The phase behavior of organically passivated 20-75 Å diameter Ag and Au nanocrystals is investigated by examining surface-area isotherms of Langmuir monolayers and transmission electron micrographs of Langmuir-Blodgett (LB) films. The effects of temperature, organic passivant chain length, and nanocrystal size and composition are studied. Three distinct types of phase behavior are observed and may be classified in terms of the extra (conical) volume (V e ) available to the alkyl capping group as it extends from a nearly spherical metal core. For V e > 350 Å 3 , the phase diagram is dominated by extended, low-dimensional structures that, at high pressures, compress into a two-dimensional foamlike phase. This behavior is rationalized as originating from the interpenetration of the ligand shells of adjacent particles. For V e < 350 Å 3 , dispersion attractions between the metal cores dominate particle condensation. For 350 Å 3> V e > 150 Å 3 , the particles condense to form closest packed structures, which, for sufficiently narrow particle size distributions, are characterized by crystalline phases. For V e ≈ 30 Å 3 , the particles irreversibly aggregate into structures similar to those expected from a diffusion-limited-aggregation (DLA) model. Optical properties of certain LB films of the closest packed phases are reported.

Bacterial viral capsids in aqueous solution can be opened in vitro by addition of their specific receptor proteins, with consequent full ejection of their genomes. We demonstrate that it is possible to control the extent of this ejection by varying the external osmotic pressure. In the particular case of bacteriophage , the ejection is 50% inhibited by osmotic pressures (of polyethylene glycol) comparable to those operative in the cytoplasm of host bacteria; it is completely suppressed by a pressure of 20 atmospheres. Furthermore, our experiments monitor directly a dramatic decrease of the stress inside the unopened phage capsid upon addition of polyvalent cations to the host solution, in agreement with many recent theories of DNA interactions.V iral capsids are often highly pressurized. In the case of double-stranded DNA bacteriophages, for example, there are large forces acting to push the genome out through the tail of the viral particle. Recent experiments (1) and theory (2, 3) have established that the forces inside these capsids reach very high values (of order 50 pN) when the genomes are fully packaged. These forces are estimated to correspond to pressures of the order of 50 atm (1 atm ϭ 101.3 kPa; refs. 1-4). Such high values of stress result from the DNA being strongly bent and confined. Specifically, the inner diameter of the capsid is comparable to the DNA persistence length (50 nm), and the typical interaxial spacings are small enough (2.5 nm; ref. 5) that neighboring chains experience strong repulsions. It is precisely these forces that drive the ejection of the genome into the host cell after the tail of the capsid has been bound to its receptor protein in the outer cell membrane; equivalently, it is these forces that must be exerted by the motor protein responsible for packaging of the viral genome (1).Previous in vitro experiments have demonstrated that DNA ejection is fast (occurring on a time scale of seconds) and complete when phage capsids are opened by their receptors in aqueous solution (6). The capsids are permeable to water and salt ions, so there is no difference in hydrostatic pressure between the inside and outside of the capsid, and osmotic equilibrium is also maintained (7). The pressure difference between the capsid and the solution is therefore associated only with the confinement of the DNA. When the capsid is opened, ejection proceeds until this pressure difference falls to zero. Clearly, at this point the force driving the ejection has also dropped to zero. For the in vivo situation, however, ejection does not occur into a simple salt solution. The DNA is ejected into a highly concentrated colloidal suspension, the cell cytoplasm, which is characterized by high concentrations of proteins and other macromolecular structures. In general, then, one expects this colloidal solution to give rise to a force that resists the entry of DNA (8). More explicitly, because the ejection force associated with stress inside the capsid drops monotonically as the ejection proceeds, there will be a...