## 3 Results and Discussion
### Wettability of the Membranes  
The interfacial energy and wettability difference of the hydrocarbon and water on the solid surfaces provide a significant route for fabricating superhydrophilic and underwater superoleophobic membranes [37,66]. Controlling these properties minimizes oil adhesion underwater, which is quantitatively described by the work of adhesion. The work of adhesion, defined as the energy required to separate oil when it adheres to a surface in water, is calculated using a simplified form of the Young-Dupre equation (Equation (6)):  
\[W=\gamma_{23}(1+\cos\theta_{0}) \tag{6}\]  
where \(\gamma_{23}\) is the interfacial tension between crude oil and water (N/m), and \(\theta_{0}\) is the oil contact angle (OCA) on the surface, as illustrated in Figure 9g. Thus, \(W\) becomes zero when the \(\theta_{0}\) reaches 180\({}^{\circ}\), signifying that a larger OCA correlates with lower adhesion work [67,68]. Based on this principle, superhydrophilic and underwater superoleophobic membranes can be fabricated by incorporating hydrophilic functional materials on a PVDF substrate.  
The untreated PVDF membrane displayed hydrophobicity with a WCA of 120\({}^{\circ}\) and underwater oil contact angle (UOCA) of 98\({}^{\circ}\) (see Figure 9a,d). To measure the UOCA, a droplet of crude oil was injected from below the submerged membrane using a microsyringe and raised to contact the membrane surface, as depicted in Figure S4. Finally, the three-phase contact angle of the membrane/crude oil/water at room temperature was measured. Plasma surface treatment decreased the WCA to 47\({}^{\circ}\), and the UOCA increased to 148\({}^{\circ}\) (see Figure 9b,e), which is related to increased surface wettability. This change is attributed to the introduction of polar groups and increased surface ro  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Membrane**} & \multicolumn{2}{c}{**C1s**} & \multicolumn{2}{c}{**O1s**} & \multirow{2}{*}{**O/C**} \\ \cline{2-2} \cline{4-6}  &
\begin{tabular}{c} **Binding** \\ **Energy (eV)** \\ \end{tabular} & & & **Atomic (\%)** & **Binding** \\ \hline PVDF & 286.23 & 52.03 & 531.74 & 4.03 & 0.077 \\ \hline LEP/PVDF & 285.54 & 86.8 & 532.15 & 10.57 & 0.122 \\ \hline AM-NaA/PVDF & 284.89 & 60.54 & 531.29 & 25.87 & 0.427 \\ \hline \hline \end{tabular}
\end{table}
Table 2: The atomic composition of H-1, LEP-H1, and H-6, as determined by XPS.  
Figure 9: Contact angles: (**a**–**c**) WCA of H-1, LEP-H1, and H-6, respectively; (**d**–**f**) UOCA of H-1, LEP-H1, and H-6, respectively; and (**g**) the schematic shape of a crude oil droplet on a membrane surface underwater.  
treatment, which amplifies hydrophilicity on the LEP-H1, consistent with the [PERSON] model [69]. These results confirm that the surface treatment was successful, and the surface is ready for the subsequent coating step. H-6 exhibited a WCA of 0\({}^{\circ}\) and a UOCA exceeding 151\({}^{\circ}\) (see Figure 9c,f), confirming its superhydrophilic and underwater superoleophobic characteristics.  
The enhanced wettability of the membrane surface caused by plasma treatment significantly improved the water flux and the separation efficiency of crude oil-in-water emulsions by PVDF membranes coated with a crosslinked AM/NaA copolymer (i.e., when V-50 = 0.0 g). Figure S5 demonstrates that the permeate of crude oil-in-water emulsion increased to 1435 L/m\({}^{2}\cdot\)h at 0.01 MPa after 4 min of plasma treatment, and the oil removal ratio reached 99%.