Interactions of highly charged polyelectrolytes with monolayers of oppositely charged amphiphiles

Abstract

Poly(vinylbenzylammonium chloride) (PVBAC) polyelectrolytes possessing defined chain lengths, narrow chain length distributions, uniform charge density, but substituents of different hydrophilicity and size at the quaternary ammonium group were used as model compounds to study the influence of the chain length and the substituent type on the interaction with oppositely charged monolayers. Langmuir monolayers were formed from dimyristoylphosphatidic acid (DMPA). Pressure-area and pressure-time isotherms revealed an increase of both the area per amphiphile molecule and the surface pressure at constant area as a function of time if the polyelectrolyte adsorbs on the amphiphile. The compressibility of the DMPA/PVBAC complex layers was found to be much higher than the compressibility of the DMPA monolayer. Moreover, the results clearly indicate the influence of the chain length and the substituent type. Larger and more hydrophobic substituents at the quaternary ammonium group significantly increase the area per amphiphile molecule, from 0.42 nm² for the amphiphile up to 0.97 nm² for the complex. This parameter was also found to increase with decreasing chain length. Incorporation of the hydrophobic substituents into the monolayer as well as end group effects are proposed to be the reason for the monolayer extension.

Acknowledgements: We thank Werner Jaeger, Fraunhofer Institute of Applied Polymer Research, Golm, Germany, for supporting the polymer synthesis and Olaf Helmecke, Technical University of Braunschweig, Germany, for the help taking BAM images. The Swiss National Science Foundation is acknowledged for financial support (grant 2100-611314).