A spectroscopic approach to probe macromolecular conformational changes at interface under different environmental conditions: A case study with PAA adsorbed at oil-water Interface

Abstract

Knowledge of the behavior of adsorbed macromolecules at liquid-liquid interface under different physio-chemical conditions is extremely important in different scientific disciplines such as colloidal formulations, food, cosmetic materials, drug delivery systems, anti-bacterial surfaces etc. The conformational changes of a weak polyelectrolyte, polyacrylic acid (PAA), adsorbed at a magnetically controllable oil-water interface under different local environmental condition is manifested from the Bragg peak shifts in the reflected light, after the droplets are assembled into a periodic 1D array with the aid of a weak magnetic field. The change in Bragg peak wavelength was large when the adsorbed PAA molecular weight was high. Further, the conformational changes are also confirmed from force-distance and light scattering measurements. The intermolecular force measurement shows that the adsorbed PAA undergoes a reversible conformational change from a compact globule at pH~3 to an extended structure at pH~11, with a huge change in the interdroplet spacing due to an enhanced ionization of surface groups. With further increase in pH, the extended PAA is fully collapsed due to counter-ion condensation, resulting in an irreversible aggregation. The force and light scattering measurements further confirm the conformational change from globule to extended transition. This new spectroscopic approach offers promising applications in probing macromolecular conformation at interfaces and to obtain new insight into colloidal stability.