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# Aquaporin transport by dielectrophoresis from interfacial water
## Introduction
Aquaporin-1 (AQP1) transports water across cell membranes at rates of ~10^9 molecules per second per channel, driven by osmosis. But what is the physical mechanism behind osmosis, and how does it act on water inside the channel? Charged adsorbed water on both sides of the membrane generates an electric field. Where a channel passes through, it creates an opening in this charged surface - and the field becomes non-uniform at the opening, exerting a dielectrophoretic force on water dipoles in the pore.
## 1. Water at surfaces
At hydrophilic surfaces, water forms a thin adsorbate - roughly 1 nm of water molecules held by the surface. Where this bound phase meets the free liquid, autoionization occurs: H2O -> H^+ + OH^-. The OH^- remains in the adsorbate; the H^+, far more mobile, escapes into the bulk. Separated between two phases, they are less likely to recombine - creating a persistent charge separation: negative at the surface, positive in the liquid. The electric field from this charge separation polarizes the adjacent water, reducing its ability to dissolve solutes - including H^+. This further suppresses recombination, which increases ionization, which strengthens the field, which polarizes more water: a positive feedback loop (Nygren, 2026).
## 2. Apertures in charged surfaces create field gradients
The adsorbate's charged surface (OH^-) covers the entire membrane. Any pore through the membrane creates a hole in this surface - a region of missing charge. At the hole, the electric field dips: charge is absent, so the field is weaker than in the surrounding surface. This is the aperture effect, well known in electrostatics and particle optics, where it forms the basis of electrostatic lenses.
In the case of two charged surfaces (adsorbate on each side of the membrane), each with a co-axial hole, the field profile along the channel axis is non-uniform. The field is weakest near each aperture (where charge is missing) and stronger in between. A particle with a permanent dipole moment in this field experiences a force: dielectrophoresis.
## 3. Asymmetric adsorbate drives net flow
With equal adsorbate on both sides, the field gradient is symmetric - the dielectrophoretic force at each aperture is equal and opposite. No net flow.
With asymmetric adsorbate - thicker on the low-salt side (more OH^-, stronger charge), thinner on the high-salt side - the symmetry breaks. The stronger adsorbate creates a deeper field minimum at its aperture. The weaker adsorbate creates a shallower one. The field is strongest near the weaker side.
Water dipoles are pushed away from the deeper minimum and toward the shallower one - from low salt toward high salt.
The force is estimated at a few pN per water molecule at aquaporin dimensions - sufficient to drive ~10^9 molecules/s.
## 4. Aquaporin geometry
AQP1 has an hourglass-shaped pore - wide vestibules (~6 A radius) narrowing to a central tunnel (~1.5 A radius) that extends ~20 A, channeling flow into single file. The vestibule opening interrupts the adsorbate's charged surface, creating the field gradient that drives dielectrophoresis.
The force scales inversely with aperture radius: smaller holes create steeper gradients and stronger force per molecule. AQP1's vestibule (~6 A) sits in the range where the dielectrophoretic force exceeds thermal noise even at modest adsorbate asymmetry. The narrow tunnel channels the driven water into single file.
At symmetry (equal salt on both sides), the forces at the two apertures cancel - no net transport. This is equilibrium. At asymmetry (salt gradient), the imbalance drives flow. When the gradient dissipates, flow stops. The osmotic pressure acts through a mechanical force: dielectrophoresis along the electric field gradient created by the adsorbate apertures.
## 5. The structure of adsorbed water
When a large fraction of H^+ has left the adsorbed water, the only stable structure is one where the layers are collapsed onto one another, each shifted sideways by half an oxygen atom so that the negative charges of one layer face the positive charges of the other. This is the structure Gerald Pollack calls the "fourth phase" of water, with ~50% ionization (Nygren, 2026). This ionization rate determines the surface charge density of the adsorbate, and therefore the strength of the dielectrophoretic force acting on water in aquaporin.
## 6. Quantitative estimate
Consider a stylized model: two parallel charged surfaces separated by d ~ 50 A (the plasma membrane), each with a circular hole of radius R = 6 A (representing the aquaporin vestibules). Side A (low salt) has surface charge density sigma_A; side B (high salt) has sigma_B < sigma_A. Both are negatively charged (OH^- in the adsorbate).
The axial field from a uniformly charged sheet with a circular hole is:
E(z) = sigma / (2epsilon) * z / sqrt(z^2 + R^2)
where z is the distance from the sheet along the axis. Superposing the two sheets gives the total field at any point in the channel.
The field is non-uniform along the axis. At side A, the hole removes more charge (higher sigma), so the field dips deeper. At side B, less charge is removed, so the dip is shallower. The result: |E| is weaker near A and stronger near B. The field grows from A toward B along most of the channel. The force on a water dipole (F = p * d|E|/dz) therefore points toward B - from low salt toward high salt.
For sigma_A = 2.9 C/m^2 and sigma_B = 1.4 C/m^2 (corresponding to four and two molecular layers of adsorbate at 50% autoionization), the force is tens of pN per molecule, directed toward side B. This is sufficient to drive ~10^9 molecules/s through the single-file channel - matching the measured AQP1 transport rate.
## 7. Relation to conventional osmotic theory
Van 't Hoff's equation (Pi = iMRT) describes osmotic pressure as a colligative property - dependent on solute concentration, not on any specific mechanism. This model does not contradict van 't Hoff - it provides the mechanism that produces the force van 't Hoff's equation describes.
The adsorbate thickness depends on salt concentration. Salt concentration determines the asymmetry. The asymmetry determines the force. The force determines the flow. The proportionality between salt concentration and flow - van 't Hoff's linearity - falls out naturally from the linear relationship between salt concentration and adsorbate asymmetry at low gradients.
## 8. Interfacial water as the cause of osmosis
Asymmetry in interfacial water between two sides of a membrane can cause osmosis through several mechanisms. Other than dielectrophoresis, an electric field can be produced from H^+ transfer by diffusion across the membrane (Zhao et al., 2009). This field drives OH^- directly, or - if O2 is available on the receiving side - electrons from OH^- oxidation, which combine with H^+ and O2 to form water. In all cases, the driving force originates in the same source: asymmetric interfacial water.
## References
Nygren, J. (2026). Diffusion potential of autoionization at adsorbate-liquid interface in polar liquids. *Phanerography Certification*. BTC/BSV/ETH/ETC/LTC/DOGE/XTZ/EOS. hashkey:2f61914ae079f073dccabeed03bbfd558ecde0ec761f9a49645edb32b174b295 (sha256).
Zhao, Q., Ovchinnikova, K., Chai, B., Yoo, H., Magula, J., & Pollack, G. H. (2009). Role of proton gradients in the mechanism of osmosis. *J Phys Chem B*, 113(31), 10708-10714.