The adsorption of water-soluble polymers (e.g., flocculants, coagulants) onto mineral particles is typically characterized by “trains” of adsorbed polymer segments with “loops” and “tails” of unadsorbed polymer segments that extend into the solution. Ex situ spectroscopic studies carried out in the past have been complicated by the way in which the unadsorbed segments of the polymer interact with the surface of the substrate upon drying. In this study, an in situ Fourier transform infrared attenuated total reflection technique was used to examine the interaction of poly(acrylic acid) and hematite at pH 2. A hematite colloid was deposited onto a ZnSe crystal, and the poly(acrylic acid) solution was subsequently pumped across the coated crystal at a known rate. Polymer adsorption was irreversible and could be described by a Langmuir isotherm, the rate of curvature of which suggested that the adsorption was weak. The mode of adsorption was shown to be bidentate chelate complexation by the carboxylate functional group to a surface ferric ion. Pimelic acid, a simple model compound effectively representing a polymer consisting of only two monomer functionalities, adsorbed onto hematite in the same manner at pH 2 and gave no infrared spectral peaks associated with unadsorbed carboxylate groups, thereby supporting the proposed adsorption mechanism. The technique could also discriminate between the adsorbed and the unadsorbed segments of the adsorbed polymer molecule, however; the fraction of adsorbed segments was difficult to quantify. When a number of assumptions were made, it was found that at most 9% of the carboxylate functional groups of the polymer were adsorbed onto the hematite.

Conformational changes of poly(vinylamine) (PVA) upon adsorption onto mica were determined by AFM. The polyelectrolyte charge can be tuned through the solution pH. At pH 3, where the PVA is highly charged, an extended coil conformation is observed. When the pH is increased, the polyelectrolyte charge decreases and induces a segmental collapse through pearl-necklace structures. Above pH 9, where the PVA is only weakly charged, one can observe a globular conformation. By analyzing the volume of the imaged objects, it could be clearly demonstrated that single molecules have been observed. By examining PVA adsorbed in its most globular conformation, the molecular mass distribution has been determined. The PVA molecules could be also detected after being grafted to an epoxy terminated surface, which has been obtained through a silanization reaction. The observed polyelectrolyte collapse through a sequence of pearl-necklace structures is likely related to analogous conformations in solution, which are predicted on theoretical grounds.

Interactions between preadsorbed films of poly(vinyl amine) (PVA) of two different line charge densities on silica substrates were studied with the colloidal probe technique based on the atomic force microscope (AFM). The preadsorbed films were prepared by adsorption of PVA from a pH 4 solution without any added salt. The highly charged PVA adsorbs in a flat configuration and in laterally heterogeneous layers, while the more weakly charged PVA analog adsorbs in thicker and more homogeneous films. As revealed by reflectivity measurements, such preadsorbed PVA films are stable in polyelectrolyte-free solutions. However, force measurements with the colloidal probe reveal that their interactions depend strongly on the ionic strength. Upon approach, interactions are dominated by electrostatic diffuse layer overlap forces. Both PVA films have very similar diffuse layer charge densities of about 1.5 mC/m2. Since these values are substantially lower than what would be expected from the total charge of the adsorbed polyelectrolytes measured by reflectivity, we infer that coadsorption of anions represents the principal mechanism in charge neutralization. Upon retraction, the adhesion between the films is dominated by bridging forces due to single polymer chains. Such bridging adhesion becomes progressively important with increasing ionic strength, whereby their range and frequency increase. The work of adhesion due to bridging is about 0.3 mN/m. At low ionic strengths, the films behave differently. While the highly charged PVA shows unspecific adhesion at small distances, the more weakly charged PVA analog shows few adhesion events occurring at long distances.

FTIR-ATR was used to examine in situ the interaction of polyacrylate and hematite at pH 13. Static light scattering and mobility measurements were used to assess solution polyacrylate dimensions and hematite surface charge, respectively. Polyacrylate adsorption occurred only with the addition of electrolyte (e.g., NaCl), and it was found that excess cations, up to approximately 1 M, facilitated adsorption, above which the effect was found to plateau. At pH 13 and at low ionic strength, adsorption of polyacrylate onto hematite is facilitated by cations in solution shielding both the negative acrylate functionality of the polymer and the negative hematite surface. The shielding of the hematite surface continues to increase with increasing salt concentration up to a measured 3 M. Similarly, the shielding of the polymer increased with electrolyte concentration up to approximately 1 M salt, beyond which no further increase in shielding was observed. At this concentration the polymer assumes a finite minimum size in solution that ultimately limits the amount adsorbed. The dimension of the polymer in solution was found to be independent of monovalent cation type. Thus, at high pH and high ionic strength adsorption is determined by the degree of hematite surface charge reduction. The cation-hematite surface interaction was found to be specific, with lithium leading to greater polyacrylate adsorption than sodium, which was followed by cesium. The stronger affinity of lithium for the hematite surface over sodium and cesium is indicative of the inverse lyotropic adsorption series and has been rationalized in the past by the "structure-making-structure-breaking" model. These results provide a useful insight into the likely adsorption mechanism for polyacrylate flocculants at high pH and ionic strength onto residues in the Bayer processing of bauxite.