Improved Synthesis of Nanosized Silica in Water-in-Oil Microemulsions

Present contribution describes modified Stöber synthesis of silica nanoparticles in oil-in-water microemulsion, formulated using heptane, 2-ethylhexanol, Tween 85 nonionic surfactant, and tetraethyl orthosilicate (TEOS). After some specified incubation time, ammonium hydroxide was added and the reaction mixture was stirred for 24 hours at room temperature. Prior to synthesis, pseudoternary diagram was created for oil-rich area and Winsor IV region was identified. These microemulsions were used for synthesis of silica particles. Resulting particles were characterized by dynamic light scattering, electrokinetic measurements, specific surface area measurements, and powder diffraction. Particles’ diameter was ranging between ca. 130 and 500 nm; usually monodisperse distribution was obtained. The specific surface area of nanoparticles was ranging between 250 and 300m/g. Notably, productivity per unit volume of solution was 3 to 5 times higher than for previously reported procedures. Our method can be extended, because polymeric materials can be added to dispersed aqueous phase. In our studies, β-cyclodextrin and hydroxyethylcellulose have been used, giving particles between 170 and 422 nm, with the surface area larger than 300m/g.


Introduction
Colloidal silica is an inorganic material of high industrial and scientific importance [1].Its structure consists of dense, amorphous particles of SiO 2 ; the building blocks of these particles are randomly distributed [SiO 4 ] tetrahedrons (Figure 1).The surface of the material is coated with silanol Si-OH groups; their density depends strongly on the manufacturing process.Generally, one can distinguish five main methods to synthesize colloidal silica [2]: (a) Ion exchange, using sodium or potassium silicate [3,4].(b) Neutralization of aqueous alkali silicates or hydrolysis of alkoxysilanes, followed by condensation [5,6].(c) Peptization of silica gel [7].(d) Direct oxidation of silicon metal [8,9].(e) Pyrogenic method, by burning of silicon tetrachloride in hydrogen/oxygen flame [10,11].
An annual production of silica by the above methods was estimated as 1.2 million metric tons [12] (data from 1990).
More recent data for Western European countries show the production was 392.1 thousand tons, while the sales in 2000 were 367.95 thousand tons [13].Approximately 80% of the above amounts are produced by method (e), and ca.10% are produced by method (b).Stöber method is the most important example of the hydrolysis-condensation (sol-gel) methods, widely employed to obtained silica particles of different size [5].Typical reaction mixture includes tri-or tetralkoxysilane, alcohol, usually ethanol, water, and acid or base, the latter being usually aqueous ammonium hydroxide.For tetraethoxysilane, the reaction can be written as [14] Si where  is usually equal to 4. Originally, micrometer-sized particles have been obtained, but nanosized ones can be synthesized using microheterogeneous medium; microemulsions seem to be particularly useful for such purpose [14,15].According to Danielsson and Lindman [16], the microemulsion is a system of water, oil, and an amphiphile which is a single optically isotropic and thermodynamically stable liquid solution.Generally, microemulsions form spontaneously upon mixing of amphiphile (surfactant), aliphatic alcohol (cosurfactant), hydrocarbon (oil), and water.One can consider microemulsions as a sort of emulsions, that is, droplet type dispersions either of oil-in-water (O/W) or of waterin-oil (W/O), with a size ranging between 10 and 100 nm in drop diameter, albeit there are significant differences between microemulsions and ordinary emulsions (which can be then called macroemulsions).Contrary to macroemulsions, which are kinetically stable, but thermodynamically unstable, microemulsions are thermodynamically stable and thus do not require high-energy processes to be formed [17].
A well-known classification of microemulsions is that of Winsor [18], who identified four types of phase equilibria: Winsor I.The surfactant is mainly in an aqueous phase and oil-in-water (O/W) microemulsions form.The surfactant-rich water phase coexists with the oil phase where surfactant is only present as monomers at small concentration.
Winsor II.The surfactant is mainly in the oil phase and water-in-oil (W/O) microemulsions form.The surfactant-rich oil phase coexists with the surfactantpoor aqueous phase.
Winsor III.It is a three-phase system where a surfactant-rich middle-phase coexists with both excess water and oil surfactant-poor phases.
Winsor IV.It is a single-phase (isotropic) micellar solution that forms upon addition of a sufficient quantity of surfactant and alcohol.
Although synthesis of silica nanoparticles in water-in-oil microemulsions is well-known process [15,34], only one type of microemulsions is usually used to carry out the process.Standard composition consists of cyclohexane, alkylphenol ethoxylate, water, and tetraethoxysilane (TEOS), and the sol-gel process is induced by the addition of an aqueous ammonium hydroxide; in many cases, medium-chain alkyl alcohol (pentanol or hexanol) was added as cosurfactant [35][36][37][38][39][40].Only Aubert et al. mention use of heptane with Brij530 (polyoxyethylene(4) lauryl ether), to form core-shell silica particles [14].There is also known example of using ionic surfactant, sodium salt of bis(2-ethylhexyl) sulfosuccinate (AOT) [41], as well as hexadecyltrimethylammonium bromide (CTAB) or sodium dodecyl sulfate (SDS) [42].Salabat et al. used dodecane and heptane/dodecane mixtures with AOT to tune stability of silica particles but did not carry out the synthesis, using commercial nanoparticles [43].
During the Stöber process in microemulsion, partial expulsion of water from the microemulsion phase may occur, resulting in formation of second phase of bulk water which, causing bimodal size distribution [44].Detailed research on the kinetics of the process has been presented by Osseo-Asare and Arriagada [38,45].
Present contribution describes modification of standard synthetic process.Oil-in-water microemulsion was formed using heptane, 2-ethylhexanol, Tween 85 (polyoxyethylene sorbitan trioleate), and TEOS, followed by delayed addition of aqueous ammonium hydroxide.This specific system has never been studied by any researcher, regarding the synthesis of nanoparticles.In order to find optimal conditions for the synthesis, pseudoternary diagrams were investigated for oil-rich area and Winsor IV region was identified.These microemulsions were used for synthesis of silica particles.Resulting SiO 2 particles were characterized by dynamic light scattering, electrokinetic measurements, and specific surface area measurements (see Scheme 1).
Our method can be extended, because polymeric materials can be added to dispersed aqueous phase.In our studies, addition of hydroxyethylcellulose (HEC) and -cyclodextrin (-CD) has been examined.

Identification of Isotropic Region in Phase Diagrams.
Nonionic surfactant was weighted into 30 mL glass vial; then an appropriate amount of n-heptane and 2-ethylhexanol was added, to obtain total volume between 10 and 20 cm 3 ; due to the limitations of measuring instrument the volume cannot exceed 24.5 cm 3 .The mixture was vortexed to dissolve surfactant; subsequently, weighted amount of water was added and the dispersion was shaken vigorously for 180 s and then  analyzed using Turbiscan LAb Expert instrument (Formulaction), operating in both transmission and backscattering mode.If the sample remained transparent, another portion of water was added, and the procedure was repeated until permanent turbidity has been achieved.Next portion of surfactant was then added to create isotropic solution, and then the titration with water was repeated.Results were plotted using ProSim Ternary Diagram software (ProSim).

Synthesis of
Nanoparticles.An appropriate amount nonionic surfactant has been weighted into 50 cm 3 polypropylene tube, followed by heptane, 2-ethylhexanol, and tetraethyl orthosilicate.Resulting mixture was shaken for 60 s with vortex, then water or 2% aqueous solution of HEC or -CD was added, and the mixture was vortexed for 15 s, followed by vigorous magnetic stirring (800 rpm).25% aqueous ammonium hydroxide was added after 15 minutes, tube was closed tightly, and the content was stirred magnetically (800 rpm) for 24 hours.Samples were diluted with ethanol (40 cm 3 ) and precipitated nanoparticles were centrifuged at 4500 rpm, then washed with acetone (40 cm 3 ), centrifuged again, and dried at 1 Torr/50 ∘ C for 6 h.The summary of experiments is given in Table 1.Three series were run, the first one with water (1W-5W), the second one with hydroxyethylcellulose (1H-5H), and the third one with -cyclodextrin (1C-5C).

Characterization of Nanoparticles.
Particle size measurements have been performed using two dynamic light scattering instruments: Nicomp 380ZLS (Particle Sizing Systems) and Photocor Complex (Photocor Instruments).The former apparatus was equipped with 532 nm/50 mW laser, and the measurements were carried out in 12.5 × 12.5 × 45 mm square polystyrene cuvettes, while the latter one was equipped with 646 nm/28 mW laser and measurements were carried out in  14.8 mm round cells submerged in a toluene as an indexmatching liquid.Refractive index of heptane-2-ethylhexanol mixture was determined for sodium  line using D5000 refractometer (Atago); density was measured with DMA4000 instrument (Anton Paar).Electrokinetic potentials have been measured by means of electrophoretic light scattering, using Zetasizer 2000 apparatus (Malvern Instruments).Immediately before the measurement, dispersions of nanoparticles were diluted with pH 7.45 isotonic phosphate buffered saline (PBS).The buffer was prepared according to Sorensen, using analytical grade NaH 2 PO 4 , Na 2 HPO 4 , NaCl, and ASTM Type I deionized water and then filtered through 0.45 m membrane.Measurements were carried out at 25.0 ± 0.1 ∘ C, using rapid mode.The value of zeta potential reported for each sample corresponds to the average of 5 measurements.
The powder diffraction measurements have been taken with D8 Advance (Bruker) X-ray powder diffractometer with CuK radiation ( = 1.54183Å).XRD data were collected over a 2 range of 10-40 ∘ with a step width of 0.01 ∘ and a counting time of 0.2 s/step.
The surface area was measured at 77 K by Brunauer-Emmett-Teller (BET) method [46] for helium/nitrogen mixture [47], using single-point FlowSorbII apparatus (Micromeritics).Prior to the measurement, samples were washed with acetone; precipitated nanoparticles were centrifuged, and the supernatant was discarded.This operation was repeated four times, and then the nanoparticles were dried in vacuo (1 Torr/50 ∘ C) for 24 hours.The mass of sample used was 1.00 g for each experiment.All experiments were run in triplicate.

Choice of Microemulsion System.
We have decided to examine completely new microemulsion, composed of nheptane, water, polyoxyethylene sorbitan trioleate (Tween 85), and 2-ethylhexanol.According to our unpublished research, heptane-based emulsions are much more stable and of better solubilization capacity.Polyoxyethylene sorbitan trioleate seems to be better choice than Triton5 X-100 surfactant, because of its lower HLB [48,49]; the literature value is 11 for Tween 85, and 13.5 for Triton X-100.Lower HLB value seems beneficial when water-in-oil system is formed, albeit it requires higher amount of surface-active agent, comparing to higher HLB compound, to obtain dispersion of the same total HLB.We believe that larger amount of surfactant may increase solubilization, making the system more capable of dispersing aqueous phase and stabilizing synthesized particles.Another factor is an environmental safety.Alkylphenol ethoxylates are consider to be harmful for environment [50]; moreover, they exhibit estrogenic activity [51,52].Contrary to this, sorbitan esters are considered to be much more environmentally friendly.
We have decided to use of 2-ethylhexanol as a cosurfactant [53], rather than n-pentanol and n-hexanol, commonly employed in the microemulsion synthesis of silica nanoparticles.This aliphatic chain of this alcohol is longer than the alcohols mentioned above and branched; we expected it to promote formation of water-in-oil microemulsions.

Identification of Isotropic Regions of Pseudoternary Diagrams.
Determination of isotropic regions is important to avoid Winsor I-III systems.As mentioned in Introduction, synthesis should occur in Winsor IV system.This assumption led us to determination of oil-rich isotropic systems, composed of water, nonionic surfactant Tween 85, and cosurfactant 2-ethylhexanol.Measurements were carried out using combined Multiple Light Scattering (MLS) system.MLS consists of near-infrared laser (880 nm), illuminating the sample.The photons, after being scattered many times by the droplets in the dispersion, emerge from the sample and are detected by the 2 detectors: transmission () for nonopaque samples (incident angle 0 ∘ ) and backscattering (BS) for opaque ones (incident angle 135 ∘ ).Analysis of both  and BS is carried out as a function of the position of the laser (0-55 mm, with the step of 0.040 mm), and the analysis time.Normally, transmission should be used, when at least 1-2% of the illuminating light passes through the sample; in other cases, backscattering is employed.These experiments give insight into many properties of the colloidal system, including stability of the particulate system, mean size, and concentration (volume fraction) of the dispersed phase.In our case, we have determined regions of high transparency, and the borderline between Winsor IV and Winsor II.Our research was not aimed at giving detailed insight into the ternary diagram, just to determine some useful conditions to carry out sol-gel synthesis.
Typical picture for the analysis of the pseudoternary diagram is shown in Figure 2. Transparent Winsor IV region has been defined as the one with at least 90% transmission, that is, 0, 0.1, and 0.2 g of water; 0.3 g of water gives decrease transparency to 50%, while 0.4 and 0.5 g results in complete turbidity.Typical destabilization pattern is shown in Figure 3, for the sample containing 10 g of heptane-2-ethylhexanol mixture (75 : 25, w/w) and 0.5 g of water; the total volume was ca.13.1 cm 3 .An aqueous phase starts to separate almost immediately, to form finally 4 mm thick layer (1.8 cm 3 ); this gives volume fraction of aqueous phase close to 0.135.The pseudoternary diagrams are shown in Figures 4-7.
For the sake of clarity, only parts of diagrams have been shown, for the mass fraction of Tween 85 below 0.35.In every case top corner representing pure Tween 85 is omitted.2-Ethylhexanol is abbreviated as 2EH.
One can see that, in the absence of 2-ethylhexanol, isotropic region is very narrow (Figure 4).Such system is not real microemulsion but rather contains swollen micelles.In the case of heptane to 2-etylhexanol mass ratio between 8 : 2 and 7.5 : 2.5 (w/w), relatively wide isotropic area is observed (Figures 5 and 6, resp.).Increasing amount of 2-ethylhexanol, isotropic region becomes narrow again (Figure 7), thus unusable for the synthesis of nanoparticles.

Synthesis and Analysis of Nanoparticles.
Nanoparticles were synthesized similarly to literature procedure [38].Delayed addition of ammonium hydroxide has been done according to the results of Abarkan and coworkers [40].The work states that using this procedure, silicon species are not present in the oil phase, because the amphiphilic character of the prehydrolyzed TEOS monomers introduced in the system drives them into the droplets of dispersed phase.In our case, 15-minute delay between the addition of TEOS and ammonium hydroxide has been applied.The stirring rate between 800 and 1200 rpm was tested, but with no significant    1, w/w)/Tween 85 system.Green points: isotropic systems, red points: two-phase system, and yellow points: turbid solution ( < 10%), but aqueous phase did not separate after 1 h.effect on the size of particles; thus we set the stirring at 800 rpm.The purification method has been taken from [40].
The yield of particles are given in Table 2.It was calculated basing on the equation given in Introduction, assuming all C 2 H 5 O-groups were replaced by -OH.The yields were moderate-to-good, with an average 77%, standard deviation of 5.3%.Some losses were probably caused by too low centrifugation speed, resulting in the incomplete sedimentation of the nanoparticles.All particles have been examined by dynamic light scattering.Typical DLS pattern is shown in Figure 8 and summary of all results is presented in Table 3.For water and HEC series, monomodal distribution is usually achieved, while in the presence of -CD, bimodal distributions are always achieved.Moreover, the presence of -CD affects mean size of the particles.In the case of water and HEC solution, the sizes ranged between approximately 130 and 430 nm, while for -CD samples the mean size was 205-294 nm.
Minor component is always of smaller diameter, as shown in Figure 8. Calculations indicate that the mas or volume contribution of this minor component ranges between 5 and 11%.The diameter of smaller particles corresponds roughly to the size microemulsion droplets, as measured by dynamic light scattering (Figure 9).One can conclude that primary silica particles are of the same size as microemulsion droplets, but some coalescence occurs, resulting in the formation of much bigger particles.This observation is consistent with the mechanism proposed by Finnie and coworkers [39].They  reasoned that in basic solution particle growth occurs via coalescence growth of colliding droplets (Figure 10), resulting in formation of spherical and dense particles by the ripening of potential aggregates formed during the collision of dropletscontaining nuclei.According to this paper, such mechanism gives the particles with strong mesoporous contribution and the specific area of particles below 100 m 2 /g.The same effect was observed by Abarkan and coworkers [40].Our research shows that the particles' surface area is much higher (Table 4), which indicates the presence of micropores rather than mesopores.This effect is quite unusual and should be attributed to the microheterogeneous system used.On the other hand, the specific surface area is smaller than in the case of particles formed at pH 1.05, when it reaches 510 m 2 /g [39]; thus one can conclude that micropores are probably larger than 1 nm.It is noteworthy that the presence of hydroxyethylcellulose results in the substantial growth of specific surface area, between 50 and 120 m 2 /g, compared to the samples obtained with water.Powder diffraction experiments revealed all particles were completely amorphous.An exemplary result is shown in Figure 11.Broad peak, centered at ca. 24 ∘ , indicates absence of any crystalline forms.Electrokinetic measurements did not show any difference between samples.All samples were measured at pH 7.45 in highly concentrated isotonic buffer; the results are given in Table 5.As one can observe, no difference between samples can be observed.Relatively low negative values are due to the high ionic strength of the continuous phase [54].

Conclusions
Silica nanoparticles have been synthesized in novel water-inoil microemulsion composed of aqueous phase/n-heptane/ polyoxyethylene sorbitan trioleate/2-ethylhexanol, where    aqueous phase was water or 2% aqueous hydroxyethylcellulose or 2% aqueous -cyclodextrin.Oil-rich Winsor IV region has been identified for several n-heptane/2ethylhexanol ratios and then used to synthesize nanoparticles; the diameter was 131.5-497.4,146.9-421.7,and 205.1-294.2nm for water, 2% aqueous hydroxyethylcellulose, and 2% aqueous -cyclodextrin, respectively.In some cases smaller fractions were observed; their size corresponded to the diameter of microemulsion particles.This supports observations of Finnie et al., explaining mechanism of formation of silica nanoparticles in basic solution.Surface area of the particles was much higher than expected for silicas with predominant mesopores; thus we concluded that micropores predominate.Addition of hydroxyethylcellulose resulted in significant increase of surface area, compared to the samples made in the presence of water or -cyclodextrin.

Figure 10 :
Figure10: Mechanism for particle formation under acidic and basic conditions.Reprinted with permission from[39].Copyright 2015, American Chemical Society.

Table 1 :
Summary of experimental conditions for the synthesis of silica nanoparticles.
a Pure water for W series, 2% aqueous hydroxyethylcellulose for H series, and 2% aqueous -cyclodextrin for C series.

Table 3 :
Particle size diameter of synthesized silicas a,b .
a Data for the most populated peak, values in nanometers.b Diameter ± standard deviation.c Monomodal distribution.

Table 4 :
Specific surface area of synthesized silicas a .
a Value ± standard deviation.

Table 5 :
Electrokinetic potentials of silica nanoparticles at pH 7.45 a .
a Value ± standard deviation.