[gmx-users] Surface tension calculation for organic monolayer

[gozde ergin]

Yes but my acid is in protonated form.
> On 13 Jul 2016, at 17:42, Evan Lowry <evanwlowry at gmail.com> wrote:
> 
> Dr. van der Spoel is correct. If you don't take into account the
> protonation state, your calculation may likely be incorrect. The pH can
> have a notable effect on interfacial/surfactant systems.
> 
> Evan L.
> Dear Spoel,
> 
> I am using the protonated form of cis-pinonic acid which means head group
> is COOH.
> I tried many other surface active molecules but never manage to estimate
> the lower surface tension than water.
> Is there anyone here that did the MD simulations with surface active
> molecules on water and estimated the reduced surface tension?
> Any suggestion would be really helpful.
> I tried some other force field (GAFF, AMBER, OPLSAA) but results did not
> change.
> I am doing something wrong but I do not know what.
> 
> 
>> On 07 Jul 2016, at 17:51, gozde ergin <gozdeeergin at gmail.com> wrote:
>> 
>> Dear Spoel,
>> Thanks for your respond.
>> For this simulations I used acids however I got the similar results when
> I used alcohol (pK values are lower than acids).
>> I doubt this error coming from the pH effects.
>> But how could i get the protonated molecules configuration?
>>> On 07 Jul 2016, at 17:11, David van der Spoel <spoel at xray.bmc.uu.se>
> wrote:
>>> 
>>> On 07/07/16 16:06, gozde ergin wrote:
>>>> Dear users,
>>>> 
>>>> I simulated three different systems in cubic box and calculated their
> surface tension as shown below;
>>>> 
>>>> 1. Pure water , surface tension = 61.5 mN/m
>>>> 2. Water with 3M NaCl salt, surface tension = 66.5 mN/m
>>>> 3.Water surface covered with cis-pinonic organic, surface tension =
> 63.5 mN/m
>>>> 
>>>> For system 1 and 2, the results are almost correct however the result
> for system 3 is not correct.
>>>> 
>>>> Cis-pionic is an surface active molecule and should lower the surface
> tension.
>>>> However in my simulations I could not capture this trend.
>>>> I used GAFF force field and LINCS for the constraints.
>>>> Does anyone has any idea?
>>>> Or is there anyone that calculated the surface tension for organic
> coated surfaces with GAFF force field?
>>>> 
>>>> Thanks
>>>> 
>>> Maybe you should consider pH effects? Since this is an acid I would
> guess part of these are deprotonated in solution, did you consider the pK?
> You probably need to mix different amounts of protonated and deprotonated
> molecules and plot the surface tension as a function of the protonation
> state.
>>> 
>>> --
>>> David van der Spoel, Ph.D., Professor of Biology
>>> Dept. of Cell & Molec. Biol., Uppsala University.
>>> Box 596, 75124 Uppsala, Sweden. Phone:       +46184714205.
>>> spoel at xray.bmc.uu.se    http://folding.bmc.uu.se
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