Effect of Surfactants in Aqueous Solutions on Oil-Resisting Performance of Membrane Surfaces with Charges by Atomic Force Microscopy

The state of the surface of oil droplet can be changed obviously by adding a small amount of surfactant in water. The oil-resisting performance of membranes for water/oil separation has a significant influence on filtration efficiency and life span. The atomic force microscope (AFM) is commonly utilized to explore interface interaction. In this paper, the probe modified by an oil (hexadecane) droplet was used as a force sensor to detect adhesion forces between oil and the charged membranes in fluid. Under low concentration of cationic surfactant in water, the adhesion between oil and the nanofiltration (NF) membrane with negative charges was intensified, increasing from 91.04 nN in pure water to 152.33 nN. On the contrary, the low concentration of anionic surfactant resulted in lower adhesion, which decreased from 5.10 nN in pure water to 3.13 nN in this situation. The measured force behavior agreed well with experimental observation from contact angle measurement. However, the high percentage of ionic active agent caused lower interaction in the above two cases. Moreover, the effect of the nonionic active agent on the adhesion between oil and two kinds of charged membranes was also discussed. Results show that the interaction would be weakened with the aid of the nonionic active agent in water, regardless of the positive or negative charges on the membrane surface.