Molecular Weight Dependent Adsorption of Polyacrylic Acid on Porous and Nonporous Alumina Influenced by pH and Surface Accessibility

The adsorption of polymers at solid-liquid interfaces is fundamental to processes such as flocculation, dispersion, and surface modification.While polymer adsorption density typically increases with molecular weight, this study demonstrates that substrate porosity can invert this trend.The adsorption behavior of polyacrylic acid (PAA) with molecular weights of 5000, 150000 and 800000 g mol -1 was investigated on two types of alumina powders: porous Linde A and non-porous AKP-50.Comprehensive characterization using X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), zeta potential measurements, particle size analysis, Brunnauer-Emmett-Teller (BET) nitrogen adsorption, and mercury intrusion porosimetry confirmed the highly porous structure of Linde A and the compact, non-porous nature of Sumjitomo AKP-50.Adsorption experiments performed at pH 4, 7, and 10 revealed a complex interplay between polymer molecular weight, substrate porosity and pH.On non-porous AKP-50, adsorption density increased with molecular weight, consistent with established theory.In contrast, on porous Linde A, the lowest molecular weight PAA exhibited the highest adsorption density, attributed to greater accessibility within narrow pores.This reverse trend was most pronounced at low pH (4), where the polymer chains are coiled.At high pH, where the chains are elongated due to ionization, adsorption density became almost independent of molecular weight, as larger molecules could also enter the pore structure.A correlation between polymer molecular size and alumina pore size distribution was established, and a model of selective pore accessibility is proposed.These findings challenge conventional understanding and highlight the critical influence of substrate morphology and environmental conditions on polymer adsorption, providing valuable insights for the design of tailored polymer-based surface treatments and dispersants.