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For adsorption of three different allylether-based PCE superplasticizers on CaCO3 surface, the thermodynamic parameters ΔH, ΔS and ΔG were determined experimentally. The GIBBS standard free energy of adsorption ΔG0ads, the standard enthalpy of adsorption ΔH0ads and the standard entropy of adsorption ΔS0ads applying to an unoccupied CaCO3 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 CaCO3 surface loaded already with polymer (isosteric conditions) were determined using a modified CLAUSIUS–CLAPEYRON equation. For all PCE molecules, negative ΔG 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 CaCO3 surface. With increased surface loading, however, ΔGisosteric decreases and adsorption ceases when ΔG becomes 0. The presence of Ca2+ 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 Ca2+, 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. |
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