|
ScienceWeek
PHYSICAL CHEMISTRY: ON ELECTRIC FIELDS IN COLLOID SUSPENSIONS
The following points are made by Patrick Warren (Nature 2004 429:822):
1) It might be thought that the last word on sedimentation or centrifugation had been said long ago. But over the past decade there have been persistent indications from experiment(1), theory(2), and simulations(3) of an unusual phenomenon in the sedimentation equilibrium of suspensions of charged colloidal particles when the ionic strength is low. Colloids are small particles, typically less than one micrometer in size, that have many technological applications as well as relevance to fundamental science. Ra and Philipse(4) have presented convincing evidence that a macroscopic electric field, generated when the charged colloid is centrifuged to equilibrium, is behind the strange effect.
2) In 1926, Jean Baptiste Perrin (1870-1942) was awarded the Nobel Prize in Physics "for his work on the discontinuous structure of matter, and especially for his discovery of sedimentation equilibrium"(5). What Perrin had discovered was that the same law that governs the rarefaction of Earth's atmosphere with height -- its barometric profile -- also governs the distribution of suspended colloid particles undergoing brownian motion. At the time, Perrin's measurements of the barometric profile in a painstakingly prepared gamboge (gum resin) suspension gave an independent determination of Avogadro's number, thus contributing to the establishment of the atomistic hypothesis as experimental fact. In centrifugation -- which is now well established as a means of determining molecular mass --the modest effects of Earth's gravity are replaced by a strong radial centripetal acceleration, as a sample is whirled around at high speed in a specialist device.
3) Piazza et al(1) found that for charged colloidal particles at reduced ionic strength the barometric profile is inflated, as though the particles weigh less than they should do. Biben and Hansen(2) have since suggested, using density functional theory, that the sample column behaves like a condenser, in the sense that imbalanced charges accumulate at the top and bottom. The resulting electric field spans the whole column and acts to lift the colloid particles up against the force of gravity.
4) A simple explanation, first discovered by van Roij (2003), relates the phenomenon to a now-classic piece of physical chemistry. In 1911, Frederick Donnan (1870-1956) considered the equilibrium across a semi-permeable membrane separating a salt solution from a suspension of charged colloidal particles or macromolecules. The membrane is permeable to the small ions of the salt but impermeable to the colloids. Naively, one might expect that the small ions would distribute themselves so as to have the same concentration on both sides of the membrane. However, Donnan discovered that this is not the case. Rather, the ions become redistributed; for example, ions with the same charge as the colloids tend to be expelled from the colloid-containing compartment -- the Donnan common-ion effect. The effects arise from a microscopic charge imbalance that builds up in the vicinity of the membrane, creating a potential difference between the compartments, now known as the "Donnan potential".
References (abridged):
1. Piazza, R., Bellini, T. & Degiorgio, V. Phys. Rev. Lett. 71, 4267-4270 (1993)
2. Biben, T. & Hansen, J. -P. J. Phys. Condens. Matter 6, A345-A349 (1994)
3. Hynninen, A. -P. , van Roij, R. & Dijkstra, M. Europhys. Lett. 65, 719-725 (2004)
4. Raa, M. & Philipse, A.P. Nature 429, 857-860 (2004)
5. Nobel Lectures on Physics 1922-1941 (World Scientific, Singapore, 1998)
Nature http://www.nature.com/nature
--------------------------------
Related Material:
PHYSICS: COLLOIDS AND EINSTEIN
The following points are made by Wilson Poon (Nature 2004 304:830):
1) Colloid science is important for applications ranging from drugs to dairy products. Less well known is that it can also illuminate basic physics questions, because in certain crucial respects, colloids behave as "big atoms", The interface between a liquid and a vapor can be studied with a colloidal model.
2) Beginning with his doctoral thesis, Albert Einstein (1879-1955) demonstrated that the incessant, random jiggling of colloidal particles, the jiggling known as "Brownian movement", was the visible manifestation of the "graininess" -- the molecular nature -- of the surrounding liquid. One consequence is that the density of particles as a function of height in a dilute suspension in sedimentation equilibrium is given by an equation that depends on the particle's buoyant mass, the gravitational acceleration, Boltzmann's constant, and the absolute temperature. It turns out that this equation also expresses the distribution of gas molecules in a constant-temperature atmosphere in gravity, where it is known as the "barometric distribution". In other words, colloids made up of relatively large particles can behave in the same way as much smaller counterparts in the molecular world; for some purposes, colloids behave as "big atoms". The experimental verification by Jean-Baptiste Perrin (1870-1942) of the "colloidal barometric distribution" contributed toward his 1926 physics Nobel Prize and the widespread acceptance of the reality of molecules.
3) Today, the study of colloids is throwing new light on fundamental problems of condensed matter physics, from the kinetics of crystallization (2) to the nature of glassy states [(3-5)). Aarts et al (1) have used colloids to study the vapor-liquid interface. At conditions far from the critical point (the temperature and pressure beyond which vapor and liquid do not exist separately), such interfaces are macroscopically flat. Microscopically, however, thermal energy excites ripples in the interface. These capillary waves (which, like capillary rise, are governed by the surface tension) are important in diverse fields such as oceanography (where wind-excited capillary ripples amplify to giant waves) and the rupture of polymer films (which is bad for coatings). After a century of study, these ripples still hold surprises. Thus, recent x-ray scattering from capillary waves in organic liquids demonstrates that as we move down to molecular length scales, the surface tension first decreases and then increases again; the decrease is probably caused by the long-range nature of the dispersion (that is, van der Waals) forces between the molecules.
References (abridged):
1. D. G. A. L. Aarts, M. Schmidt, H. N. W. Lekkerkerker, Science 304, 847 (2004)
2. S. Auer, D. Frenkel, Nature 409, 1020 (2004)
3. E. Weeks et al., Science 287, 637 (2000)
4. K. N. Pham et al., Science 296, 104 (2002)
5. W. C. K. Poon, Mater. Res. Bull. (Feb. 2004), p. 96
Science http://www.sciencemag.org
--------------------------------
Related Material:
CHEMISTRY: COLLOIDAL MOLECULES
The following points are made by Alfons van Blaaderen (Science 2003 301:470):
1) Approximately a hundred years ago, the experiments of Jean Perrin (1870-1942) on colloids -- particles a few nanometers to a few micrometers in diameter -- convinced even the skeptics that matter consists of atoms and molecules. More recently, the analogy between atoms and colloids has led to insights into crystal nucleation and growth, the glass transition, and the influence of the range of particle-particle interactions on phase behavior. Moreover, the ability to manipulate colloid crystallization has led to advanced materials such as photonic crystals.
2) To date, most studies of colloids have used spherical particles or particles with simple shapes, such as rods and plates. Syntheses of nonspherical colloids generally yield a broad size distribution. Manoharan et al (Science 2003 301:483) report a method for making large quantities of identical colloidal particles with complex shapes consisting of equal-sized colloidal spheres. The authors made these particles by drying the oil out of an oil-in-water emulsion in which spherical colloids were adsorbed to the surface of the oil droplets. Subsequent centrifugation yielded an intriguing sequence of colloidal structures. Each structure consists of 2 to 15 spheres. For a given particle size, the spheres are arranged identically in all particles.
3) Mathematical theories of the packing of spheres find application in fields such as error correction and data compression. Only 6 years ago, it was proven that the densest way in which spheres can be arranged in three dimensions -- also known as Kepler's problem -- is in stacks of hexagonal layers. External constraints also affect sphere packings. For example, the packing of colloidal spheres between two walls results, for certain wall separations, in prism-shaped structures (see the figure, top right). Other constraints, such as minimizing the volume or optimizing the energy of spheres that attract each other at short distances, yield different structures.
4) Perrin proved the atomic reality of matter with colloidal spheres. The results of Manoharan et al have now brought the investigation of the collective behavior of more complex colloidal molecules within reach. Because the method requires stability of the initial colloidal spheres in both the oil and the water phase, it will not be easy to extend the method to colloids with different compositions. But the prospects for model studies and new materials will provide a strong impetus for many groups to try.
Science http://www.sciencemag.org
ScienceWeek http://scienceweek.com
|