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Diffusion potential of autoionization at the adsorbate–liquid interface
In polar liquids, autoionization at an adsorbed surface produces charge carriers with asymmetric mobility — generating an electric field that organizes a macroscopic three-layer system from a molecular-scale process.
Diffusion potential
One ion mobile, one immobilized — maximum Nernst asymmetry
Donor · dielectric · acceptor
Self-organized capacitor-like interface in polar liquids
~50% ionization
Adsorbate vs 10⁻⁷ M bulk autoionization in water
§1–2 · Mechanism
A 250-year-old idea, applied where it had not been named
The diffusion potential arises whenever charge carriers differ in mobility. At an adsorbate–liquid interface, only one ion can cross — the strongest possible diffusion potential.
Classical diffusion potential
Both ions are mobile. Potential scales with the difference in their mobilities (Nernst). Familiar at solid–solid, solid–gas, and membrane interfaces — and in semiconductor junctions.
Adsorbate–liquid autoionization
The retained ion is immobilized in the adsorbate; only the donated ion is free in the bulk. Configuration analogous to a transistor junction — a single mobile carrier against an immobile counterpart.
| Year | Figure | Term |
|---|---|---|
| 1789 | Bennet | Adhesive electricity |
| 1800 | Volta | Contact tension |
| 1889 | Nernst | Potential difference |
| 1922 | Michaelis | Diffusion potential |
| 1902 / 1949 | Bernstein / Shockley | Membrane potential · transistor |
| This work | Adsorbate–liquid | Autoionization interface |
§3 · Structure
Three-layer organization in polar liquids
Donor, dielectric, acceptor — a self-assembling structure whenever a polar autoionizing liquid adsorbs to a hydrophilic surface.
Cross-section
Select a layer
Ionized adsorbate
Thin adsorbed phase at the surface, enriched in the immobile counter-ion (OH⁻ in water).
The adsorbate does not dissolve ions, and the retained ion cannot leave it. Its degree of ionization is pushed far beyond bulk autoionization equilibrium because the dielectric layer suppresses recombination — estimated at ~50% ionization in water, versus 10⁻⁷ M in bulk.
Charge carriers
OH⁻ immobilized
§4 · Dynamics
Self-organization by positive feedback
How a ~1 nm adsorbate organizes liquid hundreds of micrometers from the surface: recombination is suppressed, ionization rises, the field strengthens, and the dielectric extends — until currents balance.
Positive feedback
Step 1/5
Autoionization
§5 · Water
A century of observations, one quantitative picture
Particle exclusion, weak potential, H⁺ release, and ordered interfacial structure — unified by the diffusion-potential framework without new physics.
- 01 · 1930s
Bound / non-solvent water
Particle-excluding regions at hydrophilic surfaces described as bound water (Gortner), non-solvent water, and later vicinal water.
- 02 · 1970
Direct thickness
Green & Otori measure unstirred layers; later work establishes exclusion thickness up to ~0.3 mm.
- 03 · 2003–2006
Exclusion zone & potential
Zheng & Pollack map long-range forces from gels; potential across the region measured at millivolt scale.
- 04 · 2009
H⁺ release quantified
Chai et al.: ~12 nmol H⁺ released from a 4 cm² hydrophilic membrane into 4 mL water (pH → 5.5).
- 05 · 2010
Other polar liquids
Solute-free interfacial zones and potentials observed in ethanol and several other polar liquids.
H⁺ released
12 nmol
Chai et al. 2009 · pH 5.5
Adsorbate volume
0.4 nL
1 nm × 4 cm²
OH⁻ in adsorbate
30 mol/L
vs bulk 10⁻⁷ M autoionization
Ionization fraction
~50%
30 M OH⁻ in ~60 M water
Explore the estimate
Adsorbate ionization calculator
From published H⁺ release and estimated adsorbate thickness. Area fixed at 4 cm² (Chai et al., 2009).
Adsorbate volume
0.40 nL
[OH⁻] retained
30.0 mol/L
Ionization (of ~60 M water)
50%
Expected vs measured field
Nine orders of magnitude — and why that is evidence
Charge separation of ~12 nmol across 4 cm² with ~300 µm separation should, by Coulomb's law in vacuum, produce a potential on the order of 100 MV. The measured potential is on the order of millivolts. The discrepancy is exactly what polarized water as a dielectric predicts: aligned dipoles cancel nearly all of the applied field.
Vacuum / bare Coulomb estimate
~100 MV
Measured residual (Zheng et al., 2006)
~mV~10⁻⁹ of bare estimate
Separation
~300 µm EZ thickness
Charge
~12 nmol H⁺ released
Interpretation
Polarized dielectric water
§6 · Generality
Not unique to water
Any polar liquid capable of autoionization that adsorbs to a hydrophilic surface should produce the same three-layer organization.
Water
Primary quantitative case; EZ 0.1–0.3 mm
Ethanol
Measured interfacial potential
Methanol
Particle-excluding region
Isopropanol
Particle-excluding region
Acetic acid
Particle-excluding region
DMSO
Particle-excluding region
Requirement 1
Forms an adsorbed phase at the surface
Requirement 2
Autoionization across adsorbed / bulk phases
Requirement 3
Asymmetric carrier mobility
Requirement 4
Polar — polarizable by the field
The explanation has been hidden in plain sight since 2009.
Polarization of molecules in electric fields has been understood since Faraday and Maxwell. The diffusion potential from asymmetric charge carrier mobility has been understood since Nernst. The key empirical datum — release of H⁺ from interfacial water — has been available since Chai et al. (2009). The quantity of H⁺ released should generate an enormous field; that this field polarizes adjacent water was not recognized.
No new physics is required — only the application of well-characterized electrochemistry and semiconductor principles to adsorbate–liquid autoionization in polar liquids.
Read the full manuscript