Colloids and Surfaces A: Physicochem. Eng. Aspects 270–271 (2005) 189–194
Ceramic particles obtained using W/O nano-emulsions as reaction media M. Porras a,∗ , A. Mart´ınez a , C. Solans b , C. Gonz´alez a , J.M. Guti´errez a a
Department d’Enginyeria Qu´ımica i Metall´urgia, Universitat de Barcelona, Barcelona, Spain b Department Tecnologia de Tensioactius, IIQAB-CSIC, Barcelona, Spain Available online 18 July 2005
Abstract Monodisperse ceramic particles can be produced from water-in-oil (W/O) nano-emulsions by hydrolysis and condensation of ceramic alkoxides into aqueous droplets, thereby yielding nanoparticles of controlled size and shape. This study addressed both the formation of W/O nano-emulsions and the resultant ceramic particles obtained in reaction media. Nano-emulsions were prepared by adding water or catalyst aqueous solution to a mixture of decane and surfactants. Droplet size was determined by dynamic light scattering, with mean sizes ranging from 30 to 120 nm. Higher water concentrations resulted in larger droplets. Ceramic nanoparticles were prepared by adding ceramic alkoxides in W/O nano-emulsions. Tetraethyl orthosilicate and tetraisopropyl orthotitanate were used to obtain silica and titania nanoparticles, respectively. Ceramic nanoparticles were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), and dynamic light scattering (DLS). Particles with average size from 30 to 230 nm were obtained. Particle sizes correlated with droplet sizes of those nano-emulsions were used as reaction media. © 2005 Elsevier B.V. All rights reserved. Keywords: Span/tween; W/O nano-emulsion; Non-ionic surfactant mixture; SiO2 nanoparticles; TiO2 nanoparticles
1. Introduction Over the last several years, a number of studies on the formation, characterization and application of emulsions have been carried out. Emulsions are thermodynamically unstable liquid/liquid dispersions that are stabilized, in general, by surfactants, polymers or solids particles [1]. As non-equilibrium system’s emulsion properties depend not only on physicochemical variables (nature of components, composition, temperature and pressure), but also on preparation methods and the order in which components are added [1–4]. More recently, a new class of emulsions, with droplet sizes ranging in the nanometers and similar to microemulsions has been reported [5–7]. These emulsions, termed nano-emulsions, mini-emulsions or ultra-fine emulsions, are transparent or translucent (with droplet sizes between 50 and 200 nm) or milky (up to 500 nm) [8–12] and exhibit high kinetic stability.
∗
Corresponding author. Tel.: +34 645975305; fax: +34 93402 1291. E-mail address:
[email protected] (M. Porras).
0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.06.018
Nano-emulsions are a subject of increasing interest in both theoretical discussions and practical applications due to their singular properties, namely extremely small droplet size, kinetic stability and transparency. In addition, they present several advantages over conventional emulsions, owing principally to their similar characteristics to microemulsion ones [12]. For example, nano-emulsions offer the possibility of using microemulsion-like dispersions without need of high surfactant concentrations. Additionally, nano-emulsions boast a wide variety of diverse applications in the chemical (polymerization), cosmetic, and pharmaceutical industries, etc. [13]. One of the earliest chemicals applications of O/W nano-emulsions was in the preparation of latexes [5,10,14–19] by polymerization. Ugelstad et al. [14] found that the mechanism involved in miniemulsion polymerization was quite different from that of emulsion polymerization, suggesting that the main locus of nucleation for the latter was monomer droplets versus micelles [14]. While so-called miniemulsion polymerization is a broad term used to designate all polymerization processes performed in nano-emulsion (miniemulsion) media, it is also used in a more restrictive sense, referring to instances when
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the polymerization of nano-emulsion droplets is equal to the number of polymer particles and to particle size distribution [10]. The preparation of monodisperse particles has drawn considerable attention in recent years [20]. Several methods have been developed not only to synthesize nano-sized particles, but also to calibrate their size, including Langmuir–Blodgett films [21], vesicles [22], reverse microemulsions [23] and surface-active supports [24]. In fact, silica (SiO2 ) and titania (TiO2 ) submicron particles have been prepared by the controlled hydrolysis of metal alkoxides in alcohol and water mixtures [25–27]. However, as reported by Bogush et al., one limitation of this method remains a high polydispersity when the size of the particle is less than 100 nm [26]. Reverse microemulsions provide an effective medium for synthesis of monodisperse nanoparticles (typically less than 100 nm) [24]. Indeed, the synthesis of nanoparticles using microemulsion reactions was first described by Boutonnet et al. [28], who obtained monodisperse metal particles by reduction of metal salts in water-in-oil (W/O) microemulsions. Since then, there have been numerous reports on the use of microemulsions as a reaction media for nanoparticle synthesis. One major disadvantage remains in the fact that microemulsion formation requires higher amounts of surfactant than conventional emulsions, typically over 20 wt%. However, nano-emulsion formations require smaller surfactant concentration. Nevertheless, there remain many techniques for obtaining size information. Indeed, emulsion droplet or nanoparticles size data may be compiled in various ways: (1) in terms of the number N of droplets; (2) by specific particle length, namely the diameter D, using optical or electron microscopy; (3) as diameter versus droplet or particle projected area S by employing turbidity techniques; (4) as diameter versus surface area A utilizing light scattering measurements; (5) as number versus volume V by electrical resistance counting; and (6) in terms of diameter versus mass M by X-ray transmission coupled with sedimentation, or perhaps, by hydrodynamic chromatography [41]. Thus, evaluation of size can involve measures of number, length (diameter), area, volume, or mass [41–43]. As different ways of expressing the average diameter of particle distribution are frequently encountered in the literature [45,46] it is important to specify which definition is being used to avoid confusion [43] While O/W nano-emulsion studies have been undertaken [12,13,29–35] in recent years, relatively few publications on W/O nano-emulsions currently exist. Indeed, until now, few scientists have investigated either W/O nano-emulsions [36,37] or W/O miniemulsions [38]. In the present study, W/O nano-emulsion formations were analyzed using nonionic surfactant mixtures. To both characterize and evaluate their potential application as nanoreactors for nanoparticles, nano-emulsion size and stability were closely scrutinized. In the second part of this study, the formation of SiO2 and TiO2 ceramic particles using W/O nano-emulsions as reaction media was explored.
2. Experimental 2.1. Materials Sorbitan ester surfactants (Span and Tween series), Sorbitan monolaurate or Span 20 (S20), Sorbitan monooleate or Span 80 (S80), PEO 20 Sorbitan monolaurate or Tween 20 (T20) and PEO 20 Sorbitan monooleate or Tween 80 (T80) technical grade were purchased from Sigma–Aldrich Chemical. N-decane (purity > 99%) was obtained from Panreac. Water was de-ionized and Millipore filtered by a Milli-Q system. 25% Ammonia in aqueous solution (G.R.), hydrochloric acid in aqueous solution (G.R.) were obtained from Panreas. The tetraethyl orthosilicate (TEOS) and tetraisopropyl orthotitanate (TTIP), 98% pure, were supplied by Merck. The systems studied were S80:T80 (51:49)/decane/water, S20:80 (62:38)/decane/water, and S20:20 (60:40)/decane/water. Optimum surfactant ratio was determined in previous experiments (data not shown), and it is the ratio that permits to solubilize highest water [39,40]. 2.2. Methods 2.2.1. Regions of microemulsion and nano-emulsion Emulsions were formed by adding water to a mixture of the other components at 25 ◦ C and using a magnetic stirrer at 700 rpm. The limit demarcating microemulsion and nanoemulsion regions was determined by observing the evolution of back-scattered light as a function of time. This study was carried out using multiple light scattering at a wavelength of 850 nm. 2.2.2. Nano-emulsion formation These were prepared by adding water or ammonia solution to a mixture of decane and surfactants (Span 20:Tween 80 and Span 80:Tween 80). The rate of addition was kept constant at 0.03 ml/min and temperature was maintained at 25 ◦ C, while the solution was mixed using a magnetic stirrer at 700 rpm. 2.2.3. Nano-emulsions characterization: nano-emulsions droplet size For this study, the average droplet and nano-emulsion distribution sizes were determined by dynamic light scattering (DLS), using a Malvern 4700 photon correlation spectrometer (Malvern Instruments, Malvern, U.K.). An argon laser (λ = 488 nm) with variable intensity was used to cover the wide size range involved. The hydrodynamic radius measurements were consistently carried out at a scattering angle of 90◦ and a temperature of 25 ◦ C. When measurements had been completed, DLS data were processed via the CONTIN method [4,5,13] with a software package which permits the expression of diameter distribution in terms of intensity, number, or volume and permits to obtain the polydispersity data of the sample [52]. Intensity distribution shows
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the percentage of laser light scattered by particles of the corresponding size. For small particles (“small” in relative comparison to wavelength), the intensity scattered by a particle of diameter di is proportional to the volume squared or to the sixth power of particle size. In this case, the diameter corresponds to that obtained for the following expression [43,44]: ni d 6 d = d6,5 = i i5 i ni d i
(1)
where d is the average particle (or droplet) size. ni is the number of particles with the diameter di . Volume distribution shows the sample volume percentage occupied by particles or droplets of some size class. Number distribution shows the distribution percentage of particles of some size class. Polydispersity is considered as an index calculated from fitting a three-parameter polynomial to the log correlation function, interpreted as the variance of a supposed log-normal model [52]. 2.2.4. Nano-emulsion stability Nano-emulsion stability was studied by multiple light scattering using a Turbiscan MA 2000 (850 nm wavelength). It was additionally assessed by measuring droplet size as a function of time by dynamic light scattering. 2.2.5. Liquid crystals The presence of liquid-crystalline phases was detected by using crossed polarizers. 2.2.6. Particle formation Ceramic particles were prepared by adding ceramic alkoxides in W/O nano-emulsions, thereby catalyzing the formation of silica particles. Silica particles were then prepared by adding tetraethyl orthosilicate to nano-emulsions, which were formed using ammonia solution as an aqueous component. By contrast, titania particles were prepared by adding tetrisopropyl orthotitanate to nano-emulsions, which were formed using water as an aqueous component. To remove reaction by-products, the nanoparticles were washed repeatedly with both water and ethanol before being calcinated at 550 ◦ C. 2.2.7. Particle characterization Particle size was determined by dynamic light scattering while particle shape was observed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM). In SEM, a Leica Instrument Model Stereoscan 360 was used. Silica and titania samples were coated by sputtering with a gold layer 25 nm thick. In AFM, a Nanoscope 3 Digital instrument was used.
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3. Results 3.1. Regions of microemulsion and nano-emulsion The main objectives of this section were to discover the appropriate compositions that form W/O nano-emulsions and to determine those compositions that give rise to microemulsion, nano-emulsion and W/O emulsion formations. The limit demarcating regions of microemulsion from nano-emulsion was established by studying system stability by means of multiple light scattering. Microemulsion systems were defined as those wherein no changes in droplet size or volume fraction were observed, and in which back scattering and transmission remained constant [47]. System showing variations in- droplet size, in terms of coalescent mechanisms, flocculation or Oswald ripening, and which maintained uniform back scattering and transmission over the entire sample [40] were considered nano-emulsions. Those systems exhibiting droplet migration phenomena (creaming or sedimentation), which can induce fractional volume changes at sample extremities, which in turn can alter back scattering and transmission in an area of the sample [47], were termed emulsions. The samples analyzed by multiple light scattering yielded data that allowed construction of triangular diagrams in (1) a phase area defined by W/O-like microemulsions, and (2) a multi-phase area defined by W/O-like nano-emulsions and W/O-like emulsions (Fig. 1). For example, Fig. 1 shows the limit demarcating regions of microemulsion and nano-emulsion for a system formed by S20:T80 (62:38)/decane/water. For the two surfactant systems (S20:T80 (62:38) and S80:T80 (51:49)) at low surfactant concentrations (less than 3 wt%), neither microemulsions nor nano-emulsions were able to form, as is evident from whitish sample colours and phase separation. Thus, the surfactant content was not sufficient to provide the necessary system stability. In samples exhibiting higher surfactant concentration (greater than 5 wt%), microemulsion. nano-emulsion
Fig. 1. Regions of microemulsion, nano-emulsion and emulsion. System formed by S20:T80 (62:38)/decane/water. Ta 25 ◦ C.
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and emulsion regions appear. Under conditions of constant surfactant:decane ratio (S:O), and when water concentration was low, microemulsions did form. However, with moderate increases in water concentration, nano-emulsions were formed, and at even higher water concentrations emulsions were formed. 3.2. Nano-emulsions droplet size Fig. 2 shows the influence of water concentration on W/O nano-emulsion droplet size. Mean size ranged between 30 and 150 nm, with higher water concentrations yielding greater droplet sizes. Droplet radius decreased with increases in surfactant concentration, probably as a result of increases in interfacial areas. Other O/W nano-emulsion studies found that decreases in droplet size observed under higher surfactant concentrations would be attributable to both increases in interfacial area and to a lessening of interfacial tension [13]. By contrast, acid or basic catalysis is often required for SiO2 particle preparation. The effects of catalyst aqueous solution as a nano-emulsion water component were addressed in one study, where the catalysts analyzed were HCI 3.6 wt% and NH3 10 wt%. In all of the W/O nano-emulsions prepared with HCl 3.6 wt%, rapid phase separation was observed. In fact, highly polydisperse samples were obtained and in some instances, two droplet size distributions were observed. However, a correlation between droplet size and dispersed phase concentration was not found. Nano-emulsions formed with NH3 10 wt% exhibited droplet sizes similar to those prepared with water (Fig. 3). Additionally, nano-emulsions with an ammonia solution concentration greater than 10 wt% were prepared. In those nanoemulsions prepared with S20:T80 (62:38), liquid crystals were observed. However, in nano-emulsions prepared with S80:T80 (51:49), liquid crystals were not observed. A separate study of microemulsion formation utilized the surfactant mixtures S20:T80 (62:38) and S80:T80 (51:49), and in the
Fig. 3. Droplet size vs. nano-emulsion water concentration or ammonia solution concentration (wt%). System (S 20/T80 (62:38))/decane/aqueous component, surfactants:oil 15:85.
case of S20:T80 (62:38), the authors attributed liquid crystal formation with ammonia to an HLB value of nearly 12 [48]. 3.3. Nano-emulsions stability Although nano-emulsions demonstrated strong stability, without phase separation, for several weeks, analysis by dynamic light scattering revealed slight increases in droplet size over longer time periods (data not shown). There are two processes by which droplet size may increase over time: coalescence and Ostwald ripening [41–49]. For those systems included in the present study, evolution of the experimental droplet radius over time suggests that nano-emulsion breakdown might be attributable to Ostwald ripening. Indeed, during a study period of 370 h, increases in droplet size followed the Ostwald ripening model, at a rate of 7 × 10−29 m3 /s [41]. 3.4. Ceramic nanoparticles
Fig. 2. Droplet size vs. nano-emulsion water concentration (wt%). System (Span 20/Tween 80 (62:38))/decane/water, surfactants:oil 15:85 and 10:90.
As discussed in the Introduction, nanoparticles have been obtained by adding ceramic alkoxide directly to a W/O nano-emulsion. Other studies on the formation of SiO2 particles in W/O microemulsion media [50] have suggested that tetraethoxysilane molecules penetrate into aqueous droplets from the oil phase through the surfactant interface laver, wherein tetraethoxysilane hydrolysis occurs. Friberg et al. [51] studied the distribution of the intermediate species Sin O2n−r (OH)2r−x (OR)x during this reaction, observing that 29 Si NMR spectra of the microemulsion detect only two species, namely, Si(OR)4 and SiO2 . They have suggested that, since in a colloidal system the tetraethoxysilane is located within the lypophilic phase, reactions tend to proceed rapidly when molecules reach the interface near the water region. It is quite possible that in W/O nano-emulsion media, reactions are effected by a similar mechanism. Thus, alkoxide would diffuse through the organic phase (decane) until reaching
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Fig. 4. SEM photograph of Silica particles obtained in the system W/S/O 12:21:67, Span 80/tween 80 51:49, and with molar ratio water:alcoxyde 6:1.
the surfactant interface layer. Once the alkoxide had crossed the interface it would then react with the water contained in a droplet. The droplets would thereby constitute nanoreactors of controlled dimension and morphology. The principal objective of this section is to study nanoparticle shape and size, comparing them with nano-emulsion droplet size. To study nanoparticle shape, micrographs have been constructed by means of SEM, TEM and AFM. Figs. 4–6 illustrate a SEM micrograph of SiO2 , an AFM micrograph of TiO2 and a TEM micrograph of SiO2 , respectively. All three micrographs show that nanoparticles are spherical. A nanoparticle size study carried out by means of the DLS yielded data expressed in terms of intensity distribution, with the average size of TiO2 particles and SiO2 particles ranging from 30 to 230 nm. These results indicate: (1) under S:O constant conditions, particle size increases as the nano-emulsion aqueous component increases (Fig. 6); and (2) in experiments where nano-emulsion water concentration is held constant, particle size decreases as the surfactant concentration used to form nano-emulsions increases.
Fig. 5. AFM photograph of Titania particles obtained in the system W/S/O 4:14.4:81.6, Span 80/tween 80 51:49, and with molar ratio water:alcoxyde 6:1.
Fig. 6. TEM photograph of Silica particles obtained in the system W/S/O 12:15:73 Span 80/tween 80 51:49 and with molar ratio water:alcoxyde 6:1.
Finally, in order to test the correlation between nanoparticle size and nano-emulsion droplet size, DLS nanoparticle experimental size values have been compared to nanoparticle size estimates based in water content of nano-emulsion droplets. Nanoparticles have been regarded as nonporous, and resulting from the reaction between alkoxide and the stoichiometric quantity of water inside the droplet. From the diameter of the nano-emulsion droplet, the volume of water contained therein can be known and then particle size calculated. For all droplet and particle size calculations, size is expressed as volume-based geometric mean diameter [41]. Fig. 7 shows that the estimated sizes for non-porous particles formed by the reaction of alkoxide with the stoichiometric quantity of water inside the droplet are similar to experimental particle sizes. This good correspondence have been observed for all systems studied (data not shown), proving that nanoparticle size is strictly determined by nano-emulsion droplet size, i.e. by water content of the nano-emulsions droplets used as microreactors.
Fig. 7. SiO2 particle experimental size and estimated size comparison. System (Span 80/Tween 80 (51:49))/decane/water, surfactants:oil 15:85.
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4. Conclusions The present study permitted the identification of regions where nano-emulsions can be formed. W/O nano-emulsions, with droplets sizes ranging from 30 to 120 nm, were obtained by adding water to a mixture of surfactants and decane. In experiments where the surfactant:oil ratio is constant, droplet size increases as water concentration increases. Additionally, under conditions of constant water concentration, droplet size decreases when the surfactant:decane ratio increases. Droplet size was not affected by the presence of ammonia solution for those concentrations used in this study. Nano-emulsions exhibited strong stability, without phase separation, sedimentation or creaming for several weeks. However, over longer time periods, slight increases in droplet size were evident. Dynamic light scattering stability studies demonstrated that nano-emulsion breakdown could be attributed to Ostwald ripening. Nano-emulsions are kinetically stable for a long time, permitting their use as microreactors. It is possible to control droplet size by varying the composition of nanoemulsions. SiO2 and TiO2 nanoparticles, with sizes ranging from 30 to 230 nm, were obtained by alkoxide hydrolysis in W/O nano-emulsions. Nanoparticle sizes can be controlled by controlling the nano-emulsion droplet sizes.
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