# [gmx-users] lipid monolayer isotherm

Andre Farias de Moura andre at qt.dq.ufscar.br
Tue Jan 18 16:01:15 CET 2005

```On Tue, 18 Jan 2005, Hector Mtz-Seara wrote:

sorry Hector, my mistake: g_energy actually displays surface tension,
the equation I referred to is meant to convert surface tension into
surface pressure, which is the most common way to present an isotherm
(surface pressure against area per molecule). anyway, a surface tension
larger than that of water means that your system is not equilibrated
yet (it gives a negative surface pressure).

> Sorry after reading again and again there is one question I'm not sure
>
> > <>
> > g_energy output gives surface pressure, not surface tension.
>
> I've been reading the gromac's manual an in 7th character , page 138 ,
> first paragraph, it speaks about surface tension, not surface pressure.
> In the option list of g_energy also speaks about #Surf*SurfTen. Then why
> you say the output is the surface pressure?. If you are right then where
> is the value of the surface tension of the water to make this output in
> the gromacs program. By the way it then means that you can only
> calculate "surface pressure" for monolayers in pure water systems?. Can
> you explain me that a little more.

no, it means that you need to know the surface tension of the subphase,
whether it is pure water or any other substance or even a solution.

>
> > <>to compute the <><>surface tension you have to use the standard
> > expression:
> >
> > gama = gama_0 - pi
> >
> > where:
> >
> > gama = surface tension
> >
> > gama_0 = pure water surface tension
> >
> > pi = surface pressure
> >
> > anyway, if you're getting surface pressures 3-fold larger than that
> > of pure water, then your surface tension is negative, isn't it? if
> > that happens to be the case, then your systems may be still away
> > from mechanical equilibrium.
> >
> > maybe you could give some more details of your simulations: how did
> > you assembled each monolayer? how long was each simulation run? if
> > you plot surface area along the simulation time, is it stable?

/-/-/-/-/-/-/-/-/-/-/-/-/-/-/-/-/-/

Dr. Andre' Farias de Moura

/-/-/-/-/-/-/-/-/-/-/-/-/-/-/-/-/-/

Laboratorio de Quimica Teorica
Sao Carlos - SP - Brasil

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Endereco Atual

Grupo de Quimica Quantica
Instituto de Quimica de Sao Carlos