Quantum-Chemical Calculations of the Charge Distribution
in Ionic Surfactants
Department of Chemical Engineering, Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139-4307
Received March 30, 1999
In Final Form: June 29, 1999
Abstract:
The charge distribution in common ionic surfactant molecules is estimated
using quantum chemical methods. Calculations are compared for four widely
accepted semiempirical methods (MINDO/3, AM1, PM3, and MNDO/d). The atomic
partial charges are calculated for surfactants with linear alkyl tails
and common headgroups, including anionic (sulfate, sulfonate, carboxylate),
cationic (trimethylammonium, pyridinium), and amphoteric (betaine, dimethylamine
oxide) classes. The headgroup charges are shown to distribute to the rest
of the molecule, with significant partial charge on the alpha-methylene
group (3-40%) and a partial charge on the remaining alkyl tail (4-11%).
The partial charge distribution influences surfactant self-assembly and
physical properties.