The role of hydration degree of cations and anions on the adsorption of high-molecular-weight nonionic polyacrylamide on glass surfaces

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This study investigates the effect of two alkali metal cations (Li+, K+) on the adsorption of high-molecular-weight, nonionic polyacrylamide on glass surfaces using a colloidal force probe. The effect of two different anions (Cl−, NO3−) on polymer adsorption is also presented. There is a general consensus that hydrogen bonding is the main mechanism by which nonionic polyacrylamides adsorb to silica and glass surfaces. In the current study, the adsorption of the polymer is found to result in repulsion on approach as early as 20min of incubation in polymer solutions using both 0.27M lithium and potassium chlorides. On the other hand, adhesion on retraction occurs in both solutions but with higher magnitudes for the polymer solution prepared in potassium chloride. Adhesion on retraction in potassium chloride solutions occurs even after 24h of incubation, whilst the adhesion disappears in lithium chloride after 24h of incubation. The force at hard contact measured in the potassium chloride solution is generally one order of magnitude lower than those measured in lithium chloride. This indicates that the adsorbed polymer molecules in potassium chloride solutions attain flatter conformation than the ones in lithium chloride. This is attributed to the higher ability of the potassium cation to increase the adsorption energy of the polymer to the glass surface. This higher adsorption energy results in the adsorbed amount of the polymer in KCl being higher than in LiCl. Because of their lower hydration and larger size compared to lithium, potassium cations decrease the hydration of the glass surface and they have the ability to bind with the oxygen atoms in the polymer molecule to the negatively-charged sites on the glass surface. With potassium nitrate, the approach–retraction measurements indicate different adsorption kinetics of polyacrylamide compared to potassium chloride. The nitrate enhances the extension of the adsorbed polymer molecules away from the surface, reflected by the longer ranged steric repulsion in the approach measurements conducted during 24h of incubation. This is probably attributed to the large size of the nitrate anion, its lower hydration and different chemical structure, which allow it to form H-bonds with the polymer. Hence, the polymer is prohibited from forming H-bonds with the surface allowing it to extend away from the surface. This also negatively affects the adsorbed amount, which is found to be lower than in LiCl.