Effect of Ionic Strength and Layer Number on Swelling of Polyelectrolyte Multilayers in Water Vapour

Abstract

The swelling behavior of polyelectrolyte multilayers (PEMs) of poly(sodium-4 styrene sulfonate) (PSS) and poly(diallyl dimethyl ammonium chloride) (PDADMAC) prepared from aqueous solution of 0.1 M and 0.5 M NaCl are investigated by ellipsometry and Atomic Force Microscopy (AFM). From 1 double-layer up to 4 double-layers of 0.1 M NaCl, the amount of swelling water in the PEMs decreases with increasing number of adsorbed double layers due to an increase in polyelectrolyte density as a result of the attraction between the positively charged outermost PDADMAC layer and the Si substrate. From 6 double layers to 30 double layers, the attraction is reduced due to a much larger distance between substrate and outermost layer leading to a much lower polyelectrolyte density and higher swelling water. In PEMs prepared from aqueous solution of 0.5 M NaCl, the amount of water constantly increases which is related to a monotonically decreasing polyelectrolyte density with increasing number of polyelectrolyte layers. Studies of the surface topology also indicate a transition from a more substrate affected interphase behavior to a continuum properties of the polyelectrolyte multilayers. The threshold for the transition from interphase to continuum behavior depends on the preparation conditions of the PEM.

Keywords:

• Continuum properties
• Interphase behavior
• Ionic strength
• Multilayers
• Polyelectrolytes
• Substrate effect
• Swelling behavior
• Threshold
• Water vapor

Acknowledgments

We appreciate the financial support by the Deutsche Forschungsgemeinschaft within the Priority Program SPP 1369 and also the financial support by the Cluster of Excellence, Unified Concepts in Catalysis (UniCat) and Nanosystems Initiative Munich (NIM).