Quantum-Chemical Calculations of the Charge Distribution in Ionic Surfactants

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 α-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.