Surface Chemical Characteristics

It is essential to perform surface chemical characterization, to complement the information that is provided by characterization techniques addressing surface morphology and physical characteristics, so that membrane performance is further understood. Surface chemical characterization techniques include techniques for providing qualitative chemical characterization and elemental compositions for points on the membrane surface such as Energy Dispersive X-ray (EDX) and X-ray Photoelectron Spectroscopy (XPS). Furthermore, the structure and phases of the membrane materials need to be examined. This can be achieved using ATR-FT1R and XRD. In addition, it is necessary to inspect the surface hydrophobicity using the contact angle technique and the surface charge using the zeta potential measurement technique, since they both affect the fouling of the membrane.

Energy Dispersive X-ray Spectroscopy (EDX)

The EDX technique is used to deliver a qualitative chemical characterization of specific points on the membrane surface. EDX can be coupled with other characterization techniques such as SEM and ТЕМ (Khulbe and Matsuura

2017). The quantity of energy that is released upon focusing an electron beam onto the sample surface depends on the electron’s starting and ending shell.

The generated X-ray is converted into an EDX plot that shows different peaks, each corresponding to different energy levels in the received X-ray. Since each chemical element has its own unique structure, each peak in the EDX plot is identified by comparing it with a peak of a known atomic structure as shown in Figure 7.5 (Ferrero et al. 2011).

Shahabi et al (Shahabi et al. 2019) characterized their membrane of graphitic carbon nitride using EDX. They observed that the membrane consisted of carbon and nitrogen without the presence of any other impurities. In another study, the EDX technique was used for mapping of Ti elements in the membrane surface of GO, to examine the dispersion of Ti0₂ nanoparticle in the GO sheets (Safarpour et al. 2015).

X-ray Photoelectron Spectroscopy (XPS)

XPS is a quantitative spectroscopic technique that is surface-sensitive. It is able to measure the elemental composition within a film; in addition, it shows the elements they are bonded to. Furthermore, the chemical state, the electronic state and the empirical formula of the elements can also be determined using this technique (Khulbe and Matsuura 2017). In XPS, a beam of X-ray

FIGURE 7.5 SEM-EDX analyses. Dry solid from surface of membrane element (Source: Ferrero et al. 2011).

is irradiated onto the material, simultaneously, analysis is made on the kinetic energy and the electrons number that escape from the surface (0-10 nm). High vacuum is required for XPS (Mather 2009). Wei et al (Wei et al. 2013) used XPS for chemical composition analysis for hollow fiber membranes that consisted of a supporting membrane of polysulfone/polyether sulfone and an active layer of polyamide. Figure 7.6 presents the results of the analysis. As shown in Figure 7.6, the two major emissions at 532 eV and 284.8 eV correspond to the binding energies of 0_ls and C_)s, respectively, while the emissions at 167 eV and 230 eV are attributed to sulfur atoms in the sulfone group. As also shown, an emission at 399 eV is only seen in the composite membrane. This indicates that the polyamide layer was successfully incorporated on the support.

Attenuated Total Reflectance - Fourier-Transform Infrared (ATR-FTIR)

FT1R is a technique that is used to identify materials. The infrared absorption bands identify the structures and the specific molecular components and give information about the functional groups of the membrane via detecting bonds between atoms. Attenuated total reflectance (ATR) is a sampling tool that is

FIGURE 7.6 XPS spectra of (a), the PS/PES-UF supporting membrane and (b), composite NF hollow fiber membrane (Wei et al. 2013).⁴

4 Reprinted from Chemical Engineering Journal 223. Wei, Xiuzhen, Xin Kong, Chengtian Sun, and Jinyuan Chen. “Characterization and application of a thin-film composite nanofiltration hollow fiber membrane for dye desalination and concentration.” 172-182, Copyright (2013), with permission from Elsevier.

used together with FTIR to measure the surface properties of the membrane (Ferrero et al. 2011, Khulbe and Matsuura 2017).

ATR-FTIR makes it possible to assess the efficiency of cleaning methods such as cleaning with NaOH, SDS, and Na-EDTA, by comparing the spectra before and after cleaning. In addition, it can be used to identify the fouling chemical nature (Ferrero et al. 2011. Khulbe and Matsuura 2017, U.S. Department of the Interior Bureau of Reclamation 2009). It can differentiate between different kinds of fouling layers, but cannot determine the thickness of fouling layers (Khulbe and Matsuura 2017). In addition, ATR-FTIR is used to confirm successful preparation or treatment of the filler in the polymer matrix (Xu et al. 2019).

The sample needs to be dried for FTIR. Samples are usually dried in an oven overnight before FTIR measurement (Xu et al. 2019).

In one study, a thin film composite nanofiltration hollow fiber membrane was characterized for chemical structure using ATR-FTIR in order to identify the functional groups, in which the amide group and the carboxylic group were identified (Wei et al. 2013). In another study, polyamide desalination membranes were grafted with MPEG-NFb, and ATR-FTIR were used to confirm grafting to the membrane surface. It was noted that there was an increase in peak intensities in regions referable to PEG. In addition, the grafted membranes had lower nitrogen and higher oxygen contents compared to the ungrafted membrane, which was consistent with a PEG-grafted surface (Van Wagner et al. 2010). Akbari et al (Akbari et al. 2010) prepared a composite of polysulfone membranes for nanofiltration application, and confirmed grafting of polyacrylic acid onto polysulfone using ATR-FTIR, in which the bands corresponding to the CO stretching at 1,713cm'¹ and OH stretching at 3,396cm'¹ increased upon grafting.

X-ray Powder Diffraction (XRD)

X-ray interacts with electrons of atoms, which causes some photons to be deflected away from their original direction. The diffracted photons are composed of sharp interference peaks, which are related to interlayer spacing according to Brag’s Law:

where n is the order of reflection; X is the wavelength of x-ray; d is the spacing between planes in the atom; and о is the angle at which the peak occurs, d-spacing is used to determine an unknown crystal.

Based on the aforementioned, XRD is a technique that is used to identify the phases and the crystal structure of different materials. XRD is usually used for characterization of membranes in order to determine the relative amounts of fouling material on the membrane surface, in addition to the various phases in which fouling material is present (Butt et al. 1997). In addition, XRD can be used to calculate interlayer spacing of the membrane material or filler to determine the degree of exfoliation that can be determined from the peak shifting (Huang et al. 2019).

XRD was used to examine the structure of graphene acid (GA) and graphene oxide (GO) membranes (Khorramdel et al. 2019). Results showed that GA had larger d- spacing compared to GO, which means that the number of layers were lower.

Huang et al (Huang et al. 2019), prepared GO membranes and rGO membranes, and also used XRD to determine d-spacing of different membranes. In addition, they were able conclude from XRD results the coexistence of both GO and rGO phases because of the existence of two peaks.

In another study, in which a hydrophilic modifier of graphitic carbon nitride nanosheets were synthesized, XRD was used to confirm the purity of the nanosheets (Shahabi et al. 2019).

Water Contact Angle

Water contact angle is measured between a tangent to the liquid surface, where the vapor - liquid interface meets the surface of the solid, and the plane of the solid surface on which the liquid settles or moves. It is the most common parameter that is used to determine the hydrophobicity of the used membrane. High contact angle (>90°) indicates high hydrophobicity (Khulbe and Matsuura

2017). Other surface properties can be known from the water contact angle, including roughness, homogeneity, and cleanness (Tretinnikov and Ikada 1994).

The goniometric technique is one way of measuring the contact angle with sessile drops. Placing a tangent to the drop plane at the contact point performs a direct measurement. However, this technique has a large error of ±2°, which makes it unsuitable for small angles (Dimitrov et al. 1991). An alternative method is the axisymmetric drop shape analysis (ADSA). This method is based on a drop profile using a computer processing techniques that uses drop height in its calculations. ADSA-CD, however, is based on the measurements of two parameters — the drop volume and the contact diameter. This technique can be applied for the measurements on imperfect solids (Dimitrov et al. 1991).

Another technique that is used to determine the contact angle is the Wil- helmy Method. In this method, the membrane sample is immersed in a liquid. Then, the measured force is used to calculate the contact angle by applying the Wilhelmy equation (Della Volpe et al. 2001)

where 0 is the contact angle; у is the surface tension of the liquid; and p is the wetted perimeter.

Albo et al (Albo et al. 2014) used a goniometer to examine the membrane wettability by sessile water drops. The contact angle was determined by specifying a circle around each drop and then recording the tangent angle that was formed at the surface.

In another study, in which graphene quantum dots were incorporated into thin film membranes, the water contact angle was measured using an automatic contact angle meter. It was noted that incorporation of graphene quantum dots had increased the contact angle because the graphene quantum dots had larger surface that consisted of hydrophobic aromatic rings, which rendered the membrane surface into a less hydrophilic surface(Xu et al. 2019).

Ren et al (Ren et al. 2015) used the contact angle to determine the effect of grafting of alumina membrane using fluoroalkylsilane. They found that the surface was converted from hydrophilic to hydrophobic with a contact angle of 133°. Furthermore, contact angle measurement was used to examine the effect of carboxylic functionalization of GO membranes on the hydrophobi- city/hydrophilicity of the membrane. It was observed that the hydrophilicity was enhanced upon functionalization when the contact angle was decreased (Khorramdel et al. 2019).

Zeta Potential Measurement Surface Charge

Zeta potential is the difference in potential between the surface of solid particle and the liquid in which it is immersed. It is an important indicator of stability of colloidal dispersion. High values of zeta potential indicate good stability of the colloid. In membranes, zeta potential is used to determine the surface charge, which is important since it affects rejection of ions through the membranes (Van Wagner et al. 2010), and significantly affects fouling (Xu et al. 2019). Increasing solution's pH changes the zeta potential of the surface from positive to negative, consequently affecting the accumulation of charged particles on the surface of the membrane (fouling) (Khulbe and Matsuura 2017).

Wei et al (Wei et al. 2013) studied the surface charge of their hollow-fiber nanofiltration membrane via surface zeta potential measurement. At pH lower than 6.6, the membrane surface had positive charge, while at pH higher than 6.6, the membrane surface had negative charge.

Moreover, zeta potential can be used to determine the cleaning effect on membrane surfaces. In a study that used both virgin membranes and fouled membranes, different cleaning agents (HC1, NaOH, SDS and mixed agents) were investigated. Zeta potential were measured for all membranes before and after cleaning. Results showed that the surface properties and performance of membranes were affected by cleaning. The permeability of both virgin membranes and fouled membranes, were higher when a cleaning agent is used. It was concluded that both permeability and charge characterization methods were useful in checking the cleaning efficiency to restore the membrane performance without affecting the material of the membrane or its surface charge (Al-Amoudi et al. 2007).

Thermal and Mechanical Properties

Thermal Gravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC)

TGA is a thermal analysis technique that shows the thermal behavior of a material under controlled heating conditions. It can be used to study the membrane material specifically to compare different membranes with different chemical compositions, and to relate it to the membrane performance, which helps in examining the membranes morphology/characterization (Khulbe and Matsuura 2017). TGA provides information about the examined membrane, such as initial decomposition temperature, thermal decomposition stages, and percentage of inorganic materials. Such information proves the existence and the percentage of any nanofiller in the membrane polymer matrix, in addition to confirming any grafting that is performed on the membrane material (Kim et al. 2013).

Mohan et al (Mohan and Kullova 2013) studied the relationship between conditions of preparations such as reactant concentrations and the performance of different polyamide thin film composites membranes, that were supported by polyethersulfone (PES). Using TGA, they found that initial decomposition temperatures of the blends of PES and polyamide were not the same for different membranes. They ranged from 20°C to 90°C lower than that of PES. Such differences in thermal behavior suggested differences in chemical composition.

DSC is another thermal analysis technique that examines the crystallization behavior of a material. It depends on measuring the heat that is required to increase the temperature of the sample. A reference sample, with a well-known heat capacity over the range of heating/cooling, is heated at the same rate as the tested sample to compare the difference in heat during melting and crystallization of the tested sample. Similar to TGA, DSC is used to study the membrane materials. It provides information about melting and crystallization onset temperatures, as well as melting and crystallization temperatures. Such information confirms different chemical compositions in addition to the presence of any nanofillers.

Furthermore, glass transition temperature (T_g) can be found using DSC. Higher T_a indicates that the membrane has a looser structure owing to more free volume fraction (Arthanareeswaran et al. 2004)

Tensile Strength Test

The tensile strength test is a mechanical test that examines the effect of tension on the tested materials in terms of stress - strain plot. Tensile modulus, tensile strength, yield point and percentage of elongation can be calculated from stress - strain plot. For membrane testing, the tensile test can be used to test the membrane strength, and to study the effects of pores shape, size and density on the mechanical properties of the membrane (Mahmoud et al. 2015). Tensile test is mainly used to test RO membranes that operate at very high pressure (Khulbe and Matsuura 2017).