Polyacrylamide at Solid/Liquid Interfaces.

Abstract

In the literature, the term polyacrylamide refers to neutral, hydrolyzed, and chemically modified polyacrylamide and in this sense polyacrylic acid may likewise be considered to consist of hydrolyzed acrylamide groups. These hydrosoluble polymers are employed in wide range of applications where the polymer characteristics at solid-liquid interfaces play an important role. The interaction with solid adsorbents is very complex since electrostatic, hydration, van der Waals, and other forces operate simultaneously. As in most applications the polyacrylamide is brought into contact with oxides, clays or soils, synthetic alumino-silicates, and aluminum oxide may be used as model adsorbents. Thus, with reference to the two systems polyacrylamide/alumino-silicate and polyacrylamide/aluminum oxide, the author attempts to define some of the interfacial processes affecting the adsorbed macromolecules and to describe certain interfacial characteristics of these complex systems. The main results are as follows. -Neutral polyacrylamide. Hydrogen bonding between acrylamide and neutral aluminol groups is responsible for adsorption and the amount of polymer adsorbed parallels the surface density of aluminol sites. At ambient temperature, the kinetics of layer formation is governed initially by the random deposition of solution macromolecules and finally by tunneling. Thermodynamic and kinetic considerations lead to a model where the majority of the adsorbed macromolecule share identical dynamic characteristics under equilibrium conditions. Departure from equilibrium induced by changing the polymer concentration in the supernatant phase greatly modifies the dynamic features of the adsorbed macromolecules. -Hydrolyzed polyacrylamide. Charge-charge interactions superimpose on the persistent effects of hydrogen bonding to alter the amount of polymer adsorbed and the adsorption kinetics. The strong affinity of hydrolyzed polyacrylamide for positively charged surfaces induces interfacial spreading of the adsorbed macromolecules and in some instances desorption of part of this population (overshoots). -Complexed polyacrylamide. The presence of positively charged, negatively charged, and neutral chain segments confers amphoteric character on the polymer chain. Strongly complexed polymers display fast adsorption, while initially weakly complexed polymers show delayed adsorption. In addition, under certain circumstances, the presence of hydrophobic microdomains confers an amphiphilic character on the macromolecules, which profoundly modifies the nature of the interaction forces. The specific interaction typical of neutral polyacrylamide progressively disappears with increasing hydrolysis until for polyacrylic acid the surface coverage corresponds to nonselective interfacial deposition. Extremely slow displacement of hydrophilic and hydrophobic groups are found to occur within the adsorbed layer. Despite the promising use of macromolecules in environmental strategies, a consideration of the complexity of the underlying interfacial phenomena points to the difficulties which may arise at short or long terms in such applications. Copyright 1999 Academic Press.