Hydrophobization of petf-surfaces for water-in-oil emulsions separations

. The technique of poly(ethylene terephthalate) track-etched membranes (PETF TMs) modification to increase of water-in-oil emulsions separations is developed. The water-in-oil emulsions separations by using PETF TMs with regular pore geometry and pore sizes 200 and 350 nm is described in the article. PETF TMs were modified with octadecyltrichlorosilane by spin-coating method to increase their hydrophobic properties. The results of changes in the pore diameters and the contact angle after PETF TMs modification are presented. The obtained samples were characterized by AFM, SEM and gas-permeability test. Chloroform–water and n-hexadecane–water emulsions have been used as a test liquid for water-in-oil emulsions separations. At an operating vacuum of 700 mbar, the specific filtration performance of chloroform: water emulsions were 51.5 and 932.0 l/(m 2 ⋅ h), hexadecane: water were 46.1 and 203.4 l/(m 2 ⋅ h) for PETF-200 / OTS and PETF-350 / OTS, respectively. The degree of purification of emulsions by modified membranes according to the refractive index is of 100 %. Obtained membranes can be used to separate oil-water emulsions in order to prevent the corrosion of pipelines and changes of crude oil viscosity, as well as the treatment of water purification from oil industry waste.

Maxim V. Zdorovets -Cand.Sci.(Physics and Mathematics), Professor, Director of the Astana Branch at Institute of Nuclear Physics of the Ministry of Energy of the Republic of Kazakhstan.https://orcid.org/0000-0003-2992-1375.E-mail: mzdorovets@inp.kzContribution of the authors: Galina B. Melnikova -concept description, experimental research design, research planning, data collection and systematization, comparative analysis, manuscript text writing; Aliaxander E. Salamianski -prepare the techniques of modification membranes; Tatyana N. Tolstaya -analysis and synthesis of literature data; Victorya M. Akulova -prepare samples of modification; Sergei A. Chizhik -interpretation of research results, manuscript text editing; Ilya V. Korolkov -instrumental research, data collection and systematization, manuscript text editing; Indira B. Muslimova -prepare of PERT TMs; Nurdaulet N. Zhumanazar -work with graphic material; Maxim V. Zdorovets -formulation of conclusions.
For citations: Melnikova G. B., Salamianski A. E., Tolstaya T. N., Akulova V. M., Chizhik S. A., Korolkov I. V., Muslimova I. B., Zhumanazar N. N., Zdorovets M. V. Hydrophobization of PETF-surfaces for water-in-oil emulsions separations.Vestsi Natsyyanal'nai akademii navuk Belarusi.Seryya fizika-tekhnichnykh navuk = Proceedings of the National Academy of Sciences of Belarus.Physical Конфликт интересов: авторы заявляют об отсутствии конфликта интересов.Информация об авторах: Мельникова Галина Борисовна * -кандидат технических наук, доцент, старший научный сотрудник Института тепло-и массообмена имени А. В. Лыкова Национальной академии наук Беларуси.https://orcid.org/0000-0003-4891-7523.E-mail: galachkax@gmail.com;Соломянский Александр Ефимович -кандидат химических наук, доцент, ведущий научный сотрудник Института химии новых материалов Национальной академии наук Беларуси.https://orcid.org/0000-0001-8355-8867.Е-mail: solasy@gmail.com;Толстая Татьяна Николаевнанаучный сотрудник Института тепло-и массообмена имени А. В. Лыкова Национальной академии наук Бе ла руси.Е-mail: tolstaya.tn@yandex.ru;Акулова Виктория Максимовна -младший научный сотрудник Института химии новых материалов Национальной академии наук Беларуси.https://orcid.org/0000-0003-3883-2921.Е-mail: SinSagi@out-Introduction.According to UNESCO data the pollution of wastewater with oil products is the ten numbers of dangerous.The separation of water-in-oil emulsions is also important in particular for prevention the corrosion of pipelines and changes of crude oil viscosity [1].The wettability of membranes is a key property that determines the separation of an oil/water emulsion.The membrane performances in contact with liquid depend on surface energy, pore size, pore density and roughness [2].Track-etched membranes (TMs) have a record narrow pore size distribution.Modification of their surface in order to regulate wettability can improve properties of separation water/oil emulsions or gases [3].The most of the above polymers, membranes have oleophilic properties [4], but the actual problem is their modification in order to form surfaces with controlled wettability.Regulation of the membrane surface wettability is achieved both by physical methods, by modifying the surface layer, and by chemical methods, at the stage of obtaining membranes [5].The modification of TMs surfaces allows formation of hydrophobic properties and significantly expands the area of their possible application due to a significant change in the surface characteristics of membranes, hydrophilic properties and the possibility of changing the pore size under the influence of external conditions.Superhydrophobic-superoleophilic membranes allow oil droplets from an oil/water emulsion to wet the membrane surface and pass through the pores without passing water.However, the surfaces of membranes intended for the separation of water/oil emulsions are easily polluted and the pores are clogged with oils, since it is not yet possible to achieve the limiting requirements for oleophilic properties.Oil contaminants due to the high viscosity are difficult to remove and affect the separation performance of the membranes when they are reused.Currently, to clean water sources from accidental oil spills, membranes are used that separate oil pollution in the aqueous phase (membranes operating under water) [6].The use of materials with low surface energy and hierarchical micro/nanostructure can contribute to the formation of superhydrophobic surfaces [7][8][9].In addition, it is possible to change the properties of the medium (pH, temperature, exposure to light, the influence of electric and magnetic fields) to expand the spectrum of separable impurities [8].Janus membranes are an emerging class of materials with opposite properties at the interface.Most of them are asymmetric in terms of wettability: one side has superhydrophilic properties (to protect against oil fouling), while the other side has superhydrophobic properties (self-cleaning) [10].Polymeric materials (for example, polyacrylonitrile, polyvinylidene fluoride, polyethersulfone, polyvinyl alcohol, polyvinyl chloride, polyethylene, polypropylene, polyamide, chitosan, etc.) [2,10,11] are of the most interest in the field of filtration and separation membranes due to their mechanical strength, chemical stability and elasticity.
Existing methods used to fabricate PTFE membranes and PTFE-modified materials are either limited by complex manufacturing equipment or relatively expensive, and are also not applicable to modifying substrates.To solve these problems, Asadi et al. [12] used the Layer-by-Layer method: this is an inexpensive method that allows the formation of multifunctional coatings of controlled thickness at the nanoscale on almost any substrate.
Another method for the production of superhydrophobic-superoleophilic or superoleophobic-superhydrophilic porous materials for separation of oil and water emulsions is the laser ablation method, which forms micro-/nano-sized structures with a certain roughness and increases the contact angle to 155.5° ± 1.5° without chemical modification [13].
One of the widely used methods for the formation of hydrophobic and superhydrophobic nanoscale coatings on the surface of a porous substrate is the method of electron beam dispersion of polymers in vacuum [14].
For water filtration PETF TMs membranes are often used.The resistance to oil population is an important criterion for membranes used for oil/water emulsion separation, since oil can easily accumulate on the top surface of the membranes, resulting in a significant reduction in flow.
In addition, there is a limited number of studies in the literature devoted to establishing a relationship between the membrane morphology and physicochemical phenomena occurring on the membrane surface [15].A correlation was established between membrane surface roughness factors, pore diameters, and membranes with high surface porosity, low microroughness, relatively high tortuosity and a small radius of pore curvature in depth, wide and relatively dense and evenly distributed density surface features, "defects" (understood as fibers, knots or even pores) are most preferred in the formation of hydrophilic/oleophobic membranes [16,17].
In this article, we present the results of PETF TMs hydrophobization by octadecyltrichlorosilane via simple spin coating method.Prepared hydrophobic PETF TMs were tested in separation of chloroformwater and hexadecane-water emulsions.
Materials and Methods.In the work, polyethylene terephthalate track-etched membranes (PETF TMs) were obtained by the Astana branch of the Institute of Nuclear Physics of the Ministry of Energy of the Republic of Kazakhstan.To obtain membranes, a PETF film of the trademark Hostaphan® RNK-12 was used, which was irradiated with 84 Kr 15+ ions with an energy of 1.75 MeV/nucleon at a DC-60 heavy ion accelerator; the irradiation density was 1 • 10 8 cm -2 .After chemical etching in 2.2 M NaOH at 85 °C [18], membranes with pore diameters 200 (PET-200) and 350 nm (PET-350) were obtained.
Preparation of the coating.OTS coating (Sigma-Aldrich, 98 % purity) was formed on PETF TMs (square of the sample is of 4.9 cm 2 ) and silicon monocrystalline substrates (size of the sample is of 2.5 to 2.5 cm) from solution of OTS in hexane (Sigma-Aldrich, ≥ 97.0 % purity) with a concentration of 1 mM by spin-coating method.All reagents were used without additional purification.
OTS solution of volume of 1 ml was applied on substrates during 10 minutes they were centrifuged at a speed of 3000 rpm for 2 min.Then OTS-modified substrates were sequentially washed with hexane and isopropyl alcohol for 1 min to remove excess OTS inside the pores.
In these experiments all reagents were used without additional purification.Study of the surface structure and mechanical properties of PETF TM.The structure of the membrane surface was studied using an atomic force microscope (NT-206, ALC "Microtestmashiny", Belarus) using standard FMG 01_SS silicon cantilevers (TipsNano, Estonia) with average rigidity of 3 N/m (accordingly passport data) and a curvature radius of no more than 10 nm.Changes in the pore diameter of PETF membranes before and after modification were estimated from the surface topography by constructing a surface profile along the image scanning line.The average value was calculated for 15 randomly selected pores.
The structure was studied by SEM using a JEOL JSM-7500F high resolution electron microscope (Jeol, Germany) with a cold (field emission) cathode.Before the study, the samples were sputtered with gold 15-20 nm thick on a JFC-1600 magnetron.
Surface wettability analysis.The contact angles were measured using a DSA 100E (KRUSS, Germany) by the sessile drop method.Distilled water and diiodomethane (Sigma-Aldrich, 99 % pure) were used as test liquids.Based on the CA values, the free surface energy was calculated according the OWRK method, according to which the energy of the surface layer of a solid includes two components: dispersive and polar.
Filtration characteristics.The pore size (r, m) was measured by the gas permeability method at a va cuum difference of 20 kPa [18].The effective pore sizes of the membranes were estimated by gas permeability using equation: Q -air permeability, m 3 /(c • m 2 ); l -film thickness, m; ∆p -applied vacuum, Pa; R -universal gas constant, J/(mol • K); M -molar mass of air, g/mol; n -irradiation density, J/m 3 ; T -temperature, K. Membrane specific performance (J) for water, chloroform, o-xylene and hexadecane were determined by solvent filtration at an overpressure created using an LVS 210 T vacuum pump operating vacuum of 700-900 mbar.An emulsion of chloroform (Merck, ≥ 99.9 % purity) -distilled water was prepared in a volume ratio of 9 : 1, n-hexadecane (Merck) -water in a ratio of 100 to 1. Mixing was carried out using IKA T 18 digital ULTRA-TURRAX dispersant for 3-4 min.The emulsion is a cloudy stable liquid for 2 h.Immediately after dispersion, the emulsion was passed through the membranes.The performance measurement was determined under vacuum from 900 to 700 mbar.After filtration, the refractive index of the solvent was evaluated on refractometer (Isolab GmbH).
Results and Discussion.According to AFM analyze, the differences in the structure of the modified membranes with OTS were established (Figure 1).
In the case of modifying the surface of OTS membranes, the pore diameter for PETF-200 membranes does not change, for PETF-350 -it decreases by 1.5-2 times.The arithmetic mean roughness, R a , decreased by 2 and 1.5 times for PETF-200 and for PETF-350, consequently (Table 1), the root meansquare roughness values change more significantly: R q decreased by 3.6 and 1.8 times for PETF-200 and for PETF-350, consequently.Gas permeability results and changes of pore size also confirm formation of OTS coating on PETF TMs surfaces (Table 2).Based on this, it can be concluded that the main contribution to the formation of the hydrophobic properties of the surface is made by the nature of modifier layer on PETF TM surface.The changes of the above parameters after modification indicated the formation of a uniform coating, as well as enveloping the membrane pore boundaries with a layer of modifier.The results of hydrophilic properties are presented in Table 3.  Accordingly, the OTS formed more hydrophobic layer on silicon substrate.The diiodomethane CA for PETF TM modified with OTS changed from 49° to 9° within 1 min, which was due to the passage of a liquid drop through the membrane pores.The low values (γ p ) of the polar component of the specific surface energy for the PETF-200/OTS samples should be noted, which indicates an increase in the oleophilicity of the surface.PETF TMs performance results.For PETF TMs with a pore diameter of 350 nm the performance values (J) for water, o-xylene and chloroform and n-hexadecane are significantly higher (Figure 2, b) compared to membranes with a pore diameter of 200 nm (Figure 2, a), which is due to the larger pore diameters.For hexadecane, the J values for PETF-200 are close to zero.

Sample
It should be noted that a change in vacuum to 800 mbar makes it possible to increase the performance for water of PETF-200 membranes from 0 to 7.9 l/(m 2 • h).
The performance of n-hexadecane (n-hexadecane is a non-polar liquid, the surface tension of which is less than that of most technical oils) for the original membranes is close to zero.The modified membranes filter this solvent, the values J of the membranes modified with OTS are found in the range from 38.2 to 51.5 l/(m 2 • h) at a vacuum of 900 to 600 mbar, respectively.These coatings are promising for the separation of water-oil emulsions.
Chloroform emulsions were used as test solutions and hexadecane in water at volume ratios of 9 and 100 to 1 (Figure 3), respectively.For the original unmodified membrane with a pore diameter of 350 nm (by gas permeability) separation of the emulsion is established, but after the chloroform is filtered out, water also seeps through the pores.Performance is of 716.4 l/(m 2 • h).
An increase in filtration flow with increasing operating vacuum has been established.At a vacuum of 700 mbar, the specific filtration performance of chloroform: water emulsions were 51.Conclusions.In this study, we have shown simple and technologically convenient method of PETF TMs hydrophobization with octadecyltrichlorosilane.Thus, the application of spin-coating meth-

Figure 1 .
Figure 1.AFM (a-f ) and SEM (g, h) images of the morphology of PETF TMs (a, d) and modified with OTS (b, c, e, f, g, h).PETF TMs with pore diameters 200 nm (a-c, g) and 350 nm (d-f, h), AFM Topography (a, b, d, e) and Torsion (c, f ) regimes

T a b l e 1 .
The values of roughness of the modified TM surfaces

T a b l e 3 .
Contact angle (CA), specific surface energy (w) and polar component of surface energy (γ p ) for hydrophobized membranes 5 and 932.0 l/(m 2 • h), hexadecane: water was 46.1 and 203.4 l/(m 2 • h) for PETF-200/OTS and PETF-350/OTS, respectively.The degree of purification of emulsions by modified membranes according to the refractive index is of 100 %.It has been established that for PET-350 membranes, the performance values do not change for five filtration cycles, for PETF-200 -for one cycle.