posted on 2013-07-25, 00:00authored byNóra Abrankó-Rideg, Mária Darvas, George Horvai, Pál Jedlovszky
The adsorption layer of five different
surfactants, namely, pentanol,
octanol, dodecanol, dodecyl trimethyl ammonium chloride, and sodium
dodecyl sulfate, has been analyzed on the basis of molecular dynamics
simulation results at two surface densities, namely, 1 and 4 μmol/m2. The analyses have primarily focused on the question of how
deeply, in terms of atomistic layers, the different surfactant molecules
are immersed into the aqueous phase. The orientation and conformation
of the surfactant molecules have also been analyzed. The obtained
results reveal a clear difference between the immersion behavior of
the alcoholic and ionic surfactants. Thus, alcoholic surfactants are
found to be located right at the water surface, their apolar tails
not being considerably immersed into the aqueous phase and the alcoholic
headgroups being preferentially located in the surface layer of water.
Ionic surfactants are immersed several layers deep into the aqueous
phase, with headgroup atoms reaching the sixth–eighth and tail
carbon atoms reaching the third–fourth subsurface layer in
several cases. The observed difference is related, on the one hand,
to the ability of the alcoholic surfactants of substituting surface
water molecules in their lateral hydrogen bonding network at the water
surface and that of their apolar tails for replacing dangling hydrogens
and, on the other hand, to the energetic gain of the ionic headgroups
if they are fully hydrated rather than being in contact with hydrocarbon
tail groups.