Understanding the structure of adsorbed layers, including their composition (the mole fraction of sorbate, xA) and thickness (dal), is of great significance for revealing the nature of adsorption and guiding its applications. Many techniques have been used to estimate the structure of adsorbed layers of organics at solid/liquid interfaces. However, there is still a lack of feasible thermodynamic models to describe the correlation between the structure (more precisely, xA and dal) and the equilibrium adsorption amount (Γe). Herein, a thermodynamic model, called the dynamic bonding equilibrium (DBE) model, was developed on the basis of the adsorption equilibrium thermodynamics with an assumption that, at adsorption equilibrium, the sorbate and solvent within the adsorbed layer both exist in different bonding states. The DBE model relates xA and dal with Γe and thus can predict or describe the structure (xA and dal) of adsorbed layers from Γe. Its rationale was confirmed by the literature-reported adsorption data of organics, including surfactants, proteins, and polymers, on hydrophilic and hydrophobic surfaces in water. This work provides a feasible approach for obtaining information about the structure of adsorbed layers at solid/liquid interfaces.
A Model for the Structure of Adsorbed Layers at Solid/Liquid Interfaces.