Adsorption of nonionic polyacrylamide on sodium montmorillonite: Relation between adsorption, ξ potential, turbidity, enthalpy of adsorption data and 13C-NMR in aqueous solution

Abstract The adsorption of nonionic polyacrylamide on sodium nontomorillonite is studied using different molecular weight macromolecules (mol wt = 4.40 × 10 4 , 1.20 × 10 5 , 3.70 × 10 5 , 3 × 10 6 ), with the monomeric unit represented by the isobutyramide molecule. All of the adsorption runs are completed by turbidimetric, electrokinetic, X-ray scattering, and calorimetric data. The adsorption of isobutyramide reveals the nature of the different adsorption sites (broked edge faces and external and internal basal faces). At the plateau of the isotherm, the amount adsorbed is approximately equal to ( Q a ) 23.5 g per 100 g of solid, i.e., one isobutyramide molecule for two hexagonal cavities. This is the maximum number of acrylamide units in contact with the solid. Our results on macromolecule adsorption seem to show that the Q B ∼ 20 g a · 100 g s −1 , the macromolecules have statistically a flat conformation. For higher Q a values, the macromolecules adsorb, making larger and larger loops into the bulk solution. This process is achieved along a destruction of the turbostratic gathering of the tactoids. The 13 C-nuclear magnetic resonance of the CO groups is the adsorbed phase and in aqueous soluton corroborate the conformation variations along the isotherms. It is demonstrated that line broadening is due to a chemical shift dispersion arising from the surface site heterogeneity or the rotational isomerism hindrance accompanying a change of macromolecular structure near the surface. If Q a is lower than 20 g a · 100 g s −1 , linewidth remains constant and equal to 420 Hz (± 15 Hz). For Q a high than 20 g a · 100 g s −1 , linewidth decreases to 106 Hz (mol wt = 3 × 10 6 and Q a = 68 g a · 100 g s −1 , indicating a higher mobility of the chains when Q a increases.