PREPARATION OF GLOBULAR NANO-AGGREGATES USING MICROEMULSION CRYSTALLIZATION. Richard DVORSKY, Petr PRAUS, Pavel MANČÍK, Jana TROJKOVÁ, Jiří LUŇÁČEK

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1 PREPARATION OF GLOBULAR NANO-AGGREGATES USING MICROEMULSION CRYSTALLIZATION Richard DVORSKY, Petr PRAUS, Pavel MANČÍK, Jana TROJKOVÁ, Jiří LUŇÁČEK VSB-Technical University of Ostrava, Ostrava, Czech Republic, EU, Abstract Aqueous microemulsions of hydrophobic organic liquids are used to be applied in some cases for separation of soft microparticles from aqueous dispersions in a regime of oil flotation. A mechanism of this separation is based on different wetting of particles by water and organic liquids. A reverse way of this process can be applied on a molecular level when different wetting is replaced by different solubility of some molecules in both phases of a microemulsion. Elemental nanoparticles C60 macromolecules are well soluble in toluene (3.2 mg/l) in contrast to water. By a method of emulsion crystallization globular nanoparticles of required size, stability in aqueous dispersion can be prepared. The 50 % solution of C60 in toluene after mixing with water by intensive turbulence in an ultrasound bath was transferred into a microemulsion with a maximal portion of droplets with the radius of nm. During continual bubbling under lower pressure this emulsion was permanently kept at the temperature of 20 o C till toluene was totally evaporated. A resulting aqueous dispersion contained mostly globular nanoaggregates nc60 with the radius of 26.1 nm. Their size predicted by a nucleation model matched the content of C60 molecules in an emulsion microdroplet and depended on the concentration of C60 in toluene and quality of the initial microemulsion. Keywords: nanoparticles, globular nanoaggregates, microemulsion, crystallization INTRODUCTION Microemulsion methods are often used for synthesis of nanoparticles by precipitation reactions [1-4]. Preparation of emulsions is based on dispersion of a solution, containing material necessary for formation of nanoparticles, into a non-miscible liquid. The dispersion is achieved by extreme interface deformation caused by conversion of mechanic energy at turbulent mixing [5,6], which is, in some cases, completed by cavitation in ultrasound field [7,8]. A dispersed part is often stabilized by surfactants [9-12] and an emulsion is formed spontaneously in some cases. Two basic types of microemulsions are water/oil and oil/water differing in a domination volume of one of miscible phases. Nanoparticles can be prepared by precipitation reactions using several types of microemulsion method. One of the methods is mixing two microemulsions containing reactants of precipitation reactions. Each emulsion droplet presents a potential space for nucleation and growth of a new nanoparticle. Coalescence of reactant droplets in a volume of new droplets leads to creation of a microreactor naturally limiting a reaction range and also a volume of solid reaction products. The second basic possibility is a method of single emulsion. In this process, one reactant of a precipitation reaction is dissolved in emulsion droplets and the second reactant exists in a liquid phase of dispersion environment. A precipitation reaction carries out on an interface of both liquids and size of creating particles is limited by amount of reactants inside emulsion droplets. Whereas in both mentioned cases nanoparticles are formed by precipitation reactions in this study a kinetic analysis of the fullerene C60 nanoparticles formation by a method of emulsion crystallization without chemical reactions was performed.

2 METHODS The method of emulsion crystallization of nc60 nanoparticles is based on nucleation and growth of fulleren C60 nanoaggregates in their solutions. From a list of some solvents given in Table 1 toluene was selected because of good accessibility, security and a relatively high saturated concentration C sat(c60) = 3.2 [mg/ml]. The toluene solution of with the concentration of fullerene C(C60) was prepared using chemicals from Sigma Aldrich. Unlike classical microemulsion methods the fullerene solution was dispersed in demineralized water without using surfactants only by an vigorous turbulent mixing in ultrasound field. After this dispersion the resulting emulsion was gradually evaporated under a lowered pressure. Table 1 Saturated concentration of fullerenu C60 in selected solvents [13] SOLVENT Csat(C 60) [mg/ml] octane dodecane tetradecane chloroform 0.17 benzene 1.5 toluene 3.2 1,2-dimethylbenzene ethylnaphthalene 33 1-chloronaphthalene 51 water Fig. 1 shows a process of solvent amount lowering in a microemulsion droplet. The initial period phase of this process is demonstrated by a droplet with the radius R 0 and a final phase is demonstrated by a solid globular nanoparticle nc60 with the radius R, which is an aggregate of n C 60 molecules with the radius R C60. During evaporation of the toluene suppersaturation was gradually increased and the radius of nc60 nuclei increased to its ritical value Rcrit and further to the final value R. A number of aggregated molecules n in the solid fullerite structure is given by a mass balance where ρ sol = 1750 kg/m 3 is the fullerite density, MC60 is the molar mass of fullerene C60 and N A is Avocado s number. R o R o R cr it R Fig. 1 Scheme of development of an emulsion droplet with the initial radius R0 to the nc60 nanoparticle nuclei with the final radius R

3 n R [nm] Fig. 2 Dependence of a number aggregated molecules C 60 on the final radius R of nc60 nanoparticles calculated for density of crystallite fullerite ρ sol = 1750 kg/m 3 The cubic dependence Chyba! Nenalezen zdroj odkazů. of a number of fullerene molecules n on the radius of final fullerite nanoparticles is shown in Fig. 2. Taking in account a basic assumption of formation of a single nucles the final nanoparticle radius can be expressed using mass balance of dissolved C 60 in toluene as While the density ρ sol corresponds to a solid nanoparticle phase and was approximated by the density of crystallite fullerite, ρ sat is the density of saturated solution of fullerene v microemulsion droplets and CC60 [%] is the percentage content of C 60 the in the real solution. On the basis of analysis of mechanical energy transport into the solution of emulsion droplets and a similarity criterion, a steady state of every emulsion at the same energy densities in both phases was assumed. At a minimal size of the volume fraction C dp [%] of dispersion of the C60 solution and at the constant intensity of dispensing the minimal radius of emulsion droplet is R oo. Then, an equilibrium radius is function of two variables according Eq. Chyba! Nenalezen zdroj odkazů. R = R + a C and the final nanoparticle radius R is a o oo dp The linear dependence in Eq. Chyba! Nenalezen zdroj odkazů. corresponds to the first approximation of hydrodynamic equilibrium of non-miscible phases with the content of dispersed phase C dp of 1 7 vol. %. At the lower concentration, this condition of structures similarity is unrealistic and preliminary measurements gave highly spread results.

4 EXPERIMENTAL RESULTS For the experimental study of preparation of nc60 nanoparticles by the microemulsion crystallization the aqueous microemulsions of C 60 in toluene in 15 various combinations (CC60, C dp) were prepared according to Table 2. Table 2 Diameters of nc60 nanoparticles measured by dynamic light scattering (DLS) method d[nm] C C60 [%] , Cdp [%] The microemulsions were prepared by highly turbulent mixing in ultrasound field without surfactants. These microemulsions were immediately after dispersion bubbled by air at lowered pressure during 90 min to remove toluene from emulsion droplets. After total evaporation of toluene relatively stable aqueous dispersions of nc 60 nanoparticles remained. Their size shown in Table 2 was measured was measured by DLS using a Malvern Zetasizer Nano ZS device. On the basis of the above given analysis (Table 2) an optimal two-parameter non-linear regression was performed as demonstrated in Fig. 3. The regression parameters A = a B = were determined with the correlation coeficient R = Fig. 3 Non-linear regression of theoretical dependence d(c dp, C C60) = 2R(C dp, C C60) according to Eq. Chyba! Nenalezen zdroj odkazů. The prepared nanoparticles were of a globular shape as obvious form Fig. 4 and polydisperse as confirmed by DLS. A number of aggregated molecules C60 changed from to according to Fig. 2. Based on preliminary measurements of FTIR spectra a combination of the fullerite crystallic and amorphous structures forming shells were found. This nanoparticles structure can be explained by a time course of its diffusion growth. While nanoparticles after their nucleation grew slowly during the initial period, a rate of volume decrease during the final period of evaporation was very high and this fast process of the C60 molecules deposition on solid nanoparticles did not facilitate sufficient configuration relaxation to the energetically favourable ordered structure of fullerite.

5 Fig. 4 TEM micrograph of globular nanoaggregates obtained by JEOL 200 CX (+ EDX EDAX 9900) CONCLUSIONS The method emulsion crystallization was used to prepare globular nc60 nanoparticles in the stable aqueous dispersion whose size can be controlled by the concentration of fullerene in the toluene phase and by a selection of the content of dispersion phase in aqueous environment. The solutions of C 60 in toluene after intensive mixing with water in ultrasound field were transferred into microemulsions. The emulsions were kept under a continual evaporation of toluene at lowered pressure and at the temperature of 20 o C to the total toluene evaporation. The resulting stable water dispersion contained a maximal portion of nc 60 nanoaggregates consisting of globular nanoparticles with size from 33 nm to 100 nm. Their size predicted by the nucleation model corresponded to a number of C 60 molecules changing from to These globular nanoparticles showed characteristics of a core-shell structure with the fullerite core and the amorphous shell. AKNOWLEDGEMENTS This research was supported by the Grant Agency of the Czech Republic under the project P107/11/1918 and the Regional Material Technology Research Centre (RMTVC) under the project CZ.1.05/2.1.00/ LITERATURE [1] Lo pez-quintela, M. A.: Synthesis of nanomaterials in microemulsions: formation mechanisms and growth control, Current Opinion in Colloid and Interface Science 8 (2003), p [2] Ethayaraja, M., Dutta, K., Muthukumaran, D., Bandyopadhyaya, R.: Nanoparticle Formation in Water-in-Oil Microemulsions: Experiments, Mechanism, and Monte Carlo Simulation, Langmuir 23 (2007), p [3] Concha, T., de Dios, M., Barroso, F.: Surfactant Effects on Microemulsion-Based Nanoparticle Synthesis, Materials 4 (2011), p [4] Malika, M. A., Wanib, M. Y., Hashima, M. A.: Microemulsion method: A novel route to synthesize organic and inorganic nanomaterials: 1st Nano Update, Arabian Journal of Chemistry 5 (2012), Issue 4, p [5] Kolmogorov, A. N., On the comminution of droplets in turbulent flow (in Russian), Dokl. Akad. Nauk SSSR 66 (1949), p [6] Holmberg, K.: Handbook of Applied Surface and Colloid Chemistry, John Wiley & Sons 2002, ISBN

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