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PHYSICAL CHEMISTRY: COLLOIDS, PROTEINS, AND CLUSTERS

The following points are made by A. Stradner et al (Nature 2004 432:492):

1) Controlling interparticle interactions, aggregation and cluster formation is of central importance in a number of areas, ranging from cluster formation in various disease processes to protein crystallography and the production of photonic crystals. Recent developments in the description of the interaction of colloidal particles with short-range attractive potentials have led to interesting findings including metastable liquid-liquid phase separation and the formation of dynamically arrested states (such as the existence of attractive and repulsive glasses, and transient gels)[1-5]. The emerging glass paradigm has been successfully applied to complex soft-matter systems, such as colloid-polymer systems and concentrated protein solutions. However, intriguing problems like the frequent occurrence of cluster phases remain.

2) A number of globular proteins have been shown to exhibit the major characteristics of colloids that interact via a short-range attractive potential. At high ionic strength, where the salt screens electrostatic repulsions, these short-range attractions increasingly dominate with decreasing temperature. This leads to a metastable liquid-liquid phase separation and related critical phenomena. In agreement with predictions from mode-coupling theory, there is also evidence for a glass or gel transition at low particle volume fractions and high interparticle attractions. Such a scenario obviously affects the ability to form the high quality crystals required for protein crystallography.

3) The authors report that their results from two very different systems unambiguously confirm that the formation of equilibrium clusters in protein solutions is not caused by protein-specific interactions; rather, the combination of a weakly screened, long-range electrostatic repulsion and a short-range attraction leads to the formation of small equilibrium clusters with a concentration-dependent aggregation number Nc. The authors suggest the findings demonstrate the general importance of residual, weakly screened charges together with short-range attractions in cluster formation, and indicate a possible means of obtaining tunable cluster formation through protein or colloid self-assembly. This mechanism might also provide an alternative route to nanostructured colloidal gel and glass phases, where the structural elements could be self-assembled colloid clusters

4) In summary: The authors report small-angle scattering and confocal microscopy investigations of two model systems: protein solutions and colloid-polymer mixtures. The authors demonstrate that in both systems, a combination of short-range attraction and long-range repulsion results in the formation of small equilibrium clusters. The authors suggest this finding is relevant for nucleation processes during protein crystallization, protein or DNA self-assembly and the previously observed formation of cluster and gel phases in colloidal suspensions.

References (abridged):

1. Dawson, K. A. The glass paradigm for colloidal glasses, gels, and other arrested states driven by attractive interactions. Curr. Opin. Colloid Interf. Sci. 7, 218-227 (2002)

2. Trappe, V., Prasad, V., Cipelletti, L., Segre, P. N. & Weitz, D. A. Jamming phase diagram for attractive particles. Nature 411, 772-775 (2001)

3. Sciortino, F. Disordered materials: one liquid, two glasses. Nature Mater. 1, 145-146 (2002)

4. Pham, K. N. et al. Multiple glassy states in a simple model system. Science 296, 104-106 (2002)

5. Eckert, T. & Bartsch, E. Re-entrant glass transition in a colloid-polymer mixture with depletion attractions. Phys. Rev. Lett. 89, 125701-125704 (2002)

Nature http://www.nature.com/nature

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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

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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

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