What Causes the Exclusion Zone in Water? Three Theories

Scientists have proposed three theories for what causes the exclusion zone in water. All three describe the same observation: a band about 100 micrometers wide next to water-loving surfaces, from which dissolved particles disappear. Gerald Pollack reads it as a fourth phase of water with a density of its own. Inside the zone, researchers have measured a refractive index of 1.46 instead of 1.33. The electrical potential there is −120 to −200 millivolts, against 0 millivolts in the surrounding water. A second theory traces the zone to coherent domains, a hypothesis from quantum field theory published in 1988. The third needs no new water structure at all. It relies on ion exchange and diffusiophoresis, meaning particles that drift along a concentration gradient. A 2020 review by Elton and colleagues weighed the experiments and found diffusiophoresis the most convincing explanation.

What is the exclusion zone in water?

The exclusion zone is a region next to a water-loving surface from which dissolved particles are pushed out over a surprisingly long distance. It reaches much farther than any known surface force. Jian-ming Zheng and Gerald Pollack described it in 2003 at the University of Washington. They brought suspensions of tiny plastic microspheres up to gels and watched under the microscope as the spheres retreated (Zheng & Pollack 2003, Physical Review E 68, 031408).

The scale is what made researchers sit up. Classical theory expects an ordered boundary layer of one or two nanometers, only a few molecular layers thick. The measured exclusion reached about 100 micrometers, roughly 50,000 times as far.

Quantity	Expected from classical theory	Observed in the zone
Range of ordering	1–2 nanometers	~100 micrometers
Lifetime of a hydrogen bond	≈ 1 picosecond	unchanged
Binding energy per hydrogen bond	0.24 electronvolts	unchanged

Several groups have confirmed independently that the zone exists. The open question is which mechanism produces it.

Is the exclusion zone a fourth phase of water?

In Pollack's reading, the fourth phase is a state between solid and liquid. Water molecules line up in it to form an ordered, negatively charged layer. Pollack points to several measured values that set this layer apart from the rest of the water.

Measured quantity	Value inside the zone	Value in the surrounding water
Refractive index	1.46	1.33
T2 relaxation time (NMR)	27.2 ± 0.4 ms	25.4 ± 1 ms
Electrical potential	−120 to −200 mV	0 mV

On top of that come a pH below 3 right at the surface of Nafion, the plastic used in the experiments, and stronger absorption of light at a wavelength of 270 nanometers. The lab observed one more effect: radiant energy expands the zone, reversibly and depending on the wavelength. The group interprets this as energy being stored in the water (Chai, Yoo & Pollack 2009, Journal of Physical Chemistry B 113, 13953–13958). The values come from measurements, and the word phase is Pollack's interpretation laid over them. That interpretation is exactly what the debate is about.

Could coherent domains explain the exclusion zone?

A coherent domain is a proposed region in which water molecules oscillate together with the electromagnetic field and therefore stay ordered over time. Emilio Del Giudice, Giuliano Preparata, and Giuseppe Vitiello formulated the idea in 1988 (Physical Review Letters 61, 1085–1088). The word coherent, which often comes up today in connection with water, goes back to this paper.

A team led by Michael Cowan measured directly in 2005 how long water holds on to a structure. According to their results, water loses the memory of its own structure within 50 femtoseconds, an order of magnitude faster than previously assumed (Cowan et al. 2005, Nature 434, 199–202). A femtosecond is one millionth of a billionth of a second. Coherent domains, by contrast, describe an order meant to last much longer, and the model is derived from theory. How the two fit together is one of the questions that physicists who study water are still working through.

How ion exchange and diffusiophoresis create the zone

Diffusiophoresis is the movement of particles along a concentration gradient of dissolved substances. Colloid chemists have known the process for a long time, and it needs no new water structure. In 2013 the physical chemist J. Michael Schurr proposed explaining the zone in exactly this way.

The sequence runs like this:

1. The surface material Nafion releases protons and takes up other ions.
2. This creates a gradient of ion concentrations in the adjacent water.
3. Charged particles in the gradient are carried by a directed flow and pushed away from the surface.

A research group at Eindhoven University of Technology tracked how the zone develops over time. A diffusiophoresis model described their data (Florea, Musa, Huyghe & Wyss 2014, PNAS 111, 6554–6559). Schurr's explanation works with physics that colloid chemistry knows well, and it assumes no new state of water.

Which experiments decide between the three theories?

A discriminating experiment is a measurement for which two explanations predict different outcomes. Measurements like these are what move a scientific debate forward. A 2020 review by Daniel Elton and colleagues examined the available experiments systematically (Elton, Spencer, Riches & Williams, International Journal of Molecular Sciences 21, 5041). These measurements take center stage:

Measurement	What it tells apart	Result
Horizontal instead of vertical surface	rules out flow driven by gravity	zone persists, time course fits the ion gradient
Time course of growth	power-law exponent	0.6 measured, 0.5 expected for a diffusion process
Force field via optical tweezers	phase or force	force falls off with distance, a phase would have a boundary
Neutron radiography	higher density	no sign of a denser phase

The authors sum up their conclusion in one sentence:

“Schurr's theory based on diffusiophoresis presents a compelling alternative explanation for the core EZ phenomenon.”

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Elton, Spencer, Riches & Williams: Exclusion Zone Phenomena in Water: A Critical Review of Experimental Findings and Theories. International Journal of Molecular Sciences 21, 5041 (2020), DOI 10.3390/ijms21145041

The review would still like to see two findings from Pollack's lab repeated by other groups: the growth of the zone under laser light and the exclusion of salt. Research on the zone is still very much in motion.

Water in the body: where the zone was actually measured

Water makes up a large share of the human body. In adult men it accounts for about 60 percent, in women about 55 percent. The brain and heart are 73 percent water, and the lungs as much as 83 percent (U.S. Geological Survey, The Water in You).

The zone itself was measured in the lab, on gels and above all on Nafion, an industrial ion-exchange plastic. How water is organized inside living cells has long been a lively topic in cell biology. Philip Ball gives a good overview of that discussion (Ball 2008, Chemical Reviews 108, 74–108). What research as a whole has measured about order in water is the subject of our article Does Water Have Memory? What Physics Has Actually Measured.

How to judge a new study on ordered water yourself

Anyone who knows the three theories and the measurements behind them can read a new report about the zone with a calm eye. At Qi Blanco we work in this field ourselves, and two simple questions usually take us most of the way. Does the report describe an observation or an interpretation? And was the finding made on Nafion in a beaker or on living tissue? With those two answers you know fairly precisely where a result stands. From there, following new papers becomes a real pleasure, because the story of ordered water at surfaces is far from finished.

Readers who want to go deeper into the water inside our cells will find the biophysics in Cellular Hydration: What Water Really Does Inside Your Cells. The research we cite on our own subjects is collected on our studies page.

Frequently asked questions about the exclusion zone

Is the exclusion zone real?

Yes. It has been described since 2003, and several research groups have reproduced it independently. Today the debate is mainly about how it forms.

Is coherent water the same as the fourth phase of water?

The two terms come from different lines of research. The fourth phase goes back to Pollack's interpretation of the zone, while coherent water goes back to the quantum field theory paper by Del Giudice and colleagues from 1988. In everyday use the two are often mixed up.

Which theory is currently considered the most likely?

The 2020 review by Elton and colleagues finds the diffusiophoresis explanation more convincing than the idea of a fourth phase. Its reasoning is that diffusiophoresis accounts for the time course, the pH gradients, and the decaying force field together.

Does the exclusion zone tell us what water does in the body?

So far the zone has been measured on plastic surfaces in the lab, mostly on Nafion. Cell biology studies how water is organized in living cells with its own methods, and Philip Ball's 2008 review is a good place to start.

Key takeaways on the exclusion zone

• The zone at water-loving surfaces is real and reaches about 100 micrometers, roughly 50,000 times the 1 to 2 nanometers that classical theory expects.
• Three explanations are under discussion: a fourth phase, coherent domains, and diffusiophoresis.
• Measurements inside the zone include a refractive index of 1.46 instead of 1.33 and a potential of −120 to −200 millivolts against 0 millivolts.
• The coherent-domain hypothesis of 1988 describes long-lasting order, while a direct measurement in 2005 found that liquid water remembers its structure for only 50 femtoseconds.
• The 2020 review backs diffusiophoresis with four findings: a horizontal surface, the growth exponent of 0.6, the decaying force field, and neutron radiography with no sign of higher density.
• The zone has been measured on Nafion and gels in the lab. An adult man's body is about 60 percent water, an adult woman's about 55 percent.

Sources and further reading

• Zheng, J.-M.; Pollack, G. H.: Long-range forces extending from polymer-gel surfaces. Physical Review E 68, 031408 (2003). DOI 10.1103/PhysRevE.68.031408
• Chai, B.; Yoo, H.; Pollack, G. H.: Effect of Radiant Energy on Near-Surface Water. Journal of Physical Chemistry B 113 (42), 13953–13958 (2009). PubMed 19827846
• Del Giudice, E.; Preparata, G.; Vitiello, G.: Water as a Free Electric Dipole Laser. Physical Review Letters 61, 1085–1088 (1988). DOI 10.1103/PhysRevLett.61.1085
• Cowan, M. L.; Bruner, B. D.; Huse, N.; Dwyer, J. R.; Chugh, B.; Nibbering, E. T. J.; Elsaesser, T.; Miller, R. J. D.: Ultrafast memory loss and energy redistribution in the hydrogen bond network of liquid H2O. Nature 434, 199–202 (2005). DOI 10.1038/nature03383
• Florea, D.; Musa, S.; Huyghe, J. M. R.; Wyss, H. M.: Long-range repulsion of colloids driven by ion exchange and diffusiophoresis. PNAS 111 (18), 6554–6559 (2014). DOI 10.1073/pnas.1322857111
• Elton, D. C.; Spencer, P. D.; Riches, J. D.; Williams, E. D.: Exclusion Zone Phenomena in Water: A Critical Review of Experimental Findings and Theories. International Journal of Molecular Sciences 21 (14), 5041 (2020). DOI 10.3390/ijms21145041
• Ball, P.: Water as an Active Constituent in Cell Biology. Chemical Reviews 108 (1), 74–108 (2008). DOI 10.1021/cr068037a
• U.S. Geological Survey: The Water in You: Water and the Human Body. Water Science School