradzielski, M.; Hoffmann, H.; Langevin, D. Solubilization of Decane into the Ternary System TDMAO/1-Hexanol/Water. J. Phys. Chem. 1995, 99, 12612−12623. (27) Gradzielski, M. Effect of the cosurfactant structure on the bending elasticity in nonionic Oil-in-Water microemulsions. Langmuir 1998, 14, 6037−6044. (28) Mitra, D.; Chakraborty, I.; Bhattacharya, S. C.; Moulik, S. P.; Roy, S.; Das, D.; Das, P. K. Physicochemical studies on cetylammonium bromide and its modified (Mono-, Di-, and Trihydroxyethylated) head group analogues. Their micellization characteristics in water and thermodynamic and structural aspects of water-in-oil microemulsions formed with them along with n-hexanol and isooctane. J. Phys. Chem. B 2006, 110, 11314−11326. (29) Strey, R. Microemulsion microstructure and interfacial curvature. Colloid Polym. Sci. 1994, 272, 1005−1019. (30) Cichocki, B.; Felderhoff, B. U. Short-time diffusion coefficients and high frequency viscosity of dilute suspensions of spherical Brownian particles. J. Chem. Phys. 1988, 89, 1049−1054. (31) Moulik, S. P.; De, G. C.; Bhowmik, B. B.; Panda, A. K. Physicochemical studies on microemulsions. 6. Phase behavior, dynamics of percolation, and energetics of droplet clustering in Water/AOT/n-Heptane system influenced by additives (sodium cholate and sodium salicylate). J. Phys. Chem. B 1999, 103, 7122− 7129. Hait, S. K.; Moulik, S. P.; Rogers, M. P.; Burke, S. E.; Palepu, R. Physicochemical studies on microemulsions. 7. Dynamics of percolation and energetics of clustering in Water/AOT/Isooctane and water/AOT/decane w/o microemulsions in presence of hydrotopes (sodium salicylate, α-naphthol, β-naphthol, resorcinol, catechol, hydroquinone, pyrogallol and urea) and bile salt (sodium cholate). J. Phys. Chem. B 2001, 105, 7145−7154. (32) Dozier, W. D.; Kim, M. W.; Klein, R. Light scattering in a dilute microemulsion. II. Radius dependence of interactions. J. Chem. Phys. 1987, 87, 1455−1456. (33) Nilsson, P. G.; Lindman, B. Water self-diffusion in nonionic surfactant solutions. Hydration and obstruction effects. J. Phys. Chem. 1983, 87, 4756−4761. (34) Garti, N.; Aserin, A.; Ezrahi, S.; Tiunova, I.; Berkovic, G. Water Behavior in Nonionic Surfactant Systems I: Subzero Temperature Behavior of Water in Nonionic Microemulsions Studied by DSC. J. Colloid Interface Sci. 1996, 178, 60−68.

REFERENCES

(1) Cao, G. Nanostructures and Nanomaterials: Synthesis, Properties and Applications, 1st ed.; Imperial College Press: London, UK, 2004; Chapter 3. Schmidt, G. Nanoparticles: From Theory to Application; Wiley-VCH: Weinheim, Germany, 2010. Cushing, B. L.; Kolesnichenko, V. L.; O’Connor, C. J. Recent advances in the liquid-phase syntheses of inorganic nanoparticles. Chem. Rev. 2004, 104, 3893−3946. Murphy, C. J. Sustainability as an emerging design criterion in nanoparticle synthesis and applications. J. Mater. Chem. 2008, 18, 2173−2176. (2) Boutonnet, M.; Kizzling, J.; Stenius, P. The preparation of monodisperse colloidal metal particles from microemulsions. Colloids Surf. 1982, 5, 209−225. (3) Pileni, M. P.; Gulik-Krzywicki, T.; Tanori, J.; Filankembo, A.; Dedieu, J. C. Template Design of Microreactors with Colloidal Assemblies: Control the Growth of Copper Metal Rods. Langmuir 1998, 14, 7359−7363. (4) Santra, S.; Tapec, R.; Theodoropoulou, N.; Dobson, J.; Hebard, A.; Tan, W. Synthesis and Characterization of Silica-Coated Iron Oxide Nanoparticles in Microemulsion: The Effect of Nonionic Surfactants. Langmuir 2001, 17, 2900−2906. (5) Giordano, C.; Longo, A.; Ruggirello, A.; Turco Liveri, V.; Venezia, M. Physicochemical investigation of cobalt−iron cyanide nanoparticles synthesized by a novel solid−solid reaction in confined space. Colloid Polym. Sci. 2004, 283, 265−276. (6) Ledo, A.; Martinez, F.; Lopez-Quintela, M. A.; Rivas, J. Synthesis of Ag clusters in microemulsions: A time-resolved UV−vis and fluorescence spectroscopy study. Phys. B (Amsterdam, Neth.) 2007, 398, 273−277. (7) Danielsson, I.; Lindman, B. The definition of a microemulsion. Colloids Surf. 1981, 3, 391−392. (8) Shinoda, K. Proc. 5th Int. Congr. Detergency, Barcelona, Spain 1968, 275. (9) Kunieda, H.; Nakano, A.; Pes, M. A. Effect of Oil on the Solubilization in Microemulsion Systems Including Nonionic Surfactant Mixtures. Langmuir 1995, 11, 3302−3306. (10) Aramaki, K.; Hayshi, T.; Katsuragi, T.; Ishitobi, M.; Kunieda, H. Effect of Adding an Amphiphilic Solubilization Improver, Sucrose Distearate, on the Solubilization Capacity of Nonionic Microemulsions. J. Colloid Interface Sci. 2001, 236, 14−19. (11) Pileni, M. P. The role of soft colloidal templates in controlling the size and shape of inorganic nanocrystals. Nat. Mater. 2003, 2, 145− 150. (12) Lopez-Quintela, M. A.; Tojo, C.; Blanco, M. C.; García Rio, L.; Leis, J. R. Microemulsion dynamics and reactions in microemulsions. Curr. Opin. Colloid Interface Sci. 2004, 9, 264−278. (13) Capek, I. Preparation of metal nanoparticles in water-in-oil (w/ o) microemulsions. Adv. Colloid Interface Sci. 2004, 110, 49−74. (14) Eastoe, J.; Hollamby, M. J.; Hudson, L. Recent advances in nanoparticle synthesis with reversed micelles. Adv. Colloid Interface Sci. 2006, 128−130, 5−15. (15) Grest, G. S.; Webman, I.; Safran, S. A.; Bug, A. L. R. Dynamic percolation in microemulsions. Phys. Rev. A 1986, 33, 2842−2845. Wang, F.; Fang, B.; Zhang, Z.; Zhang, S. Mechanism of Electrical Percolation of w/o Microemulsions Formed by Nonionic Surfactants. J. Dispersion Sci. Technol. 2008, 29, 1166−1172. (16) Eicke, H. F.; Thomas, H. Charge Transport in Percolated Water-in-Oil Microemulsions. Langmuir 1999, 15, 400−404. GarciaRio, L.; Ramon Leis, J.; Mejuto, J. C.; Pena, M. E.; Iglesias, E. Effects of Additives on the Internal Dynamics and Properties of Water/AOT/ Isooctane Microemulsions. Langmuir 1994, 10, 1676−1683. (17) Fanun, M. A study of the properties of mixed nonionic surfactants microemulsions by NMR, SAXS, viscosity and conductivity. J. Mol. Liq. 2008, 142, 103−110. 1788


Langmuir

Article

(35) Baxter, R. J. Percus−Yevick Equation for Hard Spheres with Surface Adhesion. J. Chem. Phys. 1968, 49, 2770−2774. (36) Senatra, D.; Pratesi, R.; Pieraccini, L. DSC spectra as thermal fingerprints of percolative microemulsions. J. Therm. Anal. 1998, 51, 79−90. (37) Garti, N.; Aserin, A.; Tiunova, I.; Fanun, M. A DSC study of water behavior in water-in-oil microemulsions stabilized by sucrose esters and butanol. Colloids Surf. 2000, 170, 1−18. (38) Liu, C.; Zou, B.; Rondinone, A. J.; Zhang, Z. J. Reverse Micelle Synthesis and Characterization of Superparamagnetic MnFe2O4 Spinel Ferrite Nanocrystallites. J. Phys. Chem. B 2000, 104, 1141−1145. (39) Mathew, D. S.; Juang, R.-S. An overview of the structure and magnetism of spinel ferrite nanoparticles and their synthesis in microemulsions. Chem. Eng. J. 2007, 129, 51−65.

1789


Microemulsions as Reaction Media for the ... - ACS Publications

Recommend Documents