Visualization of Charge-Carrier Propagation in Water
Andrey Klimov†and Gerald H. Pollack*
Department of Bioengineering, Box 355061, UniVersity of Washington, Seattle, Washington 98195
ReceiVed June 12, 2007. In Final Form: August 7, 2007
The electrical properties of water in the region between parallel electrodes were investigated using pH indicator
dyes. Different pH values corresponded to different colors, which could be registered by a video camera. Imposition
of electrical current was able to produce zones of constant pH around, and well beyond each electrode: extremely
low pH around the positive electrode and extremely high pH around the negative electrode. The border between
alkaline and acid zones was jagged and separated by only a narrow layer of water with neutral pH. When the water
was replaced by various salt solutions, similar zones were observed. Again, passage of current produced large zones
of extreme pH values near and beyond each electrode. Alkaline zones appeared to propagate from the negative to
the positive electrode in narrow channels through the neutral solution. When the power supply was disconnected from
the electrodes and replaced by a resistive load, a potential difference was registered, and current flowed through the
resistor for some period of time. Hence, the acid and alkaline zones appear to carry opposite charges throughout their
Previous work from this laboratory revealed an unexpected
observation: solutes were profoundly excluded from aqueous
zones in the vicinity of various charged surfaces. These surfaces
included hydrogels, ion-exchange resins, and polymers as well
as biological entities, and the exclusion zones could extend up
water in this zone was physically different from bulk water, and
appeared to be charged.3
The question arose whether similarly charged aqueous zones
might be found when nucleating surfaces were replaced by
the surface, and could conceivably be more ample.
We found indeed, that next to each electrode, the injection of
The following components were used to study electrolytic
generation of charge carriers:
(1) A universal pH Indicator (Sigma no. 36803), made from a
mixture of different dyes whose color depends on pH, added in
on thickness of the water layer.
(2) Carboxylated polysterene microspheres, 1 µm diameter, the
surfaces of which are covered with hydroxyl groups which are able
to assume negative charge in aqueous solutions at pH > 3.
(3) In one experiment a physiological salt solution was used,
containing universal pH indicator, 0.5 mM EGTA, 100 mM KCl,
20 mM potassium phosphate, 0.2 mM MgCl2, 2 mM DTT. Initial
pH ) 7.0.
In general, two electrodes, made from platinum-wire welding
rods, diameter 0.32 mm, were situated parallel to one another in a
on a table with transparent glass surface, illuminated from below
with light from tungsten bulb, scattered by white paper to produce
uniform illumination. The electrodes were connected to a power
indicator dye color, as well as the motion of negatively charged
microspheres, were captured by a color video camera (Logitech
cm. The camera was connected through USB port to a computer.
The chamber, whose floor was made of glass, had 7-mm high
Elastomer Kit Sylgard-184 (Figure 1A,B). The platinum elctrodes
were mounted in the Plexiglas and silicone walls. The chamber had
of 26 mm and were 13 mm horizontally apart from one another.
and from the nearest wall 2-3 mm. The electrodes were passed
horizontally, and then turned vertically downward to compartment
B separated by 10 mm from one from another. One of the
10 times by external resistors. In this way, external voltage could
be regulated with 10 times better accuracy than with the power
supply regulator. Current from the divider was sent to a Fluke 8020
multimeter, which was able to measure current with a precision of
10-6A. After the ammeter, the current was sent to switch SW1, 2,
* To whom correspondence should be addressed. E-mail:
†Current address: Institute of Theoretical and Experimental Biophysics,
Puschino, Russia 142290.
(1) Zheng, J. M.; Pollack, G. H. Phys. ReV. E: Stat., Nonlinear, Soft Matter
Phys. 2003, 68, 031408.
I. L., Wheatley, D. N., Eds.; Springer: New York, 2006; pp 165-174.
(3) Zheng, J.-M.; Chin, W.-C; Khijniak, E.; Khijniak, E., Jr.; Pollack, G. H.
AdV. Colloid Interface Sci. 2006, 127, 19-27.
Langmuir 2007, 23, 11890-11895
10.1021/la701742v CCC: $37.00© 2007 American Chemical Society
Published on Web 10/16/2007
Hence the high-pH zone, which is negatively charged, may be
similar in character to the negatively charged solute-exclusion
zonesthe latter being physically different from ordinary bulk
When the current was applied through vertically rather than
in that the established zones did not mix. They remained
separated by a small layer of neutral-pH water (Figure 5). Here,
the dye formed jagged boundaries with triangular emanations
from the start, moving progressively closer toward one another
When physiological solutions containing various salts were
substituted for distilled water, application current produced
qualitatively similar results, but with some notable differences
(Figure 6). Whereas the low-pH zone that formed around the
positive electrode was similar to those formed in the absence of
salts, the events around the negative electrode were unexpected.
but it soon gave rise to a finger-like protuberance extending
toward the positive electrode, a structure that implied a more
features might be characteristic, particularly in physiological
solutions. Although such patterns were noted, they were not
studied in detail as they were not the main focus of the study.
The experimental results also showed that excess positive
charge in the low-pH zone, and excess negative charge in the
flow could persist for minutes. In the absence of the resistive
load, however, the zones were maintained next to one another
for an extended period, in spite of the huge pH and charge
gradients. Almost no mixing occurs. The thin, neutral zone
of uniform color for an extended period.
The key question emerging from these results is why these
regions with different pH do not immediately neutralize one
another. Either the charges must somehow be neutralized by
be driven through the resistor, or the charges must somehow be
stabilized within a matrix, as they are for example, in p- and
n-type semiconductor materials. Future experiments will need
to address these important electrochemical questions.
Charge-Carrier Propagation in Water Langmuir, Vol. 23, No. 23, 2007 11895