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<DIV><FONT face=Arial size=2>Hi.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>I was trying to simulate the behaviour of
N-isopropyl propionamide, CH3-CH2-CO-NH-CH-(CH3)2 (NIPPA) in aqueous
solution.</FONT></DIV>
<DIV><FONT face=Arial size=2>This molecule has an inverted solubility behaviour
with temperature and it displays a miscibility gap in water for temperatures
higher than about 30°C.</FONT></DIV>
<DIV><FONT face=Arial size=2>Experiments show that at 293 K and 1 atm the
water/NIPPA omogeneous solution phase is stable at any composition.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>I performed a few NPT MD simulations of NIPPA in
water SPC using the 45A3 force field, at different NIPPA concentrations,
293 K (Nose Hoover) and 1 atm (Parrinello Rahman), PME for electrostatic. Two
typical systems contained [288 NIPPA molecules and 1680 water molecules] or [166
NIPPA and 4140 water molecules].</FONT></DIV>
<DIV><FONT face=Arial size=2>In the starting configuration the NIPPA molecules
were randomly placed and solvated.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>I was very surprised to see that after 2 ns (
time step 2 fs) the NIPPA molecules aggregated, in several ways (layer,
cylindrical tube, spherical object) depending on the NIPPA concentration,
whereas I was expecting a complete solubility in water.</FONT></DIV>
<DIV><FONT face=Arial size=2>In a further run at 273 K ( a temperature
about 30 degrees lower than the critical temperature) I found the same
aggregation effect.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>What do you think? Am I beyond the possibilties of
the simulation?</FONT></DIV>
<DIV><FONT face=Arial size=2>Any comment is welcome. Thanks in
advance</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Ester</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>P.S. I found in the literature that the DPPC
self-assembly was modeled in MD simulations ( de Vries et al JACS 2004) at 323
K, a temperature higher than the transition temperature of DPPC. Why did the
Authors use a T where the organized phase is unstable? I was wondering since
also for my system the temperature is a crucial parameter.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV> </DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Ester Chiessi<BR>Dipartimento di Scienze e
Tecnologie Chimiche<BR>Università di Roma Tor Vergata<BR>Via della Ricerca
Scientifica<BR>00133 Roma<BR><A
href="http://www.stc.uniroma2.it/cfmacro/cfmacroindex.htm">http://www.stc.uniroma2.it/cfmacro/cfmacroindex.htm</A><BR>tel:
*39*6*72594462</FONT></DIV></BODY></HTML>