2,750 research outputs found
Hydration of an apolar solute in a two-dimensional waterlike lattice fluid
In a previous work, we investigated a two-dimensional lattice-fluid model,
displaying some waterlike thermodynamic anomalies. The model, defined on a
triangular lattice, is now extended to aqueous solutions with apolar species.
Water molecules are of the "Mercedes Benz" type, i.e., they possess a D3
(equilateral triangle) symmetry, with three equivalent bonding arms. Bond
formation depends both on orientation and local density. The insertion of inert
molecules displays typical signatures of hydrophobic hydration: large positive
transfer free energy, large negative transfer entropy (at low temperature),
strong temperature dependence of the transfer enthalpy and entropy, i.e., large
(positive) transfer heat capacity. Model properties are derived by a
generalized first order approximation on a triangle cluster.Comment: 9 pages, 5 figures, 1 table; submitted to Phys. Rev.
Effect of hydrophobic solutes on the liquid-liquid critical point
Jagla ramp particles, interacting through a ramp potential with two
characteristic length scales, are known to show in their bulk phase
thermodynamic and dynamic anomalies, similar to what is found in water. Jagla
particles also exhibit a line of phase transitions separating a low density
liquid phase and a high density liquid phase, terminating in a liquid-liquid
critical point in a region of the phase diagram that can be studied by
simulations. Employing molecular dynamics computer simulations, we study the
thermodynamics and the dynamics of solutions of hard spheres (HS) in a solvent
formed by Jagla ramp particles. We consider the cases of HS mole fraction x =
0.10, 0.15 and 0.20, and also the case x = 0.50 (a 1:1 mixture of HS and Jagla
particles). We find a liquid-liquid critical point, up to the highest HS mole
fraction; its position shifts to higher pressures and lower temperatures upon
increasing x. We also find that the diffusion coefficient anomalies appear to
be preserved for all the mole fractions studied.Comment: 8 pages, 7 figures, 1 table. In press (Phys. Rev. E
Solvent-free coarse-grained lipid model for large-scale simulations
A coarse-grained molecular model, which consists of a spherical particle and
an orientation vector, is proposed to simulate lipid membrane on a large length
scale. The solvent is implicitly represented by an effective attractive
interaction between particles. A bilayer structure is formed by
orientation-dependent (tilt and bending) potentials. In this model, the
membrane properties (bending rigidity, line tension of membrane edge, area
compression modulus, lateral diffusion coefficient, and flip-flop rate) can be
varied over broad ranges. The stability of the bilayer membrane is investigated
via droplet-vesicle transition. The rupture of the bilayer and worm-like
micelle formation can be induced by an increase in the spontaneous curvature of
the monolayer membrane.Comment: 13 pages, 19 figure
A lattice model of hydrophobic interactions
Hydrogen bonding is modeled in terms of virtual exchange of protons between
water molecules. A simple lattice model is analyzed, using ideas and techniques
from the theory of correlated electrons in metals. Reasonable parameters
reproduce observed magnitudes and temperature dependence of the hydrophobic
interaction between substitutional impurities and water within this lattice.Comment: 7 pages, 3 figures. To appear in Europhysics Letter
Adding Salt to an Aqueous Solution of t-Butanol: Is Hydrophobic Association Enhanced or Reduced?
Recent neutron scattering experiments on aqueous salt solutions of
amphiphilic t-butanol by Bowron and Finney [Phys. Rev. Lett. {\bf 89}, 215508
(2002); J. Chem. Phys. {\bf 118}, 8357 (2003)] suggest the formation of
t-butanol pairs, bridged by a chloride ion via
hydrogen-bonds, and leading to a reduced number of intermolecular hydrophobic
butanol-butanol contacts. Here we present a joint experimental/theoretical
study on the same system, using a combination of molecular dynamics simulations
and nuclear magnetic relaxation measurements. Both theory and experiment
clearly support the more intuitive scenario of an enhanced number of
hydrophobic contacts in the presence of the salt, as it would be expected for
purely hydrophobic solutes [J. Phys. Chem. B {\bf 107}, 612 (2003)]. Although
our conclusions arrive at a structurally completely distinct scenario, the
molecular dynamics simulation results are within the experimental errorbars of
the Bowron and Finney work.Comment: 15 pages twocolumn revtex, 11 figure
Simulations of a lattice model of two-headed linear amphiphiles: influence of amphiphile asymmetry
Using a 2D lattice model, we conduct Monte Carlo simulations of micellar
aggregation of linear-chain amphiphiles having two solvophilic head groups. In
the context of this simple model, we quantify how the amphiphile architecture
influences the critical micelle concentration (CMC), with a particular focus on
the role of the asymmetry of the amphiphile structure. Accordingly, we study
all possible arrangements of the head groups along amphiphile chains of fixed
length and 16 molecular units. This set of idealized amphiphile
architectures approximates many cases of symmetric and asymmetric gemini
surfactants, double-headed surfactants and boloform surfactants. Consistent
with earlier results, we find that the number of spacer units separating
the heads has a significant influence on the CMC, with the CMC increasing with
for . In comparison, the influence of the asymmetry of the chain
architecture on the CMC is much weaker, as is also found experimentally.Comment: 30 pages, 17 fgure
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