Adsorption of Polyelectrolytes on Calcium Carbonate - Which Thermodynamic Parameters are Driving This Process?

Polyelectrolytes such as dispersants are frequently used to modify the properties of wet preparations applied in the ceramic industry. The working mechanism of those admixtures relies on adsorption on mineral surfaces. Here, an analysis of the thermodynamic parameters which influence adsorption and effectiveness of two cement and clay dispersants was performed. As polyelectrolytes, linear beta-naphthalenesulfonate formaldehyde (BNS) polycondensate, and comb-shaped alpha-allyl-omega-methoxy poly(ethylene glycol)-maleic anhydride polycarboxylate copolymer (PC) were selected. The standard free energy, enthalpy, and entropy of adsorption on CaCO3 as model substrate were determined at pH 9 and in alkaline CaCl2 solution (pH 12.6 plus 1 g/L Ca2+). From temperature dependent adsorption isotherms the thermodynamic parameters Delta H-0 and Delta S-0 were established. Additionally, the Gibbs free energy of adsorption was calculated. For PC, the gain in entropy resulting from adsorption is consistently higher than for BNS which exhibits a higher release of enthalpy. Thus, adsorption of comb-shaped PC occurs as a result of entropy gain owed to the release of counterions and water molecules whereas linear BNS adsorbs because of strong electrostatic attraction to the CaCO3 surface. Obviously, the specific molecular architecture and anionic charge amount of the polyelectrolytes strongly impact the portion of enthalpic and entropic contribution to adsorption.