Emulsion vs Microemulsion

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Synthesis and Concentration of Organosols of Silver Nanopartcles Stabilized by AOT: Emulsion vs Microemulsion Alexander I. Bulavchenko, Aida Turusbekovna Arymbaeva, Marina G. Demidova, Pavel Sergeevich Popovetskiy, Pavel E Plyusnin, and Olga Alexandrovna Bulavchenko Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b04071 • Publication Date (Web): 27 Jan 2018 Downloaded from http://pubs.acs.org on January 27, 2018

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Synthesis and Concentration of Organosols of Silver Nanopartcles Stabilized by AOT: Emulsion vs Microemulsion Alexander I. Bulavchenko1, Aida T. Arymbaeva1, Marina G. Demidova1, Pavel S. Popovetskiy1*, Pavel E. Plyusnin1, Olga A. Bulavchenko2 1

Nikolaev Institute of Inorganic Chemistry SB RAS, 3 Acad. Lavrentiev Ave., Novosibirsk,

630090, Russiа 2

Boreskov Institute of Catalysis SB RAS, 5 Acad. Lavrentiev Ave., Novosibirsk, 630090,

Russia

In this work we tried to combine the advantages of microemulsion and emulsion synthesis to obtain stable concentrated organosols of Ag nanoparticles, promising liquid-phase materials. Starting reagents were successively introduced into a micellar solution of AOT in n-decane in the dynamic reverse emulsion mode. During the contact of the phases, Ag+ pass into micelles and Na+

pass

into

emulsion

microdroplets

through

the

cation

exchange

AOTNaOrg+AgNO3Aq=AOTAgOrg+NaNO3Aq. High concentrations of NaNO3 and hydrazine in the microdroplets favor an osmotic outflow of water from the micelles, which reduces their polar cavities to ~2 nm. As a result, silver ions are contained in the micelles, and the reducing agent is present mostly in emulsion microdroplets. The reagents interact in the polar cavities of micelles

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to form ~7 nm Ag nanoparticles. The produced nanoparticles are positively charged, which permitted their electrophoretic concentration to obtain liquid concentrates (up to 30% Ag) and solid Ag-AOT composite (up to 75% Ag). Their treatment at 250°C leads to the formation of conductive films (180 mOhm per square). The developed technique makes it possible to increase the productivity of the process by ~30 times and opens up new avenues of practical application for the well-studied microemulsion synthesis.

Introduction Stable highly concentrated organo- and hydrosols of metal, oxide, and salt nanoparticles are promising as inks for printing of microcircuits,1-4 heat transfer fluids,5-7 optically active media for random lasers,8 and starting components for the formation of various functional nanomaterials. 9,10 Nanoparticles in sols are free, i.e., are not bound to each other by coagulation and coagulation-crystallization (phase) contacts. Therefore, they are evenly distributed in formed hybrid nanomaterials. In addition, only in this case the size effects are manifested in full measure. One of the most common and popular methods for the production of organosols and ultradispersed powders is microemulsion (micellar) synthesis.11-13 Microemulsion is a thermodynamically stable and optically transparent system. It consists of “swollen” micelles with solubilized aqueous solutions of the reagents. The micelles are not larger than 100 nm.14 Microemulsion synthesis makes it possible to regulate the size of resulting nanoparticles and produce organosols of particles with a narrow size distribution.15 Solutions of starting reagents, most often water-soluble metal salts and reducing agents, are injected (solubilized) into two different parts of a solution of a micelle-forming surfactant (0.1-10 wt%) in organic solvents. Then the microemulsions with reagents are mixed to form optically transparent solutions.


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Nanoparticles form through intermicellar exchange. The growth of nanoparticle is limited by the adsorption layer of the surfactant; the particle size is close to or slightly exceeds the size of the micelle cavity.16 The drawback of the injection-microemulsion synthesis is the low concentration of nanoparticles in the obtained dispersions. Solubilization capacity drastically decreases with increasing concentration of injected salts. Therefore, this synthesis usually runs when the solubilization capacity does not exceed 1 vol%. Moreover, low solubilization capacities are responsible for a small size of nanoparticles (2-10 nm) and a high degree of monodispersion. Thus, the concentrations of reagents decrease 100 times during micellar synthesis as compared with “water” synthesis. The produced samples are often too small for their characterization, research, and, especially, practical use. Numerous attempts were made to increase the productivity of microemulsion synthesis.17,18 However, most of them were aimed at the precipitation of solid aggregates and agglomerates of nanoparticles during synthesis and their subsequent redispersion. Synthesis of microparticles in reverse emulsions is also well known. It is especially popular for the preparation of polymer particles (so-called emulsion polymerization19-22). Emulsions, in contrast to microemulsions, are thermodynamically unstable systems. Water droplets in emulsions are of micron size; however, the content of the aqueous phase with reagents in emulsions can reach 90 vol%. For this reason, emulsion synthesis is applied even on an industrial scale.23 The emulsion always contains a microemulsion as a subsystem, when micelle-forming surfactants are emulsion stabilizers. In this case, microdroplets of the aqueous phase coexist with “swollen” micelles.


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The aim of this work was to prepare stable highly concentrated silver organosols in the dynamic reverse emulsion mode and to compare the obtained results with the results of ordinary microemulsion synthesis. Sodium bis-(2-ethylhexyl)sulfosuccinate (Aerosol OT, AOT), an anionic surfactant, was used as a micelle-forming agent and an emulsion stabilizer. Experimental Section Reagents. Silver nitrate (99.9%) and hydrazine monohydrate (99%) were used as starting reagents for synthesis of nanoparticles; n-decane (99%) was used as a solvent. The AOT content in the Aldrich reagent was ≥97%. Synthesis. Emulsion synthesis was performed as follows. An aqueous solution of AgNO3 (4 mL, 0.01-1 mol L-1) was injected dropwise into a solution of AOT in n-decane (20 mL, 0.25 mol L-1) with agitation of the mixture with a magnetic stirrer (500 rpm). Hydrazine (4 mL, 10 mol L1

) was added immediately after silver nitrate. In some experiments the volumes of these

chemicals were increased (16:16:80 mL). The mixture was stirred for 1 h at room temperature. Then, the aqueous and organic phases were separated by centrifugation (1500 rpm, 10 min). Part of silver precipitated as an ultradispersed powder. The reaction yield was calculated from the content of silver in the stable organosol, which was determined spectrophotometrically from absorption at a wavelength of 400 nm. The separated aqueous phase was analyzed for cations by flame photometry (Na+) and atomic-absorption spectroscopy (Ag+) on a Z-8000 Hitachi spectrometer. The concentrations of hydrazine in the aqueous and organic phases were measured by titration with a 0.1 mol L-1 HCl solution. A mixture of bromocresol green and methyl red (3:1 w/w) was used as an indicator. Hydrazine in the aqueous phase was determined by direct titration. Hydrazine in the micellar phase was determined by back titration, until the bright red solution turned green.


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Microemulsion synthesis was performed in a conventional way: aliquots of aqueous solutions of silver nitrate (0.1 mL, 0.05-1 mol L-1) and hydrazine (0.1 mL, 10 mol L-1) were introduced by the injective solubilization method to two portions of a AOT solution in n-decane (10 mL, 0.25 mol L-1). Then, the micellar solutions of the reagents were mixed for several minutes and left for 16-20 h. In both cases, the synthesized organosols were dehydrated by stirring in an open beaker for 3 h.24 A specified quantity of water in them was solubilized before electrophoresis. Electrophoretic concentration. Electrophoretic concentration was performed following the technique described in.25 The produced organosol (20 mL) was placed into a 5×5×5 cm electrophoresis glass cell with horizontally oriented copper parallel-plate electrodes spaced 1 cm apart. In the experiments with a large volume of organosol (up to 100 mL), a 7×7×7 cm cell with 1.5-2.0 cm interelectrode gap was used. Voltage of 600 V was applied to the electrodes. About 2 h after, a nanoparticle concentrate was deposited on the bottom electrode. The electric field was turned off. The cell was tilted to one of its corners to make the concentrate flow down. Then, the field was turned on again for ~30 min. The resulted concentrate was sampled with a syringe. Part of the concentrate was dried in a weighing bottle at room temperature for 24 h. The content of Ag in the solid composite was determined by atomic-absorption spectrophotometry. For this purpose, 6-8 mg of the solid phase was decomposed by heating in a mixture of concentrated H2SO4 and HNO3 (1.5 mL, 2:1, v/v). The solution was transferred into a graduated flask and diluted with deionized water (to 50 mL). The concentration of silver was estimated by atomic absorption spectroscopy. The error of determination of silver content did not exceed 5%.


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The content of AOT in the concentrate was determined by CHNS analysis on EUROEA 3000 (EuroVector, Italy), with two parallel runs (m = 2-3 mg) for each sample. The analytical error was no more than 5%. Methods. The size of micelles was determined by photon correlation spectroscopy (NanoBrook Omni, Brookhaven Inst. Corp., USA) with a monomodal cumulants analysis and a multimodal NNLS (Non-Negatively Constrained Least Squares) analysis. Solid-state laser with a wavelength of 640 nm had power of 35 mW; scattered photons were detected at an angle of 90° to the radiation source. The photon accumulation time for one measurement was 10 s. The electrokinetic potential (ζ-potential) of nanoparticles was measured on the same device by phase analysis light scattering (PALS). Particle-scattered photons were detected at an angle of 15°. The measurements were performed in a special SRR2 cell (Brookhaven Inst. Corp., USA) resistant to organic solvents. Plane-parallel palladium electrodes (S ~ 45 mm2) with the interelectrode gap of 3.45 mm were used in the cell. All organosols were cleaned from dust before the photon correlation spectroscopy measurements. We used five-fold cyclic filtration through a Teflon membrane filter with a pore diameter of 0.2 µm (Sartorius, Germany). The hydrodynamic diameter was determined as an average of 30-50 measurements. The organosols were also cleaned from dust before the PALS measurements. The electrodes were cleaned from dust by continuous filtration of a solvent through them for 20 min, using a BISFS filtration system (Brookhaven Inst. Corp., USA). The rate of solvent flow was 7.8 mL min-1. The velocity of nanoparticles was measured in the manual mode (the automatic mode does not ensure the required sensitivity). The field was varied from 150 to 550 V cm-1; the particle velocity was determined from 10–20 measurements. The velocity–field dependences were linear


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for all systems. This indicates the fulfillment of the “true” nonaqueous electrophoresis criteria.26 Electrophoretic mobility was determined from the slope of the velocity–field plot. Zeta potential was calculated by the Hückel–Onsager formula.27,28 The viscosity of organosols was measured with a MicroVISC viscometer (ReoSense Inc., USA). The measurements were carried out in a MicroVISC TC air temperature controller at 20°С, with an accuracy of ±0.02°С. The surface tension was evaluated from the shape of a hanging drop on an OCA 15Pro tensiometer (DataPhysics, Germany). The tension was calculated by the Laplace–Young equation. The density was measured by weighing the known volumes of organsols on a ViBRA HTR220CE analytical balance (Sinko Denshi, Japan). Electron absorption spectra of nanoparticles (surface plasma resonance spectra) were recorded on a UV-1700 spectrophotometer (Shimadzu, Japan) in the region 300-700 nm. Quartz cells with the optical path length of 1 and 0.2 cm were used. The metal content was determined from the optical density of organosol in the region 395-405 nm. TEM images of nanoparticles were obtained using a JEM-2010 microscope (JEOL, Japan) with the maximum point resolution of 0.2 nm and line resolution of 0.14 nm. The nanoparticle concentrate was diluted 500-4000 times with hexane. A drop of the solution was applied to a membrane and dried in a vacuum. The nanoparticle size was determined as an average of 100 measurements of the size in images of different magnifications. Synchronous TG (thermogravimetric)-DTG (differential thermogravimetric)-DSC (differential scanning calorimetry) analysis was performed on an STA 449F1 Jupiter thermal analyzer (NETZSCH, Germany). The experiments were carried out in closed Al2O3 crucibles heated from


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30 to 600°C in synthetic air (80 vol% Ar, 20 vol% O2), with the heating rate of 10 K min-1, the argon flow rate of 40 mL min-1, and the oxygen flow rate of 10 mL min-1. The experimental data were processed using the Proteus analysis software. High-temperature diffraction experiments were carried out on a high-precision X-ray diffractometer mounted on the output channel of a VEPP-3 electron storage ring.29,30 The diffractometer comprises a monochromator, a collimation system, and a position-sensitive detector. The Ge(111) single-reflection monochromator crystal made it possible to deflect a monochromatic beam by ~30° in the vertical plane, providing the wavelength selection ∆λ/λ~(2÷3)·10-4. The operating wavelength was λ = 1.731 Å. The diffractometer was equipped with an HTK-1200 high-temperature reactor (Anton Paar, Austria) placed in such a way that a monochromatic beam of synchrotron radiation would fall on the sample surface at an angle of ~15°. X-ray patterns of the samples were recorded in the angle range 28-60° 2θ; the integration time was 0.5 min. The rate of temperature rise was 6 K min-1. Electroconductive films were prepared as follows: the concentrate (20 µL) was applied to a 2 × 2 cm glass plate using the Doctor Blade method. The dried film was dark brown to nearly black; the electronic absorption spectrum of silver nanoparticles in the film was identical to the spectra of the liquid organosols. The plate was kept in a laboratory drying oven (250°C, 2 h). SEM images of films were obtained with a JSM 6700F scanning electron microscope (JEOL, Japan). Results and discussion Distribution of Ag and Na ions and hydrazine between the micelles and aqueous emulsion microdroplets. The distribution of Ag+ and Na+ between the micelles and aqueous emulsion microdroplets was determined immediately after the introduction of the first reagent (AgNO3). The mixture was stirred for 2 min. The results are listed in Table 1. Silver passes from aqueous


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microdroplets into the micelles almost immediately. Recovery degree R is ~90% with AgNO3 concentration lower than 0.3 mol L-1. The recovery degree decreases to 61% with AgNO3 concentration equal to 1 mol L-1. This means that ~40% of silver nitrate remains in the emulsion microdroplets. Increasing the stirring time to 30 min does not cause additional transition of silver ions into the micelles. This indicates the attainment of the equilibrium concentration of Ag. At the same time, sodium ions pass from the micelles into the aqueous emulsion microdroplets. This ion exchange is equivalent. It is evidenced by the same molar quantities of Ag+ in the microemulsion and Na+ in the aqueous microdroplets: mAgOrg/mNaAq ~ 1. Next we consider how much water passes into the micelles during the introduction of the first reagent and how their size changes. When pure water of the same volume is introduced into the micellar solution (the first row in Table 1), the solubilization capacity reaches 16.7 vol%, and the hydrodynamic diameter increases from 3.0 nm (diameter of “dry” AOT micelles) to ~24.4 nm. On the addition of 0.1-1 mol L-1 AgNO3 solution, the diameter of micelles decreases to 21.5-8.1 nm. This is a result of the transition of large amounts of strong NaNO3 electrolyte into the microdroplets. We believe that the NaNO3 solution in the microdroplets “extracts” water from the micelles because of osmosis. We changed a 0.3 mol L-1 AgNO3 solution by a 0.3 mol L-1 NaNO3 solution to prove this role of NaNO3. The obtained results (data are provided under "a" footnote in Table 1) are slightly understated as compared with those for AgNO3. The difference in values is apparently because the total concentration of the former AgNO3+NaNO3 electrolyte in aqueous microdroplets is 0.44 mol L-1, which is somewhat higher than 0.3 mol L-1. The distribution of hydrazine was also investigated. We used hydrazine of constant concentration (10 mol L-1) to reduce different amounts of AgNO3. For this reason, we studied the


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distribution of hydrazine between the emulsion microdroplets and the micelles only at this concentration. Its concentration in the aqueous phase and microemulsion was ~4.0 and ~0.2 mol L-1, respectively. Taking into account the low solubilization capacity of the microemulsion and the double dilution of hydrazine with the first reagent, we conclude that most of hydrazine (~80%) remains in the emulsion microdroplets. Nevertheless, the hydrazine content in the micelles is sufficient for the reduction of silver ions. The hydrodynamic diameter of the micelles was 5 nm; thus, hydrazine “squeezes” many water molecules out of the micelles. Note that these measurements were made in the absence of AgNO3 (it was changed by 0.3 mol L-1 NaNO3), because the optical density of Ag organosols is extremely high. Almost all laser radiation is absorbed even in a special microcell with a laser beam path length of 0.1 mm. Thus, the preliminary experiments have showed the following facts. With medium AgNO3 concentrations (≤0.3 mol L-1), silver ions are almost totally exchanged for “micellar” sodium ions initially bound with AOT-. As a result, NaNO3 electrolyte appears in the aqueous emulsion microdroplets, which limits the solubilization of water by micelles. Most of hydrazine remains in the emulsion microdroplets and additionally “extracts” much water from the micelles. This ensures ideal conditions for synthesis of stable nanoparticles: small micelles with a high content of silver and a huge reservoir with a reducing agent gradually passing into the micelles. The content of Ag+ in the emulsion microdroplets an optimal concentration of AgNO3 is low. Therefore, the reduction reaction runs predominantly in the micelle cavities, which ensures a high yield of nanoparticles and prevents the precipitation of solid Ag. Starting reagents in the microemulsion. For microemulsion synthesis, reagents are injected into two different parts of the same microemulsion, which are then mixed. Figure 1 shows the dependence of the hydrodynamic diameter of AOT micelles on the solubilization capacity of


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starting reagents. The hydrodynamic diameter grows regularly as the solubilization capacity increases. The diameters of AOT micelles with different reagents are virtually the same with the solubilization capacity of 1 vol%. They are close to the diameter of micelles during the emulsion synthesis after the introduction of hydrazine. Moreover, preliminary experiments showed that the yield of silver in microemulsion synthesis drastically decreases with the solubilization capacities more than 1 vol%. Taking into account these facts, we performed synthesis using reagents with solubilization capacity of 1 vol%. Synthesis. Then, emulsion and microemulsion syntheses of organosols were carried out in starting aqueous solutions with different silver contents, and the yield of the product was determined (Figure 2a). As expected, the yield of the product of emulsion synthesis drops at low and high silver concentrations. In the first case, all silver passes into micelles, but the high solubilization capacity and the large size of micelles hamper the formation of stable organosols. The large size of micelles is due to the low content of NaNO3 electrolyte (passed from the micelles) in aqueous microdroplets. Subsequent addition of hydrazine almost immediately causes reduction of silver ions, preventing compression of the micelles to a small size. Part of silver remains in the emulsion microdroplets at high concentrations of Ag+ (Table 1). This part of silver is reduced and coagulated to form a precipitate. The maximum yield of the product (~80%) was achieved at an Ag+ concentration of 0.3 mol L-1. The yield of silver in stable organosols on microemulsion synthesis varies insignificantly and is about 50%. The concentration of silver in organosol produced by emulsion synthesis is much higher (Figure 2b). Then, both syntheses were carried out with two concentrations of AgNO3 corresponding to the maximum yield of the product and to the maximum effectiveness of emulsion synthesis (0.3 and 0.9 mol L-1).


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Characteristics of nanoparticles in organosols. The nanoparticles in organosols produced by emulsion and microemulsion syntheses with 0.3 mol L-1 AgNO3 have almost the same parameters: maximum wavelength of surface plasmon resonance absorption (405 and 398 nm), molar extinction coefficient (1.5×104 L mol-1 cm-1), electrokinetic potential (18 mV), hydrodynamic diameter (8.1 and 12.4 nm), metallic core size (6.6 and 8.0 nm) and high stability (Figure 3). These parameters did not change in organosols for a year or even more. Electrophoretic concentration of organosols. Earlier we showed that the concentration of silver nanoparticles in AOT organosols produced by microemulsion synthesis can be increased to 1 mol L-1 by nonaqueous electrophoresis.31 Later we studied the possibility of electrophoretic concentration of organosols obtained by emulsion synthesis. The organosol was separated from the emulsion water and was additionally dehydrated by agitation with a magnetic stirrer in an open beaker for 3 h. A specified amount of water was added after dehydration to evaluate the influence of water content on the efficiency of concentration. The best results (3-4.5 mol L-1 Ag in the concentrate) were obtained with the water solubilization capacity of 1 to 3 vol% when an aqueous 0.3 mol L-1 AgNO3 solution was used for emulsion synthesis. With increasing the concentration of AgNO3 in the aqueous solution to 0.9 mol L-1, the Ag concentration in concentrate reached 8 mol L-1 with the solubilization capacity of 2.0 vol%! The maximum concentration of Ag in the concentrate for organosol produced by microemulsion synthesis did not exceed 1 mol L-1 (Figure 4). The dependence of the density and viscosity of the concentrates is obtained by emulsion synthesis on the silver concentration is shown in Figure 5. The surface tension of the obtained organosols changed insignificantly from initial 0.25 mol L-1 АОТ in decane: 23.7±0.2 and 23.6±0.4 mN m-1, respectively.


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Electrophoresis of organosols produced by both syntheses yields different volumes of concentrates with different Ag contents. Therefore we compare the effectiveness of these syntheses based on the mass of silver in concentrates obtained by electrophoresis from 100 mL organosols. Emulsion synthesis is 32 times more effective at the maximum yield (507 mg instead of 16 mg) and 24 times more effective at the maximum productivity (1025 mg instead of 43 mg) than microemulsion synthesis. Thus, the organosol after emulsion synthesis is concentrated perfectly in the electrophoresis cell. All obtained electrophoretic concentrates are characterized by phenomenal resistance to coagulation, which was controlled for a year by periodic measurements of the hydrodynamic diameter of nanoparticles and recording of surface plasmon resonance absorption spectra. These parameters stayed constant. Note that we had to dilute the obtained concentrates 1000 and more times before the measurements because of their extremely high optical density. Production of solid composites and conducting films. We have shown that a liquid hydrophobic electrophoretic concentrate can be used to inject into furniture varnish, polystyrene, and polyethylene matrices. Silver nanoparticles do not coagulate and retain surface plasmon resonance absorption. Complete evaporation of the solvent from the electrophoretic concentrate results in a dry solid Ag-AOT composite with a silver content of 55-75% and 45-25% AOT. When stored in an open vessel at room temperature, the silver nanoparticles in the solid composite are not subject to coagulation either. The produced solid Ag-AOT composite is easily redispersed in saturated hydrocarbons, benzene, toluene, chloroform, carbon tetrachloride, diethyl ether, and isopropanol even after prolonged (one year) storage. The concentration of silver, the size of the nanoparticles, and the absorption spectra do not change.


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Thermolysis of the solid composites was studied by thermogravimetric analysis and diffraction with synchrotron radiation. According to the TG-DSC data (Figure 6), the solid Ag-AOT composite in the synthetic air is stable up to ~140°C. Insignificant loss of mass (1.8%) accompanied by an exothermic effect takes place above this temperature. The second and third stages of decomposition of the organic matrix of the composite proceed within 190-240°C and 240-340°C and are accompanied by endothermic effects. The fourth stage of decomposition, at 340-560°C, is accompanied by an exothermic effect and leads to complete oxidation (combustion) of intermediate carbon-containing products. The first stage of Ag-AOT decomposition is the spontaneous process of coarsening (sintering) of silver nanoparticles. Sintering proceeds with the release of heat. It leads to a partial decomposition of the organic matrix, as evidenced from the slight loss of mass in the TG curve. Based on the DSC curve (Figure 6a), the thermal effect of the “exothermic sintering” is estimated at ~17.5 J g-1. Note that such effects are not observed in the composites obtained by microemulsion synthesis (Figure 6b). The results of thermogravimetric analysis are confirmed by X-ray diffraction (including in situ) data. Figure 7 shows 3D (Figure 7a) and 2D (Figure 7b) X-ray patterns of the Ag-AOT sample recorded during the in situ experiment on heating in the air from room temperature to 200°C at a rate of 6°C min-1. There is a significant jumpwise narrowing of the diffraction peaks of silver [JCPDS 040783] in the temperature range 160-180°С, which indicates the beginning of sintering of the particles. In addition, there is a specific change in the shape of reflection 111. As seen from Figure 7, the reflection shifts toward smaller angles at 140-180°C; the intensity of the shoulder of the peak located at 45° decreases. Note that in the initial state, the presence of this shoulder and the shift of reflections 111 and 200 toward each other relative to the literary values


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(d111=2.31Å, d200=2.06Å vs d111=2.36Å, d200=2.04Å) apparently indicate the presence of stacking faults.32 The concentration of stacking faults decreases during the thermolysis of the Ag-AOT composite. This leads to sintering of nanoparticles. Figure 7c shows a change in the full width at half maximum (FWHM) of the diffraction peak of reflection 111 of Ag and in the particle size with temperature. The particle size was evaluated from reflection 111 by the Scherrer equation, with ignorance of the broadening of diffraction peaks due to microdistortions and stacking faults. At temperatures below 60°C, the reflection width is constant, ~5°; in the temperature range 160-180°С it drastically decreases to 0.6°. Within 160-200°С, the average size of the coherent-scattering region of Ag drastically increases from 2.5 to 23.0 nm as a result of sintering of nanoparticles. The investigated thermal properties and high resistance to coagulation make it possible to use a concentrate of organosol synthesized by the emulsion method to obtain stable electroconductive inks for microcircuit printing.33 Figure 8 shows SEM images of the obtained conductive films. As seen at the image top, the film is uniform over its area and is virtually free of defects. It consists of sintered crystallites smaller than 100 nm. Their electric conduction is no higher than 180 mOhm per square. This indicates that the crystallites are connected by phase contacts, although they do not form a granular structure typical of bulk metals. Such systems are promising for the production of flexible conductive coatings. The side view of the film cleavage shows that the film is also uniform throughout its volume. The film is 2.5–3 µm thick. Conclusions A new approach to the production of stable silver organosols is proposed. It combines the advantages of emulsion synthesis (the possibility of using high concentrations of reagents) and microemulsion synthesis (production of stable organosols with small nanoparticles). The


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emulsion version based on microemulsion synthesis permits even the laboratory production of the gram quantities (against tens of milligrams in the case of microemulsion synthesis) of a stable concentrate and a solid composite containing 55-75 wt% nanoparticles from only 100 mL of the starting micellar solution. Undoubtedly, the new approach to microemulsion synthesis will significantly expand its potentialities, especially in terms of its practical use. According to the proposed mechanism, the injection of emulsion reagents must be accompanied by the transition of at least one of them into micelles. The transition of water from emulsion microdroplets into micelles must be negligible for the creation of the smallest polar regions. The small size of micelles limits the growth of nanoparticles; therefore, the energy of their van der Waals attraction will be minimum, the nanoparticles do not coagulate, and stable organosols form. Using both a stabilizer and a micelle-forming AOT surfactant shows the following advantages: (1) most of elaborated microemulsion syntheses are based on AOT; (2) AOT is a known “charging” agent for nano- and microparticles, which permits additional electrophoretic concentration and production of organosols and solid composites with a high content of “free” metal nanoparticles.


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Table 1. Compositions of aqueous emulsion microdroplets and microemulsion after separation of the aqueous phase CAg [mol L-1]

VS/V0 [%]

d h* [nm]

CAgAq [mol L-1]

CNaAq [mol L-1]

mNaAq [mol]

CAgOrg [mol L-1]

mAgOrg [mol]

RAg [%]

mAgOrg/ mNaAq

0

16.7

24.4±0.6

-

-

-

-

-

-

-

0

10.3a)

16.8±0.3a)

-

-

-

-

-

-

-

0.1

11.7

21.5±0.7

-

-

-

-

-

-

-

0.2

9.9

18±1

3.4·10-2

0.30

5.4·10-4

5.4·10-4

6.4·10-4

91

1.2

0.3

8.1

14.2±0.9

6.2·10-2

0.38

8.7·10-4

8.7·10-4

9.5·10-4

88

1.1

0.5

6.1

10.3±0.8

0.14

0.53

1.4·10-3

1.4·10-3

1.5·10-3

80

1.1

0.7

4.8

11.1±0.9

0.24

0.66

2.0·10-3

2.0·10-3

2.0·10-3

74

1.0

1.0

2.9

8.1±0.5

0.43

0.77

2.6·10-3

2.6·10-3

2.3·10-3

61

0.88

1.0b)

3.8

9.1±0.5

0.44

0.79

2.5·10-3

2.5·10-3

2.4·10-3

62

0.96

* - P=0.95, n=30 (for the data in column 3). The error range for the data in column 4-8 was 5%. a) A 0.3 mol L-1NaNO3 solution was introduced instead of AgNO3 solution; b) Stirring time 30 min.


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AUTHOR INFORMATION Corresponding Author E-mail: [email protected] Phone: +7-952-908-4559 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This work was supported by the Russian Science Foundation (project № 15-13-00080). ACKNOWLEDGMENT The authors thank Z. S. Vinokurov for in situ XRD measurements, Dr. E. A. Maximovskiy for SEM experiment, Dr. E. N. Tkachev for conductivity measurement. The authors also thank Siberian Synchrotron and Terahertz Radiation Center for allocation of synchrotron radiation beamtime.


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(13) Bagwe, R.; Hhilar, K. C. Effects of Intermicellar Exchange Rate on Formation of Silver Nanoparticles in Reverse Microemulsions of AOT. Langmuir. 2000, 16, 905–910. (14) Lemyre, J.-L.; Lamarre, S.; Beaupre, A.; Ritcey, A. M. A New Approach for the Characterization of Reverse Micellar Systems by Dynamic Light Scattering. Langmuir. 2010, 26, 10524–10531. (15) Kim, D.-W.; Oh, S.-G.; Lee, J.-D. Preparation of Ultrafine Monodispersed Indium-Tin Oxide Particles in AOT-based Reverse Microemulsions as Nanoreactors. Langmuir. 1999, 15, 1599–1603. (16) Smetana, A. B.; Wang, J. S.; Boeckl, J.; Brown, G. J.; Wai, C. M. Fine-Tuning Size of Gold Nanoparticles by Cooling During Reverse Micelle Synthesis. Langmuir. 2007, 23, 10429–10432. (17) Noritomi, H.; Umezawa, Y.; Miyagawa, S.; Kato, S. Preparation of Highly Concentrated Silver Nanoparticles in Reverse Micelles of Sucrose Fatty Acid Esters through SolidLiquid Extraction Method. Adv. Chem. Eng. Sci. 2011. 1, 299–304. (18) Sosa, Y. D.; Rabelero, M.; Trevino, M. E.; Saade, H.; Lopez, R. G. High-Yield Synthesis of Silver Nanoparticles by Precipitation in a High-Aqueous Phase Content Reverse Microemulsion. J. Nanomater. 2010, 2010, Article ID 392572. (19) Johnson, D. W.; Sherborne, C.; Didsbury, M. P.; Pateman, C.; Cameron, N. R.; Claeyssens, F. Macrostructuring of Emulsion-templated Porous Polymers by 3D Laser Patterning. Adv. Mater. 2013, 25, 3178–3181. (20) Ikem, V. O.; Menner, A.; Horozov, T. S.; Bismarck, A. Highly Permeable Macroporous Polymers Synthesized from Pickering Medium and High Internal Phase Emulsion Templates. Adv. Mater. 2010, 22, 3588–3592. (21) Lovell, P. A.; El-Aasser, M. S. Emulsion Polymerization and Emulsion Polymers. Wiley, Chichester, West Sussex, England, 1997, 826 pp. (22) Chern, C.-H. Principles and Applications of Emulsion Polymerization. Wiley, Hoboken, NJ, USA, 2008, 272 pp. (23) Eliseeva, V. I.;

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Figure 1. Dependence of the hydrodynamic diameter of AOT micelles on the injection solubilization of water, 0.3 mol L-1 AgNO3, and 5 and 10 mol L-1 hydrazine solutions


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Figure 2. Dependence of the yield of nanoparticles as stable organosols (a) and of concentration of silver in organosols (b) on the concentration of silver nitrate in the starting aqueous phase for emulsion and microemulsion syntheses. The starting aqueous solution contained 10 mol L-1 hydrazine.


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Figure 3. Characteristics of nanoparticles in organosols produced by emulsion (a, b, c) and microemulsion (d, e, f) syntheses. Surface plasmon resonance spectra (a and d), ζ-potential (a and d, insets), efficient hydrodynamic diameter (b and e, 10 parallel measurements and averaged distribution function (wide line)), and TEM images (c and f) with distribution functions (insets).


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Figure 4. Dependence of silver concentration in electrophoretic concentrate on the volume of injected water. Silver concentration in the starting aqueous solution is 0.3 and 0.9 mol L-1.


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Figure 5. Density (a) and viscosity (b) of concentrates depending on silver content.


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Figure 6. TG, DTG, and DSC curves of Ag-АОТ produced by emulsion (a) and microemulsion (b) syntheses.


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Figure 7. Results of in situ diffraction studies with heating of solid Ag-AOT composite (53 wt.% Ag) to 200°C: 3D XRD patterns (a) and 2D XRD patterns (b); changes in the width of diffraction reflection 111 of Ag and in the average size of the coherent-scattering region of Ag calculated from reflection 111 by the Scherrer equation (c).


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Figure 8. SEM image (top view and side view) of conductive Ag film obtained from the electrophoretic concentrate.


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SYNOPSIS Graphic for the Table of Contents.

A new emulsion approach to the production of silver organosols is proposed. It combines the possibility of using high concentrations of reagents and production of stable small nanoparticles. Emulsion approach permits an increase in the process productivity and is well combined with electrophoretic concentration. The injection of emulsion reagents must be accompanied by the transition of at least one of them into micelles.


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Emulsion vs Microemulsion

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