Langmuir 2003, 19, 933-938
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Ca(OH)2 Nanoparticles from W/O Microemulsions Alessio Nanni† and Luigi Dei*,‡ Department of Chemistry and Consortium CSGI - University of Florence, Polo Scientifico via della Lastruccia, 3 I-50019 Sesto Fiorentino (FI), Italy Received August 19, 2002. In Final Form: November 21, 2002 The aim of this study was the synthesis and physicochemical characterization of Ca(OH)2 nanoparticles. The synthesis was carried out in ternary water-in-oil microemulsions where the oil phase was cyclohexane; two different microemulsion systems were investigated changing the nonionic surfactant from the tetraethylene-glycol-monododecyl ether (C12E4) to the pentaoxyethylene-glycol-nonyl-phenyl ether (IgepalCO520). Phase diagrams of the two microemulsion systems were studied in the presence of high concentration of Ca2+ or OH- in the aqueous pool, allowing the determination of the optimum concentrations of the three components (surfactant, cyclohexane, CaCl2, and NaOH aqueous solution) employed to carry out the nanoparticles’ synthesis. The microemulsions were characterized by Quasi-Elastic Dynamic Light Scattering (QELS) obtaining a linear relationship between the hydrodynamic diameter and the wo-parameter both for the pure (NaOH and CaCl2) and the mixed systems. The nanoparticles’ synthesis was performed in w/o microemulsions with different values of the wo-parameter varying in the ranges 1.5-12 and 2-5 for the Igepal-CO520 and C12E4 systems, respectively. The particles obtained were characterized by Transmission Electron Microscopy (TEM), X-ray Diffractometry (XRD), and Fourier Transform Infrared Microspectroscopy (FTIRM). The nanoparticles’ size varied in the range 2-10 nm, and the average dimension depended on the wo-parameter. The synthesized Ca(OH)2 nanoparticles were shown to be highly reactive against carbonatation by atmospheric CO2; therefore, the method was shown to be very effective also to prepare ultra-fine CaCO3 particles.
Introduction The synthesis and characterization of nanoparticles of inorganic materials as oxides, hydroxides, metals, and sulfides from w/o microemulsions or reverse micelles have been the subject of many papers.1-10 Most of these compounds are strongly insoluble in water and, therefore, the concentration of the reactant species in the microdroplets can be kept quite low. Instead, the analogous preparation of nanoparticles of substances with moderate solubility in water is rather problematic, due to the high ionic strength inside the droplets, which complicates the microemulsion/micellar system formulation. Recently, a first attempt to synthesize nanoparticles of high Ksp materialssCaSO4shas been performed in different types of w/o microemulsion.11 The Authors succeeded in obtaining calcium sulfate nanoparticles with different shapes working with [SO42-] and [Ca2+] inside the aqueous pool of the w/o microemulsion varying in the range 0.0250.20 M. When the nanoparticles that have to be prepared * Author to whom correspondence should be addressed. Fax: + 39-0554573036. E-mail:
[email protected]; http://www.csgi.unifi.it. † Present address: Sago S. p. A., via Odorico da Pordenone, 32 I-50127 Firenze, Italy. ‡ University of Florence. (1) Lisiecki, I.; Pileni, M. P. J. Phys. Chem. 1995, 99, 5077. (2) Boutonnet, M.; Kizling, J.; Stenius, P. Colloids Surf. 1982, 5, 209. (3) Yamauchi, H.; Ishikawa, T.; Kondo, S. Colloids Surf. 1989, 37, 71. (4) Tina, Y.; Wu, C.; Fendler, J. H. J. Phys. Chem. 1994, 98, 4913. (5) Bowers, C. R.; Pietrass, T.; Barash, E.; Pines, A.; Grubbs, R. K.; Alivisators, A. P. J. Phys. Chem. 1994, 98, 9400. (6) Haram, S. K.; Mahadeshwar, A. R.; Dixit, S. G. J. Phys. Chem. 1996, 100, 5868. (7) Nair, P. K.; Garcia, V. M.; Fernandez, A. M. J. Phys. D: Appl. Phys. 1991, 24, 441. (8) Vogel, R.; Hoyer, P.; Weller, H. J. Phys. Chem. 1994, 98, 3183. (9) Kortan, A. R.; Hull, R.; Opila, R. L.; Bawendi, M. G.; Steingerwald, M. L.; Carroll, P. J.; Brus, L. E. J. Am. Chem. Soc. 1990, 112, 1327. (10) Tojo, C.; Blanco, M. C.; Rivadulla, F.; Lopez-Quintela, M. A. Langmuir 1997, 13, 1970. (11) Rees, G. D.; Ewans-Gowing, R.; Hammond, S. J.; Robinson, B. H. Langmuir 1999, 15, 1993.
are hydroxides with moderate water solubility, further difficulties arise from the strong pH conditions inside the water pool. This can be a crucial factor for the surfactant stability: some surfactants can be hydrolyzed at high pH values, with possible subsequent phase separation of the microemulsion systems. This is the reason colloidal particles of hydroxides with moderate water solubilitys i.e., Ca(OH)2, Mg(OH)2sare usually obtained by means of other synthetic routes, as homogeneous hydrolytic reactions,12,13 or synthesis in diols at high temperature.14 However, particles with very low dimensions, i.e., in the range of a few nanometers, cannot be produced with these procedures. Moreover, it has been recently shown that nanosized Ca(OH)2 and Mg(OH)2 can have important applications in the field of cultural heritage conservation and safeguard, in particular in restoration of wall paintings and de-acidification of ancient papers.13,15 The aim of the present study was to develop and optimize a synthetic procedure to produce Ca(OH)2 nanoparticles utilizing the w/o microemulsions route. Two microemulsion systems were selected and the nanoparticles’ synthesis was carried out at various values of the woparameter (wo ) [water]/[surfactant]). The microemulsions were studied by dynamic Quasi Elastic Light Scattering (QELS) performing experiments before and after the mixing of the two microemulsions containing the reactant species (Ca2+ and OH-, respectively) in the water pool. The obtained Ca(OH)2 nanoparticles were characterized by Transmission Electron Microscopy (TEM), X-ray Diffractometry (XRD), and Fourier Transform Infrared Microspectroscopy (FTIRM). The reactivity of the Ca(OH)2 particles toward CO2 was investigated in order to extend (12) Ambrosi, M.; Dei, L.; Giorgi, R.; Neto, C.; Baglioni, P. Langmuir 2001, 17, 4251. (13) Arai, Y. Chemistry of Powder Production; Powder Technology Series; Scarlett, B., Ed.; Chapman & Hall: London, 1996; Chapter 4. (14) Salvadori, B.; Dei, L. Langmuir 2001, 17, 2371. (15) Baglioni, P.; Ceccato, M.; Dei, L.; Giorgi, R.; Schettino, C. V. Langmuir 2002, 18, 8198.
10.1021/la026428o CCC: $25.00 © 2003 American Chemical Society Published on Web 12/24/2002
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Figure 1. Behavior of the cloud point for the system C12E4/ cyclohexane/aqueous solution as a function of the wo-parameter: (0) NaOH 0.2 M, (9) NaOH 0.1 M, (O) CaCl2 0.05 M, and (b) CaCl2 0.1 M.
the synthetic procedure to the production of nanosized CaCO3 crystals. Experimental Section A. Materials. Sodium hydroxide, NaOH, potassium hydroxide, KOH, calcium chloride dihydrate, CaCl2‚2H2O, silver nitrate, AgNO3, nitric acid, HNO3 (65 wt %), and propan-1-ol (purity > 99.5%) were supplied by Merck, Darmstadt, Germany, and were used without further purification. Water was purified with a Millipore Milli-RO 6 and a Milli-Q (Organex system) water system (water resistance g 18 MΩ cm). Standard copper grids (200 Mesh 3 mm L) with a carbon-supported film were obtained from Taab Chemicals & Equipment for Microscopy Ltd, England. Cyclohexane (purity > 99.5%) and tetraethylene-glycol-monododecyl ether (C12E4) were purchased from Fluka, Buchs, Switzerland; pentaoxyethylene-glycol-nonyl-phenyl ether (Igepal-CO520) was supplied by Aldrich-Chemie, Steinheim, Germany. B. Synthesis of the Particles. The synthesis of the Ca(OH)2 particles was carried out by mixing equal volumes of two w/o microemulsions containing, respectively, Ca2+ and OH- in the appropriate concentration inside the aqueous pool. Microemulsions containing water, NaOH, and CaCl2 solutions in the dispersed phase were prepared and the phase diagrams were determined in order to choose the suitable range of wo. In a first attempt the synthesis of Ca(OH)2 nanoparticles was carried out using Na(AOT) and Ca(AOT) based systems, but it was not possible to collect any particles. Moreover, the Na(AOT)/decane/ water microemulsions were strongly affected by the addition of Ca2+ ions, due to the shrinkage of the microemulsion domain in the phase diagram.16,17 Therefore, nonionic surfactants were selected in order to overcome the problem of counterion substitution at the microdroplet interface. The surfactants chosen were C12E4 and Igepal-CO520 and the concentrations, kept constant for both microemulsions, were 0.2 M for C12E4 and 0.15 M for Igepal-CO520. The microemulsion stability against temperature was determined investigating the phase stability map: while for the Igepal-CO520 all the microemulsions selected were stable at 25 °C, a different behavior was detected for C12E4 (see Figure 1). In particular, the systems containing NaOH had to be kept at a temperature below 15 °C in order to prevent phase separation; therefore, the synthesis was carried out at 10 °C. The range of wo selected for the synthesis was 2-5 and 1.5-12 for the C12E4 and Igepal-CO520 systems, respectively. Both microemulsions systems were characterized by Quasi-Elastic Light Scattering (QELS) measurements in the dynamic mode by means of a Brookhaven apparatus (BI200SM with BI9000AT correlator) (16) Capuzzi, G.; Pini, F.; Giambi, C. M. C.; Monduzzi, M.; Baglioni, P.; Teixeira, J. Langmuir 1997, 13, 6927. (17) Caboi, F.; Capuzzi, G.; Monduzzi, M.; Baglioni, P. J. Phys. Chem. B 1997, 105, 10205.
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Figure 2. Hydrodynamic diameter (from QELS measurements) as a function of the wo-parameter for (9) Igepal-CO520/ cyclohexane/CaCl2 0.25 M aqueous solution, and (O) IgepalCO520/cyclohexane/NaOH 0.5 M aqueous solution. using a Nd:YAG (532 nm) diode pumped laser with a power attenuated in order to avoid sample heating; power stability was (0.5%. The scattered light was collected by a Thorn-Emi 96350 photomultiplier. The data were analyzed using the cumulant technique18,19 from which it was possible to evaluate both the mutual diffusion coefficient D and the degree of polydispersity; from the mutual diffusion coefficient the hydrodynamic diameter was calculated. QELS measurements were carried out at 25 or 10 °C depending on the system. C. Physicochemical Characterization. The particles obtained were characterized by Transmission Electron Microscopy (TEM), X-ray Diffractometry (XRD), and Fourier Transform Infrared Microspectroscopy (FTIRM). The TEM apparatus was Philips EM201C; the samples for TEM measurements were prepared according to the procedure reported in the literature.11 FTIRM measurements were carried out by means of a Bio-Rad FTS-40 spectrometer coupled with an UMA-500 Bio-Rad microscope equipped with a MCT detector. FTIRM spectra were collected on a 250 × 250 µm region putting a few microliters of the mixed microemulsions on a gilded plate and allowing the evaporation of the liquid phase under nitrogen; background spectra were collected from the gilded surface. XRD spectra were obtained on dried particles using a Philips PW1050/70 apparatus with a Co KR X-ray (λ ) 1.78 Å) source.
Results and Discussion The Igepal-CO520-based microemulsions were prepared adding different amounts of water or aqueous solutions of NaOH/CaCl2 to a 0.15 M solution of the surfactant in cyclohexane. The addition was made at 20 °C referring to the triangular phase diagram along the line of the constant ratio [surfactant]/[cyclohexane]. The samples were stable up to wo ) 32.0, whereas the microemulsions containing NaOH or CaCl2 became stable, after mixing, up to wo ) 11.7. This decrease of the singlephase microemulsion region in the presence of salts has already been reported for other systems.11,20 Figure 2 shows the behavior of the hydrodynamic diameter as a function of the wo-parameter for the two microemulsions to be mixed. The trend was linear in both cases, as reported in the literature for other systems.21,22 No strong differ(18) Koppel, D. E. J. Chem. Phys. 1972, 57, 4814. (19) Brown, J. C.; Pusey, P. N.; Dietz, R. J. Chem. Phys. 1975, 3, 1136. (20) Steytler, D. C.; Jenta, T. R.; Eastoe, J.; Heenan, R. K. Langmuir 1996, 12, 1483. (21) Vasilescu, M.; Caraghergheopol, A.; Almgren, M.; Brown, W.; Alsins, J.; Johannsson, R. Langmuir 1995, 11, 2893. (22) Natrajan, U.; Handique, K.; Mehra, A. M.; Bellare, J. R.; Khilar, K. C. Langmuir 1996, 12, 2670.
Ca(OH)2 Nanoparticles from W/O Microemulsions
Langmuir, Vol. 19, No. 3, 2003 935 Table 1. Experimental Conditions for the Ca(OH)2 Nanoparticles’ Synthesis samplea
wo
[NaOH]/M
[CaCl2]/M
t/°C
A1 A2 A3 A4 A5 A6 B1 B2 B3 B4 B5 B6 B7
2.0 3.5 5.0 2.0 3.5 5.0 1.7 2.6 3.5 4.4 5.8 7.3 11.7
0.10 0.10 0.10 0.20 0.20 0.20 0.50 0.50 0.50 0.50 0.50 0.50 0.50
0.05 0.05 0.05 0.10 0.10 0.10 0.25 0.25 0.25 0.25 0.25 0.25 0.25
15 10 10 10 10 10 25 25 25 25 25 25 25
a A ) C E based-microemulsion, B ) Igepal-CO520-based 12 4 microemulsion.
Figure 3. Hydrodynamic diameter (from QELS measurements) as a function of the wo-parameter for the microemulsion system obtained by mixing the Igepal-CO520/cyclohexane/ CaCl2 0.25 M aqueous solution and Igepal-CO520/cyclohexane/ NaOH 0.5 M aqueous solution: (9) immediately after mixing; (O) 1 week after mixing.
ences were observed in passing from NaOH to CaCl2, apart from a slight increase in the average size of the microdroplets. The nanoparticles’ synthesis was performed mixing equal volumes of the microemulsions containing NaOH and CaCl2 in the aqueous pool. After 48 h, the obtained particles were collected and characterized. Figure 3 shows the behavior of the hydrodynamic diameter for the mixed microemulsions. QELS data revealed that the mixing of the two microemulsions did not affect the microdroplets’ size in the whole range of wo-parameter investigated, indicating that neither increase nor decrease of the aqueous pool where Ca(OH)2 precipitation occurs was originated. This allowed the preparation of Ca(OH)2 particles able to increase in size to the microdroplets’ dimension. The only difference compared to the “pure” microemulsions was represented by the behavior of the hydrodynamic diameter values, which deviated slightly from the linear trend for wo higher than 8.0, and by the polydispersity whose value became larger than 15% (see Figures 2 and 3). Moreover, keeping the mixed microemulsions for a week at 25 °C did not alter the values of the hydrodynamic diameter, confirming that the mixed system was extraordinarily stable. The C12E4-based microemulsions were selected following the procedure reported by Rees et al.11 working in a narrower range of the wo-parameter (2-5 against 2-25 used by these Authors11 and 1.5-12 of our Igepal-C0520based microemulsions) with two different concentrations of the reactants NaOH and CaCl2. The experimental conditions for the Ca(OH)2 nanoparticles’ synthesis are reported in Table 1. The synthesis of samples “A” was carried out keeping the mixed microemulsions at 10 °C for 48 h (except for sample A1 for which the temperature was 15 °C) by means of a thermostatic bath. After this “incubation time”, no phase separation occurred and the samples appeared isotropic and transparent. It is interesting to notice that QELS measurements on this system before and after the mixing (immediately and after a week) gave results similar to those found for the Igepal-CO520 system, although the investigated wo-range was much narrower. In particular, time did not affect the stability, as observed for Igepal-CO520. Figure 4 shows the TEM results obtained
Figure 4. TEM pictures showing the Ca(OH)2 nanoparticles obtained from the C12E4 microemulsions’ system; top, wo ) 3.5; bottom, wo ) 5.
for the samples at high wo. The particles obtained were nanospheres and their size varied in the range of few nanometers in perfect agreement with the microdroplets hydrodynamic diameter that ranged in the interval 6-8 nm.21 Analogous behavior was found for samples A1-A3. It is worth noting that the particles shown in Figure 4 are relative to some small portions of the analyzed copper grids (ca. 0.25-0.30 µm2). Thus, to rigorously assess the particle size distribution, the histograms of the particle diameter distribution was evaluated according to the usual image analysis procedure.23 Figure 5 indicates that, for both systems examined, the particle size varied in the range 2-10 nm. It is worthwhile to note that the increase of the water content (i.e., the wo-parameter) resulted both in a more enhanced polydispersity, and in an increase of the average diameter. These results are in agreement with the QELS experiments that had shown that at high wo the average size of the microdroplets was higher than the expected lineartrend value. The polydispersity determined was the highest for the set of the examined samples. It is noteworthy that our results are comparable with those reported by Rees at al.11 for their systems at low wo-values. (23) Bagwe R. P.; Khilar, K. C. Langmuir 1997, 13, 6432.
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Figure 5. Histograms of the diameter of the nanoparticles obtained from the C12E4 microemulsions’ system; top, wo ) 3.5 - total number of particles analyzed ) 163 - average diameter ) 3.8 nm; bottom, wo ) 5.0 - total number of particles analyzed ) 237 - average diameter ) 4.8 nm.
Figure 6. TEM pictures showing the Ca(OH)2 nanoparticles obtained from the Igepal-CO520 microemulsions’ system; top, wo ) 2.6; bottom, wo ) 7.3.
In particular, the shape (nanospheres)11 and the dimension (3-5 nm at wo ) 2)11 determined by these Authors are in good agreement with our results at wo ) 2 and wo ) 3.5. These results suggest that, at least at low water content, the particle shape and dimensions are strictly driven by the properties of the micro-compartmentalized system rather than by the growing particles. In the range of wo investigated, no particular effects associated with the reactant concentration were observed. Concerning the synthesis from Igepal-CO520-based microemulsions, the range of wo studied was much wider, due to the higher stability of these microemulsions at the same NaOH concentration. Figure 6 reports the nanoparticles obtained working at a very low wo-value (top) and at wo ) 7.3. For all the syntheses performed, the nanoparticles were spherical with size below 9-10 nm, in agreement with the hydrodynamic diameter determined by QELS mea-
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Figure 7. Histograms of the diameter of the nanoparticles obtained from the Igepal-CO520 microemulsions’ system; top, wo ) 2.6 - total number of particles analyzed ) 70 - average diameter ) 3.4 nm; bottom, wo ) 11.7 - total number of particles analyzed ) 193 - average diameter ) 4.9 nm.
surements (see Figures 2 and 3). The histogram reported in Figure 7 shows that the behavior of the average diameter agreed with the results found for the previously described microemulsion system, i.e., the dimension increased with increasing the wo-parameter value. On the contrary, the polydispersity of the diameter distribution exhibited an opposite behavior compared to that found for the C12E4 system: the polydispersity decreased with increasing the wo-parameter. However, it should be noticed that for the Igepal-CO520 system the range of the wo-parameter investigated was much higher and, therefore, the decrease of the polydispersity at very high wo was not perfectly comparable with that observed for the C12E4 system, where the highest value of wo was less than 1 /2 of that for Igepal-CO520 (11.7 for Igepal-CO520 against 5.0 for C12E4). The analysis of the histograms reported in Figures 5 and 7 indicated that at high wo values the size distribution became wider and, in some cases, a bimodal distribution was detected. This behavior could be explained in terms of nuclei fusion among different microdroplets, which results in a broader nanoparticles’ distribution. The fusion process is possible only at high values of wo, due to the elasticity of the interface (vide infra). An interesting feature determined for the Ca(OH)2 nanoparticles obtained with both microemulsion systems was the tendency of the nanosized crystals to form aggregates having either quasi-spherical structures (see Figure 8, C12E4 system) or quasi-spherical and threadlike morphologies (see Figure 8 bottom). A deeper analysis of the behavior of the average particle diameter as a function of the wo-parameter exhibited the trend reported in Figure 9 (we carried out this analysis only for the Igepal-CO520 system due to the wider wo range investigated). This behavior is usually explained in terms of occupancy number and nucleation and growth of the particles.23-25 At the beginning of the process (wo ) 2.6, very low occupancy number), the Poisson distribution of the reactant species in the microdroplets26,27 states that only a very small fraction of microdroplets can originate (24) Boakye, E.; Radovic, L. R.; Osseo-Asare, K. J. Colloid Interface Sci. 1994, 163, 120. (25) Nagy, J. B. Colloids Surf. 1989, 35, 201. (26) Atik, S. S.; Thomas, J. K. J. Am. Chem. Soc. 1981, 103, 3542. (27) Towey, T. F.; Khan-Lodhi, A.; Robinson, B. H. J. Chem. Soc., Faraday Trans. 1990, 86, 3757.
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Figure 10. FTIR spectrum in the microreflectance mode (spectrum A) on a thin layer of the Igepal-CO520 microemulsion 48 h after mixing and FTIR spectrum in KBr pellet (spectrum B) of the powder obtained as specified in the text. Figure 8. TEM pictures showing the aggregates of the Ca(OH)2 nanoparticles: top, C12E4 system, wo ) 2.0; bottom, Igepal-CO520 system, wo ) 4.4.
Figure 9. Average diameter of the Ca(OH)2 particles obtained from the Igepal-CO520 microemulsions’ system as a function of the wo-parameter.
Ca(OH)2 nuclei, which can then grow slowly by intermicellar collision. The result is the formation of a few particles able to grow considerably. At wo ) 4.4 (second point in the graph of Figure 9), the Poisson distribution foresees that the fraction of microdroplets that can originate Ca(OH)2 nuclei is significant. Thus, many nuclei can be formed. Due to the high consumption of reactant species during the nucleation step, the nuclei cannot grow remarkably. Since at low wo values the nuclei fusion is still strongly inhibited by the rigidity of the interface, the particles formed are smaller than for the case of wo ) 2.6. For wo-values above 5.8 (third point in the graph of Figure 9) the increase in the average diameter as a function of wo depends strictly on the possibility of nuclei fusion among different microdroplets due to the presence of a less rigid interface. At very high values of wo (for example at wo ) 1523), a decrease of the particle size is often observed and this is attributed to intramicellar nucleation;23 we did not detect this effect, probably because our largest value of wo (11.7) was not sufficiently high.
The nanoparticles were characterized by FTIRM spectra collected directly (under the microscope) on the mixed microemulsion after the incubation time. The presence of the hydroxide is confirmed by the narrow peak at 3643 cm-1 (Figure 10, spectrum A) assignable to the OH stretching in the Ca(OH)2 crystals.28 An attempt of determining the XRD patterns of the obtained nanoparticles was carried out collecting about 1 mg of the powder by preparing 10 mL of the IgepalCO520 mixed microemulsion and destabilizing it after 48 h of incubation by adding ca. 80 mL of continuous phase (cyclohexane). The latescent phase was centrifuged at 9000 rpm for 10 min and the residue was washed with cyclohexane. This procedure was repeated 4 times in order to remove completely the surfactant, and the final residue was dried in an oven at 60 °C for 1 day to obtain about 1 mg of a white powder. The XRD patterns recorded on this powder showed only the significant peaks of calcite. This result, coupled with the FTIRM findings reported in Figure 9, revealed that the manipulation required for the preparation of the sample for the XRD measurements resulted in a complete carbonatation of the Ca(OH)2 nanoparticles by atmospheric CO2. The presence of calcite was also ascertained by FTIR measurement on a KBr pellet where the sample was prepared by an analogous procedure reducing the number of both centrifugations and washings. Figure 10 shows the FTIR spectrum: the absorption at 874 cm-1 (CO32- out-of-plane bending28) is clearly evident. Many other peaks are also detected attributable to the surfactant, result of the less accurate purification. The origin of the high reactivity of Ca(OH)2 nanoparticles toward CO2 resides on the very high activity of the particles’ surface. Colloidal Ca(OH)2 particles with greater average size synthesized in previous studies12,14 had shown only partial carbonatation, even after long exposure to the atmosphere. Therefore, the high reactivity observed in this work seems to be attributable to the very small size of the microemulsion particles that, not only produces a highly active surface, but does not also allow the formation of a protective CaCO3 layer on the Ca(OH)2 particle surface. In fact, for such very small particles it is practically impossible to distinguish between surface and bulk carbonatation. (28) Farmer, V. C. The Infrared Spectra of Minerals; Mineralogical Society: London, 1974.
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Conclusions The present work showed that, despite their moderate solubility, Ca(OH)2 nanoparticles can be easily prepared in w/o microemulsions. The major problem was represented by the choice of the suitable microemulsions’ composition. We succeeded in preparing w/o microemulsions with NaOH and CaCl2 concentration in the water microdroplets up to 0.50 and 0.25 M for NaOH and CaCl2, respectively. The crucial factors for the successful preparation of these Ca(OH)2 nanoparticles were the following: (i) choice of a nonionic surfactant to prevent possible counterion interactions, (ii) selection of the microemulsion system with a large one-phase region even in the presence of high ionic strength and pH, and (iii) choice of a wo-range well below the maximum value compatible with the microemulsion stability in order to avoid wide and/or bimodal particle distribution. In the selected systems cyclohexane was the oil (dispersant) phase, while two different nonionic surfactantsstetraethylene-glycol-monododecyl ether and pentaoxyethylene-glycol-nonyl-phenyl etherswere shown to be efficient in stabilizing these w/o microemulsions characterized by a very high ionic strength in the aqueous pool. Temperature was another critical parameter that had to be kept under control in order to avoid phase separation during the synthesis. In particular, for the synthesis with C12E4 as surfactant, the temperature had to be always below 15 °C. The Igepal-CO520 microemulsions containing NaOH, CaCl2, and NaOH plus
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CaCl2 species in the water microdroplets were characterized by QELS measurements providing the average value of the hydrodynamic diameter. In the range of the wo-parameter investigated, these microemulsions exhibited a linear trend of the hydrodynamic diameter versus wo. TEM data showed that the Ca(OH)2 nanoparticles synthesized by w/o microemulsions possessed size in the range 2-10 nm for both microemulsion systems. Interestingly, the behavior of the particle average size as a function of the wo-parameter matched that found for the synthesis of more insoluble inorganic compounds. The characterization of the particles by FTIR microspectroscopy allowed both the detection of the typical spectral feature of Ca(OH)2, and the establishment of the very high reactivity of these particles toward atmospheric CO2. XRD patterns confirmed the presence of CaCO3 ultrafine particles. Acknowledgment. The authors express their gratitude to Prof. Piero Baglioni for invaluable comments and discussions, to Dr. Giulia Capuzzi and Dr. Moira Ambrosi for the useful suggestions, and to Mr. Patrizio Nuti for the assistance during TEM measurements. Consorzio Interuniversitario per lo Sviluppo dei Sistemi a Grande Interfase, CSGI, is gratefully acknowledged for financial support. LA026428O
