# Easy Fabrication of Ultrafiltration Membrane via Polyethersulfone-Fumed Silica
## 2 Materials and Methods
### Membrane Characterization  
In this study, the performance of the blend membranes was evaluated using pure water as the test fluid. The experiment was carried out for 30 min to achieve a stable flux in a stirred dead-end cell (HP4750 Stirred Cell, Sterlitech Corporation, Kent, WA, USA) with an effective membrane filtration area of 13.4 cm\({}^{2}\), as illustrated in Figure 1. To maintain consistent pressure in the pure water chamber, nitrogen gas was introduced into the dead-end-cell unit at a constant pressure of 2 bar. Throughout the experiment, the permeating water weight that penetrated through the tested membrane was recorded using a Weighing Environment Logger (A&D Co. Ltd., Tokyo, Japan). The equations below were utilized to determine the volumetric flux (\(J_{v}\)) and permeability flux (\(L_{p}\)) [22]:  
\[J_{v}=\frac{Q}{A\times\Delta t} \tag{1}\]  
\[L_{p}=\frac{J_{v}}{\Delta P} \tag{2}\]  
where \(Q\) is the permeate water quantity during the sampling time, \(\Delta t\) is the sampling time, \(A\) is the area of the membrane, and \(\Delta P\) is the pressure difference.  
After conducting the water flux test, a membrane selectivity test was performed using a 1 g/L protein solution under identical conditions. For the protein separation test, three distinct protein types with concentrations of 0.1 wt% were used: bovine serum albumin (BSA, Mw = 66.5 kDa), pepsin (Mw = 40 kDa), and lysozyme (Mw = 14.3 kDa). These proteins were prepared in a phosphate-buffered solution with a pH of 7.2. The protein separation test was conducted using a dead-end cell filtration test, applying a fixed pressure of 2 bar. The concentration of proteins in the supernatant solutions, or the permeated liquid, were determined using a N4S UV-Visible Spectrophotometer (Ningbo Hinotek InstrumentCo., Ltd., Ningbo, China). The operational wavelength was set at 280 nm. This specific wavelength corresponds to the absorption peak of amino acids [23]. The solute rejection (SR) was determined by [24]:  
\[\%SR=\left[1-\frac{C_{p}}{C_{f}}\right]\times 100 \tag{3}\]  
where \(C_{p}\) and \(C_{f}\) are the protein concentrations of the permeated and feed solutions, respectively. The data presented in this study represent the mean values of triplicate samples for each membrane.  
The membrane's hydrophilicity was evaluated by measuring the contact angle between a water droplet and the membrane surface. A total of 5 \(\upmu\)L pure water was carefully dropped on the surface of dried membrane using a micropipette. Subsequently, a digital microscope (Dino-Lite Edge 3.0 AM73915 MZTL, Dino-Lite, New Taipei City, Taiwan), captured an image of the water's droplet contact angle. CAD software (AUTOCAD 2022) was used to measure the angle between the membrane and the droplet surface. To ensure precision, the contact angles were measured at four different locations, and the average value was calculated. The membrane surfaces and cross-sectional morphologies were examined using scanning electron microscopy (SEM Phenom ProX, Thermo Fischer, Waltham, MA, USA). Cross-section micrographs were acquired by fracturing the wet membrane samples after rapidly freezing them in liquid nitrogen.