Nonionic surfactants (Figure 8.6) do not ionize in water because they have no charge on their heads. The solubility of nonionic surfactants is affected by two main interactions: van der Waals forces and hydrogen bonding. Nonionic green surfactants can be extracted from biological sources and plants (roots, leaves, etc.). Therefore, they are cheap and biodegradable. Table 8.4 lists some examples of nonionic surfactants used in EOR. A common nonionic natural surfactant produced from renewable and natural sources is the alkyl polyglycoside (APG) [132]. This is a carbohydrate-based surfactant [132]. The hydrophilic part of APG is a sugar structure, which is formed by connecting d-glucose through glycosidic bonds, and its hydrophobic part comes from fatty alcohols. In addition to bio-based surfactants, the presence of alcohol resulted in an ultra-low IFT.non ionic surfactant

Saponin is a natural surfactant that can be used as an alternative to synthetic surfactants. In other words, saponins are the main source of plant-based natural surfactants [33]. Sea life and plant species are good sources of saponins. It is derived from plants belonging to the Sapindaceae family [52, 57]. The good solubilization, foaming and emulsification of saponins are the main reasons for their wide application in industry [52,53,57,58]. The surface activity of saponins is due to the simultaneous presence of sugar chains and aglycones. The nature of saponins is considered to be slightly acidic, and the main reason for this property is the hydrolysis of glycosides [56]. Saponins are high molecular weight glycosides with detergent properties and are considered to be a natural surface active substance. It can also produce stable foam in water [133]. Table 8.5 [53, 134–136] lists the chemical formulas of saponins obtained from Sapindus berries reported in the literature. Saponins contain 6%H, 51%O and 44%C. High tolerance to pressure and temperature is another aspect of Sapindus saponins [137].
Table 8.5. Chemical formulas, molecular ions, mass-to-charge ratios, and molecular weights of triterpene saponins from Sapindus pericarp.

Tannic acid-based surfactants are nonionic surfactants with good surface properties and can reduce IFT better than traditional anionic surfactants (i.e., SDS) [131]. bold