Molecular structure–surface property relationships for heterocyclic and dipropylamino derivatives of hexadecylphosphocholine and cetyltrimethylammonium bromide in NaBr and salt-free aqueous solutions

Interfacial area per surfactant molecule values at the air/liquid interface for heterocyclic and dipropylamino derivatives of zwitterionic hexadecylphosphocholine and cationic cetyltrimethylammonium bromide derivatives were calculated from surface tension measurements in NaBr and salt-free aqueous solutions. The interfacial area values were found to be in the range of 0.45–0.86nm2 for both zwitterionic and cationic derivatives. The interfacial area data were correlated with the structure and arrangement of surfactants molecules at the interface. In NaBr solutions, cationic cetyltrimethylammonium bromide derivatives acquire denser packing in the interfacial layer due to the repulsion reduction between cationic headgroups by bromide ions which resulted in smaller interfacial area values. In salt-free aqueous solutions, interfacial area of cetyltrimethylammonium bromide derivatives increased above the level of that of hexadecylphosphocholine derivatives due to the repulsion between positively charged headgroups of adjacent cationic surfactant molecules. The arrangement of hexadecylphosphocholine derivative molecules in the interfacial layer is tighter due to the absence of repulsive electrostatic interactions between neighboring ammonium and phosphate groups. The introduction of electrolyte ions in the hexadecylphosphocholine derivative surfactant aqueous solution results in electrostatic disruption of tight arrangement of its molecules and hence, in the interfacial area increase. The comparison of the experimentally obtained interfacial area values with the surfaces of minimum energy conformers representing the surfactants headgroup parts obtained from molecular models indicates that the arrangement of molecules of hexadecylphosphocholine derivatives in the interfacial layer can be affected more by inter- and intramolecular interactions than the arrangement of cetyltrimethylammonium bromide derivatives.