(186m) Nonionizable Surfactants as Charge Control Agents in Nonpolar Liquids

Non-polar liquids do not easily accommodate electric charges, but ionic surfactant additives are found to help raise the conductivity and stabilize the immersed solid surfaces. They avoid the large energetic barrier associated with the introduction of small ions in low dielectric media by forming charged micellar aggregates. Here, we study the rarely considered charging effects induced by surfactant molecules without ionizable groups, sorbitan oleate. Precision conductometry, light scattering, and Karl-Fischer titration reveal a distinctly electrostatic action of the nonionic surfactant. The behavior that non-ionizable surfactants induce charge in non-polar oils can be explained by charge disproportionation of molecular surfactant complexes in the sub-CMC regime and reverse micelles above the CMC. Electrokinetic studies of added colloidal particles further exhibit the micelles' ability to create and screen electric charges on the surface of suspended solid particles. Strong particle-charging effects and charge screening are found even at surfactant concentrations below the critical micelle concentration. Our findings suggest that nonionic surfactants can play a key role in the industrially important task of controlling charges in non-polar solvents.