Studies on interaction between similarly charged polyelectrolyte: fatty acid system

Abstract

Interaction between an anionic polyelectrolyte, poly(acrylamido)methylpropanesulfonate (PAMPS), and anionic fatty acid, arachidic acid (AA), has been studied at the air-water interface. Although similarly charged, the polyelectrolyte complexes with the AA monolayer are possibly driven by interactions other than electrostatic, such as hydrophobic and hydrogen bonding. The absence of electrostatic interaction between the polymer and the fatty acid was demonstrated by performing the experiments in the presence of salt, at low pH, or with a neutral surface-active molecule such as octadecanol (ODO). The effect of polymer charge density on its interaction/degree of complexation with the monolayer has also been investigated. The degree of complexation depends on the net charge on the polymer and increases with the decrease in the charge. To further supplement the role of nonelectrostatic interactions, we have studied the interaction of PAMPS with an anionic surfactant, dioctyl sulfosuccinate (AOT), by surface tension measurements. At lower polymer concentrations, we observe the presence of a critical aggregation concentration (CAC), which is typical for interacting systems. The complexation of polymer with AA has been followed by surface pressure-area (Π-A) isotherms and Brewster angle microscopy (BAM). Attempts to form Langmuir-Blodgett (LB) films of the complexes did not yield any lamellar layer by layer stacking, possibly due to the bulkiness of the polymer and the nature of the interactions used. Quartz crystal microgravimetry (QCM) and Fourier transform infrared spectroscopy (FTIR) have been used to characterize such films.