Heterogeneous collision velocity for hydrated ions in aqueous solutions is nearly 10{sup 4} cm/s
- Cornell Univ. Medical College, New York, NY (United States)
- Brookhaven National Lab., Upton, NY (United States)
The heterogeneous collision velocity (v{sub o}, units of centimeters/second) is the average velocity of a solution particle toward a surface. V{sub o} thus defines the maximum flux of the particles to the surface. Einstein argued that v{sub o} in condensed phases and in gases may be deduced in a precisely analogous manner and that v{sub o} in aqueous solution therefore should be nearly 10{sup 4} cm/s (for particles with a mass of 100 Da). Values of v{sub o} for several aqueous monovalent cations (Na{sup +}, K{sup +}, Rb{sup +}, Cs{sup +} and NH{sub 4}{sup +}) were estimated from steady-state limiting current measurements through single gramicidin A channels spanning lipid bilayer membranes. The collisional and diffusional current components were separated by making use of the different viscosities of H{sub 2}O and D{sub 2}O solutions. The transfer of ions from the bulk solution to the channel entrance is modeled using a hemispherical entrance and an extension of the classical analysis of diffusion to a (hemi)sphere in which we relax the assumption that the jump distance (associated with three-dimensional diffusion of an ion in the bulk phase) be small compared to the hemispherical capture radius. Our estimate of v{sub o} agrees well with the prediction of Einstein. We therefore conclude that ion dehydration (only partially hydrated ions can pass through the gramicidin A channel) cannot be a rate-controlling step - a conclusion that is consistent with known rate constants for water exchange for these ions. 36 refs., 7 figs., 1 tab.
- Research Organization:
- Brookhaven National Lab. (BNL), Upton, NY (United States)
- DOE Contract Number:
- AC02-76CH00016
- OSTI ID:
- 226767
- Journal Information:
- Journal of Physical Chemistry, Vol. 100, Issue 11; Other Information: PBD: 14 Mar 1996
- Country of Publication:
- United States
- Language:
- English
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