## 3 Results and Discussion  
The HP\(\upmu\)S membrane and TPC membrane were characterized in terms of support morphologies and active layer characteristics, respectively. When it comes to the active layer characteristics, the active layer thickness, roughness, and crosslinking degree were investigated to double-check whether 1 wt% DMSO worked properly to improve the water permeability without compromising salt rejection, as described previously [27]. According to our observations (Figure 1a-c), 1 wt% DMSO led to a thinner and smoother active layer with slightly more unreacted acyl chlorides (evidenced by the slightly larger intensity at 950 cm\({}^{-1}\)). The thinner active layer with a slightly lower crosslinking degree at a moderate level was expected to increase water permeability without compromising salt rejection. The as-prepared HP\(\upmu\)S membrane was confirmed to have narrow and regular columnar macrovoids (Figure 1d), as reported previously [26]. The phenomenon is attributable to the fact that the solvent mixture (NMP: DMF = 1:7) induces rapid desolvation by enhancing the thermodynamic instability of a PSf polymer solution. The HP\(\upmu\)S membrane was expected to endure high pressure ranging from 25 to 55 bar owing to the unique structural characteristics. Figure 2a,b show the as-prepared TFC-D1 membrane's water flux, salt rejection, \(A\), and \(B\) values measured at different operating pressures, respectively. The water flux and salt rejection of the TFC-D1 membrane were 5.54 LMH/bar and 98.9% at 25 bar, which looks reasonable as compared to the previous result [27]. However, as the operating pressure increased from 25 to 55 bar, the \(A\) value was reduced from 5.54 to 3.31 LMH/bar while salt rejection increased from 98.9% to 99.3%. This phenomenon is likely to arise from membrane compaction as the pressure difference across the membrane increases. It seems understandable that membrane compaction was accompanied by a decreased \(A\) value and higher salt rejection, but two points caught our eye. First, a process approach to increase salt rejection (for example, by applying a higher pressure) will not allow us to overcome the trade-off between water permeability and salt rejection since the trade-off is intrinsically linked with the properties of the polyamide layer, according to empirical evidence [13; 37]. Consequently, the process approach of increasing operating pressure will entail a higher energy consumption (i.e., SEC will increase). Second, unlike variable \(A\) value and salt rejection which change with applied pressure, \(B\) value was nearly constant throughout the entire operating pressure range. This phenomenon is ascribed to the fact that ion transport is mainly governed by the concentration gradient, which would be almost constant at the same recovery rate regardless of applied pressure. This result suggests that a process approach could not make a fundamental change in the improvement of the selectivity of RO membranes, although it could result in a bit higher salt rejection apparently at the expense of a lower \(A\) value. Figure 1: (**a**) Cross-sectional SEM images (Scale bar: 500 nm) and (**b**) AFM images of TFC-D0 (top) and TFC-D1 (bottom) membranes with the average thickness and roughness values of the active layers, respectively. (**c**) FTIR spectra of TFC-D0 and TFC-D1 membranes observed at 950 cm\({}^{-1}\). (**d**) Cross-sectional SEM image of the HP\(\upmu\)S membrane (Scale bar: 50 \(\upmu\)m). Our observation implies that a material approach is required to achieve a fundamental improvement in the selectivity of RO membranes. For instance, the high-selectivity RO membrane was recently developed using porous nanomaterials called cucurbit[6]uril (CB[6]; it features 0.39 nm pores with a partially negative charge) and demonstrated that it could significantly reduce salt permeability to half of its original value (1.29 to 0.66 LMH) without significant flux decline (4.30 to 4.01 LMH/bar) owing to selective water transport over monovalent ions [2].