# Negative interfacial tension in water-oil system
**URL:** https://gromacs.bioexcel.eu/t/negative-interfacial-tension-in-water-oil-system/12913
**Category:** User discussions
**Created:** [November 13, 2025, 4:19pm UTC](https://gromacs.bioexcel.eu/t/negative-interfacial-tension-in-water-oil-system/12913 "2025-11-13T16:19:04Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@A.Chekalov](https://gromacs.bioexcel.eu/u/A.Chekalov)
#### Post date: [November 13, 2025, 4:19pm UTC](https://gromacs.bioexcel.eu/t/negative-interfacial-tension-in-water-oil-system/12913/1 "2025-11-13T16:19:04Z")
GROMACS version: 2025.3
GROMACS modification: No
Good afternoon all, I am performing a simulation of interfacial behavior of water-oil phases in presence of ionic surfactant.
For this purpose i’ve created two slabs of water with a slab of oil (7 different types of hydrocarbons) between them and put 150 surfactant molecules on each of the formed interfaces. Total system size is 8x8x20 nm, so the average surface area per one surfactant molecule is around 0.43 nm2.
I started with energy minimization, then short 200 ps NVT run and 1 ns isotropic NPT run. Finally ive done a production NVT run (40 ns) with the following .mdp:
title = AEC\_300\_40ns
cpp = cpp
include =
define =
integrator = md
dt = 0.001
nsteps = 40000000 ; 40 ns
comm-mode = Linear
nstcomm = 200
nstxout = 5000 ;trajectory file
nstvout = 5000 ; velocity file
nstfout = 0
nstlog = 5000 ;log file
nstenergy = 1500 ; energy file
nstxtcout = 1500 ;compressed trajectory file
nstlist = 10
pbc = xyz
coulombtype = pme
pme\_order = 4
fourierspacing = 0.12
rlist = 1.2
rcoulomb = 1.2
vdw-type = cut-off
cutoff-scheme = Verlet
rvdw = 1.2
Tcoupl = V-Rescale
tc\_grps = System
tau\_t = 2.0
ref\_t = 298.15
gen\_vel = no
gen\_seed = 94839
constraints = h-bonds
constraint-algorithm = lincs
lincs\_iter = 1
lincs\_order = 4
Dispcorr = EnerPres
At the end I calculated the interfacial tension by pressure tensor and got negative value (-198.6 bar\*nm\*).\* Firstly i thought about interface oversaturation, so I decreased the number of molecules to 125, 100 and 75 and got -114.8, -27.8 and 149.3 bar\*nm respectively, so only at 75 molecules at interface value was positive. My second suggestion was about pressure, so i used semi-isotropic 1ns NPT instead of isotropic before production run, but got the same negative values of negative IFT. In the absence of surfactant (water - oil - water) and with use of another surfactant (non-ionic) IFT values seem to be realistic.
OPLS-AA force field was used. I would be glad for any possible suggestions, may be somebody has encountered same issues?
Thanks
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### Author:  [@hess](https://gromacs.bioexcel.eu/u/hess)
#### Post date: [November 13, 2025, 8:53pm UTC](https://gromacs.bioexcel.eu/t/negative-interfacial-tension-in-water-oil-system/12913/2 "2025-11-13T20:53:11Z")
Your area per surfactant is extremely small, so I would guess that you indeed have oversaturation.
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### Author:  [@A.Chekalov](https://gromacs.bioexcel.eu/u/A.Chekalov)
#### Post date: [November 14, 2025, 8:02am UTC](https://gromacs.bioexcel.eu/t/negative-interfacial-tension-in-water-oil-system/12913/3 "2025-11-14T08:02:20Z")
Dear Berk, thank you for your opinion, I find it strange that for another surfactant, this area per molecule is more than acceptable.
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### Author:  [@hess](https://gromacs.bioexcel.eu/u/hess)
#### Post date: [November 14, 2025, 9:40am UTC](https://gromacs.bioexcel.eu/t/negative-interfacial-tension-in-water-oil-system/12913/4 "2025-11-14T09:40:19Z")
I don’t know what surfactants you have, so I can’t judge that.