Graphene Oxide Nanocapillary Water Filtration

Membrane

Interlayer spacing d 9.0 Å

Laminate thickness N 4 sheets

Applied pressure ΔP 3.0 bar

Dissolved species (hydrated Ø)

Live readouts

Effective opening

2.5 Å

Water permeance

—

Solute rejection

—

Passed / Blocked

0 / 0

Legend

Water (2.8 Å) — always slips through Na⁺ hydrated ion Mg²⁺ hydrated ion Dye / organic solute

How it works

Stacked graphene-oxide (GO) sheets form a laminate whose oxidised regions hold the layers apart by an interlayer spacing d, creating slit-shaped 2D nanocapillaries between them. Each capillary wall carries a bound monolayer of ordered water roughly t_w ≈ 3.5 Å thick (evidence from Nair et al., Science 2012), so only the space left over is free for a solute to pass:

Effective opening:   h_eff = d − t_w
Sieving rule:        species passes only if hydrated diameter δ < h_eff
Slit (Poiseuille) flow: J ∝ ΔP · h_eff³ / (12 μ N L)

This reproduces the two hallmark GO-membrane behaviours reported by Joshi et al. (Science 2014) and follow-up work:

Unimpeded water permeation — water's own diameter (~2.8 Å) is smaller than h_eff at almost any achievable spacing, so it flows through the near-frictionless capillary largely unrestricted by pore size, its rate set instead by capillary height cubed (h_eff³) and pressure.
Hydrated-size molecular sieving — ions and dyes carry a shell of bound water that sets their effective size. Shrinking d (e.g. by drying or capillary pressure) below an ion's hydrated diameter blocks it even though the bare ion itself is far smaller than the channel.
Laminate thickness N — a thicker stack means the flow must cross more series capillaries, so the modelled permeance falls roughly as 1/N even though the sieving cutoff itself is set only by d.

Drag d down and watch Mg²⁺ (larger hydration shell) get excluded first, then Na⁺, while water keeps flowing — the same size-selectivity that makes GO laminates candidates for desalination and dye-removal membranes.

d = 9.0 Å · N = 4 h_eff = 5.5 Å

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