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At this work, the magnetite nanoparticles were synthesized by coprecipitation of Fe (II) and Fe (III) ions in presence of ammonia. The average size and size distribution of particles was determined by particle analyzer instrument using Dynamic Light Scattering method. The average diameter was determined to be 26.5 nm. In TEM images, nanoparticles were spherical and size distribution was narrow. The specific surface area of nanoparticles was determined to be 44.36. The surface charge density, active site density and surface potential measured by titration method.
Keywords: Magnetite Nanoparticles, Hydroxylic Groups
Recently, Nanotechnology, is one of the most important advancements in science and technology of the past decade, is associated to the manipulation of materials and systems at the nanometer scale (typically less than 100 nm.) .
The most important groups of nanomaterials, are nanoparticles. nanoparticles's small size gives them a high specific surface area for novel application. nanoparticles are used in various domains. This is, to some extent, due to their novel properties of them, which differ from both the isolated atoms and the bulk phase [2-5].
Among various nanoparticles, magnetic nanoparticles such as magnetite, maghemite, due to their magnetic properties, have much attracted for their unique properties and potential applications in various fields, especially in biomedicine and bioengineering for example magnetically assisted drug delivery , cell isolation , MRI contrast agents [4-5], catalyst [7-9], Disease diagnosis and remediation [10-12], heavy metal removal [13-21], bio macromolecule purification[22,23], sensor [24,25] etc.
Many methods have been reported for the synthesis of the magnetic magnetite nanoparticles, such as reduction of hematite (Fe2O3) by CO or H2, coprecipitation of ferrous/ferric by ammonia solution, oxidation of the ferrous hydroxide gels using KNO3, X-ray radiation, microwave plasma synthesis, micro emulsion etc [26-28]. The common way of magnetite synthesis is the alkaline hydrolysis of iron (II) and iron (III) salts[29-32]. The size of produced particles depends on the relative over saturation, pH, temperature and ionic strength of synthesis medium . Spontaneous processes known as ageing, take place in time, in any colloidal dispersion which increase the size of nanoparticles .
Increase in the primary size due to the size dependent solubility of solid particles, the aggregation, i.e., the adhesion of colloidal particles because of Van der Waals and electrostatic attraction, magnetic attraction in the case of magnetite and other magnetic nanoparticles, chemical transformation of nanoparticles (the surface Fe (II) cations of Fe3O4 reacts with the adsorbed oxygen to form a rim of maghemite Fe2O3) are the examples of eaging in nanoparticles suspension .
So the usage of fresh nanoparticles dispersion is recommended.
Analytical grade solutions of Na2SO4, FeCl2.4H2O, FeCl3.6H2O, NH3, HCl, NaOH were used. All chemicals used were of analytical grade and used without further purification. High purity water with a resistivity of 18 Mâ„¦cm was used throughout all the experiments (Milli-Q, Millipore).
2.2. Preparation of Magnetite Nanoparticles
Solutions of 1M iron (III) chloride hexahydrate (FeCl3.6H2O) and 2M iron (II) chloride tetra hydrate (FeCl2.4H2O) were prepared by dissolving FeCl3.6H2O (10.8120 g, 0.04 mol) in 40mL H2O and FeCl2.4H2O (3.9762 g, 0.02 mol) in 10mL 2 M HCl, respectively. The two solutions were then mixed together prior to their addition to 500mL of 0.7M aqueous ammonia solution with continuous 'mechanical' stirring. After stirring for 30 min, the precipitate was washed twice with water (1 L), by magnetic Sedimentation and was added to 500 ml deoxygenated water .
2.3. Characterization of Magnetite Nanoparticles
The iron oxide nanoparticles were imaged with a transmission electron microscopy (TEM, CEM 902A; Zeiss), at an accelerating voltage of 80 kV. The average size and size distribution of nanoparticles was determined by Dynamic light scattering (DLS) method in Malvern particle analyzer Instrument. The specific surface area of the products was determined from the nitrogen adsorption isotherm using the BET method with a Gemini 2370 instrument (Micromeritics). The determination of the pHpzc (pH of Point of Zero Charge) of the samples was carried out as Saifuddin et al described : 50 cm3 of 0.01 M Na2So4 solution was placed in a closed Erlenmeyer flask. The pH was adjusted to a value between 2 and 12 by adding HCl 0.1 M or NaOH 0.1 M solutions (pHinitial.). Then, 2 g of magnetite nanoparticles was added and the final pH (pHfinal.) measured after 48 hrs under 200 rpm agitation at room temperature. The pHpzc is the point where the curve pHfinal vs. pHinitial crosses the line pHinitial. = pHinitial. The pHpzc is the pH value of solution which the charge density of magnetite nanopaticles surface is zero .
3. Result and Discussion
3.1. Average Size and Size Distribution
The particle size of the oxides used in this work was determined by using DLS and TEM techniques. The determined values were in the range of 10-50 nm. Fig. 1 and Fig. 2 show the particle size distribution and TEM image of the colloidal particles respectively. Jing Hu et al , Mayo et al , Mak et al , Uheida et al , have synthesized the magnetite nanopaticles in 10-20 nm by coprecipitatiojn method.
Fig.1. The TEM micrographs of magnetite synthesized by coprecipitation method.
Fig.2. The size distribution of magnetite nanoparticles, synthesized by coprecipitation method.
3.2. Surface Active Site
The specific surface area of the magnetite nanoparticles was determined to be 44.36m2g-1 by BET method.
The functional hydroxyl groups,, surround magnetite nanoparticles surface. These functional groups are amphoteric, protonated at pH below the pHPZC, and deprotonated above the pHPZC as follow [36-39]:
The alternative form of eq.1 is
The symbol refers to magnetite surface.
The titration curve is shown in fig. 3.The Point of Zero Charge of magnetite in 0.01M Na2So4 solutions by titration method was found to occur at pH 6.45.
The pHPZC value magnetite, have determinated 3.8-9 by other researchers , but the the most values are near the neutral pH .
and could be calculated form and balance before and after titration.
Fig.3. The magnetite nanoparticlrs titration curve. pHfinal vs. pHinitial
As mentioned above, in acidic condition, the functional hydroxyl groups,, react with solution to produce according to Eq.1, so the acidic hydroxylic group concentration could be calculated from proton concentration differences before and after titration as follows 
In basic condition, the functional hydroxyl groups,, react with solution to produce according Eq.3, so the basic hydroxylic group concentration could be calculated from differences before and after titration as follows
Due to amphoteric properties of magnetite functional hydroxyl groups, the total concentration of available surface sites (Nn) can calculated as average of acidic and basic hydroxyl groups as follows:
By using the specific surface and concentration of magnetite, the surface density of hydroxylic group is 
In this way, Nm for magnetite calculated to be 1.14-10-5 molL-1. The maximum loading capacity of magnetite can be calculated theoretically according to the number of Fe sites based on the unit cell on the surface as follows ,
Where nFe is the number of Fe ions per unit cell on one side, a, is the lattice parameter for the unit cell of the oxide and Na is Avogadro's number. The maximum surface loading capacity of magnetite was calculated to be 1.7-10−5 molm-2 respectively.
The experimental and theoretical values are in same order and approximately close. The experimental value lightly is less than theoretical value, implies a few percent of magnetite surface hydroxylic groups has not toke part in titration. This may attributed to corruption of them or competitive adsorption of other ions.
3.3. Surface Charge Density
According to the IUPAC recommendation, H+ and OH- ions are considered to be the potential-determining ions. If H+ is the only specifically sorbing ion present, the net surface charge density on the oxide (C m-2) is given by [41-45]:
where CA the molar concentration of added acid, CB the molar concentration of added base and F is the Faraday constant. The molar concentrations of H+ and OH- are calculated from the pH measurements.
The pH dependent surface charge density of magnetite suspension is shown in Fig 4.
Fig.4 - pH-dependent surface charge development of magnetite at 0.01M Na2SO4 ionic strengths.
3.4. Surface Potential (Zeta potential)
When surface charge development occurs by direct proton transfer from the aqueous phase, the surface potential (Zeta potential) () can be calculated analogously to the Nernstian surfaces as follow [42,46].
Where T is absolute temperature and R is gas constant.
In non electrostatic models (NEM), the electrostatic interactions are assumed to be negligible and the surface potential is considered to be zero. In the diffuse-layer model, the surface potential could be calculated as following expression [37,44,45,47].
With I the ionic strength and z the ion charge.
The surface potentials of the magnetite nanoparticles under different solution pH values are shown in Fig. 5 and Fig. 6 by DLM and Nernst models respectively. In both figures, it is observed that magnetite nanoparticles had positive potentials in acidic solutions (pH<pHPZC) and negative potentials in basic solutions (pH>pHPZC). From the electrostatic interaction point of view, the positive potential of magnetite nanoparticles under acidic solution conditions would favors the adsorption of negatively charged species and the negative potential of the magnetite nanoparticles under basic solution conditions favors the adsorption of positively charged species.
Therefore the adsorption of positively charged ions as Ni+2, Cs+1,Yb+3 , Co+2, Pd+2 , Pt+4 , Rh+3 , etc is done in pH value above pHPZC and in reverse the adsorption of negatively ions as , , , etc is well done at bellow pHPZC.
Fig.5. pH-dependent surface potential of magnetite at different ionic strengths by Double Layer Model.
Fig. 6. pH-dependent surface potential of magnetite by Nernst Model.
From the present study, we have synthesized magnetite Iron oxide nanoparticles successfully using coprecipitation methods. The pH of point of zero charge determinated to be 6.65. the surface density calculated theoretically by titration data and consequently the surface potential calculated by Nernest and Double Layer models.
This study was partly financed by Iranian Nanotechnology Initative.