[gmx-users] Re: Forming a micelles

Alif M Latif prism_dead at yahoo.com
Fri Sep 26 10:54:41 CEST 2008

>Date: Wed, 24 Sep 2008 10:15:44 -0700
>From: "Chih-Ying Lin" <chihying2008 at gmail.com>
>Subject: [gmx-users] Forming a micelles
>To: arun.target at gmail.com
>Cc: gmx-users at gromacs.org
>    <5777f3840809241015l2f0e85eo34dddc8f401864e1 at mail.gmail.com>
>Content-Type: text/plain; charset=ISO-8859-1
>Would you please say more about your system?
>How do you design / decide your simulation size of 100 surfactants +100
>co-surfactants + 4000 water molecules??
>How many surfactants will form a micelle?
>How many atoms does one surfactant have?
>How many atoms does one co-surfactant have?
>Do you start from the critical micelle concentration?
>You mentioned that it took 3 x 600 ps to see the micelles.
>Do you mean that you have 3 computers to do the parallel simulation?
>How long does it take to simulate 600 ps?
>Do you visualize the whole 600 ps-image and SEE the micelle?
>Or, other technique to KNOW the surfactants forming the micelles
>without visualizing the system?
>Thank you
Dear Lin,
I have a suggestion:

Micelles should (theoretically) form above the CMC. So if you want a micelle, the concentration should be >>CMC
I suggest that you find the CMC experimentally, then convert the concentration to nm^3 (let say: mv/1000nm^3)
The number of molecules should be able to obtained by multiplying with Avogadro number: (6.023x10^23). If your system contain co-surfactant, then you'll need to get the experimetal CMC for at least one of the two surfactants.Take a look at this paper: 
Bourov G. K. and Bhattacharya A., Brownian dynamics of mixed surfactant micelles, Journal of Chemical Physics, 123 (2005) 204712(1)-204712(6)
How long 600 ps simulation is depends on your system size, box type, parameters and computational power (see manual).
One other way to KNOW the surfactants formed micelle is by measuring the surface tension (use g_energy). But g_energy gives surface*surface tension value, so you'll need to work on that. (HINT: Surface tension value drop drastically at CMC)

Hope these helps,

Muhammad Alif Mohammad Latif 
Department of Chemistry
Faculty of Science
Universiti Putra Malaysia
43400 UPM Serdang, Selangor

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