Methyl methacrylate (MMA), Methyl acrylate (EA), and Acrylic acid (AA), (AR grade) bought from National Chemicals Co., Ltd., are kept in brown bottles and stored in a refrigerator at below 5˚C. Ammonium persulfate (APS), divinylbenzene (DVB), methylbenzene all are bought from Tianli Chemicals Co., Ltd (Tianjing, China). Sodium alkylphenol ether sulfosuccinate (MS-1) and po1yoxyethylene octylphenol ether (OP-10) are supplied by Hainan Petrochemical Company (Jiangsu, China).

An emulsion polymerization method is used to prepare P (MMA-AA-EA) with MMA, AA and EA as the monomers, DVB as the cross-linker and APS as the initiator. First of all, 0.20 g MS-1 and 0.20 g OP-10 are added to 70 g deionized water in a 250 mL three-mouth flask which is properly assembled for heating under reflux, dropping funnel, thermometer. Afterwards, the flask is heated to 75˚C, 10 mL APS initiator solution is gradually added into the flask which is prepared by dissolving 0.2 g APS into 20 mL deionized water. Subsequently, the mixture of 0.5 g EA, 8.0 g MAA, and a certain amount of AA and DVB are dropped into flask with a constant pressure funnel. Finally, the reaction temperature is maintained at 75˚C for 3 h, and then gradually cooled to room temperature. And the correspond products is purified by using deionized water and vacuum freeze-drying. Figure 1 shows reactions schematic of as-prepared P (MMA-AA-EA).

Crude oil is supplied by Block115 in Fushan Oilfield. And the physical analysis of the crude oil was shown in Table 1. W/O emulsion is prepared by adding 70-vol% water to the crude oil and then the two phases are mixed at 50˚C. And then crude W/O emulsion is completely stirred at high speed for 30 min by CJSS-B12K type frequency conversion high-speed mixer. At last, the emulsion is kept stable for 24 h without water or oil separated.

The demulsification performance test is evaluated according to SY/t5281-2000 (Industrial standard). The polymer is dissolved in an organic solvent and prepared into a demulsifier solution with concentration of 10 g/L. The test is carried out in 25 ml graduated prescription bottles. Firstly, an amount of demulsifier is added into 20 mL of emulsion, and then the bottles are shaken vigorously 200 times by hand within 2 minutes. Afterward, the bottles are immediately returned into the as-settled water bath. The demulsifying efficiency is measured by calculating the amount of water removed from the emulsion sample.

D E = C o − C d C o × 100 % (1)

where C_o is initial water concentration before water separation, C_d is water concentration after water separation.

The infrared spectrometer is equipped with a 15 cm optical path and a ZnSe gas pool. The scanning spectrum range is 4000 - 400 cm⁻¹. Each sample is scanned by mid-infrared DTG detector for 16 times. The samples are made by KBr compression. The IR spectra of the copolymer are shown in Figure 2. It is obvious that the absorption peaks at 2952.35 cm⁻¹ and 2848.36 cm⁻¹ are ascribed to the C-H stretching vibrations of -CH₂. The characteristic peaks of the ester at 1739.32 cm⁻¹ (C=O stretching) and 1145.00 cm⁻¹ (C-O stretching) are ascribed to carbonyl groups in MMA and EA. In addition, the peaks at 1450.10 cm⁻¹ and 1224.83 cm⁻¹ belong to bending vibration absorption peak of C-H and the asymmetric stretching peak of C-O-C, respectively. The absorption peak at 987.86 cm⁻¹ is the characteristic peak of ethyl acrylate. These results have confirmed that the polymerization of vinyl monomers occurs.

The surface tension of the polymer is tested by hanging piece methods (platinum plate size: 19.9 mm × 10 mm × 0.2 mm, accuracy: 0.01 mN/m) at 20˚C. The sample is synthesized by using 1.4 g AA, 8.0 g MMA and 0.5 g EA as monomers. The relationship of surface tension and concentration of P (MMA-AA-EA) is shown in Figure 3. The surface tension is reduced with the increase of the amount of polymer. When the amount of polymer is critical micelle concentration (CMC), the surface tension will no longer be changed. With the increase of the amount of polymer, the polymer molecule is gathered into micelles. The interaction between molecules is equilibrium. The surface tension reduction (π_cmc) is 10.66 mN/m (π_cmc = γ₀ − γ_cmc). It means that the surface tension of a solution was reduced by the polymer. The parameters such as Gibbs free energy of micellization (ΔG_mic) and Gibbs free energy of adsorption (ΔG_ads) can be obtained by Gibbs absorption equations [18] . The result of calculations indicates that ΔG_mic is −26.79 KJ/mol and ΔG_ads is −37.83 KJ/mol.It means that the polymer tends to be adsorbed to the interface of emulsion rather than micelle. Then the adsorption will be accompanied with micellization at last. This means that the micellization of these polymeric esters in the solution is inhibited more than the adsorption at the liquid/air interface, or the adsorption is facilitated more than micellization.

The size of the polymer is tested at Malvern nano ZS laser particle size analyzer at 25˚C. The particle size distribution of P (MMA-AA-EA) is shown in Figure 4. The polymer is distributed at the size of 30 - 210 nm. And the average size is 76.99 nm. It is believed that the particle size and distribution of the polymer are smaller and narrow. Therefore, it has larger surface area, and it would improve the solubility of polymer. Furthermore, the polymer can be dispersed in the emulsion more uniformly and adsorbed on the water/oil interface rapidly to replace the nature emulsifiers.

When the amount of AA is 1.4 g, different amount of demulsifiers are added into the W/O emulsion at 70˚C. The results are shown in Figure 5. When the concentration of emulsifier is 100 ppm, there is no demulsifying phenomenon because the emulsifier is not enough to replace the nature emulsifiers. Therefore, the interfacial film cannot be ruptured and the coalescence of oil droplets cannot be triggered. With the concentration of emulsifier increasing from 200 to 400 ppm, demulsification efficiency also increases. Furthermore, the adsorption rate of emulsifier increases with its amount at the same time. When the concentration of emulsifier is 400 ppm and the emulsion is heated for 60 min, the demulsification efficiency is 96%. With the increase of the concentration of emulsifier, more natural emulsifiers are replaced. And then the demulsification efficiency is increased.

The demulsification of different amount of polymers in W/O emulsion is shown in Figure 6. The definition of water phases is influenced by different amount of polymers. The most clear water phase separated from emulsion is the cylinder in Figure 6(d). Kokal et al. pointed that demulsifiers was also surface-active agents like the emulsifier [19] . And high concentration of demulsifier may prone to overdosing. It is obvious that the water phase of Figure 6(e) was less clear than Figure 6(d).

It is shown in Figure 7 that the demulsification efficiency presents an obvious increasing trend with the demulsification temperature increased from 60˚C to 75˚C. As the result demonstrates that the demulsifying efficiency is only 58.26% at 60˚C. In contrast, as the temperature is reached up to 70˚C, it is found that the efficiency of demulsification is about 94.78%. On the one hand, the rise of the temperature would cause the reduction of viscosity and the mechanical strength of oil/water interface. One the other hand, the rise of the temperature would promote the kinetics of water dropt and increase the difference of water-oil phase density (Brownian motion). It is demonstrated that the above consequence may contribute for effective coalesce and aggregation of water drops and the separation of crude oil emulsions.

Polymers of 300 ppm are added into W/O emulsion at 70˚C for 10 minutes. A drop of the emulsion is magnified 100 times under the microscope. The bright spot in the picture is the water phase. Pictures of the emulsion are taken every two minutes three times. It is shown in Figure 10 that water droplets are aggregated and merged into larger ones. The little water droplets are “eaten” by larger ones [22] . Furthermore, two large water droplets are gathered into a huge one marked with red arrows in Figure 10. The results elucidate that the interface of the emulsion is destroyed by the demulsifier during the initial stage of emulsification. It means that the droplet has a process of aggregation and the water is accumulated in large quantities under the action of demulsifier. At the later stage of the demulsification process, the water droplets are accumulated in large droplets and then settled under the action of gravity, and the nearly and completely oil-water phase separation could be achieved.

A drop of emulsion with demulsifier of 300 ppm is magnified 100 times under the microscope. It is heated at 70˚C for 5, 10, 15 and 20 min, respectively. As it is shown in Figure 11, the size of water droplets is increased obviously at different moments. Kang et al. pointed out that the action of the flocculation between water droplets overcame the inhibition of the W/O film to coalesced [23] . And then the water droplets which were separated from W/O emulsion were well-distributed in the emulsion. After that the water droplets were coalesced completely and the transparent water phase could appear on the bottom. On the other hand, the coalescence of the water droplets in the remaining emulsion could promote the quick sedimentation.