2.2. Experimental Procedures

The synthesis of oleic acid diethanolamide (OADA) was performed by the esterification reaction of methyl oleate (1) with diethanolamine (2) in a 1:3 ester:amine molar ratio, as shown in Figure 1 and described in the literature [17,28]. Under nitrogen atmosphere, the system was heated until it reached 170 °C and then, 5 (w/w) % of catalyst (K₂CO₃) were added. After that, it was refluxed for 8 h at 170 °C under constant magnetic stirring.

OADA synthesis scheme where the main chemical species involved were highlighted as 1: Methyl oleate, 2: diethanolamine and 3: OADA.

To eliminate unreacted species, the mixture was purified using solvent extraction (saturated NaCl solution and hexane). The obtained product was characterized through FTIR, ¹H NMR and ¹³C NMR.

Based on the systems studied by Avila et al. [6], styrene (10 mL), a specific amount of DVB and an OADA aqueous solution were added to a three-necked flask containing 250 mL of deionized water under nitrogen flow, equipped with a condenser and under magnetic stirring. Here, OADA was used both as the emulsifier for the emulsion polymerization and as the surfactant to be carried. Due to that, the OADA content used was based on an excess of its critical micelle concentration, which was determined by the surface tension method [29,30]. After homogenization of the system, 50 mg of KPS were added as initiator. Then, the mixture was stirred for 6 h at 80 °C. The obtained emulsion was purified by separation and enrichment based on centrifugal ultrafiltration (filter membrane Amicon Ultra NMWCO 20K, Merck Millipore Brasil, Barueri, Brazil). After each centrifugation cycle, deionized water was added, allowing the nanoparticles to be washed three times to eliminate unreacted species and free surfactant that remained in the aqueous media.

A certain amount of styrene (St) was partially substituted by sodium 4-styrenesulfonate (St-S) under the same conditions, and the proportions between total monomer, initiator and surfactant amounts were maintained [31]. The main formulations for the nanoparticles polymerization reactions studied in this work are listed in Table 1. This was accomplished by keeping constant the total monomer amount (87.2 mmol), while the mol % for the two monomers was altered. The crosslinking agent employed was DVB, in 0.3 mL for all formulations. The obtained emulsion was purified by separation and enrichment as described in the previous section.

Formulations for nanoparticles polymerization reactions.

¹ OADA: oleic acid diethanolamide. ² St: styrene. ³ St-S: sodium 4-styrene-sulfonate. ⁴ CMC: critical micelle concentration.

PSNP and SPSNP were characterized by infrared spectrometry (FTIR) in the 400–4000 cm⁻¹ range, using a Fourier transform infrared spectrometer (Nicolet 740 FTIR from Thermo Fisher Scientific, Waltham, Massachusetts, EUA) with a DTGS KBr detector and a beam splitter. Conventional preparation techniques of KBr pellets for solids were employed. Particles number-average diameter and polydispersity index (PdI) results were obtained by photon correlation spectroscopy (PCS, Malvern Zetasizer–MAL 1013334 from Malvern Panalytical Brasil, São Paulo, Brasil).

The nanoparticles stability during storage and agglomeration tendency in deionized water at room temperature was investigated by monitoring particle size (hydrodynamic diameter obtained by PCS) for 60 days.

The surface tension of the nanoparticles/water systems was measured using the Wilhelm plate method, and the interfacial tension (IFT) between suspensions with different nanoparticles concentrations and mineral oil, through the Du Nouy Method at constant temperature (28 ± 0.5 °C). For all these measurements, the apparatus used was a Krüss K100 tensiometer (Hamburg, Germany). Also, through IFT reduction with time, it was investigated the occurrence of the surfactant (OADA) release at the water/oil interface. For low IFT systems, like the ones obtained by surfactant solutions in water and some SPSNP suspensions in water, the Krüss SITE 100 spinning drop tensiometer was used at 28 °C. The rotating tube of the system was filled with these prepared solutions/suspensions, and, during the analysis, about 3 µL of mineral oil were added. Then, the tension was measured along the time, until it reached equilibrium. All measurements were performed in triplicate.

To quantify the amount of OADA that remained on the nanoparticles′ structure after the synthesis process, it was measured the total organic carbon in the supernatant after each centrifugation cycle by a Total Organic Carbon (TOC) Analyzer (TOC-L from Shimadzu Brasil, Barueri, Brazil), considering that the carbon present was from the free surfactant. As a result, the difference between the OADA amount added before the reaction and the one quantified by TOC analysis was considered to be the one that corresponds to the OADA immobilized or encapsulated in the nanoparticles, according to Equation (1) [32].

C_i and C_e are the initial and equilibrium (in the supernatant phase) concentrations (mg/L), m is the nanoparticle mass (g) and V the solution volume (250 mL).

The retained surfactant percentage (Retention%) was calculated from Equation (2).

The sand was washed with a 0.1% HCl solution and later thoroughly washed with deionized water to remove any soluble impurities. To remove any organic compounds still present, it was calcined at 600 °C for 12 h. The resulting sand presented 30% of porosity and size distribution between 362 and 635 µm.

The mineral composition of the sand used as porous media was determined by X-ray diffraction (XRD) using a RIGAKU Ultimate IV X-ray diffractometer (Rigaku Corporation, Tokyo, Japan) which recorded the 2θ range 10–80° at a scan rate of 0.02°/min, using CuKα (λ = 1.54 Å) radiation. Also, thermogravimetric analysis (thermogravimetric analyzer TGA-51 from Shimadzu Brasil, Barueri, Brazil) was carried out in a heating rate of 10 °C/min and temperature range of 25–800 °C under a O₂ atmosphere in order to evaluate the existence of organic matter adsorbed.

To ensure that the prepared nanoparticles suspensions could be transported through a porous medium, their transport and adsorption behavior were analyzed by experiments in an unconsolidated sand porous medium column test. The porous medium was constructed in a liquid chromatography column (diameter 2.5 cm, length 15 cm), filled with sand and sealed with two PTFE (polytetrafluoroethylene) end fittings, connected to a peristaltic pump (Masterflex^® L/S peristaltic pumps, Cole-Parmer, Vernon Hills, Illinois, USA). The test consists of subsequent fluid injections in the column: 1 pore volume (PV) of distilled water, 3 PV of surfactant solution or nanoparticle suspension and finally 3 PV of distilled water. The concentration of the active substance (surfactant or nanoparticle) in the injected fluid was 0.1 (m/v) %, and the pumping flow rate was 0.1 mL/min. During the experiment, every ten ml of effluent were collected and analyzed by TOC detection for the surfactant-only injections or by UV–Vis spectrophotometry at 400 nm in the case of nanoparticles injection. The active substance content in the effluents allowed to plot the breakthrough curves, i.e., the relative concentration C/C₀ (C—concentration in the effluent and C₀—initial concentration injected) as a function of the pore volume [11,33]. Besides, to evaluate the content of adsorbed material in the medium, a graph of cumulative content recovered versus the pore volumes injected was obtained.

These tests were conducted using the same system described in the previous section to evaluate the efficiency of the surfactant delivery system employed in recovering the oil, as shown in Figure 2. Initially, 1 PVs of a mixture of 50% crude oil, 25% mineral oil and 25% heptane, here used to simulate a paraffinic oil, was added and then, a sand presenting the same characteristics as the one used in the previous experiment was used to fill the column. Subsequently, 3 PVs of deionized water were injected at 1 mL/min as displacing fluid and afterwards 1 PV of the aqueous nanoparticles′ suspensions, which corresponds to tertiary oil recovery. At the end, 2 PVs of deionized water were injected. The aqueous systems used were a surfactant solution (0.006% OADA aqueous solution) or 0.1 wt % SPSNP solutions with nanoparticles of different sulfonate contents.

Oil displacement set-up scheme.

Effluents were collected in 10 mL graduated cylinders and, based on the oil volume measured from every effluent collected, materials balance calculations were carried out to evaluate the oil recovery as a function of fluid injected [11,14,34].