There is a great need to improve synthetic membrane filtration performance (increased permeation flow rates and higher selectivity) for pressure-driven membrane systems for water, energy, pharmaceutical, and biotechnology applications like desalination, fuel cells, small molecule purification, and protein separation, respectively. The limiting problem is membrane fouling due to adhesion of foulants to the membrane surface and subsequent inhibition of filtration performance (flux and selectivity). To date, the fouling process is still poorly understood due to its complex and random nature. Recently, we have utilized Sum Frequency Generation (SFG) spectroscopy to study water structure and interactions at a series of chemically modified interfaces. SFG allows one to measure the vibrations and structure of interfacial water specifically through a nonlinear optical process, where IR and visible light interact with the sample resulting in the emission of a photon at the summed frequency. These measurements allow one to study how water interacts at different surface chemistries and gives insight into what is occurring during the fouling process.
Since studying the buried interphase between a functionalized surface and water with SFG requires a transparent substrate, an analogous system to a conventional opaque polymer membrane was developed. Poly(ether sulfone) was spin-coated onto CaF2 wafers and then the surfaces were activated using Atmospheric Pressure Plasma (APP). The monomers were added to the activated surfaced and polymerized via free radical polymerization from the PES surface. The surfaces were modified with various different monomers, such as PEG, zwitterions, and hydrophobic alkanes. A study was also performed to determine the effect of monomer concentration on interfacial water structure. The new surfaces were then characterized using Atomic Force Microscopy (AFM) and then analyzed using SFG. Preliminary results showed that water is extremely ordered at a surface modified with PEG when compared with an unmodified control PES membrane. This supports the rule that hydrophilic surfaces are protein resistant and this could be due to the structure of water at the membrane surface during fouling.