Nav Button - Home

This issue...

  Brieflies

  View from the Inside

  Hot Algae, Pale Coral

  Freezing Proton Motion

  Working Science

  People

  About

  Subscribe Free















This issue...

  Brieflies

  View from the Inside

  Hot Algae, Pale Coral

  Freezing Proton Motion

  Working Science

  People

  About

  Subscribe Free

Freezing Proton Motion

by Michaela Mann


Researchers at the Pacific Northwest National Laboratory have experimental evidence that has overthrown the Grotthus model of proton transport in crystalline water (such as ice)—a model used by the scientific community for the past three decades. The Grotthus model portrays protons, in the form of hydronium ions, moving steadily from one H2O molecule to the next in a complex process of charge transfer and structural reorientations called "tunneling". The model assumed the apparent proton movement was temperature independent, and could occur even at zero K.

The theory questioned: Theoreticians had reservations about the model. The polarity of a water molecule changes when a water molecule accepts a proton and becomes a hydronium ion, causing the molecule to reorient itself. In theory, neighboring molecules would reorient in such a way that the negatively charged ends would all point to the positively charged proton (and hence each other), creating a negatively charged well from which the proton could not escape. Energy would be required to reorient the water molecules back to a state that facilitated proton transport. Thus, the theorists argued, the tunneling could not take place at very low temperatures.

Improvements in experimental methodology: The methodology previously used to support the Grotthus model involved high power pulsed lasers, which the PNNL researchers suspected introduced too many variables in the measurement of proton movement. Instead, the PNNL researchers used a "soft landing" technique. They employed a very low voltage (1 eV) ion laser to deposit hydronium ions on ultra-thin layers of ice at 30 K. This method ensured that variables such as counter-ions, planar defects, and traps that would affect polarity and proton transport were not generated by the deposition method. They also sandwiched the hydronium between ice films to make sure the ions were not immobilized by a surface trap unrelated to the tunneling effect. The ions generate a macroscopic voltage according to their position in the film, which the PNNL team used to follow their migration through the ice. The research builds on earlier infrared experiments by John Devlin at Oklahoma State University, which hinted at limits to the Grotthus mechanism. The PNNL research unequivocally shows that the hydronium motion was much more restricted than the Grothus model predicted.


Hydronium motion in water ice is sensed by drops in the film voltage as the temperature is slowly increased, for samples dosed with ions at 30 K (left) and 160 K (right). The drops seen are due to evaporation of film (200 K), dielectric constant changes (150 K), or hydrogen bond defect motion (124 K)...no ion motion is seen from 30 to 190 K.

No proton movement below 190K: The PNNL experiments detected no voltage change due to hydronium movement between 33 K and 190 K. These observations demonstrate that temperature plays a large role in proton transport, and suggests that much work still needs to be done in refining the physical model of water ice. The research is particularly pertinent to environmental and biological scientists who are deeply interested in proton movement in crystalline water. Atmospheric researchers study ice particles interacting with acids (e.g., hydronium) in understanding both cloud formation and Antarctic ozone holes. Biologists seek to understand the nature of hydronium transport along "water wires" within cells and through cell walls. Water is bound so tightly within proton channels in the cell walls that it is often described as "crystalline" or "ice."

The research was reported in Nature, Vol. 398, April 1, 1999. The work was supported by the Office of Basic Energy Sciences, Chemical Sciences Division.
Contact: James Cowin, PNNL, (509) 376-6330, jp.cowin@pnl.gov

Contents
Search
Comments
previous    next










Freezing Proton Motion

Click for larger image with explanatory caption.

   previous     next


• Energy Science News • Energy Science News • Energy Science News • Energy Science News •
www.pnl.gov/energyscience/