Synthesis of Baso4 Nanoparticles by Precipitation Method Using Polycarboxylate as a Modifier

Problem statement: Barium Sulfate (BaSO 4) is suitable for many applications because of its whiteness, inertness and high specific gravity. Approach: Nanoparticles of barium sulphate (BaSO4) have been synthesized from barium nitrate by prec i itation method in the presence of water soluble organic polycarboxylic polymer as a modifyi ng agent. Transmission electron microscopy, Scanning electron microscopy, Fourier transform inf rared spectroscopy and X-ray powder diffractometry were used to characterize the produc ts as well as laser grainmeter. Results: The results indicate that spherical BaSO 4 nanoparticles are obtained with poor crystalline an d diameters ranging from 30-35 nm. Conclusion: The organic polycarboxylate shows as good modifier agent. So, this method can be employed to synthesize higher yield o f BaSO4 nanoparticles.


INTRODUCTION
Syntheses of inorganic powders with ultrafine size, controlled surface properties and controlled morphology attract increasing interest because of its important use in various fields. Barium Sulfate (BaSO 4 ) is suitable for many diverse applications because of its whiteness, inertness and high specific gravity (Bala et al., 2006). Barium sulfate is a kind of important inorganic chemical product as packing and additive in painting, coating, plastics and medicines fibers (Shen et al., 2007;Wu et al., 2007;Kucher et al., 2008). Nanometer barium sulfate has more scientific advantages of size reduction. Crystallization and precipitation processes are widely used in the chemical industry. One of the most well-known crystallization processes is the reaction of barium (Ba 2+ ) and sulphate (SO 2− 4 ) ions into barium sulphate (BaSO 4 ), as shown in Eq. 1 (Oncul et al., 2006;Kieffer et al., 2009): Ba 2+ (aq) + SO 2− 4 (aq) → BaSO 4 (s) ↓ Liquid/liquid reaction is the main method of the preparation of nano-BaSO 4 . This method has some divisions such as direct precipitation, micro emulsion (Adityawarman, 2005) membrane separation (Wu et al., 2007), microchannels reactor (Wang et al., 2009). Preparation of BaSO 4 particles has been widely studied in order to assess the effect of mixing, precipitation models, agitator speed and feed position on particle size distribution, crystal growth and morphology (Judat and Kind, 2004;Bala et al., 2005).
Many different approaches have been reported for preparation of BaSO 4 nanoparticles including the addition of different additives (Jones et al., 2003;Wang et al., 2005) induction by Monolayer and microemulsion (Nagaraja et al., 2007) The addition of additives and induction by LB monolayer could causea significant change in morphology, but the size was generally in the micron scale rather than nanometer scale. The size and corresponding morphology obtained in W/O microemulsions (Chen et al., 2005) or reverse micelle approach could be controlled well by adjusting the molar ratio of water to surfactant. However, the product yield was rather low because of the poor solubility of salts in conventional microemulsions. There were also some reports about preparation of organo-modified BaSO 4 , but the BaSO 4 obtained was in micron scale. Although preparation of organocapped BaSO 4 was earlier described, (Sui et al., 2004) the condition was not suited to the industry. Preparation of barium sulphate nanoparticles by use of tetradecanoic acid, hexadecanoic acid and stearic acid as modifier was studied (Shen et al., 2007).
When inorganic fillers with surface functional groups reach the polymer matrix, it can be expected to enhance stiffness and impact properties simultaneously (Li et al., 2002;Bala et al., 2006). However, the surface of inorganic particles (e.g., BaSO 4 ) is hydrophilic, so it is not easy to disperse these particles in a lipophilic matrix. For this reason, it is very important to conduct surface modification of BaSO 4 . One of the several ways to achieve a good dispersivity is to modify the hydrophilic particle surface so that it is lipophilic in nature. For this purpose various coupling agents, which are capable of introducing a certain functional group onto the particle surface, have been employed: Titanate coupling agents (He et al., 2005), silane coupling agents (Bala et al., 2006) and organophosphonic acids (D'Andrea and Fadeev, 2003;Sheng et al., 2004). This study aims to synthesize nanoparticles of barium sulphate (BaSO 4 ) from barium nitrate by precipitation method in the presence of water soluble organic polycarboxylic polymer as a modifying agent.

Chemicals and instruments:
The starting materials used in this study were Barium nitrate, ammonium sulphate and polycarboxylates. They were used without further purification. Double distilled water was used in all experiments. The prepared barium sulfate was characterized by using (SEM :JEOL JSM 6360 DLA, Japan), Transmission Electron Microscope ((TEM; Hitachi, H-800), Fourier transform Infrared (FT-IR) spectra were measured using a Perkin Elmer 880 FT IR spectrometer by incorporating samples in KBr (1: 99 mg) disks to confirm the characteristic vibrational bands, with resolution of 4 cm −1 , X-ray Diffraction (XRD) pattern of BaSO 4 was recorded on (M/S. Shimadzu Instruments, Japan) diffractometer XRD 7000 with Ni filtered Cu Kα as a radiation source at 2θ scan speed of 4• min −1 and the particle size distribution was analyzed by L4 submicron particle size analyzer He-Ne Laser beam, Fluoride, USA.
Preparation of the samples: Direct Precipitation (DP) procedure: 50 mL 0.1 M Ba(NO 3 ) 2 was added to 50 mL 0.1M (MH 4 ) 2 SO 4 in the presence of water soluble organic polymers as modifier agent. The solution was added drogpwise into the flask while stirring at room temperature with dispersant at strong mechanical stirring 2000-2500 rpm. The steady drop rate was 20 drops min −1 . Gelatinous white precipitates were formed instantly. The precipitates were separated from the mother liquid by centrifuged at 3000 rpm for 20 min. and then the supernatant solution was discharged and the solid was redispersed in deionized water. This process was repeated three times in order to rinse the particles. After the last centrifugation, sedimented particles were dried in the microwave oven. The products were slightly grinded for analysis (Bala et al., 2005;Gupta et al., 2010). Figure 1 show the IR spectra of polycarboxylates 1 and 2. Note the band around 3400 cm −1 for to H 2 O. The band at near 3000 cm −1 for carboxylic acid O-H stretch also for, in the same region, the C-H stretching bands of both alkyl and aromatic groups. The carbonyl stretch C = O of a carboxylic acid appears as an intense band from 1760-1690 cm −1 . The C = C stretch band is at 1640 cm −1 . The C-O stretch appears in the region 1320-1210 cm −1 and the O-H bend is in the region 1440-1395 cm −1 and 950-910 cm −1 from C-H bending bands in the same region (Khenifi et al., 2007).

RESULTS AND DISCUSSION
TEM micrographs: The TEM micrograph (Fig. 2) shows that the BaSO 4 particles are nearly elliptical and have a round shape. The particle size is between 6 and 26 nm, with an average size of 18 nm. In addition, there are some uniform mesopores in the particles and the diameter of the cavities is about 6-8 nm. These regions appear brighter because they have absorbed fewer electrons than their surroundings. The larger particles contain more pores and the number of pores decreases as the diameter of the particles decreases, especially in small particles that only contain one pore. The high dispersibility of the powder in water might be related to the mesopores of the particles. The formation of the mesoporous structure is related both to nucleation and to the growth mechanism of the BaSO 4 particles. Judat and Kind (2004) and Nagaraja et al. (2007) investigated the particle morphology and internal structure (the cavities contained within the particles) of precipitated BaSO 4 . They showed that BaSO 4 grows according to a combined mechanism involving molecular and aggregative growth. The pore size increases with increasing supersaturation.

SEM Micrographs:
The SEM images (Fig. 3-c) shows that the spherical shell types nanoparticles are distributed uniformly. The larger particles contain more pores and the number of pores decreases as the diameter of the particles decreases, especially in small particles that only contain one pore. This reveals that the high dispersibility of the powder in polycarboxylate might be related to the mesopores of the particles and the high rate of adsorption of polycarboxylate onto their surfaces. The formation of the mesoporous structure is related both to nucleation and to the growth mechanism of the BaSO 4 particles (Nagaraja et al., 2007).
XRD spectra: Figure 4 shows the XRD pattern of nano barium sulfate. It is shown that the nano barium sulfate composed mainly from barite. All of the peaks could be indexed as a typical orthorhombic structure of BaSO 4 , with crystalline cell constants a = 7.144 Å, b = 8.865 Å, c = 5.445 Å, which were basically in agreement with the reported values (JCPDS No. 80-0512) (Salah et al., 2009;Shen et al., 2007). The crystallite sizes of the sample are estimated from the line width of the (212) XRD peaks.   FTIR spectra: Figure 5 shows the FT-IR spectra of BaSO 4 nanoparticles. The bands centered at 1073-1192 cm -1 and the shoulder at 982 cm -1 were the symmetrical vibration of SO 4 -2 . The peaks at 610 and 638 cm -1 corresponded to the out-of-plane bending vibration of the SO 4 -2 . The peaks at 2851 and 2920 cm -1 could be assigned to the symmetric and asymmetric vibrations of -CH2-and-CH3 groups. The peaks at 1436 and 1402 cm -1 were ascribed to the scissoring of -CH2-or the symmetric deformation of the -CH3 group. The FT-IR spectra showed that the particles contained polycarboxylate and there were no corresponding peaks in XRD patterns. This may be because the polycarboxylate was absorbed on the surface of BaSO 4 rather than entering the BaSO 4 crystal to form composite (Shen et al., 2007;Gupta et al., 2010).
Partical size distribution: The particle size distributions of the prepared BaSO 4 nanoparticles are given in Fig. 6 and also Table 1. It can be found that the average particle size is 17.35 nm.

DISCUSSION
In this study we have exposed nanoparticles of barium sulphate (BaSO 4 ) from barium nitrate by precipitation method in the presence of water soluble polycarboxylate polymer as a modifying agent. The reason that the average diameter of BaSO 4 particles in the presence of polycarboxylate polymer was smaller than that obtained without polycarboxylate polymer (Saraya et al., 2010) was that when the polycarboxylate polymer was added into the reaction system, the growth of BaSO 4 was prevented due to the steric hindrance of polycarboxylate polymer on the surface of BaSO 4 particle. But the steric hindrance was so weak that the difference was negligible. The possible reactions were as follows: PRCOONH 4 + H2O → PRCOO -+ NH + 4 PRCOO -+ Ba 2+ → P(RCOO) 2 Ba (soluble) P(RCOO) 2 Ba (soluble) + SO 4 2-→ BaSO 4 ↓ When polycarboxylate polymer was first added into water the PRCOOwas fully released. Ba 2+ was relatively surplus because SO 2-4 was dropped into Ba 2+ in the process. Polycarboxylate P(RCOO -) acted as an inducer to attract Ba 2+ via electrostatic forces. With the addition of SO 4 2-, BaSO 4 crystals formed gradually. PRCOOstill covered the surface of the BaSO 4 crystals, resulting in the inhibition of further growth of BaSO 4 nanoparticles, which was the reason for smaller size and uniform distribution of particles. During the process, polycarboxylate two roles: inducer and inhibitor. The average diameter of BaSO 4 nanoparticles decreased in the order of addition of tetradecanoic acid, hexadecanoic acid and stearic acid, which was possibly related to the chain length of organic acid. The longer the chain length, the greater the steric hindrance (Shen et al., 2007).

CONCLUSION
We have synthesized BaSO 4 nanoparticles using water soluble organic stabilizing agent, polycarboxylate. Nanoparticles crystallize in the orthorhombic structure. FT-IR and electron microscopy studies confirm that the nanoparticles free from the surfactant and they are in the range of 30-55 nm. This method can be employed to synthesize higher yield of BaSO 4 nanoparticles.