UC Berkeley

Membrane-based separations have been increasingly applied to address the challenges of water scarcity and pollution facing humanity. Two-dimensional (2D) nanomaterials, including graphene, graphene oxide (GO), reduced GO (rGO), and their composites have exhibited many unprecedented properties that can be promisingly exploited to make novel membranes with enhanced performance. Analogous to the cylindrical pores of traditional polymeric membranes, the 2D nanochannel formed between horizontally restacked GO nanosheets enables size exclusion by rejecting unwanted species from water and provides a pathway for water permeation across the membrane.

The aqueous-phase separation capability of a layer-stacked GO membrane can be significantly limited by its natural tendency to swell, that is, absorb water into the GO channel and form an enlarged interlayer spacing. In order to fully understand the nanostructure of the GO membrane in the liquid phase, we developed an in-situ monitoring approach by combining quartz crystal microbalance with dissipation (QCM-D) and liquid-phase ellipsometry to simultaneously monitor the mass and thickness change of the GO membrane (Chapter 2). This novel method allows the precise quantification of the interlayer spacing when the GO membrane is set in the real working conditions such as in the aqueous solutions and organic solvents. Using this method, the interlayer spacing of GO in solutions with different ionic strength and pH was measured, which provided profound insight into the swelling of GO layers in aqueous solutions. Moreover, for the first time, the density change of water in the confined GO nanochannel was experimentally detected. Molecular simulations were conducted to understand the structure and mobility of water in the GO channel, and a theoretical model was developed to predict the interlayer spacing. It was found that, as a dry GO membrane was soaked in water, it initially maintained an interlayer spacing of 0.76 nm, and water molecules in the GO channel formed a semi-ordered network with a density 30% higher than that of bulk water but 20% lower than that of the rhombus-shaped water network formed in a graphene channel. The corresponding mobility of water in the GO channel was much lower than in the graphene channel, where water exhibited almost the same mobility as in the bulk. As the GO membrane remained in water, its interlayer spacing increased and reached 6 to 7 nm at equilibrium. In comparison, the interlayer spacing of a GO membrane in NaCl and Na2SO4 solutions decreased as the ionic strength increased and was ∼2 nm at 100 mM.

To gain insight into the potential applications of GO membranes in the process of organic solvent nanofiltration (OSN), the swelling behavior of GO was also investigated in organic solvents (Chapter 3). To understand the swelling mechanism, the solubility parameter of GO was experimentally determined and used to mathematically predict the Hansen solubility distance (Ra) between GO and solvents, which is found to be a good predictor for GO swelling and the interlayer spacing. Solvents with low solubility distance (e.g., dimethylformamide, n-methyl-2-pyrrolidone) tend to cause significant GO swelling, resulting in an interlayer spacing of up to 2.7 nm. Solvents with high solubility distance (above 10) such as ethanol, acetone, hexane and toluene only cause minor swelling and are thus able to maintain an interlayer spacing of around 1 nm. Correspondingly, GO membranes in solvents with high solubility differences exhibit better separation performance, for example > 90% rejection of small organic dye molecules (e.g., rhodamine B and methylene blue) in ethanol and acetone without crosslinking due to insignificant swelling. Additionally, solvents with higher solubility distance results in a higher slip velocity in GO channels and thus higher solvent flux through the GO membrane.

To prevent GO swelling in aqueous solutions, a layer-by-layer (LbL) technique was employed to synthesize the GO membrane, where GO was used as a special polyanion to electrostatically bind with positively charged polyethyleneimine (PEI). The effects of synthesis conditions (i.e., ionic strength, pH, and dielectric constant) on the membrane properties and performance were investigated (Chapter 4 and 5). The ionic strength was found to significantly affect the charge and size of GO (and less so for PEI) in deposition solution, increasing the mass deposited in each LbL cycle. The pH of GO and PEI solutions could be used to adjust the charge distribution within GO-PEI multilayers, which offers a unique opportunity to make membranes with an internal positive charge for improved removal of multivalent cations regardless of membrane surface charge. Moreover, organic solvent was used to tune the dielectric constant of the synthesis media. Membranes synthesized in water–ethanol mixture was found to have more than doubled mass loading and enhanced stability in solutions with ionic strength of up to 100 mM.

It is also important to understand water and solute transport in the 2D nanochannels of GO membranes because they are fundamentally different from that in traditional polymeric membranes. The removal of three representative pharmaceuticals and personal care products (PPCPs) by diamine-crosslinked GO membranes was studied. The decoupled diffusion and partition coefficients for three representative PPCPs were quantified by adsorption experiments performed on a QCM-D sensor. The results suggest that the diffusion coefficient of caffeine, acetaminophen and carbamazepine in the confined GO nanochannels are 1.13x10-9 cm2/s, 2.05x10-9 cm2/s and 0.48 x10-9 cm2/s, respectively, which are more than four magnitude slower than that in bulk solutions. The activation energy for the diffusion and partition process of carbamazepine were calculated to be 42.8 kJ/mol and -33.6 kJ/mol, respectively, indicating the molecular transport is dominated by the diffusion process while high petitioning is the main cause of lower-than-expected rejection. By adjusting the surface chemistry (i.e., GO reduction) and interlayer spacing of GO membrane, we found that the molecular diffusivity in the GO membranes is closely related to the interfacial interactions and the size ratio between the solute and GO channel.