Precise and Ultrafast Molecular Sieving Through Graphene Oxide Membranes

Abstract

Graphene-based materials can have well-defined nanometer pores and can exhibit low frictional water flow inside them, making their properties of interest for filtration and separation. We investigate permeation through micrometer-thick laminates prepared by means of vacuum filtration of graphene oxide suspensions. The laminates are vacuum-tight in the dry state but, if immersed in water, act as molecular sieves, blocking all solutes with hydrated radii larger than 4.5 angstroms. Smaller ions permeate through the membranes at rates thousands of times faster than what is expected for simple diffusion. We believe that this behavior is caused by a network of nanocapillaries that open up in the hydrated state and accept only species that fit in. The anomalously fast permeation is attributed to a capillary-like high pressure acting on ions inside graphene capillaries.

Graphene membranes for water desalination

“…However, at the gap between the edges of non-interlocked neighboring GO sheets and, at the cracks and holes of the sheet, they can diffuse perpendicular to the GO sheets and permeate through the membrane. 18,55 Thus, such structural defects supplement the role of the interlayer nanochannels in the separation of the ions and molecules. Moreover, oxygen-containing functional groups on the GO sheets are ionized in the wet state and thus negatively charged.…”

“…55 In this regard, the interlayer spacing of the GO sheets has a key role. Although in graphite, which is composed of pristine graphene sheets, the interlayer spacing in dry state is only 3.4 Å, the presence of oxygen-containing functional groups on the GO sheets increases this spacing to 6-7 Å.…”

“…18 Such promising features make multilayer GO structure an ideal candidate for the production of advanced ionic and molecular sieving membranes for desalination. 1,4, [53][54][55][56] Theoretical considerations. According to molecular simulations, water flows at a very high rate in planar graphene nanochannels because of the very large slip length and low friction on graphene sheets.…”

Graphene-based membranes for organic solvent nanofiltration

“…For example, it was found that sub-micrometer-thick graphene oxide (GO) membranes were almost impermeable to liquids, vapors, or gases but allowed unimpeded permeation of water [1] because of the nearly frictionless flow of water molecules through the two-dimensional hydrophobic nonoxidized capillaries. Moreover, it was further demonstrated that GO membranes could act as molecular sieves to block all solutes with hydrated radii > 0.45 nm when immersed in water [2], in which size exclusion appears to account for this behavior. For smaller sizefitted solutes that can enter in nanochannels, GO membrane could achieve more accurate ion selectivity via the diverse interactions between ions and sp 2 clusters or oxygen-containing functional groups on GO basal planes [3].…”

Nanotechnology Solutions for Global Water Challenges

“…A schematic representation of the desalination of salt water is shown in Figure 6. R. K. Joshi et al studied the permeation of water through micrometer thick GO laminates prepared using a vacuum filtration technique (112). The GO laminates act like a molecular sieve which block all solute particles possessing a hydrated diameter greater than 4.5 Å.…”