Synthesis of Iron Nanoparticles by Thermal Decomposition of Diironnonacarbonyl in Ionic Liquid and Their Potential Use as Nanotracers for Mixer Studies in Liquids Feeds

Iron nanoparticles with dynamic light scattering median diameter around 10 nm have been prepared by thermal decomposition under a nitrogen atmosphere from diironnonacarbonyl (DINC) dissolved in n-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF). The effect of temperature changes in the range of 170 ̊C 200 ̊C and changes in concentration of DINC in BMIMBF in the range of 0.1% 0.9% on the properties of obtained iron nanoparticles has been investigated. The stable dispersion of iron nanoparticle in ethanol has been prepared after separation of nanoparticles from ionic liquid by centrifugation following by their re-dispersion in ethanol. The possibility of quantitative analysis of iron content in ethanol dispersion by deposition of ferromagnetic nanoparticles on the surface of plastic-protected neodymium magnet, dissolution of iron in hydrochloric acid and addition of ammonium thiocyanate solution following by spectrophotometric determination of iron cations at wavelength of 490 nm has been investigated. The feasibility of using the same approach in case of addition of ethanol dispersion of iron nanoparticles to the liquid animal feeds for evaluation of efficiency of their mixing has been discussed.


Introduction
In recent years, the ionic liquids (ILs) have been extensively evaluated as environmental-friendly or "green" alternatives to conventional organic solvents for synthesis of nanoparticles [1]. Generally speaking, ILs refer to a specific class of molten salts which are liquids at temperatures of 100˚C or below [2] [3] [4]. ILs have very low vapor pressure and generate virtually no hazardous vapors.
Moreover, ILs are composed of charged species, which provide a highly polar medium useful in various applications, such as extraction, separation, catalysis and chemical synthesis medium [5] [6]. The large number of ILs, containing 1-alkyl-3-methyl-imidazolium cation, where alkyl group is butyl, hexyl or nonyl, and anions, such as chloride, bromide, tetrafluoroborate and hexafluorophosphate, have been used for dissolving cellulose [7].
Previously iron nanoparticles have been prepared by thermal decomposition under a nitrogen atmosphere from diironnonacarbonyl (DINC) dissolved in n-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF) [8]. DINC has an almost nonexistent vapor pressure at room temperature. Nonetheless, Fe 2 (CO) 9 has been successfully used to dope carbon nanotubes with iron and to construct iron nanowires used for magneto-resistance and spintronic devices [9] [10]. As reported in these sources, the average atomic percentages of iron in these deposits range from about 50% -80% with Pfeiffer et al. showing a purity as high as 95% [11] [12].
In this study the iron nanoparticles with dynamic light scattering median diameter around 10 -11 nm were synthesized under similar conditions ( Figure 1).
The authors followed several narratives in this investigation: 1) influence of the temperature conditions on the size and properties of formed nanoparticles; 2) influence of concentration of DINC in BMIMBF on those parameters; 3) possibility to create a stable dispersion of iron nanoparticles in the liquid animal feeds; 4) feasibility of quantitative analysis of iron nanoparticles magnetically retrieved from the liquid feeds and its applicability for evaluation of the quality of mixing.

Synthesis and Redispersion of Nanoparticles
Diironnonacarbonyl, Fe 2 (CO) 9 , DINC, (Cat # 225460, 98% purity) and BMIMBF (Cat # IL-0012-HP, H 2 O content, 100 ppm; Cl− content, 50 ppm) were purchased from Sigma-Aldrich and IoLiTec, respectively. All manipulations were done using Schlenk techniques under dry nitrogen. The BMIMBF was dried under high vacuum (10 −3 mbar) for several days. Thermal decomposition was carried out under nitrogen or air in a vessel which was connected to an oil bubbler by method described in publication [8]. In a typical experiment the certain amount of Fe 2 (CO) 9 (from 6 to 54 mg) was dissolved at room temperature in 6.0 g of the ionic liquid to give a 0.1% -0.9% solution. The solution was slowly heated to 170˚C -200˚C for over a period of 12 h under stirring. After the mixture was cooled to room temperature under nitrogen, an aliquot of the ionic liquid was collected for microscopic and dynamic light scattering measurements.
Iron nanoparticles were separated by centrifugation (15 min at 2500 rpm under nitrogen) from the BMIMBF, and the ionic liquid can be reused. Two different procedures were used for adding nanoparticles to liquid feeds. According to the first procedure, the collected nanoparticles were redispersed in ethanol at room temperature (about 50 mg of Fe nanoparticles per 10 ml of ethanol) and added to 200 g of molasses-based liquid feeds, following by evaporation of ethanol in vacuum at temperature 40˚C -50˚C. According to the second procedure, the nanoparticles were redispersed in oleic acid at temperature 35˚C (about 40 mg of Fe nanoparticles per 10 ml (9.0 g) of oleic acid (Sigma-Aldrich, Cat. #364525, 90% purity). Prepared dispersion was added to 200 g of molasses-based liquid feeds containing ~4.9 g 28% aqueous solution of ammonium hydroxide which was sufficient for complete conversion of oleic acid in ammonium oleate.

Characterization of Nanoparticles
UV-VIS absorbance spectra were measured on Cary 60 Agilent spectrophotometer. Transmission electron microscopy (TEM) photographs were taken using a Zeiss LEO 912 transmission electron microscope. Dynamic light scattering measurements were performed using Zetasizer Nano-ZS from Malvern Panalytical.
Four-step procedure has been used in the spectrophotometric method for evaluating stability of iron nanoparticles' dispersion in molasses-based liquid feed: 1) Retrieving iron nanoparticles from prepared suspension with a plastic coated Neodimium magnet; 2) Dissolving nanoparticles in 20% aqueous solution of HCl; 3) Adjusting pH of prepared solution to 5.0 (by adding 28% solution of NH 4 OH) and mixing it with 1% aqueous ammonium thiocyanate solution; 4) Dilution of prepared solution with a mixture of 0.1 M aqueous HCl and ac-etone (1:1, v/v) and determination of optical density of formed iron thiocyanate solution at 490 nm.

Results and Discussion
According to data presented in paper [8], the nanoparticles of iron can be prepared from Fe 2 (CO) 9 only in case of thermal decomposition under atmosphere of dry inert gas (argon or nitrogen). Performance of this reaction under air leads to almost complete oxidation of formed nanoparticles of Fe into Fe 2 O 3. Table 1 shows the size of nanoparticles (Fe or Fe 2 O 3 in case of thermal decomposition under nitrogen or under air, respectively) for two concentrations of Fe 2 (CO) 9 in ionic liquid. It should be noted that data presented in Table 1 show that the size of iron nanoparticles prepared in this study is slightly larger than the size of same nanoparticle synthesized by decomposition Fe 2 (CO) 9 in BMIMBF reported in paper [8] (10.1 -10.7 nm). The same conclusion can be made regarding the size of Fe 2 O 3 nanoparticles: 6.9 -7.5 nm in current study) and 6.2 -6.4 nm reported in paper [8]. Our results showed the absence of significant influence of the starting concentration of Fe 2 (CO) 9 in ionic liquid on the size of formed nanoparticles in case of thermal decomposition in nitrogen atmosphere. In case of thermal decomposition under air atmosphere the size of formed iron oxide nanoparticles is slightly larger at higher starting concentration of Fe 2 (CO) 9 (7.5 nm vs 6.9 nm). Figure 2 shows TEM photograph of Fe nanoparticles prepared from Fe 2 (CO) 9 .
Black iron nanoparticles are magnetic and are able to agglomerate as a result of their superparamagnetic properties [13].  Figure 3 shows the series of absorbance spectra of various ferric chloride solutions mixed with 0.1 M aqueous hydrochloric acid and acetone after addition of excess amount of 1% aqueous ammonium thiocyanate solution with a range of iron concentration from 1.20 ppm to 2.0 ppm. The value of absorbance at wavelengths of 490 nm has been used for creation of calibration graph ( Figure  4) which has a linear character in the chosen range of iron concentration.  Table 1).

Application of Ferromagnetic Iron Nanoparticles
Potential applications of prepared stable suspension of ferromagnetic iron nanoparticles are related to validating mixing of liquid feeds, as well as to coding liquid additives, such as enzymes, and evaluation of their distribution in premixes and final feeds. Comparison between two different procedures used for adding nanoparticles to liquid feeds (see Experimental Part) shows that redispersing them in ethanol before addition to molasses based feeds, following by evaporation of ethanol in vacuum at elevated temperature leads to relatively unstable suspension. Particularly after one hour of storage at room temperature a majority of added iron nanoparticles forms relatively large aggregates which are not equally suspended in the volume of liquid feed. ln contrary, redispersing nanoparticles in oleic acid following by addition of prepared suspension to molasses based liquid feeds containing a certain amount of ammonium hydroxide leads to formation of stable suspension which doesn't show any sign of precipitation after storage for several days at room temperature.
In our previous studies [16] [17] [18] we demonstrated that ammonium oleate (which is product of interaction oleic acid in ammonium hydroxide in the current study) is very efficient surfactant for forming stable suspension of magnetic iron oxide nanoparticles.
Here we report the use of iron nanoparticles as components of magnetically retrievable liquid marker (tracer) have been tested in laboratory trials. The laboratory trial with an addition of liquid tracer to molasses based liquid feed followed by retrieving the ferromagnetic nanoparticles and analyzing the content of Advances in Chemical Engineering and Science iron in them using the calibration graph ( Figure 3) showed approximately 82% tracer recovers. Table 2 shows the results of one of several laboratory trials where an addition of 40 mg iron nanoparticles has been used for evaluation of mixing quality for several liquid ingredients in 200 g of molasses based liquid feed mix. The data presented in Table 2 were interpreted using Poisson and Chi squared statistics [19]. Treating a series of counts (absorbance values) as a Poisson distribution, the mix is judged "complete" in case if the counts would occur by chance in more that 5 of 100 studies. If the counts would occur by chance from a "perfect" mix in fewer than 5 of 100 studies, the mix is judged "probably incomplete". If the counts would occur by chance from a "perfect" mix in fewer than one of 100 studies, the mix is judged incomplete.

Conclusions
1) Method for making iron nanoparticles by thermal decomposition of Fe 2 (CO) 9 has been developed and their stability in several liquid systems has been evaluated.
2) Possibility of using prepared stable suspension of ferromagnetic nanoparticles for validating mixing of liquid feeds has been demonstrated.
3) Further study will include the evaluation of feasibility to use ferromagnetic nanoparticles for validating mixing of liquid feeds containing some amount of solid mineral additives such as zinc complexes of amino acids traditionally added to liquid animal feed.