Synthesis of magnetic composite nanoparticles enveloped in copolymers specified for scale inhibition application

Maleic anhydride, sodium 2-acrylamido-2-methyl-1-propanesulfonate (AMPS), ferrous chloride tetrahydrate (FeCl₂·4H₂O), ferrous chloride hexahydrate (FeCl₃·6H₂O), oleic acid (98%), ammonium hydroxide (30%), hydro chloride acid, sodium lauryl sulfate, cyclo hexane, sorbitan monooleate (Span-80) and disodium ethylenediaminetetraacetiate (EDTA) were purchased and used as-received from Merck Chemicals, azobisisobutyronitrile from Sigma Aldrich.

Carbonate and sulfate based seawater were prepared according to the NACE standard method TM 03-074-95¹ but modified to simulate the produced brine composition of the desired brines.

2.2.1. Synthesis of oleic acid coated iron oxide nanoparticles (OMNPs).

Oleic coated iron oxide nanoparticles were synthesized by co-precipitation under ultrasound irradiation support. Briefly, FeCl₂·4H₂O (4.0 g, 0.02 mol), FeCl₃·6H₂O (12.96 g, 0.01 mol) were dissolved in distilled and deoxygenated water (200 ml), then ammonium hydroxide (120 ml) was quickly added to mixture under ultrasound irradiating at 20 kHz at room temperature. After 30 min, oleic acid (5 ml) was continuously added into the mixture of resulting black dispersion. The reaction temperature was raised to 91 °C in an hour. The modification was ended by adding 100 ml of HCl (0.01 M) in order to exclude base catalyst and unreacted oleic acid. Thereafter, the black dispersed particles were collected by a magnetic field and washed several times by distilled water till the pH of the washing water was around 7. Finally, the magnetic product was dried in an oven at 65 °C for 8 h. (figure 1).

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2.2.2. Synthesis of OMNPs

encapsulated copolymer of maleic acid and sodium 2-acrylamido-2-methyl-1-propanesulfonate (MA-co-AMPS). MA-co-AMPS coated magnetic iron oxide nanoparticles were prepared via inverse mini-emulsion polymerization in a 250 ml three neck-jacketed reactor fitted with an ultrasound transmitter and mechanical stirrer under nitrogen atmosphere. Following a representative synthetic procedure for inverse mini-emulsion polymerization, 10.0 g of AMPS, 5.0 g of maleic anhydride were dissolved in 80 ml of distilled and deoxygenated water as a discontinuous phase. Then 5.0 g of magnetic iron oxide nanoparticles, 1.5 g of Span-80 were dissolved in 80 ml of cyclohexane. This aqueous solution was poured into the reactor. The mixture was ultrasound irradiated at 20 kHz and stirred at 400 rpm; the temperature was maintained at room temperature for 30 min. After that, the reaction temperature was raised to 70 °C; 150 mg of azobisisobutyronitrile in 2.0 ml of water was slowly injected to initiate polymerization. After 2 h of ultrasound irradiating and stirring, the composite product was taken out of the reactor. The resulting product was washed in an ethanol/water (9/1) solution and dried at 50 °C for 6 h.

We have implemented the similar synthetic route for MA-co-AMPS based copolymer nanoparticles without magnetic iron oxide inside in order to determine the scale control ability in preventing the precipitation of calcium carbonate and calcium sulfate from solution. (figure 2).

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2.2.3. Scale inhibition test.

The magnetic iron oxide nanoparticles were synthesized by co-precipitation of Fe²⁺ and Fe³⁺ ions in an aqueous solution upon addition of ammonium hydroxide and ultrasound irradiation assisted. Due to strong magnetic interactions, the MNPs tend to aggregate in order to reduce their surface energy. In this work, oleic acid was coated on the MNP surface to stabilize and enhance the activeness of obtained MNPs for a further reaction of polymerization. It is well known that oleic acid provides a high affinity with iron oxide through the chemical interaction between their –COO–groups and Fe atoms. As a result, the hydrophobic tails of oleic acid molecule face outwards and generate a non-polar shell.

The presence of oleic acid on the surface of MNPs was confirmed by FTIR (figure 3). The bands at 2862 and 2923 cm⁻¹ were observed according to the stretch modes of –CH₂– and –CH₃ of oleic acid. The stretching vibration of ${\rm C}=\!\!\!={\rm O}$ at 1710 cm⁻¹ was clearly detected, and the band at 1438 cm⁻¹, 1518 cm⁻¹ were clearly recognized and attributed to the asymmetric and symmetric stretching vibrations of –COO–functional group, indicating that the layer of oleic acid was successfully coated on the surface of MNPs. Furthermore, the band at 587 cm⁻¹, corresponding to the vibration of the Fe–O bonds in the structure of Fe₃O₄ was observed. The results corresponded well with other previous data.

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The MA-co-AMPS-OMNPs composite was prepared by inverse mini-emulsion polymerization of OMNPs, MA and AMPS. Using this procedure, the formation of core–shell structure can be made through three possible principles: (i) well dispersion by ultrasound irradiation assistance; (ii) electrostatic interactions between OMNPs with NH-, SO₃⁻,–COO⁻ of maleic acid and AMPS and (iii) the functional group of oleic acid on the surface of OMNPs not only makes the OMNPs more stable but it can also be a temporary monomer in the polymerization reaction.

The final product was characterized by FTIR, TEM and XRD.

The FTIR spectroscopy from figure 4 was employed to characterize the chemical functional groups present in the MA-co-AMPS-OMNPs composite comparing to those of the original MNPs, OMNPs, MA-co-AMPS copolymer. The FTIR spectrum of the original MA-co-AMPS copolymers showed the characteristic signals at 3200–3500 cm⁻¹ (–NH–stretching), 2928 cm⁻¹ (alkyl- stretching), 629 cm⁻¹ (S–O stretching) 1116 cm⁻¹ (${\rm S}=\!\!\!={\rm O}$ stretching) and 1033 cm⁻¹ (C–S stretching) present in the chemical structure of AMPS. In addition, the signal at 1665 cm⁻¹ and 1189 cm⁻¹ belong to ${\rm C}=\!\!\!={\rm O}$ and C–O stretching vibrations of maleic acid in copolymer structure. As we can see, the FTIR spectrum of the composites seemly presents all the signals of the original components, including those at 3344 cm⁻¹ (NH–stretching vibration), 2923 cm⁻¹ and 2862 cm⁻¹ (CH– and CH₃–stretching vibration), 1711 cm⁻¹ and 1537 cm⁻¹ (${\rm C}=\!\!\!={\rm O}$ stretching vibration), 1039 cm⁻¹ (${\rm S}=\!\!\!={\rm O}$ signal), 632 cm⁻¹ (C–S signal), 581 cm⁻¹ (Fe–O signal).

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Figures 5 and 6 demonstrate the TEM micrographs of OMNPs and MA-co-AMPS- OMNPs, respectively. In both micrographs, MNPs and MA-co-AMPS- OMNPs were spherical in shape with the average size of 20–30 nm. The MNPs monodispersed particles were better obtained and observed than MA-co-AMPS-OMNPs particles due to the amorphous structure of the copolymer on the surface of composites and the adhesive ability of this composite is essentially higher than those of MNPs and OMNPs.

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In order to identify and interpret the morphology of the composite product, XRD spectra of the samples were taken and the results were shown in figure 7. In figure 7, XRD spectrum related to the composite which revealed that the crystallographic structure of MNPs present in MA-co-AMPS copolymer matrix after polymerization did not change compared to that in OMNPs. In addition, XRD spectrum of composite shows the low intensity peak at 2θ less than 25° which is related to the presence of the polymer in the nanocomposites. The presence of the peak at 30°, 36° and around 40° can exclude the existence of magnetic iron oxide.

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Basing on XRD data we can calculate the average crystallite size of obtained particles from the Debye–Scherres equation existence

$$\begin{eqnarray*}&&D = \frac{{0.9\lambda }}{{\beta \cos \,\theta }},\end{eqnarray*}$$

where D is average crystallite size (Å), λ is x-ray wavelength (Cu-Kα, λ = 1.5418 Å), β is full-width at half-maximum and θ is the Bragg diffraction angle. As calculated, the average size of obtained OMNPs and MA-co-AMPS-OMNPs particles were at the range of 20–30 nm. These results were similar to those in TEM micrographs.

Thermal characteristics of coated iron oxide nanoparticles was investigated by thermogravimetric analyzer (TGA) and the resultant thermograms were presented in figure 8. The composite product showed a considerable thermal stability under inert conditions. At above 250 °C, a remarkable weight loss derives from the thermal decomposition of the side groups and polymer backbone and it continues up to 900 °C. This thermogravimetry (TG) analysis showed that weight percentages of inorganic Fe₃O₄ and polymeric constituents are 68 and 32%, respectively.

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According to previous studies [14 ,15], it is strongly believed that the inhibitor molecules must be adsorbed at the active growth sites on the surface, which may be crystal defects, thus preventing further crystal growth by interfering with the growth process. The inhibition of scale formation is affected by both the location of the adsorbed inhibitor at the crystal surface, and the extent of chemical bonding with the surface. Therefore, the scale efficiency is determined by the ability of complex creation between the scale inhibitors and the multivalence cations of the seed solution.

In terms of morphology, with the absence of an inhibitor, obtained calcium carbonate precipitates were in calcite crystal form with more angular shapes, very high adhesion surface and were difficult to wash out from the deposited surface. In contrast, in the presence of inhibitors, calcium carbonate crystals were obtained in the form of agaronite with smooth surfaces and easily washed away with hydraulic lines or other washing methods.

In our study, calcium carbonate crystals obtained from testing samples got similar results, with the presence of inhibitors based on copolymers or composites; obtained calcium carbonate precipitates were also in agaronite form, but when inhibitors were absent, in calcite forms. Both pure polymers and composites had the same impact on the morphology of the obtained calcium carbonate crystals through similar mechanisms which were previously mentioned (figures 9 and 10).

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Calcium inhibition efficiency was determined according to the NACE standard TM 03-074-95 testing method. (See footnote ¹). Inhibition efficiency is calculated according to the following equation:

$$\begin{eqnarray*} &&I_{\rm Ca} (\% ) = \frac{[{\rm{Ca}}^{{\rm{2 + }}} ]_{{\rm{in}}} - [{\rm{Ca}}^{{\rm{2 + }}} ]_{{\rm{non - in}}} }{{[{\rm{Ca}}^{{\rm{2 + }}} ]_{\rm{i}} - [{\rm{Ca}}^{{\rm{2 + }}} ]_{{\rm{non - in}}} }} \times 100,\end{eqnarray*}$$

where I_Ca(%) is calcium inhibition efficiency, [Ca²⁺]_in is soluble calcium concentration of inhibited sample, [Ca²⁺]_non−in is soluble calcium concentration of the uninhibited sample and [Ca²⁺]_i is initial soluble calcium concentration.

Soluble calcium concentrations are obtained by sample titration with standard EDTA solution and ammonium purpurate indicator. The results are shown in the charts below.

As shown in figures 11 and 12, at 90 °C and 5 ppm of used dose, I_Ca of inhibitor based on composite and polymer products reached highest values. Namely, the best I_Ca value was 63.64% accounting for composites and 65.00% for pure copolymers. At 120 °C, I_Ca of copolymers was likely higher than those of composites but not much: 22.22 and 25.03% attributing to composite and copolymers, respectively. This result could be related to the unusual high molecular weight of composite, due to heavy Fe₃O₄ core, compared to pure polymers. So that the same concentrations, the active copolymer component contained in composite samples was obviously less than those of copolymers.

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Besides the inhibition activity of magnetic composites as high as that of the parent copolymer, the most promising advanced characteristic of this new inhibitor is that it can be easily detected in produced water by magnetic monitoring devices after being used in oilfields. This is a very important requirement for oilfield scale inhibitors, and so far do not have any commercial polymer based scale inhibitors that meet the requirement. For this reason, we should try to further synthesize and develop effective magnetic inhibitors.