Modeling the Adsorption Behavior of Linear End-Functionalized Poly(ethylene glycol) on an Ionic Substrate by a Coarse Grained Approach

ABSTRACT. The rheology of cement pastes can be controlled by polymeric dispersants such as branched polyelectrolytes that adsorb on the surfaces of silicate particles. In the present work, we analyze the adsorption behavior of ad hoc-prepared end-carboxylated poly(ethylene glycol), or PEG, on CaCO3 particles as a model of cement in an early hydration stage. The experimental adsorption isotherms form the base of a theoretical study aimed at unraveling polymer conformational aspects of adsorption. The study was carried out with Monte Carlo simulations using a coarse-grained bead-andspring model of linear end-charged polymer chains adsorbing on a flat, continuous, uniformly charged surface. The adsorption driving force was introduced by a Debye-H€uckel electrostatic screened potential to describe the interaction between the negatively charged end group of PEG and the positively charged CaCO3 surface empirically. With a suitable length-scale conversion between real PEG and the coarse-grained model, the calculated and experimental adsorption isotherms can be semiquantitatively compared. The theoretical results reproduce the fundamental aspects of polymer adsorption, in essential agreement with analytical approaches relating the isotherm shape to the polymer conformational properties. The conformational transition mushroom-brush of the adsorbed polymer is located on the isotherm and is related to the molecular shape. The solvent quality effect and the solution ionic strength are also considered, and their implications on the isotherms are discussed.