Specific ion effects in the adsorption of carboxymethyl cellulose on cellulose: The influence of industrially relevant divalent cations

The adsorption of carboxymethylcellulose (CMC) on cellulose surfaces is of relevance from both academic and industrial perspectives as it facilitates resource-efficient modification of cellulose fibres that allows them to carry negative charges. It is known that, compared to monovalent ions, Ca²⁺ ions are superior ions in facilitating CMC adsorption and the subsequent introduction of charge on cellulose fibres. However, the formation and deposition of calcium oxide involved in this process necessitates the search for alternative cations. Magnesium ions form one of the more promising candidates since they are already used in the pulping process to prevent cellulose degradation during peroxide bleaching. This work aims at elucidating the effects of the industrially relevant alkaline earth metal divalent cations Mg²⁺ and Ca²⁺ on the CMC adsorption process onto cellulose surfaces. Quartz Crystal Microbalance (QCM-D) technology was used to follow the adsorption in model systems in real time, whereas the adsorption of CMC on commercial fibres was studied using polyelectrolyte titrations, total organic carbon (TOC) analysis and conductometric titrations. This study shows that the presence of Ca²⁺ ions was more favourable for the adsorption of CMC to both types of cellulosic surfaces than Mg²⁺ ions. The distinction in the adsorption behaviour in the presence of Mg²⁺ and Ca²⁺ is suggested to be due to the differences in the polarizability of the ions. The findings are decisive in designing efficient industrial processes for the adsorption of polyelectrolytes to cellulose surfaces of similar charge.