Recent experimental results [1] indicate that phosphorus, a single-component system, can have two liquid phases: a high-density liquid (HDL) and a low-density liquid (LDL) phase. A firstorder transition between two liquids of different densities [2] is consistent with experimental data for a variety of materials [3,4], including singlecomponent systems such as water [5][6][7][8], silica [9] and carbon [10]. Molecular dynamics simulations of very specific models for supercooled water [2,11], liquid carbon [12] and supercooled silica [13], predict a LDL-HDL critical point, but a coherent and general interpretation of the LDL-HDL transition is lacking. Here we show that the presence of a LDL and a HDL can be directly related to an interaction potential with an attractive part and two characteristic short-range repulsive distances. This kind of interaction is common to other single-component materials in the liquid state (in particular liquid metals [14-21,2]), and such potentials are often used to decribe systems that exhibit a density anomaly [2]. However, our results show that the LDL and HDL phases can occur in systems with no density anomaly. Our results therefore present an experimental challenge to uncover a liquid-liquid transition in systems like liquid metals, regardless of the presence of the density anomaly.Several explanations have been developed to understand the liquid-liquid phase transition. For example, the two-liquid models [4] assume that liquids at high pressure are a mixture of two liquid phases whose relative concentration depends on external parameters. Other explanations for the liquid-liquid phase transition assume an anisotropic potential [2,[11][12][13]. Here we shall see that liquid-liquid phase transition phenomena can arise solely from an isotropic pair interaction potential with two characteristic lengths.For molecular liquid phosphorus P 4 (as for water), a tetrahedral open structure is preferred at low pressures P and low temperatures T , while a denser structure is favored at high P and high T [1,6,8]. The existence of these two structures with different densities suggests a pair interaction with two characteristic distances. The first distance can be associated with the hard-core exclusion between two particles and the second distance with a weak repulsion (soft-core), which can be overcome at large pressure. Here we will use a generic three dimensional (3D) model composed of particles interacting via an isotropic soft-core pair potential. Such isotropic potentials can be regarded as resulting from an average over the angular part of more realistic potentials, and are often used as a first approximation to understand the qualitative behavior of real systems [14][15][16][17][18][19][20][21][22]2]. For Ce and Cs, Stell and Hemmer proposed a potential with nearest-neighbor repulsion and a weak long-range attraction [15]. By means of an exact analysis in 1D, they found two critical points, with the high-density critical point interpreted as a solid-solid transition. Then analytic calc...

Using event-driven molecular dynamics simulations, we study a three-dimensional one-component system of spherical particles interacting via a discontinuous potential combining a repulsive square soft core and an attractive square well. In the case of a narrow attractive well, it has been shown that this potential has two metastable gas-liquid critical points. Here we systematically investigate how the changes of the parameters of this potential affect the phase diagram of the system. We find a broad range of potential parameters for which the system has both a gas-liquid critical point C 1 and a liquid-liquid critical point C 2 . For the liquid-gas critical point we find that the derivatives of the critical temperature and pressure, with respect to the parameters of the potential, have the same signs: they are positive for increasing width of the attractive well and negative for increasing width and repulsive energy of the soft core. This result resembles the behavior of the liquid-gas critical point for standard liquids. In contrast, for the liquid-liquid critical point the critical pressure decreases as the critical temperature increases. As a consequence, the liquid-liquid critical point exists at positive pressures only in a finite range of parameters. We present a modified van der Waals equation which qualitatively reproduces the behavior of both critical points within some range of parameters, and gives us insight on the mechanisms ruling the dependence of the two critical points on the potential's parameters. The soft-core potential studied here resembles model potentials used for colloids, proteins, and potentials that have been related to liquid metals, raising an interesting possibility that a liquid-liquid phase transition may be present in some systems where it has not yet been observed.

Surface tensions evaluated from molecular dynamics simulations of fully hydrated dipalmitoylphosphatidylcholine bilayers and monolayers at surface areas/lipid of 54, 64, and 80 A2 are uniformly lowered 4-8 dyn/cm upon addition of trehalose in a 1:2 trehalose/lipid ratio. Constant surface tension simulations of bilayers yield the complementary result: an increase in surface area consistent with the surface pressure-surface area (pi-A) isotherms. Hydrogen bonding by trehalose, replacement of waters in the headgroup region, and modulation of the dipole potential are all similar in bilayers and monolayers at the same surface area. These results strongly support the assumption that experimental measurements on the interactions of surface active components such as trehalose with monolayers can yield quantitative insight to their effects on bilayers. The simulations also indicate that the 20-30 dyn/cm difference in surface tension of the bilayer leaflet and monolayer arises from differences in the chain regions, not the headgroup/water interfaces.

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We investigate the phase behavior of a single-component system in three dimensions with sphericallysymmetric, pairwise-additive, soft-core interactions with an attractive well at a long distance, a repulsive soft-core shoulder at an intermediate distance, and a hard-core repulsion at a short distance, similar to potentials used to describe liquid systems such as colloids, protein solutions, or liquid metals. We showed ͓Nature ͑London͒ 409, 692 ͑2001͔͒ that, even with no evidence of the density anomaly, the phase diagram has two first-order fluid-fluid phase transitions, one ending in a gas-low-density-liquid ͑LDL͒ critical point, and the other in a gas-high-density-liquid ͑HDL͒ critical point, with a LDL-HDL phase transition at low temperatures. Here we use integral equation calculations to explore the three-parameter space of the soft-core potential and perform molecular dynamics simulations in the interesting region of parameters. For the equilibrium phase diagram, we analyze the structure of the crystal phase and find that, within the considered range of densities, the structure is independent of the density. Then, we analyze in detail the fluid metastable phases and, by explicit thermodynamic calculation in the supercooled phase, we show the absence of the density anomaly. We suggest that this absence is related to the presence of only one stable crystal structure.