Thermodynamics of adsorption on nanocellulose surfaces

Understanding the thermodynamic interactions of cellulose nanomaterials with their environment is important to understand the forces behind their self-organization and co-organisation with other compounds, and to be able to use self-assembly to form new functional multicomponent materials. This review analyzes published studies that determined the thermodynamic parameters of the surface interactions of and adsorption of various compounds (proteins, polymers, and small molecules/ions) onto cellulose nanomaterials. We compiled the data reported and performed a meta-analysis for better comparison and to find trends in the published data. We first introduce the methods employed and describe the adsorption isotherm models typically used to describe the adsorption thermodynamics on nanocellulose surfaces. We then discuss and analyze the published results for the interaction of the various compounds with nanocellulose surfaces. The systems that have been reported on most were adsorption of natural binding proteins and various pollutants from water, such as heavy metal ions, dyes, and drugs. Interactions between cellulose surfaces and the cellulose binding module were generally both enthalpy- and entropy-driven, where the negative binding enthalpy indicates the formation of specific interactions between peptides and the carbohydrate backbone. On the other hand, interactions with charged molecules were mostly endothermic and purely entropy-driven, indicating that the adsorption on nanocellulose surfaces can be described as an interaction between opposite charges, where the entropy increase that arises from the release of surface-structured water molecules and counterions from the electronic double layer supplies the major contribution to the free energy of adsorption. We performed a meta-analysis on all published data, and found a linear relationship between ∆H and ∆S with the slope equal to the reference temperature, irrespective of whether the interacting compound is a specific cellulose binding protein, a non-specific binding protein, a polymer, or a small molecule/ion. This indicates that the process of adsorption is the same for all compounds and takes place with a constant change in Gibbs free energy of interaction, ∆G, where a change in interaction enthalpy is offset by change in entropy change upon binding and vice versa.