## 3 Results and Discussion
### Influence of Oleanolic Acid and Polypphenols on Phosphatidylcholine Liposomes Zeta Potential  
Zeta potential measurements indicate how electrical charge varies on the membrane surface. Using this data, it is possible to extract information about changes in the membrane's biophysical properties, as well as indirectly about the presence of the analyzed molecules on the liposomes' surface. The zeta potential of a phosphatidylcholine membrane at pH 7.4 is negative and amounts to \(-49.6\) mV (Figure 7a). After the addition of OA (5-100 umol), zeta potential decreases to about \(-40\) mV (\(-39.5\) mV to \(-43.9\) mV, depending on the concentration). Further experiments considered the effect of phenolic compounds on the zeta potential of an OA (20 umol)-enriched membrane. The addition of AP (Figure 7b) resulted in a lowering of zeta potential from \(-42.5\) mV (empty liposome) to about \(-30\) mV (regardless of concentration in the range 5-100 umol of flavonoid). A similar phenomenon was recorded in the presence of rutin (decrease of potential zeta from \(-42.5\) mV to \(-29\) mV at an RT concentration of 100 umol; Figure 7c). An almost equal potential lowering (from \(-42.5\) mV to \(-29.5\) mV) was found at a resveratrol concentration of 100 umol (Figure 7d). The smallest decrease of this value was observed in samples containing FA; at a ferulic acid concentration of 100 umol, zeta potential was equal to \(-34.3\) mV (Figure 7e). According to [PERSON] and co-workers [31], the zeta potential of egg yolk phosphatidylcholine liposomes in water was about--33 mV. After the addition of cholesterol, stigmasterol and \(\beta\)-sitosterol, it increased to about \(-34\) mV, \(-37\) mv and \(-38\) mV, respectively. The authors claim that zeta potential is related to hydrogen bonding between sterols and phospholipids. Hydrogen bond strength favors the presence of electronegative atoms [39]. OA considered in our experiment influences zeta potential differently than structurally similar cholesterol, stigmasterol and \(\beta\)-sitosterol. As shown in fluorescence measurements, OA is located in the hydrophobic core of the membrane, which does not promote the formation of hydrogen bonds. On the other hand, the lowering of zeta potential observed in the presence of OA may indicate its partial presence on the membrane surface. In DSPC liposomes, apigenin slightly (from \(-4.2\) mV to \(-3.8\) mV) reduces zeta potential compared to empty membranes [34]. In the case of RT, the influence of flavonoids on liposome zeta potential is dependent on bilayer composition. In liposomes made from egg phosphatidylcholine and cholesterol, the addition of rutin slightly increased this negative value from \(-17\) mV to \(-20\) mV. For DOPC/DOPE membranes, a decrease of zeta potential from \(-32\) mV to \(-21\) mV in the presence of rutin was observed. In turn, RT changed the potential value in DOPE/DOPA membranes, shifting it from positive (42 mV) to negative (\(-46\) mV) [37]. [PERSON] and co-workers [36] found that resveratrol did not affect soybean phosphatidylcholine liposomes' surface charge. In the current study, the observed decrease in zeta potential after the addition of RS may indicate that OA modifies the effect of resveratrol on the phosphatidylcholine membrane surface charge. In membranes composed of egg phosphatidylcholine, it was observed that ferulic acid caused a decrease of zeta potential from \(-10.5\) mV to \(7.37\) mV [38]. The data presented above indicate that changes of zeta potential in liposomes can be affected by various factors, such as the type of lipids that compose the bilayer and the presence of additional substances (cholesterol, plant sterols, triterpenoids and phenolic compounds). [PERSON] and co-workers [37] claim that the value of the zeta potential helps identify the dominant compound on a particle's surface. In our study, this allows us to supplement the data provided by spectroscopic measurements.