Subject: MICELLES...Hydrophilic Tails???

From: [email-masked] (Faculty)
Date: Sat, 21 Oct 1995 17:37:17 +0500

Dear Netters :
  You may recollect that I had sent you a small mail regarding
micellar molecules. We have now completed this exciting study.
An extended abstract is enclosed. We will be grateful for your
comments and suggestions. 
  Thnaks a lot!......................................Shridhar Gadre
***************************************************************************
      An Electrostatic Investigation : How Polar is the     
          Tail-group of Ionic Micellar Molecules?
          
             Shridhar R. Gadre and Subhash S. Pingale            
            Department of Chemistry, University of Pune, 
                      Pune - 411007, India.

            A comparison of the electronic charge distribution  for 
  isolated micellar ionic molecules and the respective hydrocarbons 
  is carried out using ab-initio calculations.  The systems studied 
  are  n-dodecanoate, n- decylsulfate  and   the   corresponding 
  hydrocarbons, viz. n-dodecane and n-decane.  It is observed  that 
  the molecular electrostatic potential at the hydrocarbon ends  of 
  dodecylcarboxylate and decylsulfate is   almost  ten  times  more 
  negative than n-dodecane and n-decane molecules respectively.  In 
  other words, for ionic micellar  molecules,  the  charge  of  the 
  polar head group percolates through the molecule upto the end  of 
  the hydrocarbon tail.  In view of this, caution is  warranted  in 
  the use of charge models for  simulation  of  micelles  involving 
  charged molecular systems.

  To examine the effect of this MESP distribution on the possible
  binding energy of water as one goes down the chain, starting from the
  polar head group, we have calculated water interaction energies with
  two typical moities.

  Table for water interaction energies in kcal/mol   with n-dodecanoate
  and n-dodecane molecules, by using ab initio calculations at STO-3G 
  basis set.
--------------------------------------------------------------------
Part of molecule           n-dodecanoate        n-dodecane
where water interacts        molecule             molecule
--------------------------------------------------------------------
Head group part            -26.942                 ----
--------------------------------------------------------------------
Middle part                 -1.674                -0.168
(Typically 5 to 6 carbons away from the head group)
--------------------------------------------------------------------
Tail group part             -0.470                -0.134
--------------------------------------------------------------------
Note that there is substantial difference in the water binding energies
(between the  n-dodecanoate and n-dodecane) even 5 to 6 carbons away from
the head group.
    
   We find that this is generally in agreement with various statements 
in the literature. Some of the literature statements are summarized below.
Note that our observations are sometime not in full agreement with
these statements.
     An  amphiphilic  aggregation  is  dependent  strongly upon the 
electrostatic properties  of the water model.   In  Molecular  dynamic 
simulations of a sodium octanoate  micelle  in  aqueous  solution, the 
hydration of different carbon atoms in the chain is  largest  for  the 
carboxylic atom, the decrease along the chain, reaching a minimum  and 
then increases  again  at  the  end  of  the  chain  (Jonsson  et  al. 
J.Chem.Phys. 85, 2259(1986)).
     Beesly et al.  (J.Phys.Chem.  92,  791(1988))  concluded  from  an 
experimental  survey  that   for   cooperative   interaction   between 
amphiphilic molecules, hydrogen bonding is essential (Van  Os  et  al.  
9,  (1993)).
     The aggregation of amphiphiles  to form micelles,  vesicles,  and 
bilayers   has   been   established   in   water,   hydrazine   (Evans 
(J.Phys.Chem.87,4538(1983)),  Evans  (J.Phys.Chem. 89, 1405 (1985))
formamide  and glycols.  All of these solvents are  hydrogen  bonding, 
posses high dielectric constants, and high cohesive energies (Beesley 
et al.  J.Phys.Chem. 87, 5025(1983).
      Furthermore, micelle formation has also been observed in  other 
liquids such as hydrazine (Ramadan et al.  J.Phys.Chem 87, 4538(1983), 
Ramadan et al. J.Phys.Chem. 89,3405(1985),  ethylammoniumnitrate(Evans 
et al. 87,3537(1983)) formamide,(McDonald  et  al.  J.Pharm.Pharmacole 
22,  148(1970),  Ray  et  al.  Nature  231,  313(1971)   and   glycols 
J.Gen.Chem.USSR 32, 2845(1962).  Striking is that  all  these  liquids 
can form hydrogen bonds (Van Os et al. J.Phys.Chem.  95,6361(1991)).
      From experimental results, Menger et al. (Acc. Chem. Res. 12, 111(1979))
concluded that the water penetration inside the micelle is significant.
Casal (J. Amer. Chem. Soc. 110, 5203(1988)) studied the water penetration into
the interior of a micelle using probe molecules. These studies suggest that
althoug water can penetrate into a micelle, the central core of the micelle
contains no water.
       Watanabe et al (J. Phys. Chem. 93, 6897 (1989)) have concluded that
 .."Thus, on an average, there are 8.5 water molecules in contact with the
the hydrophobic tail of each monomer forming the micelle".