Experimental determination of the thermodynamic parameters affecting the adsorption behaviour and dispersion effectiveness of PCE superplasticizers

Abstract

For adsorption of three different allylether-based PCE superplasticizers on CaCO 3 surface, the thermodynamic parameters Δ H, Δ S and Δ G were determined experimentally. The GIBBS standard free energy of adsorption Δ G 0ads, the standard enthalpy of adsorption Δ H 0ads and the standard entropy of adsorption Δ S 0ads applying to an unoccupied CaCO 3 surface were obtained via a linear regression of ln K (equilibrium constant) versus 1 / T (VAN'T HOFF plot). Additionally, the thermodynamic parameters characteristic for a CaCO 3 surface loaded already with polymer (isosteric conditions) were determined using a modified CLAUSIUS–CLAPEYRON equation. For all PCE molecules, negative Δ G values were found, indicating that adsorption of these polymers is energetically favourable and a spontaneous process. Adsorption of PCEs possessing short side chains is mainly instigated by electrostatic attraction and a release of enthalpy. Contrary to this, adsorption of PCEs with long side chains occurs because of a huge gain in entropy. The gain in entropy results from the release of counter ions attached to the carboxylate groups of the polymer backbone and of water molecules and ions adsorbed on the CaCO 3 surface. With increased surface loading, however, Δ G isosteric decreases and adsorption ceases when Δ G becomes 0. The presence of Ca 2+ ions in the pore solution strongly impacts PCE adsorption, due to complexation of carboxylate groups and a reduced anionic charge amount of the molecule. In the presence of Ca 2+, adsorption of allylether-based PCEs is almost exclusively driven by a gain in entropy. Consequently, PCEs should produce a strong entropic effect upon adsorption to be effective cement dispersants. Molecular architecture, anionic charge density and molecular weight as well as the type of anchor groups present in a superplasticizer determine whether enthalpy or entropy is the dominant force for superplasticizer adsorption.