Heterodyne-detected sum frequency generation spectroscopy of polyacrylic acid at the air/water-interfaceface

The conformational and hydration behavior of polyacrylic acid (PAA) at the air/water interface is studied using heterodyne-detected vibrational sum frequency generation spectroscopy (HD-VSFG). The amount of adsorption at the water-air interface and the influence of PAA on the water structure at the interface are found to be highly dependent on the degree of ionization (acid dissociation) of the carboxylic acid groups. At a low degree of ionization PAA is surface active and remains in a dense packing arrangement with intrachain hydrogen bonds. The interfacial water molecules show a net orientation with their OH groups pointing to the surface. Ionization of the carboxylic acid groups leads to an increase of the negative charge and to a break-up of the intrachain hydrogen bonds. The increase in negative charge is accompanied by an increase of the VSFG signal of the interfacial water molecules, as a result of their enhanced orientation. At ionization degrees α > 0.203 PAA loses its enhanced surface propensity and is well dissolved in the bulk. Addition of NaCl to the solution is observed to lead to a recurrence of PAA at the interface.