X-Message-Number: 16151
From: "Jan Coetzee" <>

Subject: 
=?iso-8859-1?Q?Phase_In=2C_Phase_Out_-_With_a_Little_Help_From_Your_PC!_?=
Date: Sat, 28 Apr 2001 19:40:30 -0400

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The thought of determining phase diagrams has traditionally sparked fear in to 
the hearts of scientists, meaning long hours at the bench coupled with complex 
calculations. But now, thanks to Equilibria, the latest module in MSI's (soon to
become Accelrys) Materials Studio PC-based modelling software, hard-worked and 
time-starved researchers can take off their white coats and leave the bench, 
down their calculators and grab the nearest mouse. Equilibria enables users to 
easily simulate phase diagrams on the PC, releasing valuable time for additional
research or instead go to the beach! Michael Francis tells us more...



What exactly are phase diagrams and why are they so important? Phase diagrams 
are a graphical visualisation of the state of a material as a function of 
temperature, pressure and concentrations of the constituent components and give 
an understanding of solubility and phase coexistence. As a result they are 
vitally important in many fields - alloy design, petrochemicals, the packaging 
industry (in particular foodstuffs), adhesives, detergents, and pharmaceuticals 
to name a few. Their experimental determination becomes more difficult as the 
number of components increases, and involves the extrapolation of thermodynamic 
functions followed by further refinement, all of which, as expected, are hungry 
on time and resources. To further complicate matters, both the interaction 
energies and free energies are required to calculate solubilities - a 
challenging and tricky limitation [1]. In order to address these difficulties, 
any technique must be able to sample the configuration space of the molecules 
efficiently and work in conjunction with a forcefield that can work over a wide 
range of temperatures and pressures and also handle simultaneously both the 
vapor and condensed phases.


"Phase diagrams for many important components are difficult to determine. Good 
experimental data require expensive equipment and weeks of measurements" 
explains Dr Gerhard Goldbeck-Wood, Director of the MSI's Polymer Consortium. 
"Critical points for a number of systems such as long chain hydrocarbons are out
of the reach of experimental methods. Empirical methods for mixtures tend to 
perform poorly for mixtures of small and long molecules. Direct simulations 
offer an attractive alternative in these difficult cases".

Equilibria


Equilbria, a completely new software product within the Materials Studio 
environment and over a year in development, addresses all these challenges. 
Using the Gibbs Ensemble Monte Carlo method [2] a state-of-the-art technology to
simulate chemical potential equilibrium between two phases, Equilibria offers a
comprehensive set of simulation tools to determine the phase equilibria of 
hydrocarbon molecules and their binary and ternary mixtures, and is the first 
direct simulation product for phase diagrams. Such simulations provide 
thermodynamic data, which assist materials research and process design in the 
chemicals and petrochemicals industries. "Phase coexistence simulations have 
been developed and tested in academia for a number of years. Equilibria brings 
this state-of-the-art technology to the desktop of the industrial chemist and 
chemical engineer" adds Goldbeck-Wood. "Equilibria is the first commercial 
software tool which allows the industrial chemist and chemical engineer to 
determine phase coexistence data by simulation alone. Its full integration into 
the Materials Studio modeling environment ensures the ease of use of this 
technology and its accessibility from the desktop."


So how does Equilibria work?


So how does Equilibria work? The Gibbs Ensemble method used enables the direct 
simulation of vapor-liquid and liquid-liquid phase coexistence properties of 
single components and mixtures from a single. These methods allow the 
determination of the phase coexistence even for larger alkanes (e.g. C48), and 
solubilities to be determined of alkanes and a-alkenes in poly(olefins). From 
the vapor-liquid equilibrium data, Equilibria determines critical constants. The
software provides a tool to determine the second virial coefficient of single 
components. Equilibria is supported by the NERD [3] united atom forcefield, so 
named from the initials of its developers, which is carefully parameterised and 
allows the effect on phase coexistence of even small variations in molecular 
structure to be determined with confidence. This forcefield covers all linear 
and branched saturated alkanes, a-alkenes and nitrogen. The parameter files are 
open enabling the user to edit the parameters and even extend the range to other
groups, e.g. higher alkenes.


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