Advances in Interfacial Adsorption Thermodynamics: Metastable-Equilibrium Adsorption (MEA) Theory

Abstract

Interfacial processes are central to understanding many processes in environmental sciences and technologies, chemical engineering, earth sciences, ocean sciences and atmospheric sciences. Thermodynamics has been used as a classical method to describe interfacial equilibrium properties over the last century. Experimentally measurable macroscopic parameters of adsorption density and concentration are widely used as the basic parameters in many equations/models to describe the equilibrium characteristics of adsorption reactions at solid-water interfaces. For instance, methods of equilibrium adsorption constants or adsorption isotherms are commonly used to describe the equilibrium relationship between concentration in solution and adsorption density on solid surfaces. However, thermodynamics has limitations in describing the equilibrium properties for surface adsorption reactions at solid-water interfaces. A fundamental principle has been missing in the conventional theoretical system where the microscopic structures on the solid surfaces are not taken into account in the conventional macroscopic methodology such as equilibrium adsorption constants and/or adsorption isotherms. The equilibrium properties for surface adsorption were conventionally described by macroscopic parameters such as adsorption density. Unfortunately, adsorption density is not a thermodynamic state variable and is generally affected by the microscopic metastable equilibrium surface structures, which make the equilibrium properties, such as equilibrium constants and/or adsorption isotherms, be fundamentally dependent on the kinetic paths and/or the reactant concentration conditions (e.g. the “adsorbent concentration effect” and “adsorbate concentration effect”). Failure in recognizing this theoretical gap has greatly hindered our understanding on many adsorption related issues especially in applied science and technology fields where the use of surface concentration (mol/m2) is common and inevitable. With the application of spectroscopy and quantum chemical calculation techniques to solidliquid interface systems, such as synchrotron based X-ray absorption spectroscopy, it is now possible to develop new thermodynamic methodologies to describe the real equilibrium properties of surface adsorption reactions and to reveal the relationships between macroscopic equilibrium properties and the microscopic metastable equilibrium adsorption