surface pressure/tension

Date: Sun Aug 22 2004 - 02:37:11 CDT

Hi folks,

I've been using NAMD for awhile now, but this is my first post here. I
have an interest in simulating Langmuir monolayers (molecular films at
an air/water interface) and being able to make a connection with
experimentally measured isotherms (plots of surface pressure vs. surface
density for a film). Lately I've been thinking a lot about the
"SurfaceTensionTarget" parameter used in Langevin CP dynamics in NAMD,
and after repeated readings of the User Guide and some surface-physics
texts, I'm still confused.

Basically, I just don't understand how this feature is implemented. The
(unfortunately brief) description in the User Guide says:

   "The pressure specified in LangevinPistonTarget becomes the pressure
   along the z axis, and surface tension is applied in the x-y plane."

Well, first of all, it seems that what would be more useful (at least
for my purposes) would be to specify the desired surface *pressure*,
rather than surface *tension*. Or perhaps the nomenclature is just
mixed up, and this is already what NAMD does? (The two terms are
similar, and in fact both quantities have the same physical dimensions
of force/length, or equivalently energy/area, but they are not the same
thing. Basically, surface pressure is defined as the difference in the
surface tension of an interface with and without a surfactant present;
thus, while the surface tension of pure water is around 70 dyn/cm at
room temp, its surface pressure would be zero.)

But a bigger issue for me is this: as far as I know, it doesn't really
make sense to talk about either surface tension or surface pressure,
except in the presence of a *surface*, i.e. an interface between two
materials (such as water/air, or water/lipid). But in a NAMD
simulation, one could apply "SurfaceTension" to a system that doesn't
contain an interface (say, a simple box full of water). Or, even if the
simulated system does have an interface, NAMD doesn't "know" anything
about this.

So the real question is, when you use "SurfaceTension", what is NAMD
actually doing? It seems like it must just be applying a certain amount
of "X-Y pressure" (in addition to the regular isotropic pressure)
equally to all atoms; but then, how does this (a force/area) relate to
the value (a force/length) that one specifies for the "SurfaceTension"
parameter? Does the Z-dimension of the MD box come into play somehow?
In other words, if I want to simulate a Langmuir film at a surface
pressure of, say, 20 dyn/cm, do I simply use that value for
"SurfaceTension" and all will be well?

Maybe I'm missing something here, but in any case I'd love for someone
who knows better to chime in and set things straight. Thanks so much!

- Erik

C. Erik Nordgren, Ph.D.
Department of Chemistry
University of Pennsylvania

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