Polyampholyte/Surfactant Complexes at the Water-Air Interface: A Surface Tension Study

The present paper is related to interactions between strongly alternating polyampholytes, i.e., copolymers of N,N'-diallyl-N,N'-dimethylammonium chloride and maleamic acid derivatives, varying in hydrophobicity and excess charges and the oppositely charged anionic surfactant sodium dodecyl sulfate (SDS). Surface tension measurements have revealed a complex behavior with the formation of polyampholyte-SDS complexes at water air interfaces which depends on both the hydrophobic character of the polyampholyte and electrostatic attractive forces between the polyampholyte and the anionic surfactant in dependence on pH. Hereby, maleamic acid copolymers with additional carboxylic groups in the phenylic side chain show a significant lower surface tension at the critical association concentration (CAC) due to the formation of surface-active SDS complexes and multicomplexes. In the presence of only one carboxylic group in the p-position the CAC can be strongly shifted by varying the pH due to repulsive electrostatic interactions.