## 2 Results and Discussion
### Adsorption of Polyphenols Extracted from Grape Pomace onto mPEG-Grafted Silica Particles  
Figure 3 shows the adsorption capacities of the adsorbents used in this study. Bare silica showed only 10.1 mg GAE (gallic acid equivalents)/g adsorption capacity whereas surface grafting with mPEG-2000 and -5000 increased the adsorption capacity by 7.1- and 11.4-fold compared to bare silica, respectively. Higher adsorption capacities observed in mPEG-grafted silica particles compared to bare silica can be attributed to the more polyphenol binding sites. Grafting bare silica with mPEG introduces a high number of ether sites which presumably facilitates hydrogen bonding between polyphenols and mPEG ligands. A similar hydrogen bonding phenomenon can be seen between bare silica and polyphenols. Bare silica surface carries silanol groups which can potentially be involved in hydrogen bonding [39]; however, it is likely that the number of these sites are limited. Therefore, bare silica yields poor polyphenols adsorption capacity. This is in good agreement with other studies showing that bare silica surfaces showed very low quercetin adsorption capacity compared to titania-modified silica particles [40; 41]. mPEG-5000-grafted silica particles showed 1.6-fold higher polyphenol adsorption capacity compared to mPEG-2000. Although the conformation of mPEG on the silica surface is not known, it is likely that mPEG-5000 exposes more hydrogen bonding sites compared to mPEG-2000 due to the presence of higher number of repetitive PEG units. Exposing more ether sites would likely increase the probability and affinity of polyphenols to bind the PEG chains resulting in higher adsorption capacities. These results are in good agreement with literature showing that increasing the chain length of PEG from 6 to 43 repetitive units resulted in strong bonding to biomolecules available on cell surfaces, presumably through hydrogen bonding [26]. This unique ability of PEG provides surface tunable properties which regulate the polyphenol adsorption capacity with mPEG-grafted silica particles. In addition to the tunable properties, mPEG-grafted silica particles demonstrated higher adsorption capacities in comparison to traditionally used food-grade polymeric resins. [PERSON] et al. evaluated 13 different food grade resins such as Amberlites, Diaions, and SepaBeads with various structures and reported \(\sim\)1.5-2.6 mg GAE/g adsorption capacities for polyphenols recovered from winery waste [42]. It is important to note that the surface areas of these resins range from 300 to 1200 m\({}^{2}\)/g. mPEG-2000- and mPEG-5000-grafted silica particles used in our study have less surface area (Table 1) compared to the resins used in [PERSON] et al. [42] while demonstrating -28-48- and -44-77-fold higher total polyphenol adsorption capacities, respectively. These results confirm the high potential for mPEG-grafted silica particles to be used in the recovery of polyphenic compounds present in the winery waste. Since mPEG-5000-grafted silica particles showed the highest polyphenol adsorption capacity, desorption experiments were performed using these particles only.