## 3 Results and Discussion
### Membrane Characterization
Previous studies have shown that at a certain loading threshold, where the casting solution becomes too viscous, a delayed demixing occurs and membranes with lower porosity and fewer interconnecting channels are formed [47]. Interestingly, this phenomenon is not observed in our membranes. This could be due to the viscosity of the casting solution not reaching the threshold value where viscosity acts to essentially slow down the demixing process (for viscosity of casting solutions see Figure S3). The decrease in both pore size and total porosity for M9 could be explained due to the fact that M9 was cast without the addition of AG and a relatively low loading of GO (0.3 wt. %). The membrane hydrophilicity and hence, wettability was determined via contact angle measurements using the Young Laplace fitting method (see Figure 6). Figure 6a depicts the results for the average contact angle values determined for the PES/GO and neat PES membrane samples, whilst Figure 6b,c show the shapes of the water drops on different membrane samples. In general, the smaller the contact angle, the more hydrophilic the membrane surface is. The noticeable trend points to a decrease in contact angle as the GO content is increased from 0 wt. % in M1 to 5 wt. % in M8 by 36%. Interestingly, we see a 9% drop in contact angle from M1 to M2 samples and this can be attributed to the introduction of AG. This correlates well with a previous study where the authors noted a drop-in contact angle caused by the charged polysaccharide residues present in AG which act to hydrophillize the membrane surface, thereby causing a decrease in the contact angle [39]. Membranes M3 through to M8 contain both AG and GO and here we see an additive effect where both hydrophilic materials act to decrease the contact angle values. The results for the M9 sample were not as expected, with the M9 sample exhibiting a lower contact angle than the M4 sample which had the same GO loadings as M9, however with the addition of AG. The hydrophilic nature of AG leads to the expectation that the M4 sample would possess a lower contact angle value than its M9 counterpart. These paradoxical results could be due to better distribution of GO at the membrane surface for the M9 sample, which may not be the case in the M4 sample. Figure 6: Average contact angle values for synthesized membranes (**a**), sample photographs of contact angles for M4 (**b**) and M7 (**c**). The zeta potential data yielded from the pH scans for selected PES/GO membranes is presented in Figure 7. As can be seen, the neat membrane (M1) possesses the smallest negative charge across all pH values. The surface negative charge of membranes with AG alone (M2) and with AG/GO (M3 and M8) is increased, with M8 having the greatest negative charge overall. At pH 7, for instance, the negative zeta potential increases by over 100% for M8 compared to M1 samples. The increase in negative zeta potentials for the PES/GO/AG membranes in comparison to the neat PES membrane can be attributed to the presence of the hydrophilic residues of the AG which contain carboxylic groups as well as the hydrophilic functional groups that are present on the GO flakes [39]. All membranes show an increase in negative charge as the pH increases. This trend is a result of the selective adsorption of the negatively charged chlorine ions on the membrane surface which leads to charge accumulation [48]. The overall negative surface charge of these membranes renders them attractive candidates in tackling fouling and biofouling issues where the fouling agents are often found to be negatively charged [49]. The increase in hydrophilicity and negative surface charge seen thus far in the PES/GO membranes can further be explained in terms of the oxygen containing functional groups that were introduced upon the incorporation of GO into the PES membranes. The presence of such functional groups can be evidenced by means of ATR-FTIR analysis. Figure 8 shows the ATR-FTIR spectra for the neat membrane (M1) and M8 samples, which has the highest loading of GO.